The Origins of Agriculture
How long have humans been cultivating plants for food, fiber? When and where did agriculture originate? Where did modern cultivated plants originate and what are their wild relatives? How have food plants changed as a result of human cultivation?
The Origins of Agriculture 15,000 years ago all plants associated with human settlements were wild Oldest evidence of domesticated crops dates from 12,000 years ago transition from hunting/gathering to agriculture was gradual hunter/gatherer societies supplemented diet by cultivating increasing populations made hunter societies more agriculture dependent Domestication defined as the genetic alteration of plants through human activity Domestication generally accompanies cultivation, through a process of selection selection of particular variants, genotypes over multiple crop cycles leads to plant types unlike their progenitors
Evidence for the age of oldest cultivated plants 1.  Fossil evidence Plant materials associated with human settlements fossil plants, pollen Wear patterns of fossil human teeth can indicate diet Carbon isotope ratios grasses (source of grain) have distinctive isotope ratios specific C isotopes 12C:13C incorporated during photosynthesis isotope ratios in human bone can be used to infer grain consumption 2.  Carbon isotope dating Change in 14C isotope over time can be used to date age of dead plant material 14C is an unstable isotope, once incorporated by photosynthesis decays to N rate of decay is slow, half-life is 5700 years 14C ratio in fossil plant material can be used to accurately date the material range is 5,000 – 50,000 years
Evidence for the age of oldest cultivated plants Plant material associated with human settlements compared with wild plants Plant and animal material older than 15,000 years ago same as wild Therefore before 15,000 no plant domestication Hunter-gatherer societies First evidence of plant domestication at 12,000 years ago Small scale agriculture, horticulture A few plants cultivated to supplement hunting and gathering Gradual increase in intensity and variety of plant cultivation Gradual change in dependence of societies on agriculture Increasing dependence on agriculture supported larger populations Centers of Agricultural Origin developed in Asia, Africa, C & S America From 10,000 to 2,000 years ago agriculture became dominant By 2,000 years ago all major civilizations agricultural Agriculture supported large populations
Centers of Origin Concept
Nicolai Vavilov (1887-1943) Russian botanist, geneticist Director of the Soviet All Union Institute of Agricultural Sciences, 1920- 1940. Developed the theory of origin of cultivated plants Vavilov reasoned that “centers of diversity�, regions where many crop species traditionally grown and with wild relatives of the cultivated crops, likely were the centers of origins of the food crops. Published "Studies on the Origin of Cultivated Plants" in 1926, in which eight centers of origin were identified.
Centers of Origin Concept
Vavilov’s Centers of Origin: 1 China 2 India 2a Indochina 3 Central Asia (N. India, Afghanistan, Turkmenistan) 4 The Near East 5 Mediterranean Sea, coastal and adjacent regions 6 Ethiopia 7 Southern Mexico and Middle America 8 Northeastern South America, Bolivia, Ecudor, Peru 8a Chile Notably missing is Africa outside of Ethiopia
Centers of Origin Concept
J. R. Harlan (1917-1998) Harlan significantly modified Vavilov’s theory of centers of origin of agricultural crops. Harlan published “Agricultural origins: centers and noncenters” (1971) in which he noted the inadequacy of Vavilov’s centers of diversity theory. He noted that centers of diversity commonly do not coincide with centers of origin, and instead proposed that agriculture originated independently in three regions (equatorial Africa, SE Asia, and Central and S America). Each region comprised a small area of origin that was coupled with a much larger (ca. 10,000 sq km) “noncenter” in which plant domestication took place over a wider geographic area.
Where did common food plants originate? Center of Origin
Crop Plants
Near East
emmer (wheat), einkorn (wheat), barley, pea, lentil, chickpea, date, flax, fig,
West Africa
sorghum, pearl millet, African rice, African yams, cowpea, oil palm
China
buckwheat, millet, rice, soybean
SE Asia
banana, sugarcane
Andes (high elev.)
quinoa, potato
Andes (mid elev)
amaranth, groundnut, coca, common bean, lima bean
Central America
avocado, chili, cotton, maize manioc, pineapple, squash sweet potato, yams
North America
Sunflower
Geographic origins of food crops Centers of Origin are shaded
Cultivated plants from the Mesoamerican North American and South American centers of origin Mesoamerican Cereals Maize Starch plants Amaranth, Chenopodium Legumes Phaseolus beans Starchy roots & tubers Sweet potato, cassava (manihot), jĂcama Oilcrops Cotton Fiber Cotton, agave Fruits Papaya, avocado, guava Vegetables and spices peppers, squash, tomato, vanilla Stimulants Cacao, quinine
South American Starch plants Amaranth, Chenopodium Legumes Ground nut (peanut), Phaseolus beans Roots & tubers Cassava (manioc), jicama, potato Oilcrops peanut, cotton Fiber cotton Fruits & nuts cashew, pineapple, Brazil nut, papaya, avocado, guava Vegetables & spice chili pepper, squash Stimulants coca, yerba matĂŠ tobacco
North American
Oil plants Sunflower Fruits & nuts blueberry, cranberry, strawberry, pecan
Cultivated Plants from the China and Southeast Asian centers of origin China Cereals rice, millet Legumes soybean, adzuki bean Starchy roots, tubers, etc turnip, yams Oil crops rape seed Fruits & nuts chestnut, quince, persimmon, apricot, peach Vegetables & spices onion, ginger Stimulants tea, ginseng Fiber hemp
India, SE Asia and Pacific Islands Cereals rice Legumes pigeon-pea Starchy roots, tubers, etc. yam, arrowroot, taro, sugarcane Oil plants coconut, sesame Fruits & nuts banana, bread fruit, orange, grapefruit, mango Vegetables & spices cucumber, nutmeg, eggplant, plantain Fiber plants coconut
Cultivated plants from the African and Near East Centers of Origin Near Eastern & Mediterranean
Africa
Cereals barley, wheat, rye, oats Legumes pea, faba bean, lentil Starchy roots & tubers turnip, carrot, radish Oil plants rape seed, safflower, flax, olive Fruits & nuts fig, walnut, date palm, almond, grape Vegetables & spices leek, lettuce, saffron, parsley Stimulants poppy Fiber flax
Cereals pearl millet, sorghum Legumes cowpea, groundnut, Starchy roots & tubers yam Oil plants oil palm, castor bean Fruits & nuts watermelon, musk melon Vegetables & spices okra Stimulants coffee
How do plants change in domestication? Many wild growing plant populations have wide variation in various traits wild populations are subject to natural selection natural selection means change due to effects of competition, predation, etc Domestication selects plants for desirable traits domesticated populations are subject to artificial selection artificial selection means change due to selection by humans for desirable traits some traits result from deliberate selection, e.g. color, yield some traits result from unintentional selection e.g. nonshattering heads, increased seedling vigor Traits commonly associated with plant domestication Increased reproductive effort (more flowers per plant) Larger seeds and fruit Nonshattering heads (grains) Uniform, rapid seed germination (reduced seed dormancy) Uniform ripening Self-compatible pollen, selfing, inbreeding Increased palatability (reduced acids, tannins) Color changes Loss of defensive structures
How do plants change in domestication?
Planting seeds densely increases seedling competition favors earlier germinating, more vigorous seedlings result of ‘natural selection’ in domestication “Automatic” selection as a result of planting and harvesting seed favors certain traits non-shattering cereal grain heads (seeds/fruits do not disperse) determinate growth increased seed set (pollination) decreased infertile flowers Increased flower size and number Increased seed/fruit size Loss of germination inhibitors (seed dormancy) Increased seedling vigor
How do plants change in domestication? Inbreeding Most flowering plants are self-incompatible (outcrossing) gametes (pollen and egg) from the same individual do not result in a fertile zygote Inbreeding or self-fertilization refers to fertile fusion of gametes from the same individual, or between genetically very similar individuals Self-pollination can only occur if both male (stamens) and female (carpel) structures occur on the same plant (imperfect flowers) or in the same flower (perfect flowers) Monoecious plants have either perfect flowers with both stamens and carpels, or imperfect staminate and carpelate flowers on the same plant. Dioecious plants have only unisexual (imperfect) flowers on one individual. Self-comptibility tends to be a desirable trait in domesticated plants Seeds resulting from inbreeding are genetically identical Uniform germination, growth, ripening etc
How do plants change in domestication? Hybrids Many domesticated plants are hybrids, a cross of two different species Example: Domesticated strawberries originated in Europe as the result of an accidental hybridization between Fragaria chiloensis from S. America and F. virginiana from N. America when the two species were planted together in a botanical garden. Hybrids are also often the result of a cross between two inbred, homozygous varieties. Hybrid crosses of inbred lines are often larger, produce larger seed crops than either homozygous parent. This is called hybrid vigor. Hybrid seed must be produced each year, either because the resulting hybrids are sterile, or if they are fertile their offspring would be heterogeneous. Most maize is produced from hybrid seed.
A double hybrid cross for producing hybrid corn seed
How do plants change in domestication? Polyploids Ploidy refers to the number of chromosomes in the nucleus. A diploid organisim has two sets of chromosomes, one from the maternal and one from the paternal parent. Polyploids have different multiples of a set of chromosomes: tetraploids have four sets hexaploids have six sets octoploids have eight sets, etc Polyploids are often larger and have larger fruits or seeds than diploid and so may have traits that would have been selected by early farmers Polyploids can arise spontaneously from nondisjunction paired chromosomes fail to separate at first meiotic division results in diploid gametes (in diploids gametes are haploid) a cross between two diploid gametes results in a tetraploid zygote naturally occurring tetraploids are not uncommon in some plant species
How do plants change in domestication?
Polyploids Hybrid plants are often reproductively sterile Functional gametes do not form because chromosomes from different parent species do not “recognize� their homologues to pair with at meiosis Example, a cross between a diploid and a haploid gamete results in a triploid hybrid. The hybrid may be viable, but in meiosis one set of chromosomes does not have homologues, so infertile gametes are produced. But if two diploid gametes combine to form a tetraploid hybrid zygote, the plant will not be sterile because all chromosomes will have homologues and normal meiosis can occur. Gametes of tetraploids are diploid.
How do plants change in domestication?
Wheat domestication involved both hybridization and polyploidy 1. Diploid einkorn wheat (Triticum monococcum) hybridized with diploid goat grass (Triticum spp.). AB genotype 2. Spontaneous tetraploid resulted in emmer and durum wheat. AABB genotype
AB hybrid
3. Another hybridization with a different goat grass species resulted in hexaploid bread wheat T. aestivum. ABD genotype 4. Another doubling of the chromosomes resulted in hexaploid wheat AABBDD genotype
ABD hybrid
How do plants change in domestication? Polyploids Some domestic crops that are polyploid Crop
base Total chromosome chromosome number number
Ploidy
Coffee
11
44
Tetraploid
Cotton
13
52
Tetraploid
Potato
12
48
Tetraploid
Strawberry
7
56
Octaploid
Bread Wheat
7
42
Hexaploid
Durum Wheat 7
28
Tetraploid
Sugarcane
80
Octaploid
10