A study of chimpanzees gives tantalizing evidence that humans may have evolved upright walking in order to carry more food.
A team of scientists from Oxford University, Cambridge, and Kyoto University tested the theory that two-legged (bipedal) walking should occur more of the time when animals are carrying prized but rare resources.
The researchers put groups of wild chimpanzees in Bossou, Guinea, through their paces. First they provided either commonly-available oil palm nuts or both oil palm nuts and some rare coula nuts in a forest clearing.
They found that when more of the prized coula nuts were available the chimps concentrated on carrying these away in preference to the oil palm nuts.
Whilst overall the chimps still mostly used all four limbs, bipedal walking increased by a factor of four when coula nuts were present. The chimps also carried twice as many items when walking on two legs – often using not only their hands but also their mouths and feet.
The researchers also separately recorded crop raids by Bossou chimps over a 14-month period. They observed that over a third of chimp trips during these raids included bipedal strides and that the number of items carried during these bipedal bouts was significantly higher than exclusively four/three-legged ones.
A report of the work is published today in Current Biology.
‘This small population of chimpanzees at Bossou has already taught us a great deal about many aspects of chimpanzee behaviour and cognition, including the uniquely West-African chimpanzee tradition of using stone tools to crack open hard shelled nuts,’ team member Dora Biro of Oxford University’s Department of Zoology tells me.
‘We've known for a long time that chimpanzees carry items bipedally, but what our study has shown is that such transports increase dramatically when chimpanzees encounter resources that are rare or unpredictable in their availability.
‘In those times their behaviour resembles a "take as much as you can at once" strategy - a bit like people piling food on their plates at a buffet table. Bipedality helps because it allows you to increase the amount of things you can carry to a safe place at once.’
The results support the idea that variable food resources and uncertain environments may ‘fast-track’ adaptations such as bipedal walking. It’s possible that the extra calories gained from novel ways of carrying food eventually selects for gradual anatomical change: something that may have driven our ancestors to stand up on their own two feet and stay there.