Orangutan study suggests early walking on two legs

WASHINGTON (Reuters) - The way orangutans navigate fragile tree branches in search of fruit has led scientists to propose that human ancestors with similar lifestyles may have begun walking on two legs earlier than previously thought.

Three British scientists, writing on Thursday in the journal Science, suggested that bipedal walking arose in arboreal apes 17 to 24 million years ago, rather than between 4 and 8 million years ago as a leading hypothesis indicates.

The study by Susannah Thorpe and Roger Holder of the University of Birmingham and Robin Crompton of the University of Liverpool focused on a vital question in human evolution -- how human ancestors came to walk on two legs rather than four.

Upright walking on two legs is seen as a defining human characteristic.

A leading evolutionary hypothesis has held that apes ancestral to chimpanzees, gorillas and people descended from the trees and began walking on the ground on all fours. In time, this hypothesis holds, they began "knuckle walking" like modern chimpanzees and gorillas, then later evolved the upright, bipedal gait of people.

But observations of wild orangutans navigating tree branches on two legs led these researchers to propose that bipedalism arose much earlier -- perhaps shortly after apes split evolutionarily from monkeys roughly 24 million years ago, assuming a specialized niche of tree-dwelling fruit eaters.

Thorpe spent a year in Indonesia's Sumatran rain forest painstakingly recording the movements of the orangutans.

"I followed them from when they woke up in the morning to when they made their night nest in the evening," Thorpe told reporters.

Orangutans spend their lives in the forest canopy, and fashion nests every night from leaves and branches at the tops of trees.

TASTY FRUIT

She observed how orangutans walked on two legs in the trees to reach fruit on the most fragile branches, using their arms to keep balance or grasp for food. They walked on all fours mostly on larger, sturdier branches.

"Our results are important because we have shown how bipedalism could have evolved in the original ape habitat, to navigate the very smallest branches where the tasty fruits are, and the smallest gaps between tree crowns," Thorpe said.

Orangutans, found only on the islands of Sumatra and Borneo, belong to the family of great apes along with chimpanzees, bonobos and gorillas. They are collectively the closest human relatives.

The red-haired orangutans are the only strictly arboreal ape. They eat fruit, tender leaves and the occasional bug.

"As the only great ape which remains in the ancestral ape niche (arboreal fruit eaters), orangutans are therefore a vital model for the understanding of the evolution of limb adaptation in apes," Crompton said.

The researchers said as the rain forest in eastern and central Africa receded due to climate changes near the close of the Miocene epoch, which ended 5 million years ago, arboreal apes that already had acquired the ability to walk on two legs were forced to spend more time on the ground.

They proposed that apes in the evolutionary line that led to people descended to the ground, remaining bipedal.

Apes ancestral to chimpanzees and gorillas evolved differently, specializing in vertical tree climbing between the tall trees and the ground, and developing knuckle walking when dashing on the ground from one tree to another.