Hominines

Hominines

Lecture 18

Many of the crucial differences between plants and animals arise from this simple but fundamental difference: They can photosynthesize; we can’t.

The last two lectures described evolution in general, focusing on those evolutionary lines that would eventually lead to our species. Now we are ready to ask how our ancestors evolved from the primates. First we must be clear about our place in the biological world. We have seen that modern systems of biological classi cation (or “taxonomy”) build on the work of Carl Linnaeus (1707–1778). The multiple levels of a taxonomy (from superkingdom to species) allow us to de ne each species uniquely so as to reveal its position in the huge family tree of life. You and I belong to the “superkingdom” of eukaryotes (we’re made from eukaryotic cells); the “kingdom” of animals (we’re not single-celled, nor are we plants or fungi); the “phylum” of vertebrates, or “chordata” (we have backbones); the “class” of mammals (we’re furry, warm-blooded, and our young develop within the womb); the “order” of primates (lemurs and monkeys); the “family” of hominoids (great apes); the “subfamily” of hominines (bipedal apes); the “genus” Homo; and the “species” Homo sapiens. (Note that classi cation systems differ in details.) In short, we are eukaryotic, multi-celled mammals from the order of primates.

The order of primates appeared about 65 million years ago, at about the time of the Cretaceous asteroid impact. The primates include all monkeys and lemurs, from tarsiers to gorillas, as well as humans! Primates share some distinctive features. Because early primates lived in trees, they developed stereoscopic vision and grasping limbs. Perhaps because visual information requires a lot of processing capacity (for brains as for computers), primates have disproportionately large brains for their size. (Elephants have huge brains, but their bodies also use a lot of computing capacity so it’s brain size relative to body size that really counts.) Larger brains generally imply longer lives to take advantage of the brain’s capacity to learn. The sense of smell is less important, so most primates have small snouts and attish faces.

The “great apes” are a “family” within the “order” of primates. They are large, intelligent, tailless primates. The family includes chimpanzees, gorillas, orangutans, gibbons, and humans as well as many extinct species. The great apes evolved in Africa about 18 million years ago, though some eventually migrated to other parts of Eurasia. Only in recent decades have we begun to realize how complex their social lives are, through the pioneering work of researchers such as Jane Goodall (1932–) and Dian Fossey (1932–1985), who studied the great apes in their own environments. The striking similarities In short, we are eukaryotic, between us and the great apes persuaded multi-celled mammals Darwin that we were closely related. from the order of primates. The “hominines” (or “hominids”) are a “subfamily” of apes that walked upright. They evolved 7 million years ago. This date is based on comparisons of the genetic material of modern great apes, because we have few fossils from this era. Genetic evidence suggests that the human line split from the chimp line about 7 million years ago, and from the gorilla line about 8–10 million years ago. The hominines diverged rapidly in a new “radiation” that may have included 20 or 30 distinct species. However, today, we are the only survivors of this diverse group of great apes. In recent decades many hominine fossils have turned up, and these show that the rst hominines were characterized not by large brains, but by bipedalism—the ability to walk on two legs. Why bipedalism? We have no conclusive answers. The creation of the African Rift Valley beginning 15 million years ago raised mountains that left much of East Africa in a rain shadow and reduced forest cover. French researcher Yves Coppens suggested that bipedal species would have found it easier to travel through the resulting grasslands and to see dangers and opportunities as they did so. Unfortunately, the recent discovery of hominine species in forested environments has undermined this promising idea. In what follows, we will focus on just three major groups of hominines. The “australopithecines” were a large and varied group of hominines that lived from about 4 million to about 1 million years ago. They include Lucy, whose remains are described in the next lecture. Australopithecines were bipedal,

though they may still have used trees as refuges. They were shorter than humans, the tallest being less than ve feet tall. At about 400–500 cubic centimeters (cc), their brains were slightly larger than those of chimps (300–400 cc) but about one-third the size of human brains (which average about 1,350 cc). Some may have made stone tools, but there is no sign of signi cant technological ability or enhanced linguistic ability. Though we are probably descended from one branch of the australopithecines, we would probably not think of them as humans if we were to meet one in the street today.

Homo habilis (“handy man”) appeared about 2.3 million years ago. Homo habilis were probably just under 5 feet tall and had brains of 600–800 cc. Louis Leakey regarded them as the rst real humans because they made stone tools. (This is a powerful reminder of how paleontologists can shape such stories. Leakey belonged to a generation that regarded the use of tools as the key marker of humanity.) Tool use implies considerable intellectual ability and may have improved diets by making it easier to scavenge meat. But today few paleontologists share Leakey’s view that they were the rst true humans. This is partly because they had small brains, partly because other ape species have been shown to use tools, and partly because their technologies did not evolve signi cantly.

Another species, Homo ergaster, evolved about 1.8 million years ago. They were as tall as us, and their brains (at about 1,000 cc) were almost as large as ours. They made more sophisticated “Acheulian” stone tools, often in the form of carefully manufactured stone “axes.” Members of this species were the rst hominines to migrate out of Africa. As these migrants entered the colder lands of southern Eurasia, reaching as far as modern Beijing, they may have learned to use re. Nevertheless, their stone tools changed little over 1 million years, which suggests that they too lacked the technological creativity that distinguishes modern humans.

By 1 million years ago most of the features that de ne us as a species were already present, but there was no sign yet of the technological and ecological creativity that makes us so different. Clearly some sort of threshold still had to be crossed. But before we describe the crossing of that critical threshold, we must examine the evidence used to trace the evolution of our species.

Essential Reading

Supplementary Reading

Questions to Consider

Brown, Big History, chap. 3. Christian, Maps of Time, chap. 6. Fagan, People of the Earth, chap. 2.

Johanson and Edey, Lucy. Jones, The Cambridge Encyclopedia of Human Evolution. Lewin, Human Evolution.

1. What features do humans share with other primates?

2. Was Louis Leakey wrong to regard Homo habilis as humans?