Why humans walk on two legs: A close look at chimpanzees puts some old theories to the test

Early bipeds evolved when savanna grasslands became increasingly common as forests retreated 4-8 million years ago.

There’s no trait that distinguishes humans from all other mammals more clearly than the way we walk. Human habitual bipedalism – obligatory walking on two legs – has long been a defining trait of our species, as well as our ancestors as far back as 4.5 million years ago.

Science’s growing understanding of chimpanzee culture, communication and emotion may have blurred the understanding of “distinctly human”, but our obligatory bipedalism has stood the test of time.

Why, when, and where bipedalism evolved remains debated, however. Numerous evolutionary pressures have been proposed. Most are about the economics and energy use of walking on two legs (bipedalism is far more efficient than quadrupedalism). Other theories describe the advantages of carrying objects. Bipedalism frees the hands to do interesting things like make and use tools and reach for fruit. It also enables us to see over tall grass.

But almost all the theories suggest that bipedalism is an adaptation to getting around on land. It’s clear that early bipeds evolved when savanna grasslands became increasingly common as forests retreated 4-8 million years ago. Walking on two legs made it easier to forage and travel on the ground.

But there’s also evidence that contradicts this idea. Hominin anatomy, palaeo-ecology and the behaviour of some ape species present challenges to the theory. For example, early hominins had a long list of adaptations to life in the trees. These included long limbs, mobile shoulders and wrists, and curved fingers. All these features are present in our current tree-dwelling primate cousins.

Studies of what hominins ate and the animals they lived with (bushbucks, colobus monkeys) also suggest that these hominins did not live in grasslands. Instead, they inhabited mosaic landscapes, consisting most likely of a mixture of riparian forests and woodlands.

Finally, evidence from the only non-African great ape – the orangutan – suggests bipedalism was an adaptation to living in trees. It helped the apes to negotiate flexible branches high in the tropical rainforests of southeast Asia.