Tanzanian chimpanzees help British scientists solve the mystery of why humans walk on two legs

An adult male chimpanzee walking upright (Rhianna C. Drummond-Clarke)

Chimpanzees are helping scientists based at UK and US universities solve one of the greatest mysteries of human evolution – why humans walk on two feet rather than four.

This is a critical question, because having upper limbs with hands rather than just feet has allowed humans to craft tools, develop bigger brains, and achieve success in ways no other has. animal has never been able to do.

Chimpanzees from a forested area in the interior of Tanzania have provided scientists with some of the crucial data needed to better understand a key aspect of human evolution.

The origins of bipedalism have always been a mystery, with two main competing theories – that humans evolved this way by moving through trees, or that they wandered through treeless grasslands.

But observations of how Tanzanian chimpanzees use their limbs are beginning to tip the scales in favor of the first theory.

Observations also shed light on the evolutionary pressures and sequence of events involved.

Like so many evolutionary changes, bipedalism likely accelerated due to climatic and ecological changes that destroyed ancient rainforests and replaced them with grasslands and open forests in which only smaller patches of dense forest have succeeded. to survive.

Some scientists had thought that this environmental change forced early humans to leave the trees and go to savannah habitats, where they later developed bipedalism.

But, by observing modern chimpanzee communities in dense forests and open woodland environments, scientists now believe it was likely patches of forest and woodland, rather than grassland habitats, that led to early human bipedalism. .

Contrary to expectations, chimpanzees quadrupled their bipedal activity in trees when in less dense forests compared to dense forest environments.

The likely reason was that food was less easily obtained in an open woodland environment than in dense forest, and therefore they had to do more foraging in less accessible areas of the woodland canopy.

What the new observations of chimpanzees suggest is that, to access these more difficult areas of the forest canopy, they had to deploy more frequently and therefore develop their bipedalism – because their upper limbs had to be used more frequently to grasp the tree branches just above. their.

The research – led by scientists from the University of Kent, University College London and Duke University, North Carolina – began in 2020 and is expected to be completed by 2024.

“Our fieldwork is crucial because understanding why we walk on two legs is key to understanding what it is to be human,” said Rhianna Drummond-Clarke, the project’s lead researcher and anthropologist.

The team focused on observing chimpanzee bipedalism in open forests in Tanzania and compared their findings with previous observations of chimpanzees made in dense forests in Uganda by other scientists.

Drummond-Clarke, from the University of Kent, spent 10 months observing a troop of 18 chimpanzees – seven adult males, six adult females, four juveniles, three adolescents and five infants – in a mostly open mountain environment of 12 square miles in western Tanzania. .

The preliminary results were published in an article in the US open-access online journal, Science Advances.

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