Prehistoric World

The ancestors of humans began walking upright while they were still living in trees, and not out on open land, according to a theory which was published in 2007.

The traditional view up to this point had been that bipedalism evolved gradually from the four-legged 'knuckle-walking' displayed by chimpanzees and gorillas today. The new study was published in the journal Science, disputing the established idea.

The British authors of the study stated that upright walking was always a feature of great ape behaviour. Humans inherited it without ever passing through a knuckle-walking phase. They believed that knuckle-walking evolved only recently as a way of getting around on the forest floor.

Susannah Thorpe, Robin Crompton, and Roger Holder came to their conclusions after analysing the movement of wild orangutans, which spend most of their lives in trees. They found that orangutans used upright locomotion to fetch food from the small branches of trees and to cross directly from one tree to another.

Both access to fruits and crossing gaps in the trees would require an ability to navigate very thin, terminal tree branches which are liable to bend under body mass. The logical conclusion from the environmental, fossil, and experimental evidence is that upright, straight-legged walking originally evolved as an adaptation to tree-dwelling.

Selective advantages

They suggested the shift made by our ancestors to a terrestrial lifestyle came about as climate change thinned out their forest habitat.

In response, these ancient hominids may have abandoned the high canopy for the forest floor. Here, they remained bipedal and began eating food from the ground or from smaller trees. The act by orangutans of walking upright on springy branches is much like athletes running on springy tracks - they use extended postures of knee and hip to give them straighter legs.

The researchers pointed out that some of the earliest fossil human ancestors combined lower limbs which were adapted for upright walking with an upper body which seemed suited to climbing trees. There was also evidence these bipedal creatures lived in a closed forest environment, not the savannah habitat which would have required them to routinely move on the ground.

Other experts in the field welcomed the new paper but were clear that it was not going to be the last word on the subject. One problem was that the selective advantage for that first hominid to stand upright was still open to question. Very little was known about the context in which that occurred.

Also questioned was the idea that the kind of locomotion displayed by chimps and gorillas must have evolved only recently. Chimps, gorillas and humans are more closely related to one another than they are to orangutans.

The relationships between the apes were not in question unless all those similarities between chimps and gorillas had evolved independently. Then the inference would be inescapable that the last common ancestor of chimps and humans must have been like a chimp or gorilla.

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