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First, a number of newly found early human skulls, both within and outside Africa, have resolved the timescale and geographical extent of the repeated out-of-Africa movements of the past 2 million years. Other skull finds have pushed the branch between our ancestors and those of chimps back even earlier. Second, the comparative analysis of skull shape has, with the advent of fast computers, been put on a more scientific basis, with the result that key skulls of prehistoric peoples from around the world can now be placed near branches on the human genetic tree, and today’s descendants on the twigs.
As we know from the effects of our recent ice age, the worst phase of the climate cycle, although brief, can cause widespread extinctions. Only the survivors of such climatic episodes can pass on their own genetic type, holding special features that may have been selected for by the environment. Around 7-8 million years ago, a dramatic reduction in the number of ape species coincided with several million years of global cooling and grassland expansion. Some have suggested that this short ice epoch already marks the time of the split between our ancestors and the ancestors of modern chimpanzees. The most important initial physical change in our ancestors, bipedalism (walking on two legs) may have its roots in that time. At present, however, the first clear evidence for bipedalism is seen only in skeletons of Australopithecus anamensis, a walking ape dating from 4 million years ago and found on the shores of Lake Turkana, in northern Kenya, in 1995.
Many think that there was a cause-and-effect relation between the spread of the grasslands and the change from quadrupedal forest living to easy walking around the savannah. This may well be true but, judging by the toothed predators with which they shared their environment, the early bipedal apes probably did not stray too far into the plain and away from the protection of the wooded islands. In any case, other primates, such as the ancestors of baboons, managed to get along surprisingly well in the savannah on four legs (as do modern baboons).
Others have suggested alternative theories of why it was advantageous to adopt a two-legged posture, such as literally keeping a cool head or, like the African meerkats, keeping an eye out for predators on the plain. However, our ancestors’ brains, although larger than those of most other land mammals, were no bigger than that of our cousin the chimpanzee, so there was less danger of them overheating. Nor is standing upright - which many mammals do, including monkeys, chimpanzees, bears, and meerkats - the same thing as habitually walking on two legs for long periods. The idea of leaving hands free to do other mischief such as wielding heavy sticks for hunting (or more likely for defence against predators, since our ancestors were mainly vegetarian) is attractive as an evolutionary force. Unfortunately we have no direct proof, since wood is perishable and stone tools are not found from that time. Those early walking apes, for whom there is still only fragmentary evidence, were followed by the famous 'Lucy' family, Australopithecus afarensis. Lucy’s partial female skeleton was discovered by Donald Johanson in 1974 at Hadar, in Ethiopia. Living between 3 and 4 million years ago, her kind were 1-1.5 metres (40-60 inches) tall, more clearly upright and bipedal, with a pelvis more similar to ours. Above the neck the similarity ceased, for their skulls and brains were like chimpanzees’ (375-500 cm3 in volume), although they had smaller canine teeth. As with gorillas, their females were much smaller, suggesting a harem society. A different two-legged version (Australopithecus africanus) lived between 2 and 3 million years ago and, although the same size, had a slightly larger average brain size than chimps at between 420 and 500 cm3. Their teeth were also smaller and more like ours. It should be said that these two particular bipeds are not necessarily steps on a direct evolutionary sequence towards ourselves: our 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'. 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.
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Origins – 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 especially large 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), 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 cm3, but among the known habilis skulls is one 1.9 million years old with a chimp-like brain volume of only 500 cm3, which is at the top end of the range for the earlier australopithecines. 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 aspect of our behaviour - something to do with the way we faced the climatic challenge, perhaps - that gave large, energy-expensive brains survival value. The problem of finding food in an increasingly dry environment must have taxed our ancestors’ resourcefulness. Larger brains clearly helped them in some way. That behaviour must still be with us today, because over subsequent major glaciations during the past 2.5 million years, new human species with larger brains and more skills appeared in Africa. As the climate briefly warmed after each glacial maximum, the Sahara would become green for a few thousand years and the new human species would venture out to try their luck in Eurasia. By 1 million years ago, brain volumes of various human species living both within and outside Africa had increased from 400 to 1,000 cm3, and even into the modern size range. In other words, human brains had grown to three-quarters of their modern size long before we came on the scene.
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