Red-ape stroll: orangutans step into the evolutionary fray over how we became upright

Science News,  August 4, 2007  by Bruce Bower

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Look, up in the trees. A barrel-chested, long-limbed creature covered with wispy, reddish hair sits on a branch far above the ground. The animal rises to a fully erect posture, reaches up to grab an overhead branch for balance, and promenades across the precarious platform. Upon reaching a duster of hanging fruit, the animal plucks off a snack with a free hand.

Still standing, it consumes the treat with gusto. Then it saunters back the way it came, striding from one padded foot to the other while continuing to grasp branches above its head.

Witness the red-ape stroll, as practiced by an orangutan living on the Indonesian island of Sumatra. New field observations of these animals, conducted by anthropologist Susannah K.S. Thorpe of the University of Birmingham in England and her colleagues, show that orangutans, unlike knuckle-walking chimpanzees and gorillas, at times walk upright much as people do. This suggests to the researchers that two-legged walking, or bipedalism, evolved in a common ancestor of all living apes at least 20 million years ago.

Among scientists who study hominids, the fossil ancestors of people, that's a heretical notion. These investigators have long assumed that an upright stance is a unique trait of hominids, a skeletal smoking gun that separates members of our evolutionary family from other ancient primates. From this perspective, hominids that walked on two legs evolved from a chimplike ancestor with a body structure suited to scooting across the ground on all fours.

Thorpe's group turns that argument on its head. Two-legged walking first appeared in ancient, tree-dwelling apes, the researchers argue, and all ensuing apes inherited a capacity for bipedalism. Around 6 million years ago, as Africa's dense forests gave way to open space interspersed by stands of trees, hominids parlayed tree-walking skills into an upright, free-handed stride that supported ground travel. In contrast, chimps and gorillas took a less-traveled evolutionary route for primates, developing bodies suited to climbing trees and walking on all fours.

"If we're right, it means you can't rely on bipedalism to tell whether you're looking at a human or another ape ancestor," says anthropologist and study coauthor Robin H. Crompton of the University of Liverpool in England. "It's getting more and more difficult to say what's a human and what's an ape."

TREE WALKERS Orangutans lead solitary fives. They hang out in dense forests, rarely leave the trees, and generally make it difficult for ground-bound researchers to study their behavior.

To penetrate orangutans' tropical veil of secrecy, Thorpe and her coworkers spent a year tracking their behavior in a Sumatran national park. They recorded 2,811 instances of the animals moving through the trees.

Although only a minority of those instances consisted of walking erect, orangutans frequently walked on two legs when they ventured onto slender, springy branches from which they could reach out and grasp other branches with their hands for support, the investigators report in the June 1 Science. Hand-assisted walking ensures safety, they say, especially as the animals cross narrow gaps from one tree to another. In this upright posture, the apes can easily extend a hand to pick and eat fruit.

The researchers note that orangutans kept their legs straight while standing on flexible branches. People also adopt a more straight-legged stance when running on springy surfaces than when striding across solid ground. This approach may conserve energy, Thorpe's team suggests.

When traversing sturdier supports, the orangutans often slipped beneath a branch, hung by their arms, and moved by swinging one hand over the other. On particularly large branches, the animals stayed topside, bent over, and walked on all fours--gripping the branch with feet and hands.

Thorpe and her colleagues contend that, between 24 million and 5 million years ago, apes in Africa and elsewhere lived in forests and moved through the trees much as orangutans now do. As African forests grew increasingly patchy toward the end of that period, the team theorizes, human ancestors came down from the trees and used a two-legged gait when gathering food on the ground and periodically clambering into small, fruit-bearing trees.

At the same time, ancestors of chimps and gorillas evolved limbs and torsos specialized for climbing large trees and retrieving food from elevated perches. For instance, these apes developed long, curved fingers and vertically extended pelvises that keep their backs stiff. Physical alterations such as these fostered knuckle walking on the ground, with fingers bent and body weight supported on the backs of the second of three rows of finger bones.

Meanwhile, orangutans' ancestors in southeastern Asia hunkered down in shrinking forests and adapted with renewed vigor to life as tree dwellers capable of strolling along springy branches.

A scenario that portrays bipedalism as the norm among ancient apes rather than as an exclusive trait of human ancestors fits with recent reassessments from fossils of how early apes moved, Thorpe's group adds. Consider Oreopithecus, a 7-million-to-9-million-year-old apelike creature that lived on what was once a Mediterranean island. Spanish scientists argue that remains of this animal's skeleton show that it could stand up and walk. With a big toe that angled sharply away from other toes on the same foot, Oreopithecus probably shuffled only short distances to obtain fruit and other food, according to the researchers.