Out of Eden: The Peopling of the World Read Free Page A

ancestor at this time could easily have been a sister species as yet undiscovered. For instance, our nearest ancestor could have been a recently discovered species from about 3.6 million years ago called
Kenyanthropus platyops
– quite literally, ‘flat-face’ ( Figure 0.1 ). The flat face, a feature of humans, might represent a bridge between the walking apes and us. What is certain, though, is that over the few million years in which the australopithecines (‘southern apes’) and their immediate ancestors walked Africa’s grasslands, we see only a moderate, not a dramatic, increase in brain size.

     
    Figure 0.1
   The ‘untidy tree’. Over the past 8 million years of hominid evolution (including here only our nearest living relatives, chimps), several species usually co-existed at any one time, so drawing a tree of direct descent on the basis of chance fossil finds may be misleading – hence the lack of branches on this tree.
     
    Growing brains in the big dry
    Things were about to change, because 2.5 million years ago the world started getting colder. Within a million years, the wet and warm Pliocene geological period gave way to the Pleistocene ice epoch. This was a grinding cycle of repeated dry ice ages, with alternating advances and retreats of African grassland lasting right up until the most recent glaciation, which climaxed 18,000 years ago. Soon after the start of this unstable, icy, and dry period, the first humans (the
Homo
genus) with their stone tools and larger brains made their entrance on the African savannah stage. As had happened a few million years before with the split into four-legged and two-legged locomotion, this was a parting of the ways for the descendants of the walking apes. One branch, known as
Paranthropus
, developed larger jaws to cope with grinding up tough vegetable matter. The other branch,
Homo
, made stone tools, developed substantially larger brains, and set off down the road towards becoming better hunter-gatherers and then, finally, us.
    Naturally, we are inclined to think of humans as being special and set apart from the other apes. Many think that it is our especiallylarge brain that makes us what we are. Some even think that an increase in brain size led to tool-making, but this argument seems unlikely. Fashioning
stone
tools, unlike walking on two legs or manual dexterity, may be unique to humans (and possibly to
Paranthropus
3 ), but even chimpanzees make crude but effective tools out of wood, and they have smaller brains and branched off much earlier than the walking apes. Although we do not have the evidence in wood from the last 7 million years, chimps still have roughly the same sized brain as our common ancestors who lived at the beginning of that time. This does not seem to constitute a strong link between simple tool-making and achieving a critical brain size. Nor does it rule out the possibility that the common ancestors of chimps and humans were making tools so long ago.
    One of the earliest human tool-makers,
Homo habilis
, had an average brain volume of 650 cm 3 , but among the known habilis skulls is one 1.9 million years old with a chimp-like brain volume of only 500 cm 3 , which is at the top end of the range for the earlier australopithecines. 4 The small body and brain size and the other apelike features of
Homo habilis
have led some anthropologists to call for their expulsion from the
Homo
genus or ‘human club’, but in spite of the well-argued case, this seems more like size prejudice than scientific reasoning.
Homo habilis
were unlikely to have been our direct ancestors, but that can be said for most hominids; and they made stone tools.
    The idea that we somehow grew a big brain first, then decided what it was for, is a negation of Darwinian principles. Any new kind of behaviour always precedes the physical adaptation that evolves to exploit that behaviour. Well before the start of the Pleistocene ice epoch there must already have been some