Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [67]
The ‘savannah hypothesis’ took root among the scientific community, offering what seemed to be an elegant explanation for the origin of bipedalism. The consensus was that it was the shift from life in the forest to life on the savannah that had set hominids apart from apes, forcing them to move upright on two legs. Once they had emerged from the cover of trees, bipedalism enabled them to scavenge or hunt game or forage for food more easily, to walk more efficiently over long distances, to peer over high grass, to carry infants, to escape predators. All the key phases of human evolution appeared to have taken place in the open grasslands of Africa.
The flaw in the savannah hypothesis became increasingly noticeable as fieldworkers provided ever more evidence that the earliest bipedal hominids occupied wooded terrain, not open grasslands. The landscape at Aramis where ramidus lived 4.4 million years ago was a mosaic of woodlands with thick underbush, flooded grasslands and swamps. The Tugen Hills site where Orrorin lived 6 million years ago was a similar mosaic of forests and woodlands with dense undergrowth and wet grasslands. The Djurab Desert site where Toumaï was found was once a lush oasis of gallery forests, swamps and wooded islets that sustained a huge diversity of animal life along the shores of a vast inland sea. The terrain there, Michel Brunet suggested, was much like today’s Okavango Delta, a freshwater paradise that lies on the edge of the Kalahari Desert in Botswana.
But without the savannah hypothesis, the puzzle about the origins of bipedalism remained unanswered. One new theory that gained attention proposed that human ancestors came down out of the trees on their hind legs, already equipped to stand upright. Robin Crompton, an expert on locomotion at Liverpool University, claimed that Miocene apes, the forerunners of hominids, had evolved adaptations to their muscles and skeleton for bipedal movement while they still lived in trees. They had developed an upright stance, he suggested, as a means of foraging for fruit on outer branches. He pointed to the example of modern orang-utans which walk upright in trees when seeking outlying fruit, grabbing overhead branches to help keep their balance. This bipedal movement, he claimed, was an ancient, ancestral trait which had evolved among ape ancestors living in the forest canopy long before gorilla and chimpanzee ancestors had developed knuckle-walking for moving on the ground and hominids had strode out into the open on two legs. ‘Upright walking evolved in the ancestors of all apes, including humans’, Crompton maintained. ‘These techniques were later used by human ancestors to allow them to adapt to walking on two feet on the ground’.
While accepting that arboreal bipedalism was a plausible mechanism for the origins of upright walking, many scientists were sceptical about Crompton’s theory. Critics cautioned against using the activities of modern orang-utans or chimpanzees as a reliable guide to the evolution of ancestors from the deep past. Orang-utan ancestors, they pointed out, were more distantly related to human ancestors than were quadrupedal gorilla and chimpanzee ancestors. They had split from the quadrupedal ape line leading to gorillas, chimpanzees and hominids some 10 million years ago, several million years before the rest of the quadrupedal line diversified.
A further intriguing piece of evidence about bipedalism came from research by Jeremy DeSilva and his team at the University of Michigan, published in 2009. After making a close study of the way modern chimpanzees in the wild scale trees—virtually vertically and with ease—and then comparing chimpanzee ankle joints with those of some thirty hominids dating back as far as 4 million years ago, they concluded that the hominids were not nearly as well adapted to climbing trees as modern chimpanzees are; they lacked