We walk upright to carry things

A new paper reports two studies of Bossou chimpanzees which show that “wild chimpanzees walk bipedally more often and carry more items when transporting valuable, unpredictable resources to less–competitive places”.

Susana Carvalho, Dora Biro, Eugénia Cunha, Kimberley Hockings, William C. McGrew, Brian G. Richmond, Tetsuro Matsuzawa. Chimpanzee carrying behaviour and the origins of human bipedalityCurrent Biology, 2012; 22 (6): R180 DOI: 10.1016/j.cub.2012.01.052

Summary

Why did our earliest hominin ancestors begin to walk bipedally as their main form of terrestrial travel? The lack of sufficient fossils and differing interpretations of existing ones leave unresolved the debate about what constitutes the earliest evidence of habitual bipedality. Compelling evidence shows that this shift coincided with climatic changes that reduced forested areas, probably forcing the earliest hominins to range in more open settings. While environmental shifts may have prompted the origins of bipedality in the hominin clade, it remains unknown exactly which selective pressures led hominins to modify their postural repertoire to include a larger component of bipedality.  Here, we report new experimental results showing that wild chimpanzees walk bipedally more often and carry more items when transporting valuable, unpredictable resources to less–competitive places.

Previous studies have suggested that it was a change of climate causing a change of landscape from thick forest to more open terrain which led to (or allowed) bipedalism among our ancestors. This is thought to have happened at least 4 million years ago at the time of Australopithecus (which is some 200,000 generations ago). It certainly was long before the large human brain evolved and the development of stone tools. Other hypotheses point to the energy efficiency of bi-pedalism for long journeys, the cooling efficiency when standing upright, the better view of the surroundings (both in offence and in defence) when standing taller or even  monogamous behaviour where the need to carry food back to a waiting family would have been high.

evolution of bipedalism image: http://www.liv.ac.uk

More likely it was a combination of many things. Some direct selection of evolutionary advantages coupled with some more indirect selection because advantages showed up at particular times as collateral effects of other gene selection events.  It is not implausible that it was a change of terrain which led to a change of behaviour which in turn created a greater need to carry things which contributed to bipedalism. It could then have been the consequent freeing of our fore-limbs which led to the dexterity and increased use of our hands which then provided a critical evolutionary advantage over other species. This then led (much later) to the creation of stone tools …… and the rest is history.

Climate change and a consequent change of terrain some 4 million years ago would seem to give ample time for this change to have occurred in more than one species. And if this use of our hands is the critical evolutionary advantage it seems to be, it could be expected that some other “current” species would also exhibit this combination of bipedalism and use of hands.  Or perhaps this has already happened. In terms of evolutionary time, “current” species must probably include Neanderthals and Denisovans who certainly walked upright and used their hands and made tools. But they have now been “incorporated” into and superseded by modern humans.

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