One of the biggest anthropological mysteries scientists have been trying to unravel is the long put question of  how did humans develop bipedal movement. There have been many theories formulated hypothesizing why our ancestors eventually switched from four limbs walking to two – some appealing, some a bit too far the edge. A recent study performed by a joint team of biological anthropologists at University of Cambridge and Kyoto University claims, with experimental data to back-up as evidence, that our human forefathers might have switched to two legs because it made carrying valuable, scarce resources easier in one go.

The researchers sought to understand how our hominid ancestors developed bipedal movement by studying the chimpanzee’s walking behavior, our closest relative. The researchers found that chimpanzees tend to switch their movement on two limbs instead of the usual four in situations when they want to monopolize a resource and want to carry as much of it as possible in one go.

“Bipedality as the key human adaptation may be an evolutionary product of this strategy persisting over time. Ultimately, it set our ancestors on a separate evolutionary path,” said Professor William McGrew, from the Department of Archaeology and Anthropology, University of Cambridge.

To test their theory which simply states that bipedal movement resulted from the need to transport resources with maximum efficiency, the researchers performed two surveys. One was in a natural clearing in Bossou Forest, Guinea, where anthropologists exposed chimpanzees to three situations in which certain nuts were available in limited or abundant supply. The oil palm nut, is naturally widely available and the chimpanzees are fully aware of this, while the other resource used in the study, the coula nut, is considered to be a scarce or “unpredictable” resource, so the latter made for a perfect control factor.

A possible explanation as to how our early ancestors came to walk on two legs

Behavior was monitored in three separate instances:

• when only oil palm nuts were available
• when a small number of coula nuts was available
• when coula nuts were the majority available resource

In the first situation, no significant alternation in their movement behavior occurred. In the second instance, however, the chimpanzes procedeed in transporting more coula nuts in one go. They proceeded much in the same why in the third instance as well, only this time they ignored the oil palm nuts altogether, since they saw the coula nuts as a much more valuable resource, deeming the current situation as a one time opportunity, unpredictable, from which they had to profit at maximum efficiency.

(A) An adult male chimpanzee seen holding tools (anvil in left hand, hammer in left foot) and Coula edulis nuts (mouth and right hand) part of a nut-cracking session. (B) Adult male chimpanzee seen carrying three papayas (one in each hand and one in mouth) during crop-raiding. (c) W C M McGrew

During the last two instances, the frequency with which the chimps switched from four to two limbs increased by a factor of four. Bipedal movement allowed the chimpanzees to carry much more resources (~around twice as much), but even so that didn’t seem enough as some were seen carrying nuts even in their mouths. It’s important to note that most transport overall was quadrupedal.

The second leg of the study was concentrated around a 14-month long survey of Bossou chimpanzees and crop-raiding, where again high-value resources are obtained with unpredictable frequency. The researchers observed that 35% of their activity involved some sort of bipedal movement, and once again, this behavior appeared to be linked to a clear attempt to carry as much as possible in one go. When correlated with the first survey, these findings lead reseachers to claim that when faced with situations where scarced resources are available at an unpredictable frequency, most often chimpanzees will switch to bipedal movement so they might carry as much as possible.

Our former hominid ancestors were subjected to more or less similar situations, faced with both unpredictable resource frequency and changing climates. With this in mind, the researchers suggest that selection pressure towards the economically favorable bipedal movement might have lead our ancestors on a separate evolutionary path where bipedal movement became the dominant form of locomotion.

The findings were reported in the journal Current Biology.