It's common wisdom that one of the things that separate us from the other apes is our ability to walk upright on two legs. Traditional theories have it that this development happened once our ancestors left the comfort of the trees for the savannah or possibly the rivers, and bipedalism allowed us to see further over the tall grasses, spot predators and prey, and carry things while we ambled around. Chimpanzees, bonobos and other apes are quadrupeds on the ground, using both feet and knuckles to perambulate, so it was widely held that proto-humans did the same thing.
As so often happens in science, it seems that common wisdom might not be correct. A new study published in the journal Science looks at the behavior of orangutans, and suggests that walking on two legs might go back further than we thought.
The data collected by Susannah Thorpe and her colleagues at the Universities of Birmingham and Liverpool in the UK shows that bipedalism, with hands assisting for balance, is common among orangutans as they make their way through the trees. Orangutans are the most arboreal of the great apes and more distantly related to modern humans than chimps or bonobos.
It is possible that the orangutans of southeast Asia became more specialized as a result of environment or that the change in ecology of Africa during the Miocene period resulted in chimps and gorillas reverting to quadrapedal knuckle-walking. If the latter turned out to be the case, the ability to walk on two legs might have been in our early ancestors toolbox all along. This would go some way to explaining our long arms: arms that one wouldn't normally associate with a biped.
Image courtesy of SKS Thorpe