Humans’ ability to walk upright on two legs may have evolved in trees rather than on the ground, according to scientists studying wild chimpanzees in Tanzania.
This contradicts the widely held theory that prehistoric human relatives evolved to walk on two legs because they lived in an open savannah environment, according to a study published Wednesday in the journal Science Advances.
Researchers at University College London (UCL) spent 15 months studying the behavior of 13 wild adult chimpanzees in western Tanzania’s Issa Valley, which features a mix of dry open land and areas of dense forest. This type of environment is known as “savannah mosaic” and is similar to the one our earliest human ancestors lived in.
The team recorded each time the chimpanzees stood upright and whether they did so while sitting on the ground or in the trees.
They then compared this to cases of standing on two legs by chimpanzees living in densely forested areas in other parts of Africa, and found that the chimpanzees of the Issa Valley spent as much time in the trees as their forest cousins.
This means they were no longer terrestrial, as existing theories suggest they should be, given the more open environment they live in. In addition, more than 85% of the times the chimpanzees walked upright happened in the trees, rather than on the ground.
Study co-author Alex Piel, an associate professor of anthropology at UCL, told CNN that widespread theories follow a certain logic.
“A long-held assumption has been: less trees means more time on the ground, more time on the ground means more time upright,” Piel said.
However, his team’s data doesn’t confirm that, instead suggesting that more open environments weren’t a catalyst in encouraging bipedalism, Piel said. “It’s not this nice logical story,” he said.
The next question for researchers is why Issa Valley’s chimps spend more time in trees, despite being near fewer trees than other chimpanzee communities, Piel said.
One explanation could be that food-producing trees encourage them to spend time there to feed, he said, while there may also be a seasonal component.
In the rainy season, the grass in the Issa Valley grows to about 2 meters high, Piel said, meaning the chimps are more vulnerable to ambush predators like leopards if they spend time on the ground.
“It could be that there’s a dramatic seasonal signature to this,” he said.
Early human ancestors would also have experienced predation in a similar environment, according to Piel.
“It’s a true analog system,” he said.
However, Piel stressed that the study does not make a direct comparison between the chimpanzees and our early human ancestors, but instead offers theories to be tested against the fossil record to see what it tells us about the anatomy of early hominids.