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 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 Science Advances.
Researchers from University College London (UCL) spent 15 months observing the behavior of 13 wild adult chimpanzees in western Tanzania’s Issa Valley, which is home to a mix of open, dry terrain and areas of dense forest. Known as “mosaic savannah,” this type of environment is similar to what our earliest human ancestors lived in.
The team recorded whenever the chimpanzees stood upright and whether this happened while on the ground or in trees.
They then compared this to cases of standing on two legs by chimpanzees living in heavily forested areas in other parts of Africa, and found that the Issa Valley chimpanzees spent as much time in trees as their forest-dwelling cousins.
This means they were no longer terrestrial, as existing theories suggest they should be, given the more open environment they inhabit. Additionally, more than 85 percent of the time chimpanzees walked upright occurred in trees, rather than on the ground.
Study co-author Alex Piel, an associate professor of anthropology at UCL, told CNN that the widely held theories follow a certain logic.
“A long-standing assumption has been: fewer trees means more time on the ground, more time on the ground means more time on your feet,” Piel said.
However, his team’s data doesn’t confirm this, instead suggesting that more open environments weren’t a catalyst in encouraging bipedalism, Piel said. “Isn’t this pretty logical story,” he said.
The next question for the researchers is why the Issa Valley chimpanzees spend more time in trees despite being around fewer trees than other chimpanzee communities, Piel said.
One explanation might be that the food-producing trees encourage them to spend time there to eat, he said, while there might also be a seasonal component.
In the rainy season, grass in the Issa Valley grows to about 2 meters high, Piel said, meaning chimps are more vulnerable to ambush predators like leopards if they spend time on the ground.
“It could be that there’s a dramatic season signing to that,” he said.
Even early human ancestors would have faced predation in a similar environment, according to Piel.
“It’s a really analogous system,” he said.
However, Piel stressed that the study isn’t drawing a direct comparison between chimpanzees and our early human ancestors, but instead provides theories that need to be tested against the fossil record to see what it tells us about early hominin anatomy.