Washington, DC, USA. Most of us walk and carry items in our hands every day. These are seemingly simple activities that the majority of us don't question.

But an international team has discovered that human bipedalism, or walking upright, may have originated millions of years ago as an adaptation to carrying scarce, high-quality resources.


Modern-day chimpanzees use two legs as they compete for food. Researchers from England, Japan , Portugal and the United States investigated the behavior of modern-day chimpanzees as they competed for resources to understand what ecological settings would lead a large ape — one that resembles the 6 million-year old ancestor we shared in common with living chimpanzees — to walk on two legs. So, how could human bipedalism (walking upright) originate millions of years ago as an adaptation to carrying scarce, high-quality resources? Their findings appear in the journal Current Biology.

"These chimpanzees provide a model of the ecological conditions under which our earliest ancestors might have begun walking on two legs," said Dr. Brian Richmond, an associate professor of anthropology at the George Washington University (GWU) and a study co-author.

Two studies were conducted by the team in Guinea. The first study was in Kyoto University's outdoor laboratory in a natural clearing in Bossou Forest. Researchers allowed the wild chimpanzees access to different combinations of two different types of nut — the oil palm nut, which is naturally widely available, and the coula nut, which is not.

The chimpanzees' behavior was monitored in three situations:

1. when only oil palm nuts were available,
   
2. when a small number of coula nuts was available, and
   
3. when coula nuts were the majority available resource.

When the rare coula nuts were available only in small numbers, the chimpanzees transported more at one time. Similarly, when coula nuts were the majority resource, the chimpanzees ignored the oil palm nuts altogether. The chimpanzees regarded the coula nuts as a more highly-prized resource and competed for them more intensely.

In such high-competition settings, the frequency of cases in which the chimpanzees started moving on two legs increased by a factor of four. Not only was it obvious that bipedal movement allowed them to carry more of this precious resource, but also that they were actively trying to move as much as they could in one go by using everything available – even their mouths.

The second study, by Kimberley Hockings of Oxford Brookes University was a 14-month study of Bossou chimpanzees crop-raiding, a situation in which they have to compete for rare and unpredictable resources. Here, 35 percent of the chimpanzees' 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 at one time.

The research findings suggest that chimpanzees switch to moving on two limbs instead of four in situations where they need to monopolize a resource, usually because it may not occur in plentiful supply in their habitat, making it hard for them to predict when they will see it again.

Standing on two legs allows them to carry much more at one time because it frees up their hands. "Something as simple as carrying — an activity we engage in every day — may have, under the right conditions, led to upright walking and set our ancestors on a path apart from other apes that ultimately led to the origin of our kind."

Over time, intense bursts of bipedal activity may have led to anatomical changes that in turn became the subject of natural selection where competition for food or other resources was strong.