Humans walk on two legs but our nearest cousins, chimpanzees and bonobos, don’t. Therefore, sometime in our distant past, our ancestors must have transitioned from walking on four limbs to walking on two. But why? There are a few theories such as that having our hands free allowed us to carry objects and use tools, and that walking upright gave us a higher vantage point from which to survey our surroundings.
To investigate the carrying hypothesis, Susana Carvalho of the University of Cambridge, Dora Biro from the University of Oxford and an international team of anthropologists studied a group of wild chimpanzees. Chimps can choose to walk on two legs for short distances if they wish. The researches wondered if the chimps would wish to do so a lot more often if they had something precious to carry around. To that end, the researchers offered them some choice nuts.
The chimps were provided with locally available oil palm nuts and imported coula nuts (which the chimps would never ordinarily come across) in the following ratios: only oil palm nuts, mostly oil palm nuts but a few coula nuts, and a few oil palm nuts but lots of coula nuts. In addition, the chimps were supplied with stone tools suitable for opening the nuts.
The chimps were much more eager to transport coula nuts than oil palm nuts, which presumably they could eat any old time. When coula nuts were present, the chimps were four times as likely to spend time walking on two legs, a posture that allowed them to carry twice as many items as they could when using their hands to walk. No surprises there.
The authors postulate that it was the desire to collect and hoard rare items that drove our ancestors down the path of bipedalism. Could that be the case? Consider that Ardipithecus ramidus, an arboreal creature living over four million years ago, was also bipedal. Did it carry items from one tree to the next? Or perhaps, as has been suggested, hold onto items in order to give them as gifts?
I don’t doubt that the ability to carry objects played a critical role in our evolution. However, I suspect that more explanations for bipedalism will be found, especially if we can trace the rise of upright walking even further back in our history.