Human evolution an outline

Details of human evolution are controversial but some points are generally

  • The evolutionary lines that led to modern chimpanzees and modern humans probably diverged between five and seven million years ago.
  • The common ancestor of humans and chimpanzees behaved intelligently in the sense explored in chapter 16 (learning, predicting, solving problems correctly on the basis of incomplete data, exhibiting flexibility and novelty of behaviour, and so on).
  • Although many of the connections between one stage of human evolution and the next are uncertain, a plausible chronology of hominid types can be constructed (see the diagram).
  • The most dramatic feature of human evolution was a rapid increase in brain size relative to body size. The evidence for this comes from skull fragments, which enable us to calculate cranial capacity and hence brain size. Modern chimpanzees have somewhat smaller bodies than modern humans, but very much smaller brains. An adult chimpanzee's brain measures 450 cc; an adult human's measures 1300-1400 cc. In other words, the size of the adult human brain has tripled during a mere six million years or so of evolution.
  • Before cranial capacity (brain size) increased significantly, hominids became bipedal. Most people44 agree that this development was crucial. Walking on two legs liberated the hands for tool use. Tool use preceded and may have been instrumental in "causing" the expansion of the brain.
  • In the most recent evolutionary steps, from Homo habilis to H. erectus and finally H. sapiens, much of the increase of brain size probably involved the frontal lobe45.
  • From the earliest hominids onwards, our ancestors seem to have been very highly social. The genus Homo has probably produced the most social mammals ever. Individuals have always depended for their survival on close co-operation within groups.

All stages of human evolution except possibly the most recent have taken place in Africa. The very oldest Australopithecines46 - presumed to be our earliest ancestors after division from the chimpanzee line - have been found in Ethiopia and South Africa. They or similar species probably occupied the part of the continent between these two areas. Opinion is divided about whether H. sapiens (1) originated in Africa like all its forebears and then migrated into Asia and Europe, or (2) evolved from H erectus after the latter had migrated to other parts of the world. The first alternative, the "out-of-Africa hypothesis", has powerful support. A very early H. sapiens grave (about 55,000 years old) has been found in Upper Egypt, and no comparable finds of the same age have been made outside Africa. This implies that sapiens originated in Africa. Also, some H. erectus remains in Java date from a time (30-50,000 years ago) after H. sapiens had reached that part of Asia, so the two "species" presumably co-existed for a time. This finding is difficult to reconcile with the second alternative, the "multiple origins hypothesis". On the other hand, some ancient Australian art appears to be

44 One of the first people to propose this idea was Frederick Engels. Long before anything was known about the ancestry of modern humans, Engels argued (for political reasons) that technology must precede knowledge; labour is the antecedent of thought; the work of the hand leads the work of the mind. So far as human evolution is concerned, he appears to have been correct.

It is customary to regard H. habilis, erectus and sapiens as three different species. However, it is hard to be sure that they could never have interbred.

The prefix "Australo" indicates "south" rather than Antipodean. The remains of these earliest of human ancestors, up to 3-4 million years old, were first found in southern Africa.

Fig. 17-1: a possible outline of the evolution of Homo sapiens. The vertical axis is a time scale indicating millions of years before the present.

more than 50,000 years old. These creations were presumably the work of H. sapiens not H. erectus (see below), and sapiens was unlikely to have reached Australia 50,000 years ago if the "out-of-Africa" hypothesis were correct. So there is some support for the "multiple origins" hypothesis. The issue is far from settled.

At various stages in human evolution two or more distinct races, or subspecies, or closely-related species of hominid must have met each other. When this happened they would either have ignored each other, mated with one another or killed one another. Probably they followed all three options at different times and in different places.

This applies not only to sapiens and erectus, but also to the more recent interaction between modern humans and Neanderthals (30-25,000 years ago). It is still not clear how the Neanderthals were connected to the rest of the hominid line of descent. They were heavier-boned and heavier-muscled than modern sapiens but their cranial capacity was the same or slightly greater. Evidence suggests that H. sapiens entered what is now Europe around 35,000 years ago, when Neanderthals were already in residence. By 30,000 years ago the two races, or subspecies, had met at various locations in Iberia and probably elsewhere. Since the Neanderthals subsequently disappeared, consensus opinion holds that modern humans wiped them out. Mitochondrial DNA evidence supports this view: the mitochondrial DNA of Neanderthals was significantly different from that of modern humans. However, skeletons have been discovered that are unquestionably those of Neanderthal-sapiens hybrids. These skeletons date from around 25,000 years ago, i.e. some four or five millenia after the two races first made contact. Since mitochondrial DNA is inherited solely from the female line, some of us might possibly be descended from hybrids whose fathers were Neanderthals and whose mothers were sapiens.

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