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 cluster 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.”
Orangutans lead solitary lives. 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.
If upright walking originated in ancient apes, different styles of two-legged striding apparently evolved later in various hominid species, remark Paul O’Higgins and Sarah Elton, anatomists at Hull York Medical School in England, in an editorial published with the Thorpe team’s paper. That could explain why limb proportions and foot shapes vary considerably in hominids from between 4 million and 2.5 million years ago, O’Higgins and Elton say.
A shared ancestry of upright walking also explains why orangutans’ feet resemble people’s feet more than they resemble the feet of chimps and gorillas, despite a closer evolutionary link between people and the latter apes, comments anthropologist Bernard Wood of George Washington University in Washington, D.C.
“Although [people’s] excessive use of standing and walking upright is novel, that novelty was built on an ability that was probably present in the common ancestor of all great apes,” Wood says.
Anthropologist David S. Strait of the University of Albany in New York doesn’t doubt that orangutans sometimes traipse two-legged through the trees. However, the fossil record shows that hominids’ upright gait was built on an anatomical foundation provided by a tree-climbing, knuckle-walking ape ancestor, not an ancient tree-walking ape, Strait contends.
“Early hominids possessed knuckle-walking features in their wrists,” he says. “They were bipeds, but the presence of those features means that they were descended from knuckle-walking ancestors.”
In 2000, Strait and anthropologist Brian G. Richmond of George Washington University compared the wrist bones of early hominids with those of people, apes, and a couple of monkey species. Two ancient hominids from eastern Africa—the more than 4-million-year-old Australopithecus anamensis and the 3.2-million-year-old partial Australopithecus afarensis skeleton known as Lucy—possess a mechanism for stabilizing the wrist and locking it in place, the researchers reported.
The same wrist-steadying arrangement supports knuckle walking in modern chimps and gorillas, they said.
In contrast, the wrists of other early hominids—the nearly 3-million-year-old Australopithecus africanus from southern Africa and the 2.5-million-year-old Paranthropus robustus, a member of a dead-end lineage in eastern Africa—looked more like those of people and lacked the knuckle-walking trait.
In Strait’s view, these findings fit with the long-standing observation that Lucy’s kind combined humanlike skeletal characteristics, such as feet and a pelvis appropriate for upright walking, with apelike features, such as long, curved fingers, long arms, and a funnel-shaped chest. Equipped with an effective two-legged gait, Lucy and her A. afarensis kin retained from an ape ancestor a knuckle-walking feature that they didn’t use, Strait proposes.
Still, researchers have argued for more than 30 years over how Lucy’s kind got around. Some suspect that these hominids primarily walked on two legs, while others regard apelike traits as evidence that A. afarensis combined ground walking with tree climbing.
Although that debate grinds on, anthropologist David Begun of the University of Toronto endorses Strait’s view that our upright stance derived from a knuckle-walking ancestor. The legs and knees of early hominids more closely resemble those of African apes than they do those of orangutans, Begun holds.
That’s because orangutans support their body weight in a decidedly nonhuman way, he asserts. Even when orangutans walk on two feet, “it’s not bipedal, because it is hand assisted, so [physical] loads are distributed completely differently from the human pattern,” Begun says.
He adds that chimps and gorillas move more capably on the ground than orangutans and gibbons do. Gibbons, a group of apes that are much smaller than orangutans, also walk and run through trees upright as they grasp branches with their hands for balance.
Anthropologist Jeffrey H. Schwartz of the University of Pittsburgh agrees. “Orangutans walk in the trees with a very different skeleton than we have,” he says. “That’s significant for orangutans but it doesn’t have any implications for hominid evolution.”
It’s not surprising that scientists can’t agree on whether bipedalism originated in tree-walking apes or ground-striding hominids, says anthropologist Kevin D. Hunt of Indiana University in Bloomington. This new dispute joins a long-running clash of hypotheses about what circumstances led to the evolution of an upright gait in the first place.
These hypotheses generally treat two-legged walking as a unique property of hominids. That assumption is unlikely to change as a result of the new orangutan observations, in Hunt’s opinion.
Like Begun, Hunt views orangutans as moving in their own particular ways, none of which qualify as the type of walking practiced by people or any of our fossil ancestors. Ancient, tree-dwelling apes might have taken initial steps toward walking upright, Hunt says, “but I’m agnostic on that possibility.”
Hunt expresses more confidence in the idea, which he first presented in 1994, that bipedalism evolved as a feeding posture for hominids that had to survive on landscapes dotted with a mix of forests, bush-and-shrub-covered areas, and grasslands. Human ancestors such as Lucy walked across open expanses to locate small, fruit-bearing trees, Hunt proposes. Individuals would harvest fruit either by reaching into trees while standing on the ground or after leaping onto a low branch and grabbing a higher branch for balance or hanging from that branch.
Wide hips and short legs in early hominids lowered the body’s center of gravity and increased stability when standing on branches, in this scenario. Curved fingers evolved to grip small branches and hang from larger ones. Lucy’s uptilted shoulder joint and funnel-shaped torso aided one-handed hanging, Hunt adds.
Other explanations for the rise of bipedalism have attracted supporters. For instance, anthropologist C. Owen Lovejoy of Kent (Ohio) State University proposes that an upright gait in early hominids promoted monogamous relationships and boosted reproductive success. Males were able to walk long distances to collect food and bring it to favored females to forge exclusive bonds. Certain of paternity, these males aided their own offspring, Lovejoy proposes.
In contrast, Peter E. Wheeler of Liverpool (England) John Moores University suggests that hominids evolved a two-legged stance in order to avoid overheating in tropical settings. When walking, a person exposes much less skin surface to direct sunlight than a chimp or gorilla moving on all fours does, Wheeler says. Moreover, upright walkers lift their torsos into a cooler zone of air than hunched-over knuckle walkers trek through.
Such proposals are difficult to test or disprove. That doesn’t bother orangutans, though. Scientific quarrels leave them unmoved, especially when fruit beckons high in the trees. Once again, they’re ready to do the red-ape stroll.