Out of Eden: The Peopling of the World Read Free

contemporary mammals. Ten million years ago, Africa was a lush paradise with vast open forests and home to several species of ape. Even then, various primate species, not just apes, were experimenting with life on the ground and in the grassland around islands and tongues of forest. Africa’s grassland has expanded progressively since then, as the world’s climate has cooled and dried, but this has happened in cyclical fits and starts of increasing frequency and severity.
    Walking apes
    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 inthat 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. 1
    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 2 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 pelvismore similar to ours. Above the neck the similarity ceased, for their skulls and brains were like chimpanzees’ (375–500 cm 3 in volume), although they had smaller canine teeth (see Plate 1 ). 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 cm 3 . 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