Human ancestors began to walk on two legs to carry rarer resources, study finds

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One of the biggest anthropological mysteries that scientists have tried to solve is the long-posed question of how humans developed bipedal movement. Many theories have been formulated assuming why our ancestors ultimately went from four limbs to two walking – some attractive, others a little too far apart. A recent study carried out by a joint team of biological anthropologists at Cambridge University and Kyoto University claims, with supporting experimental data as evidence, that our human ancestors could have gone double-legged because it made it easier to transport precious and scarce resources all at once.

Researchers sought to understand how our hominid ancestors developed bipedal movement by studying the walking behavior of the chimpanzee, our closest relative. Researchers have found that chimpanzees tend to alternate their movements on two limbs instead of the usual four in situations where they want to monopolize a resource and carry as many as possible at one time.

“Bipedics as a key to human adaptation may be an evolutionary product of this strategy that persists over time. Ultimately, this put our ancestors on a distinct evolutionary path, ”said Professor William McGrew, of the Department of Archeology and Anthropology at the University of Cambridge.

To test their theory which simply asserts that bipedal movement resulted from the need to transport resources with maximum efficiency, the researchers carried out two investigations. 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 abundance. The palm oil, is naturally widely available and chimpanzees are fully aware of this, while the other resource used in the study, the coula nuts, is considered a scarce or “unpredictable” resource, so that the latter constitutes a perfect controlling factor.

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

Behavior was monitored in three separate cases:

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

In the first situation, no significant alternation in their movement behavior has occurred. In the second case, however, the chimpanzees transported several coula nuts at one time. They did the same, why in the third case as well, but this time they completely ignored the oil palm nuts, as they viewed the coula nuts as a much more valuable resource, seeing the current situation as an opportunity. unique, unpredictable, from which they had to make the most of their efficiency.

(A) An adult male chimpanzee holding tools (anvil in left hand, hammer in left foot) and Coula edulis nuts (mouth and right hand) during a nutcracker session. (B) Adult male chimpanzee seen carrying three papayas (one in each hand and one in the mouth) during a looting of crops. (c) WCM McGrew

In the last two cases, the frequency with which chimpanzees went from four to two limbs increased fourfold. The bipedal movement allowed the chimpanzees to carry a lot more resources (around twice as much), but even so, that didn’t seem like enough as some were seen carrying nuts even in their mouths. It is important to note that most of the transports were overall quadrupeds.

The second phase of the study focused on a 14-month survey of chimpanzees from Bossou and plundering crops, where again valuable resources are obtained with unpredictable frequency. The researchers observed that 35% of their activity involved some sort of bipedal movement, and again, this behavior appeared to be related to a clear attempt to carry as much as possible in one go. When correlated with the first survey, these results lead the researchers to state that when faced with situations where scarce resources are available at an unpredictable frequency, more often than not chimpanzees will switch to bipedal movement so that they can carry as much as possible.

Our ancient hominid ancestors were subjected to more or less similar situations, faced with both an unpredictable frequency of resources and climate change. With that in mind, the researchers suggest that the selection pressure towards economically favorable bipedal movement may have led our ancestors down a distinct evolutionary path where bipedal movement became the dominant form of locomotion.

The results were reported in the journal Current Biology.


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