The origin of bipedalism. A review

Becoming bipedal was a vital step in the path to humanity. Walking on two legs freed the hands of our ancestors enabling the crafting of tools, allowing a meat focused diet and ultimately leading to a bigger brain - the characteristic that has allowed us to become the dominant species we are today. We are the only mammal adapted to a life of walking upright; it is true that some Great apes do take to two legs but their bodies are not built for sustained periods of walking and they quickly revert to their more energy efficient movements.  

The transition to become a habitual 'upright ape' has not been an easy one; a variety of skeletal and muscular modifications were necessary to make bipedalism worthwhile, leading to back problems, hernias and extremely difficult child birth. So what factors would have made it necessary to leave a quadrupedal lifestyle which suited our ancestors for millennia?

The oldest and most well known theory to answer this question is the 'savanna hypothesis', suggested by Raymond Dart (1925). His idea being that bipedalism evolved in an open grassland setting, an environment created from receding rainforests.  It was here that ‘mammalian competition’ and lack of water provided the selection pressures required for an ape that relied on fast legs and a large brain to survive. At the time the hypothesis was proposed the earliest bipedal hominid – the Taung child was dated at 2.5 million years old (mya) (Dart 1925), ice core samples support a reduction in rainforest. They reveal that the climate at this time was indeed becoming cooler and dryer – a repercussion from the closing of the Panama seaway.

A classic image of early man in a savanna setting

A supporting hypothesis was put forward by Wheeler (1991), he proposed that walking upright would have allowed our ancestors to keep cool on the savanna due to a reduced surface area being exposed to direct sunshine and increased wind speeds away from the ground. These hypotheses seem simple and logical, however the discovery of the 4.4mya bipedal hominid –Ardipithecus ramidus pushes the origin of bipedalism back to a time when Africa was covered in thick rainforest; a finding which prompted scientists to seek new explanations.

A paper by Kevin Hunt (1996) gives a good alternative theory. It suggests that our ancestors developed their bipedalism whilst still in the trees; the idea was developed from an observational study of our closest living relatives – the Chimpanzee. These Great Apes use facultative standing to reach food items, holding onto overhead branches for support; Hunt argues that our early ancestors would have used a bipedal posture to attain food and this would have developed into efficient locomotion at a later point.

Modern Orangutans also use facultative standing to reach food items   

Unlike the Savannah hypothesis the postural feeding theory fits in with the rainforest environment that is predicted to have been present at the dawn of bipedalism, and is therefore in my opinion much more likely to be true. Other theories have also been put forward including bipedalism adapting to make sex organs more visible or to allow males to provide for their mates. The argument over which factor is the most likely is still ongoing, and this debate will be continued as new research emerges.

New research into this topic have put forward some new ideas, a 2011 paper by David Carrier proposes that sexual selection may have played a role. This is the selection of traits based on female preference rather than traits which will enhance survival, a bipedal stance would have enabled males to fight one another more effectively to gain mating rights to a female. Only males winning these fights would pass on their genes and therefore this gait would have been selected for.

Finally, a 2010 review of all bipedal hypotheses, concludes that no one hypothesis can explain our adaption towards a bipedal lifestyle, the issue is far more complex than that, an opinion I tend to agree with.

References

Dart, R.A., 1925. Australopithecus africanus: The man ape of South Africa. Nature, 115, pp.195–199.

Hunt, K.D., 1996. An ecological model for the evolution of bipedalism. South African Journal of Science, 92(2), pp.77-90.

Wheeler, P.E., 1991.The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evaporative cooling. Journal of Human Evolution, 21(2), pp.107-115.