África by Regina Zaghi

África

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La historia del continente: lo que la llevó a la actualidad Secretos de la biogeografía del pasado y presente Análisis de los componentes que hacen tan famoso al continente hoy Recopilación y edición por Regina Zaghi Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com).

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Introducción La forma en que se observa la vida en el continente africano en la actualidad se debe a una serie de eventos geológicos, climáticos y de otra índole que han ido modificando al continente a lo largo de su historia. En esta revista podremos tener un acercamiento a distintos factores que han sido importantes y ayudan al entendimiento de la biogeografía actual del continente africano.

Índice Drainage evolution in south-central Africa since the breakup of Gondwana por A.E Moore y P.A. Larkin PDF-Editor (http://www.cadkas.com). Página 3 Changed with the DEMO VERSION of CAD-KAS

Mid-Holocene and glacia-maximum vegetation geography of the northern continents and Africa por C. Prentice et al. Página 26

Tectonics and human evolution por G. King y G. Bailey

Página 39

Mammalian biodiversity on Madagascar controlled by ocean currents por J. Ali y M. Huber Página 61

Integración de los factores que forman la biodiversidad Africana de la actualidad por R. Zaghi Página 64

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Journal of Biogeography, 27, 507– 519

Mid-Holocene and glacial-maximum vegetation geography of the northern continents and Africa

Blackwell Science, Ltd

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I. Colin Prentice1, Dominique Jolly1,2,3 and BIOME 6000 participants* 1Max Planck Institute for Biogeochemistry, Postfach 100164, D-07701 Jena, Germany, 2Dynamic CAD-KAS PDF-Editor (http://www.cadkas.com). Palaeoclimatology, Lund University, Box 117, S-221 00 Lund, Sweden and 3School of Ecology, Lund University, Ecology Building, S-223 62 Lund, Sweden

Abstract

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BIOME 6000 is an international project to map vegetation globally at mid-Holocene (6000 14C yr bp) and last glacial maximum (LGM, 18,000 14C yr bp), with a view to evaluating coupled climate-biosphere model results. Primary palaeoecological data are assigned to biomes using an explicit algorithm based on plant functional types. This paper introduces the second Special Feature on BIOME 6000. Site-based global biome maps are shown with data from North America, Eurasia (except South and Southeast Asia) and Africa at both time periods. A map based on surface samples shows the method’s skill in reconstructing present-day biomes. Cold and dry conditions at LGM favoured extensive tundra and steppe. These biomes intergraded in northern Eurasia. Northern hemisphere forest biomes were displaced southward. Boreal evergreen forests (taiga) and temperate deciduous forests were fragmented, while European and East Asian steppes were greatly extended. Tropical moist forests (i.e. tropical rain forest and tropical seasonal forest) in Africa were reduced. In south-western North America, desert and steppe were replaced by open conifer woodland, opposite to the general arid trend but consistent with modelled southward displacement of the jet stream. The Arctic forest limit was shifted slighly north at 6000 14C yr bp in some sectors, but not in all. Northern temperate forest zones were generally shifted greater distances north. Warmer winters as well as summers in several regions are required to explain these shifts. Temperate deciduous forests in Europe were greatly extended, into the Mediterranean region as well as to the north. Steppe encroached on forest biomes in interior North America, but not in central Asia. Enhanced monsoons extended forest biomes in China inland and Sahelian vegetation into the Sahara while the African tropical rain forest was also reduced, consistent with a modelled northward shift of the ITCZ and a more seasonal climate in the equatorial zone. Palaeobiome maps show the outcome of separate, independent migrations of plant taxa in response to climate change. The average composition of biomes at LGM was oftenPDF-Editor markedly different from today. Refugia for the temperate deciduous and tropical CAD-KAS (http://www.cadkas.com). rain forest biomes may have existed offshore at LGM, but their characteristic taxa also persisted as components of other biomes. Examples include temperate deciduous trees that survived in cool mixed forest in eastern Europe, and tropical evergreen trees that survived in tropical seasonal forest in Africa. The sequence of biome shifts during a glacial-interglacial cycle may help account for some disjunct distributions of plant taxa. For example, the now-arid Saharan mountains may have linked Mediterranean and African tropical montane floras during enhanced monsoon regimes.

Correspondence: Professor I. C. Prentice, Max Planck Institute for Biogeochemistry, Postfach 100164, D-07701 Jena, Germany. E-mail: colin.prentice@bgc-jena.mpg.de *Members of the BIOME 6000 Project who contributed data or analysis for this paper: Afanas’eva, N. B., Ager, T. A., Anderson, K., Anderson, P. M., Andrieu, V., Andreev, A. A., Ballouche, A., Bartlein, P. J., de Beaulieu, J. L., Bengo, M., Berezina, N. A., Bezusko, L. G., Bezusko, T. V., Bigelow, N. H., Blyakharchuk, T. A., Bolikhovskaya, N. S., Bonnefille, R., Bottema, S., Brénac, P., Brubaker, L. B., Buchet, G., Burney, D., Bykova, G. V., Cheddadi, R., Chen, X., Chernavskaya, M. M., Chernova, G. M., Cwynar, L. C., Dorofeyuk, N. I., Dirksen, V. G., Edorh, T., Edwards, M. E., Eisner, W. R., Elenga, H., Elina, G. A., Elmoutaki, S., Filimonova, L. V., Glebov, F. Z., Guiot, J., Gunova, V. S., Hamilton, A. C., Han, H., Harrison, S. P., Hu, F.-S., Huang, C., Huntley, B., Jolly, D., Jonson, H., Ke, M., Khomutova, V. I., Kong Z., Kvavadze, E. V., Laarif, F., Lamb, H. E., Lézine, A.-M., Li, S., Li, W., Liew, P., Liu, G., Liu, J., Liu, Q., Liu, K.-B., Lozhkin, A. V., Maley, J., Marchant, R., Mbenza, M., MacDonald, G. M., Miyoshi, N., Mock, C. J., Morita, Y., Newby, P., Ni, J., Osipova, I. R., Panova, N. K., Perez-Obiol, R., Peyron, O., Prentice, I. C., Qiu, W., Reille, M., Ren, G., Reynaud-Farrera, I., Richard, P. J. H., Riollet, G., Ritchie, J. C., Roche, E., Saarse, L., Scott, L., Sevastyanov, D. V., Sher, A. V., Song, C., Spear, R. W., Ssemmanda, I., Straka, H., Sugita, S., Sun, X., Takahara, H., Tang, L., Tarasov, P. E., Taylor, D., Thompson, R. S., Uchiyama, T., Van Campo, E., Vilimumbalo, S., Vincens, A., Volkova, V. S., Waller, M., Webb, T., III, Williams, J. W., Xia, Y., Xu, Q., Yan, S., Yang, X., Yu, G., Zernitskaya, V. P., Zhao, J., Zheng, Z.

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Major changes in physical land-surface conditions, shown by the palaeobiome data, have implications for the global climate. The data can be used directly to evaluate the output of coupled atmosphere-biosphere models. The data could also be objectively generalized to yield realistic gridded land-surface maps, for use in sensitivity experiments with atmospheric models. Recent analyses of vegetation-climate feedbacks have focused onPDF-Editor the hypothesized positive feedback effects of climate-induced vegetation changes in CAD-KAS (http://www.cadkas.com). the Sahara/Sahel region and the Arctic during the mid-Holocene. However, a far wider spectrum of interactions potentially exists and could be investigated, using these data, both for 6000 14C yr bp and for the LGM. Keywords Pollen data, plant functional data, plant functional types, biomes, vegetation distribution, vegetation changes, biogeography, climate change, land-surface characteristics, mid-Holocene, last glacial maximum.

all continents. BIOME 6000 is distinguished from previous palaeoecological data compilations by its global scope, combined with its use of a standardized, objective method BIOME 6000 (Prentice & Webb, 1998) is a data synthesis (biomization) based on plant functional types (PFTs: Steffen project sponsored by the International Geosphere-Biosphere et al., 1992; Prentice et al., 1996) to assign palaeoecological Programme (IGBP). The project came into being to support data (pollen or plant macrofossil records) to biomes. Results the ‘6000 yr bp experiment’, an initiative of the IGBP Task are presented in the form of maps in which every site is shown Force on Global Analysis, Interpretation and Modelling and can be traced to an original pollen or macrofossil count, (GAIM). The 6000 yr bp experiment aims to use palaeodata or (if unavoidable) a record digitized from a pollen diagram. from the mid-Holocene as a benchmark to evaluate simulations This approach eliminates the subjectivity inherent in more with coupled climate-biosphere models and thus to assess the traditional palaeogeographic map reconstructions. The idea extent of biogeophysical (vegetation-atmosphere) feedbacks is to compile data of high quality whose origin and validity in the global climate system (Anonymous, 1994). The success can always be checked, and which can be built on further as of this initiative depends on global vegetation data for the new data become available. mid-Holocene being available, in a form allowing direct This paper introduces the second Journal of Biogeography comparison with model output. The primary aim of BIOME Special Feature devoted to BIOME 6000. Papers presented 6000 is, accordingly, to map global biome distributions at in the first Special Feature (Prentice & Webb, 1998) estabthe mid-Holocene (6000 ± 500 14C yr bp). lished the practicality and robustness of the biomization GAIM also envisages model experiments focused on the technique, first developed and tested for Europe (Prentice last glacial maximum (LGM), especially with regard to underet al., 1996), when applied to pollen and plant macrofossil standing the natural changes in atmospheric composition data from poor (e.g. Siberia) to rich (e.g. tropical Africa) between glacial and interglacial periods (Anonymous, 1994). DEMO VERSION CAD-KAS PDF-Editor (http://www.cadkas.com). floras and including Arctic, oceanic and continental midA secondary aim ofofthe BIOME 6000 project is therefore to latitude, arid to moist subtropical, and wet- and dry-tropical create a similar map for the LGM (18,000 ± 2000 14C yr bp). climates (Jolly et al., 1998a; Tarasov et al., 1998; Yu et al., The mid-Holocene and LGM are the two key time periods 1998). In each of these regional studies, the method was first adopted by the Palaeoclimate Modelling Intercomparison applied to modern (surface pollen sample) data and shown Project, PMIP ( Joussaume & Taylor, 1995). PMIP and GAIM to successfully reconstruct the modern biome distribution. modelling studies are based on the premise that the midThen the method was applied without change to 6000 14C Holocene (taken for modelling purposes as 6000 cal yr bp) represents an ‘orbital forcing experiment’, with perihelion in yr bp data and found to generate spatially coherent and northern summer/autumn and greater-than-present axial tilt, plausible reconstructions of biome distribution for that time. but free of major ice-sheet and CO2 effects. The LGM (taken The study for China (Yu et al., 1998) was a pilot study based on a subset of the available pollen records and taxa, relying for modelling purposes as 21,000 cal yr bp) represents by mainly on digitized pollen records. The other studies, for Africa contrast primarily an experiment on the effects of enlarged (Jolly et al., 1998a) and the former Soviet Union eastward ice sheets and low atmospheric CO2. to ≈ 130°E (Tarasov et al., 1998), were comprehensive and BIOME 6000 has provided a unique opportunity for based largely or entirely on complete taxon lists. palaeoecologists to work together towards a globally comPapers in this second Special Feature extend the geographprehensive documentation of the response of the terrestrial ical coverage of the 6000 14C yr bp reconstructions, and biosphere to specific climate changes in the past. The project is community-wide and engages palaeoecologists from equip all of the regions studied at 6000 14C yr bp with INTRODUCTION

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regions. The individual papers should be consulted for detailed biome reconstructions for the LGM. The papers featured comparisons between these patterns and modern vegetation here deal with southern Europe and Africa at LGM (Elenga distribution in each region. Taken together, these modern et al., 2000), a new and comprehensive analysis for China at vegetation reconstructions convincingly recreate the broad 6000 14C yr bp and LGM (Yu et al., 2000), 6000 14C yr bp features seen in any modern vegetation map. Surface data and LGM reconstructions for Japan (Takahara et al., 2000), are, however, sparse or absent in a few regions: part of eastern an extension of the Tarasov et al. (1998) 6000 14C yr bp DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). Siberia and the Russian Far East; the African deserts, and study to cover northern Eurasia at LGM (Tarasov et al., 2000), the miombo (a type of tropical dry forest) and equatorial and 6000 14C yr bp and LGM reconstructions for three ‘New rain forest regions of Africa. World’ biogeographic regions: Beringia (Russian Federation Occasional samples from tundra regions, especially from east of 130°E, plus Alaska and the Mackenzie Delta region; islands in the high Arctic (e.g. Banks Island, Svalbard, Björnöya), Edwards et al., 2000), the western USA (Thompson & Anderson, are misclassified as taiga or cold deciduous forest due to the 2000), and eastern North America and Canada (Williams et al., weak local pollen signal being overwhelmed by transport of 2000). This scheme for dividing up the continents is based tree pollen types from distant forest regions (Prentice, 1988). partly on practical considerations (e.g. existing co-operative In the areas of most intense cultivation (eastern China, westprojects) and partly on biogeographical considerations—for ern Europe, eastern USA), a proportion of samples from example, the overlap of flora between Alaska and eastern forest biomes are misclassified into nonforest biomes (mainly Siberia, and the distinctness of the flora of the western USA desert and steppe). Nevertheless, most samples even in these from that of the rest of North America. regions are assigned to the correct (forest) biome, confirming These papers, together with those published in the first the method’s robustness against all but the most intensive Special Feature, provide 6000 14C yr bp and LGM coverhuman impact (Prentice et al., 1996). age for Africa, Eurasia (apart from South-east Asia and the Indian subcontinent), and North America southward to the US-Mexican border. They document an evolution of THE PALAEOVEGETATION MAPS AND THEIR the methodology, with an increasing focus on obtaining PALAEOCLIMATIC SIGNIFICANCE primary data and using the information in minor taxa that may have important indicator value for biomes (especially Last glacial maximum when distinguishing nonforest biomes where pollen records The map for LGM (Fig. 2) illustrates a ‘glacial world’ that are typically dominated by a few pollen taxa of broad differs radically from that of today. The most obvious features ecological tolerance, such as Artemisia and Poaceae), and are the equatorward regression of forest types in North progress by iteration towards a global set of PFTs that America and Eurasia (in the unglaciated eastern part of will ultimately be useful for biome modelling as well as for Eurasia, as well as in Europe) and compression and fragreconstructing past biomes. Another important development mentation of the forest zones in these regions. The boreal is the use of plant macrofossil records as a major information evergreen forest (taiga) occupied a far smaller area than source. Charcoal records were already included in the today and temperate deciduous forest was very restricted. earlier African study by Jolly et al. (1998a) and tree megaExisting forest types were ousted from southern China and fossils were included to document Arctic treeline regression the south-eastern USA: there are only a few LGM records since 6000 14C yr bp by Texier et al. (1997). A more extensof broadleaved evergreen/warm mixed forests in the southive use of plant macrofossils is provided here by Thompson & eastern USA, and no LGM records of tropical forests in Anderson (2000), who use packrat midden assemblages as southern China. Only a few regions show the same biome at a major data source in addition to pollen data to reconstruct DEMO VERSION PDF-Editor (http://www.cadkas.com). LGM as today. One such region is central Asia; even there, vegetation changes inof theCAD-KAS arid south-western USA. however, Tarasov et al. (2000) point out that the persistence This introductory paper represents a preliminary synthesis, of the steppe biome hides changes in the floristic composibased on the combined results of the BIOME 6000 project tion of the steppe. In the most general climatic terms, these to date. We consider the potential significance of these results shifts document the response of the terrestrial biosphere to a from several standpoints: as a simple record of biogeographic large year-round reduction in temperature, relative to the present. shifts; as documentation of vegetation’s long-term response The biome shifts also provide evidence for drier conditions to climate change; as a historical background to understandthan present across large areas of the mid-latitudes. In central ing the shifting geographical ranges of plant taxa in the face Asia and Siberia tundra encroached much further south than of a changing climate; and as data for the evaluation and today (Edwards et al., 2000), but further large areas of what improvement of approaches to modelling climate change, as is now forest were occupied by a kind of steppe (Tarasov envisaged in the GAIM 6000 yr bp experiment. et al., 2000). As suggested by palaeoclimate-biome model experiments (e.g. Prentice et al., 1993), it appears that tundra RECONSTRUCTION OF PRESENT-DAY and steppe at LGM had a long common frontier or interVEGETATION PATTERNS gradation zone in Eurasia. This is in contrast with today, where tundra-steppe intergradation only occurs in a few very Surface samples provide a way to test the validity of the dry regions, such as along elevation gradients in central Asia method used to reconstruct past biomes. Figure 1 combines and Alaska. We have avoided using the term ‘steppe-tundra’, results obtained with surface pollen samples for each of the © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507–519

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Figure 1 Present-day biomes reconstructed from surface pollen samples. The biomes are plotted in a globally consistent order. In regions characterized by complex vegetation patterns, overprinting of some sites at the small scale of these maps may result in small apparent differences with the larger-scale regional maps.

but this concept is given some support by the existence of an

do tundra samples from Alaska today or in the Holocene

et al. (2000) show that the position of the reconstructed boundary between steppe and tundra is somewhat sensitive to the classification of certain shrub and forb taxa that are treated differently in the Beringia study (Edwards et al., 2000); further work is required to establish a universal circumpolar classification scheme. Nevertheless, the LGM vegetation in Beringia was unambiguously tundra (Elias et al., 1997; Edwards et al., 2000) while the LGM vegetation in central Asia and southern and south-eastern Europe (Tarasov et al., 2000; Elenga et al., 2000) was clearly steppe. Affinity scores for the two biomes, as used in the biome assignment algorithm, confirm that there was a gradient from tundra to steppe. For example, in Europe the more northerly LGM samples (from France) assigned to steppe also have relatively high scores for tundra (Elenga et al., 2000). In Alaska, where the LGM samples are consistently assigned to tundra, these samples nevertheless have higher scores for steppe than

Temperate deciduous forests in Europe, North America and Asia were ‘squeezed’ not only by the southward encroachment of more cold-tolerant or cold-requiring forest types, but also by the extension of nonforest vegetation (Elenga et al., 2000; Tarasov et al., 2000; Yu et al., 2000; Williams et al., 2000). This phenomenon is most marked in China, where steppe and even desert extended eastward to the modern coastline in what is now the temperate deciduous forest belt, suggesting a strongly reduced East Asian summer monsoon at LGM (Yu et al., 2000). The equatorial region and the southern hemisphere are represented here only by Africa. Although the available LGM data from tropical Africa are sparse, they too document not only cooling (as shown by the downward elevational shift of broadleaved evergreen/warm mixed forest) but also drying (as shown by the encroachment of steppe into regions now occupied by tropical forests) (Elenga et al., 2000). In southern

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Figure 2 Reconstructed biomes at LGM. The biomes are plotted in a globally consistent order. In regions characterized by complex vegetation patterns, overprinting of some sites at the small scale of these maps may result in small apparent differences with the larger-scale regional maps.

Africa, the data similarly show xerophytic woods/scrub being

1997). This effect does not, however, eliminate reductions

The one striking exception to this pattern of drying is provided by the south-western USA, where a modelled southward displacement of the jet stream in response to the presence of the Laurentide ice sheet can explain high lake levels and the observed presence of open conifer woodlands in regions that are steppe or desert today (e.g. COHMAP Members, 1988; Thompson et al., 1993). A similar though less pronounced jet-stream displacement may have occurred in Europe (Kutzbach & Guetter, 1986; Harrison et al., 1992). However, while lake levels in southern Europe were high, the vegetation was uniformly steppe—suggesting that summer precipitation, at least, was insufficient to sustain forest (Prentice et al., 1992). Low atmospheric CO2 concentration at LGM may have contributed to low plant-available moisture (PAM) by reducing plant water-use efficiency (Farrera et al., 1999) and may have played a role in the lowering of tropical treelines during the glacial period ( Jolly & Haxeltine, 1997; Street-Perrott et al.,

Indeed such reductions are a common feature of model simulations of the LGM climate for the low and mid-latitudes (Pinot et al., 1999), except in those regions (most importantly the south-western USA) where the data show increased PAM and models show increased precipitation. Thus, the broad-scale biome shifts shown by the data can plausibly be taken to reflect a climate with temperatures and precipitation lower than present over most of the northern hemisphere.

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Mid-Holocene In the northern circumpolar region, the mid-Holocene data show forests (taiga, cold deciduous forest) extended polewards at the expense of tundra, indicating greater-than-present growingseason warmth, in the Mackenzie Delta region (Canada) (Ritchie, 1985, 1987; MacDonald, 1995), the European Arctic (Hyvärinen, 1976) and western and central Siberia (Texier

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Figure 3 Reconstructed biomes at 6000 yr bp. The biomes are plotted in a globally consistent order. In regions characterized by complex vegetation patterns, overprinting of some sites at the small scale of these maps may result in small apparent differences with the larger-scale regional maps.

et al., 1997; Tarasov et al., 1998) (Fig. 3). However, this shift Explanations of the 6000 14C yr bp climate will thus was at most 200 –300 km. In the Mackenzie Delta region, have to account for a strong asymmetry of the circumpolar DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). the shift has been estimated as only about 25 km (MacDonald, warming, perhaps reflecting a changed ocean circulation in 1995). The shift appears relatively slight when viewed on the Arctic Basin. Accurate representation of the forest–tundra global maps. Furthermore it is not symmetrical around the boundary is itself important for climate modelling because pole. There was no discernible northward treeline shift in even a slight shift can have substantial consequences for Alaska despite the relatively high density of sites that would the northern-hemisphere energy balance (Foley et al., 1994; allow such a shift to be detected, if it had occurred (Edwards TEMPO, 1996). The shifts of the Arctic treeline between et al., 2000). There is no clear evidence for a northward shift 6000 14C yr bp, although slight in terms of distance, may in Keewatin, although the data from that region are too therefore have been significant in terms of climate feedbacks. sparse to allow certainty on this point (MacDonald, 1995; Our ability to model these effects today is limited. Williams et al., 2000). In Québec-Labrador, treeline was slightly The general pattern across the northern mid-latitudes of south of its present position (Richard, 1995; Williams et al., Eurasia is one of poleward shifts of the forest belts. Although 2000). In eastern Siberia, where the Arctic treeline is formed by the distances involved are small compared to the changes cold decidous forest, low pollen production by the dominant between LGM and present, many of the shifts are far more tree taxon (Larix) makes the pollen-based definition of treeline dramatic than the change in the position of the Arctic treeline uncertain (Edwards et al., 2000). Nevertheless the forest and between 6000 14C yr bp and present. In many cases these tundra can be distinguished at a broad-scale even in this shifts implicate warmer winter conditions at 6000 14C yr bp region, and there is no clear evidence for the forest being even though the orbital forcing alone would tend in the further north than present at 6000 14C yr bp. direction of colder winters (Kutzbach & Guetter, 1986). For © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507– 519

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Digerfeldt, 1993; Yu & Harrison, 1996), isotopic records (Araus example, the northward and eastward extension of temperate et al., 1997) and the palaeosalinity record of the Mediterradeciduous forest in Europe requires that winters were too nean Sea itself (Kallel et al., 1997). However, the underlying warm to satisfy the winter-chill requirements of boreal climatic mechanisms are not understood. Climate model conifers (Huntley & Prentice, 1993). Similarly, the slight northsimulations generally have produced wetter winters but ward and upward extension of the broadleaved evergreen/ not summers in southern Europe at 6000 yr bp (Harrison warm mixed forest limit in China implies that broadleaved DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). et al., 1998), whereas this vegetation shift requires increased evergreen trees at this latitude were subject to less frequent PAM at 6000 14C yr bp. Increased winter precipitation in killing frosts than they would be at the same latitude today (Yu et al., 1998, 2000). most of the Mediterranean-climate area would accrue to The paradox of warm mid-Holocene winters has been runoff, but would not increase PAM (Prentice et al., 1993). extensively discussed for Europe (Harrison et al., 1992; This problem is also relevant to future trends because many Huntley & Prentice, 1993; Cheddadi et al., 1997; Masson climate model simulations with increased CO2 show increasing et al., 1998; Prentice et al., 1998) and various mechanwinter precipitation in the Mediterranean region accomisms have been proposed to account for it. Climate models panied by summer drought (Kattenberg et al., 1996), just as however, have failed to produce any (or a strong enough) they show for the mid-Holocene (Hewitt & Mitchell, 1996). winter warming in Europe (Harrison et al., 1998). For The largest changes of all at 6000 14C yr bp are seen in China, Sun & Chen (1996) and Yu et al. (1998, 2000) have the monsoon regions, above all in northern Africa, where proposed that the changes reflect a reduced strength of the the Sahara desert was drastically reduced and the Sahelian East Asian winter monsoon. In Japan the situation is more vegetation belts (i.e. steppe, xerophytic woods/scrub, tropical complicated because where broadleaved evergreen/warm dry forest) shifted systematically northwards (Jolly et al., mixed forests grow today, the dominant vegetation at 1998a). The basic mechanism of monsoon amplification due 6000 14C yr bp was temperate conifer forest. Takahara et al. to early to mid-Holocene orbital forcing is well established (Kutzbach & Street-Perrott, 1985; Kutzbach & Guetter, (2000) interpret this difference as a consequence of summer 1986; COHMAP Members, 1988; de Noblet et al., 1997) but warming creating a more seasonal climate, close to that of the further positive feedback mechanisms, involving land-surface small areas where temperate conifers such as Cryptomeria changes (Street-Perrott et al., 1990; Kutzbach et al., 1996; thrive now. In eastern North America, by contrast, the Broström et al., 1997; Claussen & Gayler, 1997; Texier et al., shifts of the forest belts are in a direction suggesting winters 1997) and/or sea-surface changes (Kutzbach & Liu, 1997; similar to or colder than present (Prentice et al., 1991; Webb Hewitt & Mitchell, 1998), need to be invoked in order to et al., 1993; Williams et al., 2000). Thus, there is apparently account for the magnitude of the biome shifts in Africa a circumpolar asymmetry also in the response of the winter(Harrison et al., 1998; Braconnot et al., 1999; Joussaume time atmospheric circulation to orbital forcing at 6000 14C et al., 1999). yr bp, manifested in qualitatively different biome shifts on Africa was not uniformly wetter than present at 6000 14C the different continents. In the mid-continents, there is some evidence of drying yr bp. The (admittedly limited) data from equatorial Africa associated with the expanded distribution of steppe at the show that the area of the tropical rain forest biome in Africa expense of forest in the Great Lakes region and the western (excluding possible offshore locations at LGM) may be greater interior of Canada (Prentice et al., 1991; Webb et al., 1993; now than it was either at LGM or at 6000 14C yr bp Williams et al., 2000). Once again, however, the hemispheric (Jolly et al., 1998a, b). At 6000 14C yr bp, the contracted pattern is asymmetrical: south-eastern Europe and central rain forest area suggests a more seasonal precipitation climate. Asia show the opposite trend, with forest biomes encroaching Model simulations predict a more seasonal climate as a DEMO of CAD-KAS PDF-Editor on theVERSION present-day steppe (Tarasov et al., 1998). Thus, it(http://www.cadkas.com). may consequence of a more northerly location of the intertropical be that the shift of the steppe–forest boundary in North America convergence zone (ITCZ) during northern hemisphere summer is not a simple consequence of warm summers leading to (Harrison et al., 1998; Joussaume et al., 1999). The reduction higher evaporation rates and reduced soil moisture, as one of tropical rain forest may therefore be dynamically linked to might guess, but rather a reflection of a global atmospheric the expansion of moisture-demanding biomes further north circulation shift. Alternatively, the direct evaporation effect (Prentice & Sarnthein, 1993; Mommersteeg et al., 1995). may be overridden in Eurasia because of the importance of monsoonal circulations in bringing moisture into the midIMPLICATIONS FOR PALAEOECOLOGY AND continent. This problem bears on the question of possible PHYTOGEOGRAPHY future trends in mid-continental aridity (Kattenberg et al., 1996) and requires further analysis to isolate the various The palaeobiome maps presented here raise questions about possible mechanisms involved. the ecological mechanisms that have allowed plant taxa to The circum-Mediterranean region shows temperate persist in the face of climate changes on the time scale of deciduous forest encroaching southward and xerophytic woods/ glacial-interglacial cycles. One of the most important conscrub reduced in extent at 6000 14C yr bp, suggesting a tributions of Quaternary palaeoecology to evolutionary biology has been the perspective that plant species must migrate if moister climate than today (Huntley & Prentice, 1993). Prothey are to survive climate change; this migration has the effect gressive aridification of this region since 6000 14C yr bp of mixing and homogenizing gene pools, and therefore opposes is also shown by some lake-level records (Harrison & © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507–519

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eastern Europe (Tarasov et al., 2000). Similarly, a number rather than promotes speciation (Bartlein & Prentice, 1989; of tropical rain forest taxa persisted in tropical seasonal Huntley & Webb, 1989; Bennett, 1997). Palaeoecologists have forest through the LGM and Holocene in equatorial also demolished the notion that ecological communities can Africa (Elenga et al., 2000). be considered as ancient, coevolved entities, since climate Consideration of the time sequence from LGM through change evokes differential responses in the constituent taxa 6000 14C yr bp to present prompts further speculations about so that the taxonomic composition of biomes during one DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). climatic phase can be very different from that during another the distributional history of plant taxa. For example, a phase (Colinvaux, 1987; Davis, 1990; Prentice, 1992). Several long-standing phytogeographic puzzle (Quézel & Barbero, papers in this issue (Edwards et al., 2000; Williams et al., 1993) concerns species that are common to the floras of 2000; Elenga et al., 2000) demonstrate this point graphically African tropical mountains and the Mediterranean region. with respect to the composition of biomes at LGM. Even in The classic example is Erica arborea (Bruneau de Mire terms of the contributing plant functional types, it can be shown & Quézel, 1959; Quézel, 1978). Similarities between these that biomes do not maintain a fixed composition through regions in terms of vegetation structure and function are time; the same applies a fortiori to the species that make up easy to understand: African montane and Mediterranean the functional types. environments today are characterized by the occurrence of The data presented here underline these dynamic perspectthe broadleaved evergreen/warm mixed forest and xerophytic ives on phytogeography. Biomes are mapped on the basis of woods/scrub biomes, in similar bioclimates. However, it the taxonomic composition of palaeoecological communities, is less obvious how the respective floras could have been but the biome assignment is an emergent property based on connected in the past, since they are separated today by functional properties. Two assemblages, even in the same thousands of kilometres of desert. A key to this puzzle is biogeographic province, need not have any taxa in common provided by the palaeodata, indicating that earlier in the in order to be assigned to the same biome. This logical sepHolocene conditions in the Sahara region were far more moist aration of biomes and taxa is important to consider when than present and in particular that the Saharan mountains evaluating possible refugia for taxa during periods that supported woodland or scrub, including typically Mediterranean are unfavourable to particular biomes. Two biomes that are taxa such as Quercus ilex and Pistacia spp. as well as Erica seemingly absent (or nearly so) from the regions covered by spp. (Wickens, 1984), as recently as 6000 14C yr bp ( Jolly Fig. 2 at LGM are temperate deciduous forest and tropical et al., 1998a). This climatic history may also explain the rainforest; this raises the question of where the constituent occurrence today of species apparently of Mediterranean taxa were able to persist. Several possibilities exist and are origin, such as the endemic Cupressus dupreziana, in the not mutually exclusive. Saharan mountains (Quézel & Barbero, 1993). 1 These biomes existed somewhere on the present land In general, favourable conditions for the existence of surface, but palaeoecologists have not found sites in the broadleaved evergreen trees in the present Sahara desert right place. This possibility cannot be ruled out completely could have existed for millenia during every climatic cycle but it becomes less likely as more data are collected. The during those periods where orbital forcing caused strong broad-scale distributions of taxa and biomes are constrained northern-hemisphere monsoons. This argument is reinforced by climate, which imposes a degree of spatial coherence. when it is noted that 6000 14C yr bp postdates the peak Arbitrary distributions of taxa and biomes are not climaticof wetness in northern Africa during the Holocene, with ally possible, although distributions can be complex and more developed vegetation occurring in the Sahara during many climatically differentiated taxa can coexist at a regional earlier millennia (Jolly et al., 1998b); even wetter conditions scale in regions of high relief. prevailed during the last interglacial. We thus speculate that DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). 2 Larger areas of these biomes existed offshore, on the areas climatic oscillations on the Milankovitch time scale allowed of continental shelf that were exposed at LGM. This may repeated mixing of floras during some phases, while forcing be the case, for example, for temperate deciduous forest in disjunctions during others. In order to evaluate such speculaChina (Yu et al., 2000). Adequate precipitation for forest tions more critically it will be necessary to examine more growth may have been available in the continental shelf complete time sequences of taxon and biome distributions. region to the south and east, which could also have proPrentice & Webb (1998) stressed the importance of palaeovided the source for re-population of Japan with deciduous ecological data bases, which ultimately will allow palaeobiome trees after the LGM (Takahara et al., 2000). It also seems mapping to be essentially automated and therefore feasible likely that tropical tree species, now found along the southfor continuous time. ern coast of China but completely ousted at LGM, could have survived further south on the exposed South China IMPLICATIONS FOR PALAEOCLIMATE Sea shelf. MODELLING 3 The taxa in question may have persisted as minor components in other biomes. This is certainly true for a number The most straightforward use of the palaeobiome data in the of temperate deciduous tree taxa that the pollen data context of climate modelling is to evaluate standard palaeoshow to have survived the LGM in broadleaved evergreen/ climate simulations, such as those being generated by PMIP, warm mixed forests in southern China (Yu et al., 2000), and simulations in which biosphere and atmosphere are and others that survived the LGM in cool mixed forest in coupled in some way, as envisaged by the GAIM 6000 yr © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507– 519

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Approaches (1) and (3) are sensitivity experiments, whereas bp experiment. Diagnostic procedures based on biome approach (2) represents a prognostic approach that could modelling have been developed to translate climate model also be applied to predicting future climate changes or to output into predictions of biomes (Claussen & Esch, 1994; understanding climate changes during periods for which the Harrison et al., 1995; Ciret & Henderson-Sellers, 1997, 1998; available data on vegetation are limited (e.g. the start of the Harrison et al., 1998; Kutzbach et al., 1998), and such last glaciation: de Noblet et al., 1996). Coupled prognostic procedures form part of the ‘equilibrium asynchronous DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). modelling remains the ultimate goal, particularly fully coupling’ approach to the inclusion of vegetation feedbacks coupled modelling in which the atmosphere and biosphere in atmospheric models (e.g. Texier et al., 1997). Thus, model interact on the time scale of the climate model itself (Foley results obtained with or without explicit vegetation feedbacks et al., 1998). Sensitivity experiments however, are also valuable can be compared with the palaeobiome data on a site by site because they make it possible to isolate the effects of one basis. This site by site approach to data-model comparison is particular type of feedback, e.g. land-surface changes. Imposgenerally preferable to grid-based approaches, because numerous ing stylized changes results in ambiguity in interpreting the assumptions are hidden in all methods that could be used to results and could even lead to artefacts if the vegetation changes interpolate palaeodata to a grid. Broccoli & Marciniak (1996) prescribed were inconsistent with any plausible climate. For have made this point with respect to CLIMAP sea-surface this reason, it is preferable to prescribe land-surface condidata, i.e. that some of the more controversial features of tions as accurately as possible, i.e. approach (3) is preferred the much-cited gridded data set provided by CLIMAP rest to (1). Broström et al. (1997) prescribed land-surface condion extrapolation from a limited number of (ín some cases tions for 6000 yr bp, based partly on BIOME 6000 data, questionable) data points. Thus, discrepancies between CLIMAP in an experiment to assess the consequences of land-surface data and model results may have been overestimated. Similar changes in northern Africa for the strength and seasonal and equally forceful arguments can be made for terrestrial phasing of the African monsoon. Crowley & Baum (1997) palaeodata of all kinds, including palaeocological data assigned attempted a global prescription of LGM vegetation and showed to biomes. the potential for strong vegetation feedbacks at LGM; However, there are specific purposes that would call for however, this global vegetation reconstruction, based on the development of best-possible gridded data sets for terrestrial data compiled during the 1980s by the COHMAP project biomes. The conventional paradigm in climate modelling (Wright et al., 1993), differs in many substantial ways (for treated the land surface as unchanged from present. In reality, example, the extent of steppe and desert in China) from the however, the physical properties of the land surface at 6000 14 picture that we are now able to show for the LGM. The C yr bp (and even more so at LGM) were considerably matter of biogeophysical feedbacks at LGM therefore requires altered. Biomes differ in properties such as surface albedo re-examination. and roughness length that are important in controlling the We envisage the possibility of using the BIOME 6000 data fluxes of energy, water and momentum between the ground first to generate altered surface boundary conditions for an and the atmosphere, with consequences for the atmospheric atmospheric or coupled atmosphere-ocean model, and then heat balance and circulation; there is therefore a considerable (with appropriate diagnostic procedures) as a benchmark to potential for biogeophysical feedbacks (Eltahir, 1996; Melillo evaluate whether the model with altered land-surface condiet al., 1996; Prentice, 1998). Much attention has been paid tions is more nearly able to reproduce the distribution of recently to two specific cases at 6000 yr bp where this feedbiomes for the period in question. There is no circularity back appears to be positive: the effect of extended northern here. One rather asks whether the modelled climate system forests in generally enhancing northern-hemisphere warming is capable of sustaining the changes that were prescribed, or (Foley et al., 1994; Melillo et al., 1996; TEMPO, 1996; DEMO of CAD-KAS PDF-Editor (http://www.cadkas.com). whether additional processes must be invoked to account for TexierVERSION et al., 1997), and the effect of vegetation expansion in the climate and vegetational changes that took place. Although the Sahara in amplifying the enhancement of the African attention has been focused mainly on certain regions and monsoon (Street-Perrott et al., 1990; Kutzbach et al., 1996; phenomena where there is a strong prima facie case for Broström et al., 1997; Claussen & Gayler, 1997; Texier et al., biogeophysical feedbacks, we suspect that there is consider1997). These studies have adopted one of three approaches able scope for further and more subtle changes in climatic to quantifying biogeophysical feedback: (1) stylized vegetaseasonality and circulation patterns mediated by vegetation tion changes have been prescribed as part of the boundary shifts at a global scale. conditions of the climate model (e.g. Street-Perrott et al., 1990; Foley et al., 1994); (2) the climate model has been linked to a biome model using equilibrium asynchronous ACKNOWLEDGMENTS coupling (e.g. de Noblet et al., 1996; Claussen & Gayler, This paper is a contribution to BIOME 6000 (http:// 1997; Texier et al., 1997); (3) realistic vegetation changes www.bgc-jena.mpg.de/bgc_prentice/), a project sponsored have been prescribed, based on the available data with an by IGBP through its programme elements GAIM, the IGBP explicit procedure used to generalize the data to the climate Data and Information System (IGBP-DIS), Global Change model grid (Broström et al., 1997; Hoelzmann et al., 1998). and Terrestrial Ecosystems (GCTE) and PAst Global changES Various objective procedures exist that could accomplish such (PAGES). The work forms part of the core research of GCTE gridding of qualitative (biome) data in a topographically sensitand GAIM. It was supported in part by the US-EPA grant to ive manner (e.g. Guiot et al., 1996). © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507–519

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Claussen, M. & Esch, M. (1994) Biomes computed from GAIM (under a subcontract with the University of New simulated climatologies. Climate Dynamics, 9, 235 – 243. Hampshire), in part by the NSF grant to the project Testing Claussen, M. & Gayler, V. (1997) The greening of Sahara during Earth system Models with Palaeoenvironmental Observations the mid-Holocene: results of an interactive atmosphere-biome (TEMPO), and in part by the European Union Environment model. Global Ecology and Biogeography Letters, 6, 369 – 378. Programme contract for the Palaeoclimate Modelling InterCOHMAP Members (1988) Climatic changes of the last 18,000 years: comparison Project (PMIP). It was facilitated by the availDEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). observations and model simulations. Science, 241, 1043 –1052. ability of primary pollen data for Europe in the European Colinvaux, P. (1987) Amazon diversity in light of the paleoPollen Data Base (EPD), for Africa in the African Pollen Data ecological record. Quaternary Science Reviews, 6, 93 –114. Base (APD), for North America in the North American Pollen Crowley, T. J. & Baum, S. K. (1997) Effect of vegetation on an Data Base (NAPD), and for the Arctic in the Palaeoecology ice-age climate model simulation. Journal of Geophysical Research, of Arctic Lakes and Estuaries (PALE) Data Base. EPD and 102, 16 463 –16 480. APD have been supported by the European Union, NAPD Davis, M. B. (1990) Research questions posed by the paleoecoloby the Illinois State Museum and NSF, and PALE by NSF. gical record of global change. Global changes of the past (ed. by The graphics and much of the analysis were facilitated by R. S. Bradley), pp. 385 – 396. UCAR/OIES, Boulder, Colorado. software (BIOMAP and BIOMISE) developed by Ben Smith Edwards, M. E., Anderson, P. M., Brubaker, L. B., Ager, T., at Lund University. We thank Gerhard Bönisch for data Andreev, A. A., Bigelow, N. H., Cwynar, L. C., Eisner, W. R., management, Silvana Schott for technical editing and graphics, Harrison, S. P., Hu, F.-S., Jolly, D., Lozhkin, A. V., MacDonald, and Kerstin Sickel for computing support at MPI-BGC. G. M., Mock, C. J., Ritchie, J. C., Sher, A. V., Spear, R. W., Nathalie de Noblet provided helpful comments on a draft Williams, J. & Yu, G. (2000) Pollen-based biomes for Beringia manuscript. 18,000, 6000 and 0 14C yr bp. Journal of Biogeography, 27, 521– 554 Elenga, H., Peyron, O., Bonnefille, R., Prentice, I. C., Jolly, D., Cheddadi, R., Guiot, J., Andrieu, V., Bottema, S., Buchet, G., de Beaulieu, J. L., Hamilton, A. C., Maley, J., Marchant, R., Anonymous (1994) IGBP Global Modelling and Data Activities Perez-Obiol, R., Reille, M., Riollet, G., Scott, L., Straka, H., 1994– 98. Global Change Report, 30, 1– 87. Taylor, D., Van Campo, E., Vincens, A., Laarif, F. & Jonson, H. Araus, J. 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WCRP Report 92. monsoons to orbitally induced variations in insolation 126, Proceedings of the First International AMIP Scientific Conference 115 and 6 k BP. Climate Dynamics, 12, 589–603. (Monterey, California, USA, 15–19 May, 1995), 425 – 430. de Noblet, N., Prentice, I. C., Joussaume, S., Texier, D., Botta, A. Joussaume, S., Taylor, K. E., Braconnot, P., Mitchell, J. F. B., & Haxeltine, A. (1996) Possible role of atmosphere–biosphere Kutzbach, J. E., Harrison, S. P., Prentice, I. C., Broccoli, A. J., interactions in triggering the last glaciation. Geophysical Research Abe-Ouchi, A., Bartlein, P. J., Bonfils, C., Dong, B., Guiot, J., Letters, 23, 3191– 3194. Herterich, K., Hewitt, C. D., Jolly, D., Kim, J. W., Kislov, A., Pinot, S., Ramstein, G., Guiot, J., Harrison, S. P., Joussaume, S., Kitoh, A., Loutre, M. F., Masson, V., McAvaney, B., McFarlane, N., Prentice, I. C. & PMIP Participating Groups (1999) Tropical de Noblet, N., Peltier, W. R., Peterschmitt, J. Y., Pollard, D., palaeoclimates at the last glacial maximum: comparison of Rind, D., Royer, J. F., Schlesinger, M. E., Syktus, J., Thompson, S., Palaeoclimate Modelling Intercomparison Project (PMIP) Valdes, P., Vettoretti, G., Webb, R. S. & Wyputta, U. (1999) simulations and palaeodata. Climate Dynamics, 15, 857 – 874. © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507–519

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Prentice, I. C. (1988) Records of vegetation in time and space: numerical simulations for 9000 yr BP. Transactions of the the principles of pollen analysis. Vegetation history (ed. by Royal Society of Edinburgh: Earth Sciences, 81, 407–427. B. Huntley and T. Webb III), pp. 17 – 42. Kluwer, Dordrecht. Sun, X. J. & Chen, Y. S. (1996) Palynological records of the Prentice, I. C. (1992) Climate change and long-term vegetation last 11,000 years in China. Quaternary Science Reviews, 10, dynamics. Plant succession: theory and prediction (ed. by 537 – 544. D. C. Glenn-Lewin, R. A. Peet and T. Veblen), pp. 293 – 339. Takahara, H., Sugita, S., Harrison, S. P., Miyoshi, N., Morita, Y. DEMOChapman VERSION CAD-KAS PDF-Editor (http://www.cadkas.com). & Hall,of London. & Uchiyama, T. (2000) Pollen-based reconstructions of Prentice, I. C. (1998) Ecology and the Earth system. Earth system Japanese biomes at 0, 6000 and 18,000 14C yr bp. Journal of Biogeography, 27, 665– 683. analysis (ed. by H.-J. Schellnhüber and V. Wenzel), pp. 219 – Tarasov, P. E., Volkova, V. S., Webb, T., III, Guiot, J., Andreev, A. A., 240. Springer Verlag, Heidelberg. Bezusko, L. G., Bezusko, T. V., Bykova, G. V., Dorofeyuk, N. I., Prentice, I. C., Bartlein, P. J. & Webb, T., III (1991) Vegetation Kvavadze, E. V., Osipova, I. M., Panova, N. K. & Sevastyanov, and climate changes in eastern North America since the last D. V. (2000) Last glacial maximum biomes reconstructed glacial maximum. Ecology, 72, 2038 – 2056. from pollen and plant macrofossil data from northern Prentice, I. C., Guiot, J. & Harrison, S. P. (1992) Mediterranean Eurasia. Journal of Biogeography, 27, 609–620. vegetation, lake levels and palaeoclimate at the last glacial Tarasov, P. E., Webb, T., III, Andreev, A. A., Afanas’eva, N. B., maximum. Nature, 360, 658– 660. Berezina, N. A., Bezusko, L. G., Blyakhararchuk, T. A., Prentice, I. C., Guiot, J., Huntley, B., Jolly, D. & Cheddadi, R. Bolikhovskaya, N. S., Cheddadi, R., Chernavskaya, M. M., (1996) Reconstructing biomes from palaeoecological data: Chernova, G. M., Dorofeyuk, N. I., Dirksen, V. G., Elina, G. A., a general method and its application to European pollen data Filimonova, L. V., Glebov, F. Z., Guiot, J., Gunova, V. S., at 0 and 6 ka. Climate Dynamics, 12, 185 –194. Harrison, S. P., Jolly, D., Khomutova, V. I., Kvavadze, E. V., Prentice, I. C., Harrison, S. P., Jolly, D. & Guiot, J. (1998) The Osipova, I. M., Panova, N. K., Prentice, I. C., Saarse, L., climate and biomes of Europe at 6000 yr BP: comparison of Sevastyanov, D. V., Volkova, V. S. & Zernitskaya, V. K. (1998) model simulations and pollen-based reconstructions. QuaternPresent-day and mid-Holocene biomes reconstructed from ary Science Reviews, 17, 659 – 668. pollen and plant macrofossil data from the former Soviet Union Prentice, I. C. & Sarnthein, M. (1993) Self-regulatory processes and Mongolia. Journal of Biogeography, 25, 1029–1053. in the biosphere in the face of climate change. Global changes TEMPO (Kutzbach, J. E., Bartlein, P. J., Foley, J. A., Harrison, S. P., in the perspective of the past (ed. by J. Eddy and H. Oeschger), Hostetler, S. W., Liu, Z., Prentice, I. C., Webb, T., III) (1996) pp. 29 – 38. Wiley, Chichester. The potential role of vegetation feedback in the climate Prentice, I. C., Sykes, M. T., Lautenschlager, M., Harrison, S. P., sensitivity of high-latitude regions: a case study at 6000 years Denissenko, O. & Bartlein, P. J. (1993) Modelling global BP. Global Biogeochemical Cycles, 10, 727–736. vegetation patterns and terrestrial carbon storage at the last Texier, D., de Noblet, N., Harrison, S. P., Haxeltine, A., Jolly, D., glacial maximum. Global Ecology and Biogeography Letters, Joussaume, S., Laarif, F., Prentice, I. C. & Tarasov, P. (1997) 3, 67 –76. Quantifying the role of biosphere-atmosphere feedbacks in Prentice, I. C. & Webb, T., III (1998) BIOME 6000: reconstructing climate change: coupled model simulations for 6000 yr b.p. global mid-Holocene vegetation patterns from palaeoecological and comparison with palaeodata for northern Eurasia and records. Journal of Biogeography, 25, 997 –1005. northern Africa. Climate Dynamics, 13, 865–882. Quézel, P. (1978) Analysis of the flora of Mediterranean and Thompson, R. S. & Anderson, K. H. (2000) Biomes of western Saharan Africa. Annals of the Missouri Botanic Garden, 65, North America at 18,000, 6000 and 0 14C yr bp reconstructed 479 – 533. from pollen and packrat midden data. Journal of Biogeography, Quézel, P. & Barbero, M. (1993) Variations climatiques au 27, 555– 584. Sahara et en Afrique sèche depuis le Pliocène: enseignements Thompson, R. S., Whitlock, C., Bartlein, P. J., Harrison, S. P. de la flore et de la végétation actuelles. Bulletin d’Ecologie, DEMO24,VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). & Spaulding, W. G. (1993) Climatic changes in the western 191– 202. United States since 18,000 yr BP. Global climates since the Richard, P. J. H. (1995) Le couvert végétal du Québec-Labrador last glacial maximum (ed. by H. E. Wright Jr, J. E. Kutzbach, il y a 6000 ans BP: essai. Géographie physique et Quaternaire, T. Webb III, W. F. Ruddiman, F. A. Street-Perrott and P. J. Bartlein), 49, 117 –140. pp. 468 – 513. University of Minnesota Press, Minneapolis. Ritchie, J. C. (1985) Late Quaternary climatic and vegetaWebb, T., III, Bartlein, P. J., Harrison, S. P. & Anderson, K. H. tional change in the lower Mackenzie basin. Ecology, 66, (1993) Vegetation, lake-levels, and climate in eastern North 612 – 621. America since 12,000 yr BP. Global climates since the last Ritchie, J. C. (1987) The postglacial vegetation of Canada. glacial maximum (ed. by H. E. Wright Jr, J. E. Kutzbach, Cambridge University Press, Cambridge. T. Webb III, W. F. Ruddiman, F. A. Street-Perrott and Steffen, W. L., Walker, B. H., Ingram, J. S. & Koch, G. W. P. J. Bartlein), pp. 415– 467. University of Minnesota Press, (1992) Global Change and Terrestrial Ecosystems: the operaMinneapolis. tional plan. Global Change Report 21. Wickens, G. E. (1984) Flora. Sahara Desert (ed. by J. L. CloudsleyStreet-Perrott, F. A., Huang, Y. S., Perrott, R. A., Eglinton, G., Thompson), pp. 67 – 75. IUCN/Pergamon Press, Oxford. Barker, P., Ben Khelifa, L., Harkness, D. D. & Olago, D. O. Williams, J. W., Webb, T., III, Richard, P. J. H. & Newby, P. (1997) Impact of lower atmospheric carbon dioxide on (2000) Late Quaternary biomes of Canada and the eastern tropical mountain ecosystems. Science, 278, 1422 –1426. United States. Journal of Biogeography, 27, 585–607. Street-Perrott, F. A., Mitchell, J. F. B., Marchand, D. S. & Wright, H. E., Jr, Kutzbach, J. E., Webb, T., III, Ruddiman, W. F., Brunner, J. S. (1990) Milankovitch and albedo forcing of the Street-Perrott, F. A. & Bartlein, P. J. (1993) Global climates tropical monsoons: a comparison of geological evidence and © Blackwell Science Ltd 2000, Journal of Biogeography, 27, 507– 519

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since the last glacial maximum. University of Minnesota, BIOSKETCHES Minneapolis. Yu, G., Chen, X., Ni, J., Cheddadi, R., Guiot, J., Han, H., Harrison, S. P., Huang, C., Ke, M., Kong, Z., Li, S., Li, W., Colin Prentice has led BIOME 6000 since its inception Liew, P., Liu, G., Liu, J., Liu, Q., Liu, K.-B., Prentice, I. C., Qui, W., in 1993, when he was Professor of Plant Ecology at Lund Ren, G., Song, C., Sugita, S., Sun, X., Tang, L., Van Campo, E., University. He has been a director of the Max Planck Institute DEMO (http://www.cadkas.com). Xia,VERSION Y., Xu, Q., Yan,ofS.,CAD-KAS Yang, X., Zhao,PDF-Editor J. & Zheng, Z. (2000) for Biogeochemistry (MPI-BGC) since 1997. He is a member Palaeovegetation of China: a pollen data-based synthesis of GAIM and is on the advisory committee for PMIP. for the mid-Holocene and last glacial maximum. Journal of Biogeography, 27, 635– 664. Dominique Jolly was the GAIM postdoctoral fellow for Yu, G. & Harrison, S. P. (1996) An evaluation of the simulated BIOME 6000 from 1993 to 1998. He is on the steering water balance of Eurasia and northern Africa at 6000 yr BP committee for the African Pollen Data Base (APD). using lake status data. Climate Dynamics, 12, 723 – 735. BIOME 6000 is a community-wide project which was Yu, G., Prentice, I. C., Harrison, S. P. & Sun, X. (1998) Pollen-based formally inaugurated in 1994 under the auspices of IGBP. biome reconstructions for China at 0 ka and 6 ka. Journal of Biogeography, 25, 1055– 1069.

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Tectonics and human evolution Geoffrey King^ & Geoff Bailey^

1 The authors propose a new model for the origins of humans and their ecological adaptation. The evolutionary stimulus lies not in the savannah hut in broken, hilly rough country ivhere the early hominins could hunt and hide. Such 'roughness', generated by tectonic and volcanic movement characterises not only the African rift valley but probably the whole route of early hominin dispersal.

Keywords: Africa, Rift Valley, human origins, hominins, H, erectus, H. ergaster

Introduction The dominant conception of human origins during the past five decades has been one of a transition from vegetarian apes living mainly in trees to ground-dwelling humans exploiting the large game herds of the African savannah in response to increased global aridity and Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). reduction of tree cover. The modern consensus is that this process first occurred in Africa, beginning at least as early as 4.5 million years ago with Ardipithecus ramidus, or perhaps earlier in the light of the recent finds in the Chad basin (Rrunet etal. 2002) and Kenya (Senut et al 2001), evolving through the Australopithecines, and leading to the evolution of the genus Homo after 2.5 Ma. The emergence o^ Homo erectus (or H. ergaster as some prefer) at -^ 1.8 million years ago, saw a capacity to range widely over open terrain, the widespread use of stone tools, a greater dependency on animal protein whether by scavenging or hunting, and dispersal more widely within and beyond Africa (Cachel & Harris 1998; Delson et ai 2000; Klein 1999). There are, however, some longstanding difficulties with the ecology of this process. As Carl Sauer presciently observed over 40 years ago: 'The various kinds of primates can he described as to their proper habitats: for early man [sic] there is no such agreement and the most familiar assignment of him to living in savanna plains is perhaps the least likely. He was not specialisedfor predation; he was inept at flight or concealment; he was neither very strong nor fast'. {Saucr 1962: 42) The modern control of large plains is in fact a very recent development in human prehistory, dependent on the domestication of riding animals. The alliance between the horse and modern humans took place on the plains of Asia as late as 6000 years ago, and rapidly spread throughout Eurasia in subsequent millennia, followed in historical times by the conquest of the plains of the Americas and Australia (Clutton-Brock 1999; Keegan 1993). ' ^

Laboratoire Tectonitjue. Imtitut de Physique du Globe Paris, 4 place Jiissieti, 75252 Paris. France (Email: king&ipgp.Jussifii.Jr) Department of Archaeology, University of York. Kings Manor. York. YOl 7EP. UK (Email: gh502<Syork.ac.uk)

Received: 31 August 2004; Accepted: 24 October 2005; Revised: 20 January 2006

80 (2006): 265-286

265

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In more arid regions the camel played a comparable role over a similar time range. Even the hunting of large herbivore herds on the steppe-tundra of Pleistocene Europe and the grasslands of the Great Plains in North America seems to have appeared relatively late in the prehistoric sequence and perhaps in some regions only as a marginal and specialised adaptation (Dixon 2001). In historical times and in many places, indigenous populations were driven to the hills where the man-horse combination ot their enemies could nor penetrate. In modern conflict, motorised war machines have replaced the horse so that only in mountainous retreats, rough terrain or urban streets, can individuals or small bands hope for tactical advantage. An animal with our body form is disadvantaged in many ways in flat terrain but well adapted to morphologically complex environments, including evermore elaborate artificial structures. Eeaving the trees to live on the savannah plains seems a paradoxical if not improbable strategy for the survival of early human populations, exposing them to greater risks and fiercer competition with better-adapted animals. For Sauer, the resolution of this paradox lay in shorelines and coasts as environments most likely to select for early human characteristics such as bipedalism and tool-making, a proposal difficult to reconcile with the pattern of the currently available evidence (but see Eriandson 2001). For others, the persistence of trees, even as patchy resources, is cited as a potential refuge from danger. The development of bipedalism early on in the process with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). of human evolutionary divergence, faunal and palaeovegetational associations suggesting well-wooded conditions in the period from about 4 to 2.5 million years ago and anatomical features in Australopithecines indicating continued adaptation to tree living lend some support to such a notion (Reed 1997; Rogers et ai 1994). Here, we develop an alternative hypothesis, which focuses on complex topography and the role of active tectonics. The African Rift is a long-lived and active tectonic structure and one of the largest in the world, creating distinctive and topographically complex landscapes of fault-bounded basins, uplifted terrain, and ubiquitous volcanoes and lava fields that pnwided the local conditions for human survival. That it is also home to some of the most numerous and extensive finds of fossil and archaeological material relating to the earliest phases of human evolution is, we suggest, no coincidence, and indicates a relationship that deserves closer investigation {Figure 1). Many authors have noted general linkages between human evolution and tectonic activity in relation to very large-scale trends such as climate change, speciation resulting from formation of geographical barriers at a continental scale, or increased opportunities for the exposure and discovery of archaeological and fossil finds (Coppens 1994; Camble 1993, 1998; Partridge et al. 1995a, b; Partridge 1997; Redfield et ai 2004; Ruddiman & Raymo 1988; Thomas 1985; WoldeCabriel et al. 2004). Others have drawn attention to the mosaic character of African Rift environments, creating variety of potential food resources, abundant surface water and opportunities for niche diversification and specialisation (Cachel & Harris 1998; Foley 1995; Stern 1993). Here, we emphasise the creative role of underlying geodynamic processes in maintaining these mosaic environments and the distinctive topographic features associated with them. We argue that a key environmental factor in driving forward the process of human evolution is localised areas of topographic complexity resulting from active tectonics, where a ground dwelling bipedal omnivore could gain tactical advantage over faster moving quadrupeds, find protection from carnivore 266

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Volcanoes active in the last 10,000 years

Historically active volcanoes

dar Awash elka Kunture

Major hominin sites

Omo^ WestTurkana Lothagaf

ejej

st Turkana ike Baripgo;^ akapafisgat eriin^

- 500 km

Laetoli

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Makapansgat Swartkraris. \J Kromdraaii Lake Baringo Sites:Tugen !Hi!!s,Chemeron, Chesowanja,Tabarin Figure I. General indication of hominin sites in the African Rift and areas of volcanic activity.

predators and outmanoeuvre herbivore prey, thereby gaining access to the vast meat reservoir of the savannah plams, while also benefiting from other resources. Extensive areas of smooth and open terrain with their large biomass of mammal protein remain important in this hypothesis, but inaccessible except in combination with other physical features.

Environmental requirements The biological and physiological changes associated with the development of a human niche, and the sorts of environmental conditions that would have selected for those changes, have been extensively discussed (for a broad review see Foley 1995, and elaboration by Aiello & 267

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Wheeler 1995; Bunn 1994; Cachel & Harris 1998; Potts 1988, 1996; Wheeler 1991). The key trends, about which there is general agreement, are increased ground dwelling and bipedalism, diversification of diet to include a wider range of foods including more emphasis on meat, increased body size and disproportionate increase in brain size, manufacture of stone tools, adaptation to heat stress, foraging over longer distances and a longer period of childhood immaturity and learning. These in their turn would have required regular water intake, increased consumption of animal protein, access to suitable stone for artefacts, and increased parental care and cooperation. This combination of developments would have opened up a meat-eating niche by facilitating access to animals and animal products inaccessible to other carnivores and scavengers, or at places and times when competition was less severe, during the day and in dry seasons, lor example. Such behaviour would have entailed greater hazards and exposure to predation and required the creation of ground 'nests' using features such as cliffs and caves, social defence or fire, as trees became less accessible. Feedback between different elements of this package is commonly emphasised, for example between daytime hunting, upright stance and physiological adaptation to heat stress, between stone tools, meat-caring and increased brain size, between increased brain size and cognitive skills in communication and problem-solving, and between cognitive skills, spatial memory and the use of larger and more environmentally diverse home-ranges. Other Changed tendencies with the DEMO CAD-KAS PDF-Editor (http://www.cadkas.com). may haveVERSION created newofpressures. For example, the prolongation of childhood vulnerability and dependency on parental care would have imposed new pressures on the ability of females to feed themselves and their offspring independently of the males and the vicissitudes of their success in obtaining meat, and on the provision of ground nests safe from predator attack. Of course, there is not universal agreement on when or how all these changes occurred. But there is a general recognition that many of these features had become integrated in a single, stable configuration by the time of emergence o^ Homo erectus, with further amplification in the course of subsequent evolutionary development of the Homo lineage. Turning to the sorts of physical conditions that might favour these developments, the following stand out as key factors: • a varied environment with a wide range and abundance of plants and animals offering new opportunities for obtaining food • abundant and easily accessible water supplies • secure locations where vulnerable individuals could find protection from attack by predators or other hazards, and food supplies within easy reach • opportunities for the trapping of mobile or elusive animals We suggest that the distinctive tectonics of the African Rift provide a unique geological environment that meets all of these environmental requirements and may indeed have contributed powerful selective pressures favouring the human trajectory.

The extetisional and volcanic environments of the African Rift Complex topography largely results from the interplay of two agents, uplift and deformation of the earth's crust by tectonic and volcanic activity, and the smoothing of relief by erosion. 268

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Active tectonics and volcanism create features such as faults and lava flows, which with repeated motion build mountains and valleys. Although erosion smoothes and in due time destroys the creations of tectonics, steep canyons, river terraces and lakes with changing shorelines can result in transitory but dramatic local relief. jg^ The African continent show.s unusually high levels of volcanic activity and this has been M the case for 15 million years or more. This is now thought to result from the African plate SQ becoming stationary above the underlying mantle '^30 million years ago (Burke 1996; p^ Scotese & McKerrow 1991). The insulating blanket ot continental lithosphere caused a rise in both crust and mantle temperature. As a result both the mantle and crust expanded (Anderson 1982), lifting the whole of Africa by about 1km. The great African escarpment that runs all along the southern coast of Africa to the Zambezi valley on the east coast and to Luanda on the west is the most spectacular manifestation of this uplift. Deep down-cutting of the major rivers such as the Zambezi, Limpopo and Nile, and rapid progradation of their deltas, can be dated to this period (Burke 1996; Moctar Doucoure & de Wit Maarten 2003). Heating in the mantle also resulted in melting, expressed as extensive volcanism at the surface. In one school of thought, the process of magma forcing upwards from the mantle has caused the African plate to split apart along the East African Rift. The alternative view that Changed withis the VERSION CAD-KAS PDF-Editor Africa beingDEMO pulled apart by plate of motion, ascendant when plate (http://www.cadkas.com). tectonic concepts were new, is now losing adherents (Davidson et al. 2002). Irrespective ofthe long-term processes, the African Rift is associated with extension, and the normal faulting that accompanies it, in which uplift ofthe rift flanks is associated with subsidence ofthe Boor (cf Armijo et al. 1996; King & Ellis 1990) and exceptionally high levels of volcanic activity. It is in such environments that earliest humans first secured a niche. Repeated earthquakes and volcanic activity are associated with subsidence ofthe rift floor and uplift of the adjacent flanks to create internally draining basins that trap water and sediments to create fertile lake environments and alluvial plains. The resulting landscape is punctuated by numerous volcanic lava flows, traversed by rivers and circumscribed by fault scarps that can form nearly vertical cliffs adjacent to local basins of subsidence that fill with water and sediment. The result is a complex topography of sedimentary basins and physical barriers at a variety of geographical scales, ranging from the extremely localised, over distances of hundteds of metres to kilometres, to larger regional scales. These provide a diversity of resources - local concentrations of plant foods and animals attracted to the shores of lakes and rivers, drinking water and aquatic foods at the shore edge - and nearby locations enclosed by lava flows or backed by fault scarps and incised river terraces (Figures 2 and 3). Volcanoes in the African Rift erupt a range of lavas, although basalt lavas are the most common and account for the greatest volume by a large margin. Although basalt is usually very fluid and can flow for long distances, the cooled surface breaks into blocks, creating a surface of often razor sharp angular blocks known as 'Aa\ named in Hawaii, where examples are common and the ages are known in detail (Figure '5^). The one illustrated is c. 10 000 years old, indicating that such flows can last as obstacles for long periods of time, even in regions of substantial rainfall and consequent erosion. Enclosed areas resulting from lava flows and faults are called Kapuka in Hawaii. Today, some native plant species only survive 269

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Rncrs cutting jnto uHicr deposits rei^vs tomb from easier active rifts

Figure 2. A cartoon representation of features associated with an active rift. An older imoothed esciirpment is shown to the left. A river is shown cutting a narrow gorge into the escarpment. Down-cutting of uplifted rift flanks is responsible for revealing fossils in earlier rift floor sediments. The view shows a lake at the base of the older escarpment. Springs, small lakes wet areas appear on the down-dropped sides of faults,PDF-Editor which form near vertical escarpments within the active Changedorwith thecommonly DEMO VERSION of CAD-KAS (http://www.cadkas.com). rift. Lava flows form barriers and often create enclosed regions (Kapuka). Distances between the faults, volcanoes and lava flows have been contractedfhr the purposes of the cartoon. The flat areas between thefeatures are typically more extensive than indieated and exhibit savannah characteristics.

in Kapuka, where they find protection from introduced wild pigs. In the past, Kapuka were also of tactical importance in warfare and feature in Hawaiian mythology. Similar lava flows are common features of the African Rift landscape. They are, of course, important as sources of stone for artefacts, bur they also offer other advantages. They can create localised sediment and water traps enclosing small and well-protected pockets of fertility. They also form formidable obstacles to rapid movement, either as near vertical but low barriers 2-3m high consisting of angular rocks, or as more extensive lava fields. When young, these lava fields are anything but smooth, representing a jumble of jagged rocks. Even when exposed to tens of millennia of erosion and rounding of the sharper edges, they remain as densely packed bouider fields that can only be traversed slowly or with difficulty. There is no other part of the world with such a large or long-lived tectonic feature dominated by volcanism and extensional tectonics. Similar extensional environments occur in Iceland and New Zealand but extension in such places as Europe, Arabia and China is not associated with similar levels of volcanic activity. A key point about the African Rift is that tectonic activity has been continuous throughout the time span of human existence. The level of activity has varied locally and the focus has shifted but the overall level has remained high. This means that all the features described above are being continuously rejuvenated, though not necessarily in the same places in the landscape or in exactly the same configuration. Landscapes that were subjected to high levels of tectonic activity at some remote period and became quiescent would lose sharpness and detail and tend towards greater environmental uniformity. The long-term persistence of environmental variability and of sharp topographic barriers such as lava flows and fault scarps is thus a distinctive feature. 270

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Figure J. Features associated with active rifting, l-igures 3(a) to (cj urejrom the Ajar region of Ethiopia. Figiirt MJ) from Hawaii, (a) Escarpment near the eastern end of Lake Gamori, Fthiopia. (b) The Awash River cutting into the uplifted rift flank hounding the currently active rift, (c) Fhe catdera of a volcano tying on the axis of the active rift, filled with jreshwater. (d) Active faults resulting from active rifiing cutting the flank of the volcano in Figure 2(c). (e) Suhsidence associated with an active fiult on the rifi axis creating a 'well watered' region, (f) A lava flow in the humid region of Hawaii. Fhe lOka flow is only modestly eroded.

It is not difficult to see how an unspecialised ground-dwelling predator, dependent on powers of observation and intelligence rather than of speed and strength, might use Iocal barriers and enclosures created by a topography of lava flows and fault scarps as a secure place for ground nesting, for feeding at leisure on food brought in from elsewhere, as a source of local food and water, and in time as a means of actively manoeuvring and diverting live animals from the edge of more open terrain into natural enclosures and traps. With an increased capacity to make use of topography in this way, early humans would become 271

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Figure 4. Change in the iocaiion of sites with progressive rifting. The environrnait where fossils were deposited in most African sites was in the bottom of the active rift (upper diagram). Activity has since created a new rift axis uplifririg the older axis (lower diagram). Human figures are not to scale.

less dependent on trees as a protective retreat, better able to range widely over varying environments and more effective carnivores. Changed with thethe DEMO VERSION PDF-Editor (http://www.cadkas.com). Beyond southern limit ol the of RiftCAD-KAS lies the Transvaal region of South Africa. This region has produced early dated material, and appears to be part of the geographical zone within which some of the earliest species of hominins evolved, or to which they rapidly dispersed. Yet it is not subject to the extensional and volcanic activity typical of the African Rift. It is, however, a region of active tectonics resulting from the long-term uplift of the South African region associated with the stability of the African plate, as noted above, and is subjected to distinctive tectonic controls on topography and water supply. Repeated faulting results in localisation and rejuvenation of perennial water supplies and thermal springs. In elevated limestone terrain, karstic processes produce localised topographic complexity and caves with advantages of protection and tactical advantage (Chris Hartnady/>fw comrn.; Kuman 1998). In key respects, then, the region shows similarities to the Rift in terms of long-term tectonic activity and topographic complexity. But the contrasts suggest that the distinctive features of the Rift may be a subset of the more general phenomenon of tectonically created topographic 'roughness'. We shall return to this point after more detailed consideration of Rift landscapes.

Archaeological implications A schematic cross-section through the Rift shows how the morphology commonly evolves over long periods of time and alters topographic features at a local scale (Figure 4). The rift is shown as a symmetrical feature although asymmetry is more common. As the rift widens, the original valley floor gradually becomes uplifted and rotated and newvalley floor appears. In time, the rift flanks show a step-like effect, with the highest and oldest features showing less tectonic activity and a topography more smoothed by long-term erosion (Figure 5). The present-day locations of Plio-Pleistoccne archaeological sites and fossils, and the features of the immediately surrounding landscape visible today, are often a quite misleading guide to 272

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Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). Figure 5. A view looking east into the Ethiopian Rift east of Addis Ababa. This area is now to the west of ihe active rift and is now almost inaciive. However, features identified in Figure 2 can be seen but have been smoothed by erosion.

the situation at the time when the sites were active. The Hadar site is now perched on the uplifted Bank of the Ethiopian rift whereas 3 million years ago it would have been located on or close to the active rift fioor. The Olduvai Gorge shows a similar geological history, with early Pleistocene sites originally formed on the active rift floor and in lake-edge locations that have been subsequently uplifted and deeply incised by tectonic movement and river down-cutting. Uplift and erosion have well-known implications for the exposure and visibility of fossil and archaeological localities. It is the erosion of the Awash River, cutting back into earlier deposits, that has revealed the hominin bones at Hadar, for example. However, these same processes also make it more difficult to reconstruct the original detail of the local topography. Small-scale features such as the edges of lava flows and river-terrace risers a few metres high, though highly significant as barriers to movement, may be removed by subsequent erosion or buried by sediment, or survive only as fragments. It is very difficult if not impossible to reconstruct their distribution around an ancient site by working from a limited number of sections into the underlying strata. As i.s indicated in Figure 2, the underlying strata that result from ongoing processes at the surface are discontinuous and form no easily discernible or coherent pattern. Were it not for studies of active volcanic landscapes that are the modern equivalent of ancient ones, we would have little idea of the small-scale features associated with the ancient landscape. Nor would we know what to look for, or what might be missing in conventional methods of landscape reconstruction based on correlations of 273

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dateable features and sections from a limited number of stratigraphic windows (Bailey et al. 2000). Few attempts have been made to incorporate tectonics into palaeogeographical reconstructions of early hominin sites and their associated landscapes. Tectonic processes, if they are not ignored completely at this local scale, are usually treated as background events, as occasional disruption of sedimentary processes, or as sources of volcanic raw material for stone tools. There are very real difficulties of reconstructing the original landscape morphology in active tectonic settings, and we do not minimise them. One problem is that reconstructions around sites of archaeological significance typically begin with the sediments that enclose the archaeological and fossil material, and work outwards by correlation of stratigraphic intervals, sedimentary features and palaeosols to progressively larger areas ofthe surrounding basin (e.g. Brown & Feibel 1991; Feibel et al. 1991; Potts et al. 1999; Rogers et al. 1994). This is a necessary and important starting point in a context where radical geomorphological change induced by tectonic processes has occurred since the time of deposition. However, the requirement for dateable material leads to a focus on the sedimentary infill of the basin rather than to other critical topographic features such as lava fields, faults, localised barriers to movement, and blocks of land that have been tilted and rotated. Moreover, the need for stratigraphic correlation limits the area reconstructions can be of extended before PDF-Editor they encounter(http://www.cadkas.com). discontinuities between Changedover withwhich the DEMO VERSION CAD-KAS one sedimentary basin and the next. The high spatial and temporal resolution of such analyses results in a reconstruction over areas of landscape and spans of time that may be too small to allow an appreciation of their significance in a larger geographical or temporal context. Conversely, inferring tectonic processes from detailed mapping of local surface features can be highly misleading, and interpretation tends to operate over much larger areas in a process of working down from the large scale to the local level. Only over the past decade or so, with the widespread availability of satellite imagery and more recently digital elevation data, has it become possible to combine the large-scale mapping of fault lines and other tectonic features with local field data and dating control. This has revolutionised our ability to understand the mechanics of continental deformation and its relationship to surface processes (e.g. Hubert-Ferrari et al. 2002a; Tapponnier et al. 2001, and references therein). So far, this approach has not yet been applied in detail to the African Rift, with the exception ofthe Afar (e.g. Hubert-Ferrari et al. 2002b; Manighctti et al. 2001, and references therein), nor has it been combined with analysis of local areas with archaeological and fossil material. Another problem is the status of sites and the degree to which they can be considered representative of ail the activities of early hominin groups. There are good reasons for suspecting that rhe known sites are biased by factors of differencial preservation and visibility and are less than representative of all the locations of significance to early human activity in the surrounding area. The sites we know about are places where artefacts were discarded, and parts of animal carcasses and human corpses abandoned. Many if not most such sites are palimpsests representing a succession of unrelated episodes of deposition that bear no relationship to residential sites as that term might commonly be used in ethnographic studies of hunters and gatherers (cf Binford 1981; Stern 1993, 1994). Some were most probably ephemeral scavenging locations used for perhaps as a little as a few days, but we do not 274

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know whether they were also used as sleeping areas or for more prolonged periods. Most were probably not used for central place foraging or home bases as originally proposed by Isaac (1978). The great majority occur in lacustrine or alluvial basins because this is where discarded material is most likely to be covered by sediment and preserved tor future discovery. j^ While these sites give a general indication of preferred areas of activity, classic techniques of H site catchment analysis that assume a residential site at the centre of its exploitation territory % and work outwards from archaeological find spots to the wider landscape, on the assumption ptf that resources in closest proximity are of greatest importance, are likely to be misleading (Bailey 2005). Working downwards in scale from the regional distribution of resources to the local scale is considered a preferable analytical strategy in such circumstances (Sturdy etal 1997; Flannery 1976; Foley 1977). These difficulties are highlighted by an objection that is commonly raised to the tectonic hypothesis, namely that early sites in the African Rift are located in smooth alluvial plains. Undoubtedly influential here are the Lake Turkana sites. The majority of these are on the edges of stream channels in the alluvial plain of the ancient river basin or on the margins of the subsequently formed lake (Brown & Feibel 1991; Brown &: McDougall 1993; Feibel & Brown 1993; Feibel etal. 1991; Rogers et al. 1994). Pal aeo topographical reconstructions typically show an area of some 10 hectares around a site, with open savannah vegetation, Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). scattered Acacia trees and denser stands of larger trees along river courses, as at the FxJj50 site (Bunn ct al. 1980). The relationship of that area to the total that might have been used can be gauged from figures for hominin home ranges. Ant6n et al. (2002), using analogies with modern apes and hunter gatherers, suggest figures for Australopithecines and early Homo ranging from 38 hectares to 452 hectares, depending on body size and diet, but note that increased dependence on meat might lead to use of much larger areas. Foley (1987: 140), citing evidence of importation of raw materials from non-local sources, gives figures of 1256 hectares for the Turkana Basin and 45 203 hectares for Olduvai. In both areas, analysis of site distributions in relation to sources of raw materials suggests changes in the organisation of land use by Homo ereetus and progressive enlargement of the areas over which they foraged (Blumenschine & Peters 1998; Potts et al 1999; Rogers et al 1994). Areas typically shown in detailed palaeotopographical reconstructions of site surroundings might thus represent anything from as much as 26 per cent of the hypothetical home range to as little as 2 per cent. These percentages might be even lower if we take into account areas within the lifetime range less frequently visited but criticai to long-term survival, and arc some indication of the larger areas that should be analysed to place local sites into wider perspective. In other words, even though there are extensive areas of flood plain in the Turkana Basin, it is an area that has also been subject to repeated volconic activity and tectonic movement over a long period, with numerous faults and extensive areas of lava flows within daily foraging range of many sites.

Tectonics and human dispersal beyond the African Rift When we turn to tectonically active areas in other parts of Africa, or to regions outside Africa, the dominant style of tectonics is usually contractional, with convergence of continental plates and compression and uplift associated with reverse faulting, or strike-slip motion 275

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lkm

New Lake (

/jT New Lake

^ / North

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Fold axis >*>*' Extension - ^ Contraction

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Figure 6. Faulting and Like formation in the FlAsnam region of Algeria, (a) Map of the region where a new lake was formed as a result of/old uplifi during the FlAsnani earthquake, (h) Cross section along the line a-a' in Figure 6(a). Gravels tilted to nearly vertical hy repeated past earthquakes occur along the base of the anticline and contain Mousterian artefacts, (c) A photograph of the lake and anticline viewed to the NWfrom near the SF. corner of the map in Figure 6(a). (d) Filted gravels at the base of the anticline.

in which plates slide past each other, resulting in long linear valleys with complex alluvial histories, while any extension is associated with less widespread volcanism. The question arises, then, as to the nature of these tectonic environments and the extent to which their distribution may have constrained or facilitated wider patterns of human dispersal. In the contractional environment of north-west Africa, observations of the 1984 earthquake of El Asnam in Algeria demonstrated a key relationship between a major earthquake, the growth of folding at the surface and the tectonic control of the water table. The earthquake resulted in vertical displacement of about 3m on a partially buried fault over a distance of 30km (King & Vita-Finzi 1981; King & Yielding 1984). Uplift associated with the faulting and folding across the course of the Chelif River impeded water flow and rejuvenated a lake in the basin upstream of the fold axis that had dried out in preceding decades (Figure 6). The fold axis represents the cumulative uplift of successive earthqtiake events over many millennia, and tilted gravels within it contain Palaeolithic artefacts, demonstrating the attractions of the area for prehistoric settlement. Similar observations in other areas of the Mediterranean, notably in north-west Greece, a region of strike-slip and contractional tectonics, demonstrate the influence of tectonic controls on Palaeolithic settlement. Archaeological sites are associated with tectonically 276

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created and maintained local basins of fertility, which conferred a degree of insulation from the impact of climatic changes and especially late glacial aridity, and with topographic barriers that allowed control of mobile prey species and provided secluded locations where human groups could monitor animal migration routes without disturbing the animals (Bailey et al. 1993; King & Bailey 1985; King etal. 1993, 1994, 1997; Sturdy etal 1997). Outside Africa, the Red Sea coasts would have offered similar environments to the East African Rift and volcanic activity extending nearly to historic times occurs extensively on the Arabian side, although faulting is rare. The 'Syrio-Jordan Rift' extending from the Culf of Aqaba to eastern Turkey is mainly a strike-slip feature although south of Mount Lebanon significant opening and normal faulting is prevalent. Substantial volcanism over the last 2 million years has also occurred over large portions of the system, particularly in the region of the present Lake Kinnetct (Sneh et al. 1998). Between Northern Israel and Syria strike-slip motion is accommodated along faults (mainly the Yamouneh fault), which are angled at about 20' to the overall direction of the 'Syrio-Jordan Rift'. This geometry results in contraction, which over the last 10 million years has created Mount Lebanon (Freund 1965; Garfunkel et al. 1981). While volcanic activity is almost absent in Lebanon, the uplift has created spectacular topography and a diverse range of environments. The present day complexity has been enhanced by features Changedresulting with the DEMO ofalmost CAD-KAS (http://www.cadkas.com). from glaciersVERSION that extended to the PDF-Editor sea during glacial maxima. The Bekka valley and the coastal regions have in historical times provided secure refuge for humans fleeing persecution (Salabi 1988) and the same features may have proved important for prehistoric hominins. In Turkey the east Anatolian fault extends to the region of Lake Van in the Caucasus. To the east, morphology resulting from contractional and strike-slip activity extends through Iran to the Himalayas. To the west, the North Anatolia fault runs to reach the Aegean at Gallipoli at the western end of the Sea of Marmara. These major features together with associated minor structures provide tectonically complex environments with many of the characteristics we have described for the African Rift. In south-east Asia, subduction systems are associated with volcanism and with tectonic activity more intense than in Africa. Some of the earliest and best dated evidence of early human activity outside East and South Africa (summarised in Bar-Yoscf 1998; Bar-Yosef & Belfer-Cohen 2001; Dennell 2003; Rolland 1998) is clearly in tectonically active regions, in the active contractional zone of north-west Africa at 1.8 million years at sites stich as Ain Hanech (Raynal etal 2001), at Ubeidiyah and Erq El Ahmar at -^1.4 million and -^1.8 million years respectively (Ron & Levi 2001), associated with the Jordan Rift and its complex volcanic, alluvial and lacustrine history, and at Dmanisi in the southern Caucasus (Gabunia etal 2000; Lordkipanidze etal. 2000), at â&#x20AC;˘^ 1.7 million years, in a contractionai tectonic environment with evidence of lake environments and volcanic activity. Hominin finds from the Sangiran dome of Indonesia are of equivalent date, -^1.8 million years (Swisher et al 1994), and are associated with the complex tectonics and intense volcanism of the Java subduction system. Whether there was early settlement in Europe at a similarly early date remains disputed. The site of Ubeidiyah is associated with lake-^edge topography and extensive lava flows (Figure 7) and would have offered a familiar environment to early hominins adapted to the African Rift. The site of Dmanisi is associated with similar features. These sites have some 277

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Figure 7. Tbe repon around the Ubeidiyah site (red spot). The red circle has a radius oflOknh Basalt lava flows are shown with ages taken from Sneh et al, 1998.

of the earliest, best documented and most persistent evidence of human occupation outside Africa, and we consider it significant that they not only have local environments dominated by tectonic and volcanic topography, but that they also lie on a potential pathway of dispersal that provides a virtually unbroken series of similar tectonic environments linking the Afar and the southern Caucasus.

Topographic roughness and human dispersal The features we have identified exist on a range of scales and cannot be characterised in a simple way. One useful proxy measure of tectonic activity is topographic 'roughness', and it is now possible to measure this in a systematic way on a continental scale through the manipulation of digital elevation data. In Figure 8 we present three versions of digital data for Africa, Europe and Asia, together with the oldest hominin sites outside Africa. 278

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Figure 8a shows the topography, with red indicating high altitude and blue low altitude. In Figure 8b, areas of rough topography are identified, but are de-emphasised at high altitude on the assumption that early hominins could not exploit cold conditions. In Figure 8c, areas at high latitude are also excluded on the assumption that these areas would have been inaccessible to the earliest human populations because of extreme climatic condition.s. Sites can be seen to correlate with areas of rough topography. The roughness maps provide a measure of tectonically active environments together with some simple constraints to eliminate climatic extremes. This is an admittedly simplified approach, but one which allows a systematic overview over large areas, a proxy indication of the areas most favourable to human settlement and the most obvious pathways for dispersal. For the pathway out of Africa into Eurasia, the Nile Valley is ofren assumed to be the most obvious route, funnelling movements through the narrow bottleneck of the Sinai Peninsula and into the Levantine corridor. However, with the exception ofthe undated site of Abassieh, there is no evidence of Eower Pleistocene hominin activity in Lower Egypt (Vermeersch 2001). The roughness maps suggest that a more obvious route is from the Afar and along the Red Sea margins. The Red Sea is itself a rift formation, and on the Arabian side volcanic activity with extensive lava flows of Pliocene and Pleistocene date has created local environments with familiar attractions of localised fertility and complex topography. ChangedThe withpresence the DEMO VERSION of CAD-KAS PDF-Editor of stone tools of Acheulean and Oldowan type in(http://www.cadkas.com). Saudi Arabia and the Yemen, though mostly undated as yet by radiometric means, reinforces the possibility of early occupation (Petraglia 2003; Whalen &: Fritz 2004). The roughness maps also clearly bring out the northerly route out ofthe Red Sea into the SyriO'Jordan Rift and the virtually continuous line of tectonically active environments extending along the East Anatolian fault as far as the southern Caucasus, westwards along the North Anatolian fault to the tectonically active southern peninsulas of Europe, and eastwards to the rectonically active environments ofthe Iran/Iraq border leading on to the foothills of the Himalayas and thence southwards to the peninsulas and archipelagos of southern Asia. A number of indirect considerations have recently highlighted the attractions of a southerly dispersal route from Africa via the Arabian coastline to south-east Asia, at least for anatomically modern humans and perhaps also for earlier dispersals (Macaulay et al. 2005; Mithen & Reed 2002; Stringer 2000; Walters et al 2000). The southern margin ofthe Arabian Peninsula certainly provides a shorter pathway to the Indian Subcontinent than the alternative to the north and one that appears relatively attractive in topographic terms (Figure 8c), although the roughness map for these regions must necessarily remain incomplete without the incorporation ofthe now submerged topography that would have been available during periods of lowered sea level. A critical factor for this pathway is the barrier posed by the Bab el Mandeb Straits, which requires a sea crossing of 20km between Africa and the south-west corner of the Arabian Peninsula. However, at lowered sea levels the Straits would have represented a much narrower waterway, although one that was probably never bridged by dry land during the maximum sea level regressions of the earlier Pleistocene (Lambeck pers. comm.). Even so, lowered sea levels would have produced a narrow channel as little as 5km wide extending for over 100km, scarcely more of a barrier than a large river, and the availability of littoral and marine resources may have been an added attraction (Flemming et al 2003; Walters et al 2000). 279

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Figure 8. For legend see jacing page.

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North-west Africa stands out as a rather isolated island within the African continent. Indeed, from the point of view of complex topography on a large scale, the most obvious pathway leading from East Africa to north-west Africa appears to lie along the eastern and northern coastal regions of the Mediterranean, rather than across the Sahara or around the southern shore of the Mediterranean (Figure 8c). However, the case for crossing the Strait of Cibraltar at an early date, though plausible, remains ambiguous, and there is a serious lack of early-dated sites between southern Spain in the west and the southern Caucasus and the Eevant in the east {Straus 2001). Hominin dispersal out of Africa seems on current evidence to have been delayed until the emergence of Homo erectus, suggesting either some physical or ecological barrier to an earlier dispersal or lack of appropriate abilities in earlier hominin populations. Amongst the various candidates for such constraining influence are climatic and geographical barriers, lack of critical technological or social skills, competitive relationships with other carnivores and scavengers, differential resistance to disease or smaller home ranges and more limited powers of dispersal (Anton et al. 2002; Bar-Yosef 1998; Bar-Yosef & Belfer-Cohen 2001; Dennell 1998; Camble 1993; Mithen & Reed 2002; Rolland 1998; Turner 1992, 1999). Our emphasis on tectonics raises the question of the ways in which contractional and strike-slip environments outside Africa may have facilitated or impeded human dispersal. Changed with VERSION of CAD-KAS PDF-Editor Thesethe sortsDEMO of environments produce a complex topography of (http://www.cadkas.com). barriers and basins that trap water and sediment, providing concentrations of local fertility and resource diversity, and opportunities for the strategic observation or control of mobile herd animals, not dissimilar to the African Rift. But they lack the distinctive features associated with extensive lava flows and riftmg. Numerous small faults give way to less numerous larger features, and abrupt features such as those shown in Figure 3d are less common. Absence of these features appears to have been no disincentive to early occupation in South Africa, so there is no reason why they should have been a disincentive to dispersal elsewhere. However, Ben-Avraham & Hough (2003) have noted an additional factor that may be relevant here. The Syrio-Jordan Rift extending north from the Culf of Aqaba at the head of the Red Sea and along the Wadi Araba to the Dead Sea, provides an attractive stepping stone out of Africa, but it did not come into existence until some time between 3 and 2 million years ago. Hence the absence of tectonic environments on this critical pathway out of Africa may have been sufficient to impede dispersal at an earlier date.

Figure 8. Digital maps of Africa. Furope and Asia derivedfrom SRFMJO data (resolution ofc. 800 metres). In eachflgure, fllled red circles iridicnle the earliest evidence ofhominins outside Africa and open circles the next earliest evidence, folbwing Dennell (2003). with additions, notably the recently reported site ofMajuangou (Zhu a al. 2004). (a) Flevation map, with red indicating high elevation and blue low elevation, (b) Roughness map corrected for altitude. Fhe map is calculated by the following steps. (!) Fhe SRFM 30 data are smoothed using a 21x21 gaussian fllter (2) Fhis is subtracted from the original data and the result squared to remove negative values. Fins provides a measure of roughness at kilometric scales and a proxy for the smaller scales discussed in the text. (3) Since rough terrain at high altitude (e.g. Tibet) would not have provided hominin habitats, these regions are de-ernphastsed by dividing the result of step 2 by the smoothed topography from step I. In the final flgure, red indicates rough terrain at low altitude and blue indicates either smooth terrain or rough terrain at high altitude, (c) Roughness map correctedfor altitude and latitude. Fhe data are subject to the same three operations as are performedfor flgure 8(b). In a further step, the (Lita are divided by the square root of the cosine of latitude. This de-emphasises regions such as Mongolia that would have been too climatically challengingforearly occupation.

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Finally, we should note that the roughness maps highlight areas of potential attractiveness to early human settlement but which lack relevant evidence, notably in the western Rift and areas further west in Africa, and which might thus repay closer investigation for relevant evidence or for factors other than geological ones that may have deterred early human occupation.

Conclusion The active tectonics of the African Rift creates features that we believe are essential to understanditig the ecological basis of human evolution. Tectonics provides the physical basis for a diversified environment with varied food resources and abundant water supplies: the environmental mosaic so often referred to as a primary advantage of the African Rift. It offers physical protection m the form of cliffs, lava flows and topographic enclosures, and hence small-scale topographic complexity in which a relatively defenceless species can find protection from predators. It creates a larger scale topographic complexity of fault scarps, folds, lava fields and natural traps, which can provide tactical advantage in pursuit of prey. Finally, it results in geologically unstable conditions that lead to greater variability in the precise configuration of topographic variables in time and space, and thus sharpens the selective pressures in favour of multiple speciation and/or adaptable behaviour. These distinctive and unique attractions of the African I^ft and ones that are the product Changedarewith the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). of its unusual tectonic history. Tectonic environments outside the African Rift: provide comparable if less distinctive features, and the opportunity afforded by digital elevation data to map their distribution over large areas offers predictions about likely pathways for human dispersal more widely within and beyond Africa. We have noted the problems that stand in the way of testing tectonic hypotheses at the local scale and suggested the need to combine a newer generation of techniques of tectonic mapping and interpretation with established techniques of palaeogeographic reconstruction. In the absence of such work, it would be premature to be more specific about the relationship between tectonics and evolutionary events, though we suggest that topographic complexity is likely to have been a more critical agent of selection as meat became a more important food resource and the capacity for wide-ranging mobility more pressing. It can, of course, be objected that the large-scale patterns of association that we have identified are coincidental and merely reflect geological conditions conducive to the preservation and exposure of archaeological evidence. We do not minimise those effects and have discussed examples earlier of ways in which landscape deformation can differentially obscure, destroy or expose archaeological material. However, by commonly held consensus and the convergence of many independent lines of evidence, the African Rift [sensu lato) is held to be a key zone for the successful emergence and expansion of the genus Homo, and probably also for our own species Homo sapiens sapiens. It seems unlikely on current evidence that the privileged position of the African Rift in this respect can be dismissed as mere coincidence resulting from differential visibility of evidence. Our emphasis here is on the impact of tectonics at all scales of analysis but especially at smaller geographical scales, and its creative role in sustaining local environmental conditions attractive to human settlement and dispersal, rather than the destructive effects resulting from episodic disruption. The African Rift stands out as being distinctive at every scale. 282

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Above all, we emphasise that this tectonic perspective requires us to think of change in the physical landscape as a continuous and dynamic process operating at many different scales with far-reaching ecological ramifications, even in seemingly placid regions and especially as we move on to longer time scales and into tectonically more active regions. Attempts to incorporate into a study of human evolution the physical environment, which treat it as a static fixture, or a static fixture subject to occasional and episodic change, are missing a vital piece of the larger picture.

1 U

Acknowledgements We acknowledge financial support from NERC, through its EFCHPLD programme (Environmental Factors in Human Evolution and Dispersal), the British Academy. INSU-CNRS, and the Leverhulme Trust through its Major Research Fellowship scheme. We thank Vincent Courtilloc tor hi.s enthusiastic encouragement and discussion of ideas, Kurt Lanibeck and three anonymous assessors tor their critical comments, which have helped in the refinement ot our argument and the elimination of errors, and Brigitte Senut and Martin Pickford tor information on African site locations. The ideas expressed here are the result of fruitful interaction over a iong period, and while King is primarily responsible for the getilogical input, and Bailey tor the archaeological, we are equally responsible tor the resulting integration and any remaining dericiencies. This paper is II'GP contribution number 2112 and INSU contribution number 391.

Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). BAR-YOSEF, O . 1998. Eady colonizations and cultural References

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Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). Vol 463 | 4 February 2010 | doi:10.1038/nature08706

LETTERS Mammalian biodiversity on Madagascar controlled by ocean currents Jason R. Ali1 & Matthew Huber2

Madagascar hosts one of the world’s most unusual, endemic, diverse Madagascar and Africa have been separated for ,120 Myr by the and threatened concentrations of fauna1. To explain its unique, 430-km-wide Mozambique Channel19,20, the question is whether the imbalanced biological diversity, G. G. Simpson proposed the non-volant, non-aquatic migrants made their way to the island by ‘sweepstakes hypothesis’, according to which the ancestors of walking or by rafting. Madagascar’s present-day mammal stock rafted there from One hypothesis is that land bridges might have enabled fauna to Africa2. This is an important hypothesis in biogeography and evolu- walk. It has been argued that substantial portions of the Davie tionary theory for how animals colonize new frontiers1,3–5, but its Ridge—a prominent bathymetric feature running north–south validity is questioned5–9. Studies suggest that currents were indown the middle of the Mozambique Channel (Fig. 1)—may have consistent with rafting to Madagascar9 and that land bridges provided once been sub-aerially exposed and thus could have acted as a quasithe migrants’ passage5–8. Here we show that currents could have continuous causeway linking Africa and Madagascar8. Although this transported the animals to the island and highlight evidence inidea has received some support5–7,21, important weaknesses are consistent with the land-bridge hypothesis. Using palaeogeographic acknowledged because it would require a radical rethinking of regional reconstructions and palaeo-oceanographic modelling, we find that plate tectonics22. It also suffers logical flaws: if land bridges were strong surface currents flowed from northeast Mozambique and responsible, a greater variety of animals would have crossed and the Tanzania eastward towards Madagascar during the Palaeogene timing of arrivals would be correlated with the putative maximum period, as required by the ‘sweepstakes process’. Subextent(http://www.cadkas.com). of the land bridge, neither of which results is supported by Changed with exactly the DEMO VERSION of CAD-KAS PDF-Editor sequently, Madagascar advanced north towards the equatorial gyre data1,23. We further note that any islands the Davie Ridge may have and the regional current system evolved into its modern configuragenerated during the Cenozoic23 would have been small and separated tion with flows westward10 from Madagascar to Africa. This may by open-water gaps several tens to hundreds of kilometres apart24 explain why no fully non-aquatic land mammals have colonized (Fig. 1). Consequently, substantial tracts of ocean (.230 km) sepaMadagascar since the arrival of the rodents and carnivorans during rated them from the nearest land in eastern Mozambique and western the early-Miocene epoch. One implication is that rafting may be the Madagascar; thus, over-water dispersal was unavoidable. dominant means of overseas dispersal in the Cenozoic era when The over-water dispersal mechanism was first mooted by Simpson palaeocurrent directions are properly considered. nearly seventy years ago2. He proposed a ‘sweepstakes’ process by which Madagascar is home to one of the most intriguing inventories of small mammals—potentially with low metabolic rates and/or a habit of flora and fauna anywhere on Earth1. It is characterized by unusually seasonal torpor—were unwittingly rafted to the island on large logs or high levels of mammalian endemism combined with a uniquely broad vegetation mats washed off eastern Africa, either down large rivers or diversity from a limited number of orders: lemurs, tenrecs, carnivorans from the coastal strip25. Key predictions of Simpson’s argument, conand rodents1,11–13. Although our focus here is primarily on mammals, firmed by the colonization-history and geological data described above, similar patterns are observed for other terrestrial animals on the island, are the limited number of families that live on the island today, in particular the absence of large-bodied forms (for example antelopes, including amphibians and reptiles14,15, implying a broadly similar colonization mechanism. It is widely acknowledged that Madagascar’s apes, elephants or lions), and the seemingly random distribution of mammals are derived from Cenozoic migrants because they share none apparent arrivals in Madagascar from ,60 to ,20 Myr ago. of the detailed characteristics of the island’s late-Cretaceous forms16,17 The main criticism of this hypothesis is that inferred currents and and thus could not have evolved from them. The current stock’s prevailing winds, based on modern observations, are in the opposite ancestors journeyed from Africa at various times during the direction to those required. As elucidated in ref. 9, if today’s surfaceCenozoic, since 65 Myr ago, and each colonization appears to be the water currents in the region are used as a guide (Fig. 1), the strong result of a single arrival event1. The timing of apparent arrival events of south-southwest-directed coast-parallel flow of the Mozambique new taxa is a fundamental constraint on hypotheses about how they Current would have acted as barrier to eastward transport. Rafts accomplished the journey1. From molecular-clock dating estimates, it off the shore of Africa would have been entrained in the southward is possible to discern four distinct early-Cenozoic to mid-Cenozoic flow and thus could never have beached on Madagascar. Instead they events: the arrival of lemurs between 60 and 50 Myr ago, that of tenrecs would either return to the African shore or be transported north or between 42 and 25 Myr ago, that of carnivorans between 26 and 19 Myr south, but never substantially to the east. No quantitative attempts ago, and that of rodents between 24 and 20 Myr ago12,13. Thus, by have been made to estimate how currents in the region may have been ,20 Myr ago the major non-volant and non-swimming faunal groups different in the Cenozoic or what implications this may have for ocean dispersal routes. Past currents remain the major unknown in were established, with no further evidence of transfer, except for a few this controversial issue and many factors, including changing palaeolate-Quaternary arrivals such as the pygmy hippopotamus, which is semi-aquatic and known to swim significant ocean distances18. As geographic and palaeo-oceanographic setting, must be considered. 1 Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China. 2Earth and Atmospheric Sciences Department and the Purdue Climate Change Research Center, Purdue University, West Lafayette, Indiana 47907, USA.

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0°

Kenya

Seychelles Western Somali basin

Tanzania 10° S

Comoros islands

Africa

10° S South Equatorial Current

Madagascar

Mozambique

20° S

Mozambique

Madagascar Mozambique Current

Réunion

Channel Madagascar Ridge

Mozambique basin

East Madagascar Current (S)

M R

20° S

Latitude

Mascarene basin

Davie Ridge highs

South Africa

Madagascar basin

30° S

Mozambique Plateau 30° E

Seychelles East African Coastal Current

Agulhas Current

40° E

50° E

60° E

40° E

30° E

50° E

60° E

Longitude Land

200–2,000 m

0–200 m

2,000–4,000 m

Current flow

>4,000 m

Depth (m)

0 Macua seamount

1,000 2,000

Paisley seamount

Sakalave seamounts

3,000 19.70° S 41.90° E

~750 km

Figure 1 | Key geographical features/phenomena of the southwest Indian Ocean region. a, Simplified bathymetric map. b, Present-day surface-water circulation based on refs 25 and 26 (M, Mauritius; R, Re´union).

13.00° S 41.40° E

c, Bathymetric cross-section along the Davie Ridge. Even in the Eocene, when parts of the feature may have been sub-aerial, the deep troughs23 separating the peaks would have posed formidable barriers22.

Changed with the DEMO of CAD-KAS evolve (http://www.cadkas.com). according to the equations of motion and thermodynamics. The Indian Ocean basin hasVERSION altered considerably as a result ofPDF-Editor plate tectonics. For instance, since 60 Myr ago Australia and India have respectively migrated ,2,200 and ,4,000 km northwards. Globally, six major ocean gateways have either opened (Tasman–Antarctica, South America/Antarctica, Atlantic–Arctic) or closed (Panama Isthmus, Indonesian, Africa–Arabia–Eurasia). Critically, Africa and Madagascar have both moved some 15u (,1,650 km) closer to the equator20. Thus, the potential for surface-water flow in the southwest Indian Ocean to have been markedly different when the faunas are thought to have crossed to Madagascar must be considered. Today, the entire southern Indian Ocean, from Africa to Australia, lies within a large anticyclonic ‘supergyre’26. Northern Madagascar lies at the boundary between this gyre and a cyclonic gyre occupying the northern Indian Ocean26,27. The southern Indian Ocean gyre’s strongest flows are to the southwest of Madagascar, where they form the Agulhas current. A relatively strong eddying flow in the Mozambique Channel between Madagascar and Africa connects the gyre’s equatorial currents with the vigorous transports of the Agulhas system27. Another important feature of the southern Indian Ocean gyre concerns the fact that Australia spans the latitudes Madagascar does. As a consequence, the zonally integrated wind stress curl, which drives the circulation, is maximized near the southern tip of Africa27. To test the hypothesis that changes in the ocean circulation around Madagascar during the Cenozoic explain the observed pattern of land-mammal migrations to the island, an independent knowledge of surface ocean currents and surface wind stress is required. The only means of obtaining this information (as there are no independent proxy records for these variables) is from palaeo-oceanographic modelling. We performed and analysed a suite of experiments using a fully and interactively coupled ocean–atmosphere general circulation model (the Community Climate System Model, version 3 (CCSM3), of the US National Center for Atmospheric Research) with Eocene conditions (Methods). In such models, climate, wind and surface-water currents are predicted and, importantly, are free to

Thus, we have not specified the winds or sea surface temperatures that drive them, nor the ocean currents. In all of our simulations, the large-scale ocean current systems in the Eocene epoch were robustly different from modern observed and modelled circulations in several crucial ways relevant to the rafting problem. Africa and Madagascar were .10u south of their current positions, which placed Madagascar in the convergence zone at the heart of the subtropical gyre (Fig. 2). Furthermore, because Australia was also much further south, the northern part of that continent did not impede the accumulation of wind stress curl at the latitudes of Madagascar; therefore, the strongest current system in the entire Southern Ocean was just to the east of Madagascar (Fig. 2a). This vigorous eddy off the coast of Madagascar caused water-mass trajectories throughout the region to converge. As shown in Fig. 2a, the strong anticlockwise gyre directed much of the flow along the African coast eastward towards Madagascar rather than southward through the Mozambique Channel, towards the Agulhas Current, as occurs today. Peak simulated eastward velocities across the Mozambique Channel usually occur in January (Fig. 2b), and monthly mean velocities of .10 cm s21 occur for three to four months within each century of simulation. Trajectories starting in the region of northeast Mozambique and Tanzania sporadically experience enhanced eastward velocities of .20 cm s21 and could therefore have crossed the necessary distance in 25–30 days (Fig. 2c). This vigorous eastward flow was not constant, occurring for only three or four weeks within any century of simulation. Hence, on the long timescales of relevance to this problem (tens of millions of years), these velocities would have occurred many times, but not routinely. Because these are rare events, it is likely that even faster eastward currents occurred, albeit less frequently. Furthermore, flow through the Mozambique Channel was probably strongly eddying, as it is today, but our simulations do not have sufficient resolution to capture the vigorous, highkinetic-energy properties of these eddies, which today have velocities of ,100 cm s21 (ref. 27). Capturing such mesoscale variability would

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Figure 2 | Eocene ocean currents. a, Simulated annual mean vertically averaged (barotropic) currents (volume flux) and surface ocean currents (streamline vectors). The barotropic currents delineate the average positions of the major Eocene ocean gyres, with Madagascar at the heart of the strongest gyre on Earth, as described in the text. The modern location of Madagascar is shown in red outline, showing how the interplay of continental position and the gyres controls dispersal pathways. b, Ensembleaveraged monthly mean ocean surface currents for January, the month in

which climatological ocean current directions were optimal for transport towards Madagascar. c, During sporadic events, as typified by this ensemble average of the four optimal ocean current events evaluated from model output saved at a temporal resolution of three days, rapid transport directly to Madagascar from Africa was possible at rates of .20 cm s21. Currents are shown using vectors (with the scale shown), and the magnitude of the east–west current strength is shown in colour. The simulations are described in Methods. Mad, Madagascar.

only increase the estimated probability of the occasional very vigorous eastward transport from Africa to Madagascar, and would enhance transport across the Mozambique Channel in jet-like currents. Consequently, our speed estimate is almost certainly lower than the true maximum eastward rafting velocity. An additional consideration is the fact that tropical storms are known to generate the large, floating tree ‘islands’, as well as associated precipitation, that might make a successful ocean voyage of this type possible4. It is therefore noteworthy that preliminary analysis of modelled tropical cyclone activity indicates that this region was a locus of such activity, as it is today, and that the tropical cyclone season encompasses the period of highly favourable ocean currents (January). Thus, successful rafting may have involved the fortuitous coincidence of transient storms and ocean current activity. Thus, all signs point to the Simpson sweepstakes model being correct: ocean currents could have occasionally transported rafts of animals to Madagascar from Africa during the Eocene. Specifically, transport should have been from northeast Mozambique and

Tanzania to the north coast of Madagascar. Given the slow tectonic drift of the island, this configuration probably continued at least through the Oligocene epoch. However, by the early Miocene, Madagascar breached the margin of the subtropical and equatorial gyres. Thereafter, currents were perennially directed westward towards Africa, making the ocean journey for mammals to Madagascar much more difficult, if not impossible. METHODS SUMMARY We used the current version of the fully coupled (ocean/atmosphere/land/ vegetation/sea ice) global climate model CCSM3. The atmospheric resolution was set at ,3.75u ? 3.75u (T31 spectral resolution); the oceans had a nominal 3u longitudinal resolution (and variable latitudinal resolution) and 25 vertical levels. This model has been applied to a wide variety of modern28 and palaeoclimate studies, for example of the Holocene epoch and the Last Glacial Maximum29 and the Eocene30. We carried out a suite of fully coupled simulations for a simulated time of more than 3,000 yr without any acceleration until they had clearly reached equilibrium. A description of the suite of simulations is found in ref. 30; results from

655

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the particular simulation, with a concentration of atmospheric CO2 of 1,120 p.p.m., used in this study have not been previously described. Here we have concentrated on one simulation appropriate for mid-Eocene to late-Eocene conditions, although none of the results we have discussed are sensitive to the particulars of that choice, as the relevant boundary conditions and main palaeocurrents remained largely unchanged between the end of the Cretaceous period and the early-to-mid Miocene, when current systems shifted towards their modern state10. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. Received 28 May; accepted 25 November 2009. Published online 20 January 2010. 1.

2. 3. 4.

5. 6. 7.

10.

Yoder, A. D. & Nowak, M. D. Has vicariance or dispersal been the predominant biogeographic in Madagascar? Only time will tell. Annu. Rev. Ecol. Evol. Syst. 37, 405–431 (2006). Simpson, G. G. Mammals and land bridges. J. Wash. Acad. Sci. 30, 137–163 (1940). Heaney, L. R. Is a new paradigm emerging for oceanic island biogeography? J. Biogeogr. 34, 753–757 (2007). Thiel, M. & Haye, P. A. The ecology of rafting in the marine environment. III. Biogeographical and evolutionary consequences. Oceanogr. Mar. Biol. 44, 323–429 (2006). Tattersall, I. in Elwyn Simons: A Search for Origins (eds Fleagle, J. G. & Gilbert, C. C.) 397–408 (Springer, 2008). Tattersall, I. Historical biogeography of the strepsirhine primates of Madagascar. Folia Primatol. (Basel) 77, 477–487 (2006). Masters, J. C., de Wit, M. J. & Asher, R. J. Reconciling the origins of Africa, India and Madagascar with vertebrate dispersal scenarios. Folia Primatol. (Basel) 77, 399–418 (2006). McCall, R. A. Implications of recent geological investigations of the Mozambique Channel for the mammalian colonization of Madagascar. Proc. R. Soc. Lond. B 264, 663–665 (1997). Stankiewicz, J., Thiart, C., Masters, J. C. & de Wit, M. J. Did lemurs have sweepstake tickets? An exploration of Simpson’s model for the colonization of Madagascar by mammals. J. Biogeogr. 33, 221–235 (2006). von der Heydt, A. & Dijkstra, H. A. Effect of ocean gateways on the global ocean circulation in the late Oligocene and the early Miocene. Paleoceanography 21, PA1011 (2006). Goodman, S. M., Ganzhorn, J. U. & Rakotondravony, D. in The Natural History of Madagascar (eds Goodman, S. M. & Benstead, J. P.) 1159–1186 (Chicago Univ. Press, 2003). Yoder, A. D. et al. Single origin of Malagasy Carnivora from an African ancestor. Nature 421, 734–737 (2003). Poux, C. et al. Asynchronous colonization of Madagascar by the four endemic clades of primates, tenrecs, carnivores, and rodents as inferred from nuclear genes. Syst. Biol. 54, 719–730 (2005). Vences, M. Origin of Madagascar’s extant fauna: a perspective from amphibians, reptiles and other non-flying vertebrates. Ital. J. Zool. (Modena) 71 (suppl.), 217–228 (2004). Nagy, Z. T., Joger, U., Wink, M., Glaw, F. & Vences, M. Multiple colonization of Madagascar and Socotra by colubrid snakes: evidence from nuclear and mitochondrial gene phylogenies. Proc. R. Soc. Lond. B 270, 2613–2621 (2003).

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16. Krause, D. W. Fossil molar from a Madagascan marsupial. Nature 412, 497–498 (2001). 17. Krause, D. W. et al. Late Cretaceous terrestrial vertebrates from Madagascar: implications for Latin American biogeography. Ann. Mo. Bot. Gard. 93, 178–208 (2006). 18. Stuenes, S. Taxonomy, habits, and relationships of the subfossil Madagascan hippopotami Hippopotamus lemerlei and H.madagascariensis. J. Vertebr. Paleontol. 9, 241–268 (1989). 19. Rabinowitz, P. D., Coffin, M. F. & Falvey, D. The separation of Madagascar and Africa. Science 220, 67–69 (1983). 20. Ali, J. R. & Aitchison, J. C. Gondwana to Asia: plate tectonics, paleogeography and the biological connectivity of the Indian sub-continent from the Middle Jurassic through latest Eocene (166–35 Ma). Earth Sci. Rev. 88, 145–166 (2008). 21. Godinot, M. Lemuriform origins as viewed from the fossil record. Folia Primatol. (Basel) 77, 446–464 (2006). 22. Rabinowitz, P. D. & Woods, S. The Africa–Madagascar connection and mammalian migrations. J. Afr. Earth Sci. 44, 270–276 (2006). 23. Bassias, Y. Petrological and geochemical investigations of rocks from the Davie Fracture Zone (Mozambique Channel) and some tectonic implications. J. Afr. Earth Sci. 15, 321–339 (1992). 24. Krause, D. W., Hartman, J. H. & Wells, N. A. in Natural Change and Human Impact in Madagascar (eds Goodman, S. D. & Patterson, B. D.) 3–43 (Smithsonian Inst. Press, 1997). 25. Kappeler, P. M. Lemur origins: rafting by groups of hibernators? Folia Primatol. (Basel) 71, 422–425 (2000). 26. Schott, F. A., Xie, S. P. & McCreary, J. P. Jr. Indian Ocean circulation and climate variability. Rev. Geophys. 47, RG1002 (2009). 27. de Ruijter, W. P. M., Ridderinkhof, H. & Schouten, M. Variability of the southwest Indian Ocean. Phil. Trans. R. Soc. A 363, 63–76 (2005). 28. Yeager, S. G., Shields, C. A., Large, W. G. & Hack, J. J. The low-resolution CCSM3. J. Clim. 19, 2545–2566 (2006). 29. Otto-Bliesner, B. L. et al. Last glacial maximum and Holocene climate in CCSM3. J. Clim. 19, 2526–2544 (2006). 30. Liu, Z. et al. Global cooling during the Eocene-Oligocene climate transition. Science 323, 1187–1190 (2009).

Acknowledgements M. Nowak, W. de Ruijter, I. Tattersall and A. Yoder supplied reprints. J. Aitchison, R. Corlett and A. Switzer are thanked for sharing information. M.H. is supported by US National Science Foundation (NSF) grant 0927946-ATM and uses the US National Center for Atmospheric Research CCSM, which is supported by the NSF. M.H. acknowledges conversations with P. Koch and D. Raup on vicariance biogeography. All computing was performed at the Rosen Center for Advanced Computing, which is part of Information Technology at Purdue, Purdue University.

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Author Contributions J.R.A. initiated the study and was primarily responsible for the geologically related aspects. M.H. carried out the palaeo-oceanographic modelling and its interpretation. Both authors contributed to the writing of the paper. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to M.H. (huberm@purdue.edu).

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METHODS Our simulations incorporate the detailed boundary conditions (that is, topography, vegetation and bathymetry) developed previously for Eocene conditions30–32. The latitudinal position of the gyres and their strengths are very robust to boundary-condition changes appropriate to the Palaeogene as they are essentially Sverdrupian responses to wind stress changes as modulated by the slowly varying palaeogeographies31–33. As described in ref. 9, the rafting problem involves estimating the surface ocean currents with the potential additional influence of surface winds. Currents below the surface are not relevant because typical rafts, such as a tree trunk launched into the ocean from a river mouth during a flood, would not have ‘keels’ that extended below several metres. Similarly, it is unlikely they had substantial protruding elements that could have formed ‘masts’, and would thus have been unaffected by wind shear. Here we assume that the raft simply acted like a drifter embedded in the ocean surface current (the surface model level extends down to 8 m). Analysis of the modern simulation using this same model (not shown) produces ocean currents in agreement with observations and the prior work on Madagascar9. The Eocene simulation focused on in this study was carried out with an atmospheric CO2 concentration of 1,120 p.p.m. and all other boundary conditions set at near-modern, pre-industrial values. It is a branch simulation from a simulation with a higher concentration of atmospheric CO2 that was integrated for a simulated time

of more than 3,000 yr. After this spin-up time, the 1,120-p.p.m. simulation was integrated for 3,500 yr. We analysed the monthly mean output from the last 400 yr of simulation to generate Fig. 2a, b. Output at 3-d resolution for the final century of integration was used to generate Fig. 2c. Although the magnitude of the optimal eastward flow is somewhat dependent on the averaging period (peak velocity increases from ,13 cm s21 when monthly mean values are used to ,23 cm s21 when 3-d means are used), the qualitative aspects of the flow are not sensitive to averaging length. In the sampling of both the mean configuration and the extreme events, flow is always westward from Madagascar towards Africa because this is the direction determined by the large-scale gyres in the modern era. Preliminary results from a comparable Miocene (15 Myr ago) simulation show similar results to the modern-day one. All file processing and graphics were performed using the US National Center for Atmospheric Research Command Language (http://ncl.ucar.edu). 31. Huber, M., Sloan, L. C. & Shellito, C. J. in Causes and Consequences of Globally Warm Climates in the Early Palaeogene (eds Wing, S. L., Gingerich, P. D., Schmitz, B. & Thomas, E.) 25–47 (GSA Special Paper 369, Geological Society of America, 2003). 32. Huber, M. & Nof, D. The ocean circulation in the southern hemisphere and its climatic impacts in the Eocene. Palaeogeogr. Palaeoclimatol. Palaeoecol. 231, 9–28 (2006). 33. Huber, M. et al. Eocene circulation of the Southern Ocean: was Antarctica kept warm by subtropical waters? Paleoceanography 19, PA4026 (2004).

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Integración de los factores que forman biodiversidad Africana de la actualidad

María Regina Zaghi Lara Distintos eventos que varían desde la formación del continente hasta la variación de su composición hídrica han llevado al establecimiento de la biodiversidad actual. Es importante, entonces, darle énfasis a los eventos que permitieron la distribución de la vida en la actualidad, incluyendo la aspectos de la evolución humana. El sistema de drenaje actual de África surge como resultado de muchos procesos en la formación y evolución del continente africano, sucesos como el rompimiento del supercontinente Gondwana pudo influir directamente (Moore 2004) así como la acción de las placas tectónicas (King 2006). Esto implica que ha habido cambios en los sistemas hídricos del continente y ha encontrado evidencia en la composición de sedimentos y fauna asociada a los cuerpos de agua que muestran un cambio en las rutas de drenaje de varios ríos a lo largo de los años. Datos desde finales del cretácico indican que ha habido cambios importante en los sedimentos de los ríos Limpopo y Zambezi lo cual se ha generado por los distintos grados de erosión del material erosionable de los ríos y cambios en el clima y por lo tanto en el flujo de corrientes (Moore 2004). Los cambios en los grados de erosión también surgen como resultado de la actividad tectónica, ya que con la formación de montañas, volcanes y valles se determinan nuevos sitios erosionables Changed with the DEMOáreas VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com). y nuevas de acumulación de agua, las cuales pueden ir cambiando con la actividad tectónica (King 2006). Tanto el recurso de agua como el resultado del acomodamiento de placas tectónicas son factores importantes para la distribución de la vida y los distintos biomas en los continentes. En uno de los artículos se habla de la utilización de un programa para determinar el cambio y fragmentación de biomas que pudo ocurrir en la última glaciación y a principios del Holoceno (hace unos 6 millones de años. Se basaron en los registros de la vegetación para interpretaciones climáticas y biogeográficas y lograr interpretar cómo estos cambios afectan a la biodiversidad )(Prentice et al 2007). La utilización de modelos y programas también es útil para hacer verificaciones de otros factores que pueden influir como las corrientes y áreas de ciclones para la elaboración de hipótesis sobre la distribución de la biodiversidad actual en África y Madagascar (Ali 2010). La composición de gases atmosféricos también afecta los procesos de vida y van variando de acuerdo a las épocas. Sabiendo esto, se hace una diferenciación entre la cantidad de CO2 en la atmósfera, especialmente a principios del Holoceno donde había una mayor concentración del gas que en el Pleistoceno donde las concentraciones eran menores. Esto afecta directamente la producción y utilización del agua en las plantas y por lo tanto, genera cambios en la composición taxonómica de los biomas (Ali 2010) Los datos de África son escasos pero indican que en las glaciaciones, la temperatura había disminuido considerablemente, y por lo tanto, bosques de altura se encontraban en lugares más bajos; también dieron sequías que se representaban en por estepas donde hoy hay bosque tropical (Ali 2010).

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Tanto la actividad tectónica (King 2006), la distribución de los drenajes de agua (Moore 2004), la composición de gases atmosféricos (Ali 2010) y la distribución de las corrientes y actividad ciclónica de África (Prentice et al 2007) tienen una influencia directa en la distribución de los biomas en el continente, en la evolución de los humanos y en la migración de fauna a la Isla de Madagascar. La composición de los biomas se pudo ver afectada porque las plantas migran buscando adaptarse a los cambios en el ambiente, lo cual afecta el pool genético de las poblaciones existentes. Esto implica un cambio en taxa y cambios en la composición de los biomas, tomando en cuenta que especies encontrarán refugio en biomas que tienen mayor dominancia porque las condiciones ambientales les favorecen, como sucedió con bosques de altura en la última glaciación (Ali 2010). Las corrientes oceánicas favorecen ciertos patrones climáticos dentro del continente y las mismas pueden ir cambiado por diversas razones. Existen datos que muestran que durante el Cenozoico hubo periodos cortos en los que las corrientes entre África y Madagascar se encontraban de manera inversa a su movimiento actual, permitiendo el paso accidental de especies de mamíferos y plantas a la isla (Prentice et al 2007). Los movimientos de las corrientes en la actualidad siguen un patrón inverso a ese, lo cual también afecta a las poblaciones vegetales de las costas africanas (Ali 2010). En uno de los artículos se argumenta que la variedad de geoformas en el terreno africano fue uno de los factores que tuvo mayor influencia en la evolución de la especie humana. Se argumenta que el cuerpo humano está mayormente adaptado para desenvolverse mejor en áreas más complejas con árboles y montañas que en áreas abiertas. Además, se toma en cuenta que conforme el avance en el cerebro humano(http://www.cadkas.com). se van generando nuevas exigencias y Changed with the DEMO VERSION of CAD-KAS PDF-Editor necesidades por lo que es necesario ambientes con más fácil accesibilidad a los recursos (King 2006). El agua es un recurso importante que va siendo definido por los movimientos tectónicos, la facilidad de erosión de los suelos y por los cambios climáticos (Moore 2004), siendo importante entonces la cercanía a estos recursos, esta teoría concuerda con la disposición geográfica del continente. Por otro lado se argumenta que el cuerpo humano tiene una mayor disposición para vivir en zonas de bosque y composiciones vegetales más complejas para lograr acceso a recursos, evitar depredadores y cuidar a las crías (King 2006). Los cambios en la disposición de biomas muestran que a principios del Holoceno hubo una reducción significativa de las áreas desérticas del Sahara, lo cual fue provocado por el fenómeno de monzones; esto permitió la colonización e intercambio de varias especies del Sur de África y del Norte del continente incluyendo la facilitación de migración de la especie humana (Ali 2010). Diversos eventos geológicos y climáticos han definido la distribución de la vida en el continente africano hasta encontrar las distribuciones y comunidades que vemos hoy en día. Los distintos cambios que se irán generando a partir de la acción humana también es un factor importante que se añade a los anteriores para la determinación de la biodiversidad de un lugar y ciertamente todos los factores seguirán teniendo un gran impacto en la vida del continente africano.

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MarĂa Regina Zaghi Lara Carnet 09257 Universidad del Valle de Guatemala Changed with the DEMO VERSION of CAD-KAS PDF-Editor (http://www.cadkas.com).