Scientists studying wild chimpanzees in Tanzania say the human ability to walk upright on two legs may have evolved in trees rather than on the ground.
This contradicts the widely accepted theory that our prehistoric human relatives evolved to walk on two legs because they lived in an open savanna 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 adult wild chimpanzees in Wadi Isa in western Tanzania, which is home to a mix of dry open land and densely forested areas. Known as a “savanna mosaic,” this type of environment represents the environment in which our early ancestors lived.
The team recorded each time the chimpanzees were upright, and whether this occurred while they were on the ground or in trees.
Then they compared it to standing on two legs by chimpanzees that live in densely forested areas in other parts of Africa, and Wadi Isa chimpanzees were found to spend as much time in trees as their forest-dwelling cousins.
This means that they weren’t more dependent on the land, as current theories suggest they should be, given the more open environment in which they live. In addition, more than 85% of the times chimpanzees walked upright occurred in trees, rather than on the ground.
Study co-author Alex Bell, associate professor of anthropology at UCLA, told CNN that widely held theories follow a certain logic.
“The assumption was: fewer trees means more time on the ground, and more time on the ground means more time upright,” Bell said.
However, his team’s data don’t prove this, instead suggesting that more open environments weren’t a catalyst in encouraging bipedalism, Bell said. “It’s not a pretty logical story,” he said.
Bell said the next question for the researchers is why chimpanzees in Wadi Isa spend more time in trees despite there being fewer trees around other chimpanzee communities.
One explanation could be the fact that food-producing trees encourage them to spend time there to eat, he said, while there may also be a seasonal element.
In the rainy season, the grass in Wadi Isa grows to about 6.5 feet, Bell said, which means chimpanzees are more vulnerable to predators like cheetahs if they spend time on the ground.
“There could be a dramatic seasonal signature to that,” he said.
According to Bell, early human ancestors would also have faced predation in a similar environment.
“It’s a really similar system,” he said.
However, Bell stressed that the study does not draw a direct comparison between chimpanzees and our early human ancestors, but instead presents theories that need to be tested against the fossil record to see what it tells us about the anatomy of early humans.
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