Human ancestors may have started walking on two legs to forage among treetops in open habitat, researchers have suggested, contradicting the view that the behavior emerged as an adaptation to spend more time on the ground.
The origins of bipedalism in hominids around 7 million years ago have long been thought to be linked to a change in the environment, when dense forests began to give way to more open wooded and grassy habitats. Under such conditions, it has been argued, our ancestors would have spent more time on the ground than in the trees and could have moved more efficiently on two legs.
But now researchers who study chimpanzees in Tanzania say that this trait can have different origins. “I think we’ve told this very logical story for a long time, which at least our data doesn’t really support,” said Dr Alex Piel, biological anthropologist at University College London and co-author of the research.
Writing in the journal Science Advancesthe researchers tell how they spent 15 months studying 13 chimpanzees living in the Issa Valley in western Tanzania, an environment similar to that experienced by our ancient ancestors.
The results reveal that these chimpanzees spent a greater proportion of their time on the ground and on the move when in an open environment of woods and grasses than in densely forested parts of the same area.
However, even in the open, the proportion of time chimpanzees spent on the ground was similar to that previously recorded for other great ape populations living in densely forested areas, including Gombe and Mahale.
“Even though we have far fewer trees, [the chimps are] more earthly,” Piel said.
The team then combined data from different environments in the Issa Valley and analyzed how often the chimpanzees stood or walked on two feet.
The results reveal that while bipedal behavior accounted for less than 1% of recorded postures, only 14% involved chimpanzees on the ground.
“Most of the time they’re on two legs, it’s in the trees,” Piel said, adding that the behavior, at least in the middle of the branches, seems to be most often related to foraging.
Rhianna Drummond-Clarke, first author of the University of Kent research, said open forests may promote bipedalism in chimpanzees, and by extension early human ancestors, as these environments have sparser trees than forests. dense.
“[Bipedalism may help them] safely and efficiently navigate the flexible branches and access as much fruit as possible when they find it,” she said.
The team says that while the study cannot prove that our human ancestors showed the same patterns of bipedal behavior, it challenges common assumptions about how humans came to walk on two legs, and suggests that trees have continued to play a role in our evolutionary history. even as the environment changed.
“Rather than spending time on the pitch stimulating [bipedalism], he may have catalyzed it, but he was already there,” Piel said. “And that fits perfectly with the fossil record, because all of these early hominids have both arboreal and terrestrial adaptations.”
