Study reveals why our ancestors switched to bipedal power

Study reveals why our ancestors switched to bipedal power
A chimpanzee moving bipedally during the study. Credit: Prof. W C M McGrew.

( -- Our earliest ancestors may have started walking on two limbs instead of four in a bid to monopolise resources and to carry as much food as possible in one go, researchers have found.

A study published in the journal Current Biology this week, investigated the behaviour of modern-day chimpanzees as they competed for food resources, in an effort to understand why our “hominin”, or “human-like” ancestors became bipedal.

Its 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 in one go because it frees up their hands.

The joint University of Cambridge and Kyoto University team of biological anthropologists, led by PhD student Susana Carvalho and Professor Tetsuro Matsuzawa, conclude that our earliest hominin ancestors may have lived in shifting environmental conditions in which certain resources were not always easy to come by. 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.

Professor William McGrew, from the Department of Archaeology and Anthropology, University of Cambridge, said: “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.”

Lack of evidence in the fossil record means that researchers remain divided over when these ancestors became bipedal. It is widely believed that they did so because of climatic changes, which reduced forested areas and forced them to move longer distances across open terrain more often.

The new research digs deeper, however, by attempting to explain what particular pressures within that context forced those hominins to modify their posture and resort to moving on their legs.

The team theorized that the reason for this change may have something to do with the need to transport resources with maximum efficiency. Because bipedal movement is sometimes observed in modern great apes, they decided to monitor the behaviour of chimpanzees and, if possible, determine when and why they resorted to moving on two legs.

Two surveys were carried out. The first was in Kyoto University’s “outdoor laboratory” of a natural clearing in Bossou Forest, Guinea. Here, the researchers allowed the chimpanzees access to different combinations of two different types of nut – the nut, which is naturally widely available, and the coula nut, which is not, so the latter is an “unpredictable” resource.

Their behavior was monitored in three different situations: (a) when only oil palm nuts were available, (b) when a small number of coula nuts was available, and (c) when coula nuts were the majority available resource.

When the rare coula nuts were available only in small numbers, the chimpanzees transported far more in one go. Similarly, when coula nuts were the majority resource, the chimpanzees ignored the oil palm nuts altogether. Clearly, 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 survey 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% of their activity involved some sort of bipedal movement, and once again, this behaviour appeared to be linked to a clear attempt to carry as much as possible in one go.

The study concludes that unpredictable resources, like the coula nut in the field survey, are seen by as more valuable. When these resources are scarce and access to them is on a “first-come, first-served” basis, they are more prone to switch to bipedal movement, because it allows them to carry more of the resource at once.

For our early ancestors, unpredictable access to vital resources may have been a frequent occurrence because of climatic shifts and rapid environmental change. Those who resorted to bipedal movement may have had an advantage, and gradually, anatomical change may have taken place as they used this strategy again and again. Once that happened, ability to move more easily on two legs may have become a selection pressure, so that over many generations, it became the norm.

Explore further

Trap-breaking chimpanzees found in Guinea

More information: The full report, Chimpanzee carrying behavior and the origins of human bipedality, is available in the March 20 issue of Current Biology:
Journal information: Current Biology

Citation: Study reveals why our ancestors switched to bipedal power (2012, March 20) retrieved 16 September 2019 from
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Mar 20, 2012
This has been common knowledge among anthropologists for many years. Good to see some new information, though.

Mar 20, 2012
Lots of reasons to get up two legs but still no understanding why this went further and led to the innovation of doing this with postural erectness and strident walking (as with us, other Homo, and Australopiths) rather than the bent knee, bent hip style of posture and gait of chimps. Understanding this is the real and still unanswered problem

Mar 20, 2012
"Over time, intense bursts of bipedal activity may have led to anatomical changes that in turn became the subject of natural selection"
This smacks of Lamarckism to me - the original article makes no such suggestion, rather that there may have been a selection pressure for more economical bipedality.

Mar 20, 2012
It seems to me that a genetic mutation moved the foramen magnun , to a lower position on the occipital bone . That was probably preceded by some change on the jawbone and/or maxilla , since the
skull evolves as a whole . I think bipedalism emerged that way .
Chimpanzees don't walk erect basically because of their foramen magnum , not feet , knees or hips .

Mar 20, 2012
I'd say that the foramen magnum moved *after* (part-time) bipedalism became the norm. Before that, there was no reason for it to move. But when upright, it allowed the head to move in a wider and more useful set of directions.

Mar 20, 2012
"switch to moving on two limbs instead of four in situations where they need to monopolize a resource"

So, the protohumans were Republicans?

Joking aside, there is no way to pass on this trait without some sort of selective pressure. Now, if this means that the "chimp" survived better due to hording I can buy that. Foramen magnum placement changes don't require mutations only a slightly different timing for the expression of physical trait genes during development. A mutation in this case would probably select against the organism as females don't want to make sexy time with the freak.....good old "stiffy" over there who can't walk right.....he's weird looking. And I heard he likes small boy apes.

Mar 20, 2012
take the line of reasoning as to the foramen magnum to its conclusion

ie. it moved after we had part time standing
it allowed the head to move in a wider more useful, etc

which would imply that since chimps, and man are not fast runners... seeing things that are shorter and fast from farther away, would lead to faster lead times.

and in a world where you only have to outrun your brother, who then gets eaten, such would then move to favor always being able to see farther and act earlier

along the way, monopolizing becomes an emergent property of the refined system of early warning. As its exploited to its fullest its new emergent properties are plumbed and then refined to the point we are no longer sure which came first...

but i would say prairie dogs stand for the same reason, but dont monopolize... (so do lots of other animals that are food)

same benefit would also lead to seeing farther and making hunting practical. something poor sighted, slow, weak bipeds would need.

Mar 20, 2012
note that prairie dogs cant monopolize, for that you would need the emergent property of grasping, which the beings in question already had due to their living style prior.

Mar 24, 2012
Well.. In a climate period where forestal regions were tranformed into sparcely forrested long grass plains, some "lookout-priciple" could have forced our ancestors to walk upright.

Explanations 'round every corner I'd say :-)

Mar 24, 2012
The article title is just an example of tabloid journalism, because this study actually doesn't reveal anything. It just proposes another hypothesis based on local observation of few chimpanzees. For example, the savanna principle considers, the anthropoids become bipedal because of improved sight at upright position and this explanation is not worse or better than the theory above presented. For example, the ants are carrying things all the time, and they don't differ in usage of their legs from other insects.

Mar 26, 2012
Walking upright(orthograde) still seems to be consequence, not cause of Foramen Magnun rotation straight downwards. This was essential to allow speech in humans ,and bigger brains, too. A mutation on MYH16 gene,about 2.4 million years ago, affecting the masticatory muscles, probably was the dawn of the genus Homo.
The main resource risking monopoly , now, are the very large fontanelles. They are better than I.Q. Tests to show who is who.

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