Opinion – Reinaldo José Lopes: The bipedal walking of human beings may have started at the top of the trees, not on the ground

In case you haven’t noticed, I’d like to remind you that you belong to a very strange species of primate. To begin with, the peculiar conformation of our skeleton means that we spend most of our lives walking around on only two legs, instead of using all four limbs, like all other decent primates and with our heads on straight.

To top off the scandal, no one is able to say with certainty how, after all, we ended up metamorphosing into this oddity of nature. Don’t get me wrong: Fossils documenting transformations in humanity’s ancestors are increasingly plentiful and well-studied, and it’s clear that between 6 million and 3 million years ago, some of the great African apes in our lineage were ceasing to be quadrupeds and adopted bipedal locomotion with more and more determination. The problem is to understand the mechanisms that produced this change.

A hypothesis that was successful for many years correlated the origin of walking on two legs with the expansion of savannas —areas of open vegetation— in Africa. The logic behind it was simple and intuitive. First, we know that chimpanzees, bonobos and gorillas, our closest relatives, generally live in closed and humid tropical forests. Second, this type of habitat has shrunk in Africa over the last few million years, thanks to geological and climate changes that have dried out much of the continent.

It would make sense, therefore, that some great apes who faced these changes would have reacted to the retreat of forests through a style of locomotion that was more efficient in open spaces. It is believed, for example, that the biped posture minimizes the body’s exposure to sunlight in environments such as the savannah.

The oldest fossils of the hominin lineage (the one that would give rise to modern human beings) come from environments in which more “thin” forest areas seem to have predominated, halfway between a tropical forest and a savannah with low vegetation. The idea, therefore, is that bipedal locomotion could have started as a mechanism for these ancestors to move between patches of arboreal vegetation, perhaps walking along the ground in short stretches.

It turns out that today’s world still harbors a possible model for what the life of the first hominins would be like: chimpanzees who spend a good part of their time in an arboreal savannah. Researchers from the University of Kent, in the United Kingdom, decided to study in detail the locomotion of the animals in the Issa Valley (west of Tanzania). The objective was to investigate whether the presence of more open vegetation actually works as an incentive to spend more time on the ground and move around bipedally.

The results of the research, coordinated by Rhianna Drummond-Clarke, have just been published in the specialized journal Science Advances. And, you see, they go against the grain of what was expected. Chimpanzees from Issa spend less time on the ground than their fellow species in some dense forests. And they tend to assume a bipedal posture high in trees rather than on the ground.

Apparently, they do this to make the most of the fruits distributed in a concentrated way on fewer trees in the savannah environment. If the data are representative of what happened to the first hominins, bipedal life may have started, paradoxically, in the trees, before conquering the ground.