Why does man walk on his feet and not on his hands?

‘Uniquely human: there is no feature that distinguishes humans from all other mammals more clearly than the way we walk on two legs. Walking, essentially on two legs, has long been an obvious trait of our species. Our history of walking on two legs is about 4.5 million years old. Science’s growing understanding of chimpanzee life, how they live, eat and drink, how they communicate, and how they feel may have clouded the understanding of what is “distinctly human,” but our reasoning is that time has met the criteria.

However, debate continues as to why, when and where this evolved. Several evolutionary approaches have been proposed. In most cases this approach is concerned with the economics and energy use of bipedalism (bipedalism is much more efficient than quadrupedalism). Other theories describe the benefits of carrying objects.

Bipedal walking in humans frees the hands to do interesting things, such as making and using tools and reaching for fruits hanging from trees. It also enables us to see the tall grass. However, almost all theories suggest that bipedalism is an adaptation to locomotion on land. It is clear that bipedalism began at an early stage, when savannah grasslands rapidly diminished as forest cover decreased between 40 and 80 million years ago. Traveling on two legs became easier.

But, there is also evidence that contradicts this idea. Hominin anatomy, paleo-ecology and behavior of some ape species challenge this theory. For example, early hominins had a long list of adaptations to life in trees. These included long limbs, curved shoulders and wrists, and curved fingers. All of these characteristics are present in our extant primate cousins who live in trees. 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 a mosaic landscape, most likely a mixture of riparian forests and woodlands. Finally, evidence from the only non-African great ape – the orangutan – suggests that bipedalism was an adaptation for living in trees. This helped apes to settle on flexible branches in the tropical rainforests of Southeast Asia. We studied the behavior of wild chimpanzees in a savanna mosaic in the Issa Valley in Tanzania.

Issa the chimpanzee

The Issa chimpanzee lives in an environment dominated by woodland. It is surrounded by grasslands, stony plains and forests along the banks of rivers. We followed the chimpanzees for 15 months, collecting data on an individual’s positional behavior every two minutes, what type of vegetation they were in (forest, woodland), and what they were doing (foraging, resting, etc.) . We expected that chimpanzees would spend more time on the ground and stand or walk upright in open vegetation such as woodlands where they cannot easily travel through tree canopy.

We thought they would be overall more terrestrial than other animals of their genus living in forests in other parts of Africa. We found that Issa chimpanzees actually spend more time on the ground in woodlands than in forests. But, they were not more terrestrial than other (wild) communities. In short, it is not a simple rule that low trees spend more time on the ground.