Researchers find that chimpanzees learned to walk in trees, not on ground

Dec. 14 (UPI) -- Humans walking on two legs may have evolved from the trees and not from the ground, according to a new study by researchers at the University College London.

The study explored the behaviors of wild chimpanzees, the closest living relatives of humans, that were living in the Issa valley of western Tanzania.

The study found that the Issa chimpanzees spent as much time in the trees as other chimpanzees, despite their more open habitat. More than 85% of walking took place in the trees, the researchers said.

"We naturally assumed that because Issa has fewer trees than typical tropical forests, where most chimpanzees live, we would see individuals more often on the ground than in the trees," co-author Dr. Alex Piel said. "Moreover, because so many of the traditional drivers of bipedalism (such as carrying objects or seeing over tall grass, for example) are associated with being on the ground, we thought we'd naturally see more bipedalism here as well. However, this is not what we found."

"Our study suggests that the retreat of forests in the late Miocene-Pliocene era around 5 million years ago and the more open savanna habitats were in fact not a catalyst for the evolution of bipedalism. Instead, trees probably remained essential to its evolution -- with the search for food-producing trees a likely a driver of this trait," Piel added.

The researchers recorded more than 13,700 instantaneous observations of behavior from 13 chimpanzee adults. They then used the relationship between tree/land-based behavior and vegetation (forest versus woodland) to investigate patterns of association.

The authors note that walking on two feet is a defining feature of humans when compared to other great apes, who "knuckle walk."

"To date, the numerous hypotheses for the evolution of bipedalism share the idea that hominins (human ancestors) came down from the trees and walked upright on the ground, especially in more arid, open habitats that lacked tree cover," co-author Dr. Fiona Hill said. "Our data do not support that at all.