How did humans learn to walk the walk that sets us apart from our closest kin—the apes?
A new study of Sumatran orangutans in Indonesia suggests that ancient apes may have developed upright walking while still living in the trees—well before human ancestors, known as hominids, ever descended to the ground.
The study authors spent many hours observing Sumatran orangutans as they moved about the canopy of their rain forest home in Gunung Leuser National Park.
(See a map of the region.)
The critically endangered animals—which number about 7,300 in the wild—live nearly their whole lives aloft.
On sturdier branches the orangutans use all four limbs. But on thinner branches in search of fruit, the apes move on two legs and use their arms for balance.
"When they are on the very fine stuff, they are using bipedalism," said study co-author Robin Crompton of the University of Liverpool in England.
"It shows that bipedalism can be adaptive in the trees," Crompton said. "People have suspected that it evolved in the trees, but no one has been able to see a sensible reason why it should happen."
The researchers think they've uncovered that sensible reason: Upright bipedalism in human ancestors was quite likely an adaptation to moving and feeding on ripe fruit in the peripheries of trees, they say.
A Unique Ability
The ability to walk on two legs is one of the things that make humans unique and separates them from close relatives like chimps and bonobos.
Many theories seek to explain just when—and why—our ancestors first developed an upright gait.
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