Every human population living today has interbred with every other human population with which it has had extensive contact. Ecological differences among existing humans are entirely a product of childhood education: it is not the case that some of us are born with sharp teeth and equipped to hunt deer, while others are born with grinding teeth, gather berries, and do not marry the deerhunters. Therefore all modern humans belong to the same species.
On perhaps two occasions in the past, however, the human lineage split into separate species, as distinct as wolves and coyotes. The most recent such occasion, which I shall describe later, may have been at the time of the Great Leap Forward. The earlier occasion was around three million years ago, when our lineage split into two: a man-ape with a robust skull and very big cheek teeth, assumed to eat coarse plant food, and often referred to as Australopithecus robustus (meaning 'the robust southern ape'); and a man-ape with a more lightly built skull and smaller teeth, assumed to have an omnivorous diet, and known as Australopithecus africanus ('the southern ape of Africa') >The latter man-ape evolved into a larger-brained form termed Homo habilis ('man the handyman'). However, fossil bones often attributed to male and female Homo habilis differ so much in skull size and tooth size that they may actually imply another fork in our lineage yielding two distinct kahilis-like species: Homo habilis himself, and a mysterious 'Third Man'. Thus, two million years ago there were at least two, and possibly three, proto-human species. The third and last of the big changes that began to make our ancestors more human and less apelike was the regular use of stone tools. This is a human hallmark with clear animal precedents: woodpecker finches, Egyptian vultures, and sea otters are among the other animal species that evolved independently to employ tools in capturing or processing food, though none of these species is as heavily dependent on implements as we are now. Common chimpanzees also use tools, occasionally of stone, but not in numbers sufficient to litter the landscape. But by around two-and-a-half million years ago, very crude stone tools appear in numbers in areas of East Africa occupied by the proto-humans. Since there were two or three proto-human species, who made the tools? Probably the light-skulled species, since both it and the tools persisted and evolved. With only one human species surviving today but two or three a few million years ago, it is clear that one or two species must have become extinct. Who was our ancestor; which species ended up instead as a discard in the rubbish-heap of evolution; and when did this shakedown occur? The winner was the light-skulled* Homo habilis, who went on to increase in brain size and body size. By around 1,700,000 years ago the differences were sufficient that anthropologists give our lineage a new name, Homo erectus, meaning 'the man that walks upright'. (Homo erectus fossils were discovered before all the earlier fossils I have been discussing, so anthropologists did not realize that Homo erectus was not the first proto-human to walk upright.) The robust man-ape disappeared around 1,200,000 years ago, and the 'Third Man' (if he ever existed) must have disappeared by then also. As for why Homo erectus survived and the robust man-ape didn't, we can only speculate. A plausible guess is that the robust man-ape could no longer compete, since Homo erectus ate both meat and plant food, and since tools and a larger brain made Homo erectus more efficient at getting even the plant food on which his robust sibling depended. It is also possible that Homo erectus gave his sibling a direct push into oblivion, by killing him for meat.
Several branches of our family tree have become extinct, including those belonging to the robust australopithecines, Neanderthals, and possibly a poorly understood 'Third Man' and an Asian population contemporary with Neanderthals. Some descendants of Homo habilis survived to evolve into modern humans. To recognize by different names the changes in fossils representing this line, they are somewhat arbitrarily divided into Homo habilis, then Homo erectus appearing about 1.7 million years ago, and Homo sapiens appearing about 500,000 years ago. A. stands for the genus name of Australopithecus, H. for Homo.
All the developments that I have been discussing so far were played out within the continent of Africa, to which our closest living relatives (the chimps and gorilla) are still confined. The shakedown had left Homo erectus as the sole proto-human on the African stage. Around one million years ago Homo erectus expanded his horizons. His stone tools and bones show that he reached the Near East, then the Far East (where he is represented by the famous fossils known as Peking Man and Java Man) and Europe. He continued to evolve in our direction by an increase in brain size and in skull roundness. By around 500,000 years ago, some of our ancestors looked sufficiently like us, and different enough from earlier Homo erectus, to be classified as our own species (Homo sapiens, meaning 'the wise man'), though they still had thicker skulls and brow ridges than we do today.
Readers unfamiliar with details of our evolution might be forgiven for assuming that the appearance of Homo sapiens constituted the Great Leap Forward. Was our meteoric ascent to sapiens status half-a-million years ago the brilliant climax of Earth's history, when art and sophisticated technology finally burst upon our previously dull planet? Not at alclass="underline" the appearance of Homo sapiens was a non-event. Cave paintings, houses, and bows and arrows still lay hundreds of thousands of years off in the future. Stone tools continued to be the crude ones that Homo erectus had been making for nearly a million years. The extra brain size of those early Homo sapiens had no dramatic effect on our way of life. That whole long tenure of Homo erectus and early Homo sapiens outside Africa was a period of infinitesimally slow cultural change. In fact, the sole candidate for a major advance was possibly the control of fire, of which caves occupied by Peking Man provide one of the earliest indications in the form of ash, charcoal, and burnt bones. Even that advance—if those cave fires really were man-lit rather than natural—would belong to Homo erectus, not Homo sapiens.
Thus, the emergence of Homo sapiens illustrates the paradox discussed in Chapter One, that our rise to humanity was not directly proportional to the changes in our genes. Early Homo sapiens had progressed much further in anatomy than in cultural attainments along the road up from chimpanzeehood. Some crucial ingredients still had to be added before the Third Chimpanzee could conceive of painting the Sistine Chapel.
How did our ancestors make their living during the one-and-a-half million years that spanned the emergence of Homo erectus and Homo sapiens'?
The only surviving tools from this period are stone tools that can charitably be described as very crude, in comparison with the beautiful, polished stone tools made until recently by Polynesians, American Indians, and other modern stone-age peoples. Early stone tools vary in size and shape, and archaeologists have used those differences to give the tools different names, such as 'hand-axe', 'chopper', and 'cleaver'. These names conceal the fact that none of those early tools had a sufficiently consistent or distinctive shape to suggest any specific function, as do the obvious needles and spear-points left by the much later Cro-Magnons. Wear-marks on the tools show that they were variously used to cut meat, bone, hides, wood, and non-woody parts of plants, but any size or shape of tool seems to have been used to cut any of those things, and the tool names applied by archaeologists may be little more than arbitrary divisions of a continuum of stone forms.