This contradicts the widely accepted 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 scientific advances.
Researchers from University College London (UCL) spent 15 months studying the behavior of 13 wild adult chimpanzees in western Tanzania’s Issa Valley, which is home to a mix of dry open land and areas of dense forest. Known as the “savannah mosaic,” this type of environment is similar to the environment inhabited by our earliest human ancestors.
The team recorded each time the chimpanzees were upright and whether they were on the ground or in the trees.
They then compared this to instances where they were standing on two legs of chimpanzees living in densely forested areas in other parts of Africa and found that the Issa Valley chimpanzees spent as much time in the trees as their forest-dwelling cousins.
This means that they were no longer land-based as existing theories suggest, given the more open environment in which they live. In addition, more than 85% of the times the chimpanzees walked upright occurred in trees rather than on the ground.
Study co-author Alex Piel, associate professor of anthropology at UCL, told CNN that there is a certain logic to popular theories.
“A long-held assumption was: fewer trees mean more time on the ground, more time on the ground means more time upright,” Piel said.
However, his team’s data doesn’t confirm this, instead suggesting that more open environments weren’t a catalyst for promoting bipedalism, Piel said. “It’s not that nice logical story,” he said.
The next question for the researchers is why the Issa Valley chimpanzees spend more time in trees despite staying near fewer trees than other chimpanzee communities, Piel said.
One explanation could be the fact that food-producing trees encourage them to spend time there to eat, he said, while there could also be a seasonal component.
In the rainy season, the grass in the Issa Valley grows to a height of about 6.5 feet, Piel said, meaning the chimpanzees are more vulnerable to ambush predators like leopards when they spend time on the ground.
“It could be that this has a dramatic seasonal signature,” he said.
According to Piel, early human ancestors would also have been exposed to predators in a similar environment.
“It’s a really analog system,” he said.
However, Piel emphasized that the study does not draw a direct comparison between the chimpanzee and our early human ancestors, but instead provides theories that need to be tested against the fossil record to see what they tell us about early hominid anatomy.