WASHINGTON (Reuters) - Chimpanzees scampering on a treadmill have provided support for the notion that ancient human ancestors began walking on two legs because it used less energy than quadrupedal knuckle-walking, scientists said.
Writing on Monday in the Proceedings of the National Academy of Sciences, the researchers said people walking on a treadmill used just a quarter of the energy relative to their size compared to chimpanzees knuckle-walking on four legs.
The scientists equipped five chimpanzees and four people with face masks to track oxygen usage and looked at other measures to assess energy expenditure and biomechanics on a treadmill.
Bipedalism is a defining characteristic of the human lineage and marked an important divergence from other apes.
Chimpanzees are the closest genetic cousins to people. They are thought to have a common ancestor with humans dating back anywhere between 4 million and 7 million years, depending
on the estimate.
Some scientists for decades have advanced the hypothesis that millions of years ago, human ancestors began walking upright because it used less energy than quadrupedal walking, gaining advantages in things like food foraging.
But there has been scant data on this notion, aside from a 1973 study looking at locomotion energy in juvenile chimps.
“This paper provides strong support for the fact that energy savings played a role in the evolution of bipedalism,” one of the scientists, University of Arizona anthropologist David Raichlen, said in a telephone interview.
The chimpanzees were taught to walk on the treadmill both quadrupedally and bipedally, the scientists said.
“These guys are smart enough that they would hit the stop button on the treadmill when they were done. If they didn’t want to walk on the treadmill, they’d just hit the stop button or they’d jump off,” Raichlen said.
Raichlen, who worked with Michael Sockol of the University of California-Davis and Herman Pontzer of Washington University in St. Louis, said chimpanzees on occasion walk on two legs in the wild, but are not very good at it.
Overall, the chimpanzees used about the same amount of energy walking on two legs compared to four legs, but the researchers saw differences among the individual animals in how much energy they used based on their gaits and anatomy.
One, for example, used a longer stride and was more efficient walking on two legs than four, the researchers said.
They also looked at the fossil record of human ancestors and found anatomical features such as hind legs that might use less energy in locomotion, and pelvic structural changes allowing for more upright walking.