Lessons From The Orangutans: Upright Walking May Have Begun In The Trees
Science Daily  By observing wild orangutans, a research team has found that walking on two legs may have arisen in relatively ancient, tree-dwelling apes, rather than in more recent human ancestors that had already descended to the savannah, as current theory suggests.
An adult female Sumatran Orangutan resting in Gunung Leuser National Park, Indonesia. (Credit: Image courtesy of SKS Thorpe) An adult female Sumatran Orangutan resting in Gunung Leuser National Park, Indonesia. (Credit: Image courtesy of SKS Thorpe)
Upright walking, or bipedalism, has long been considered a defining feature of humans and our closest ancestors. One of the most popular explanations, known as the savannah hypothesis, suggests that the ancestors to chimps, gorillas and humans descended from the trees and began walking on the ground on all fours.
Over time, this four-legged gait would have evolved into the "knuckle-walking" that chimps and gorillas still use today and then into upright, two-legged walking in humans.
Paleontologists have conventionally used signs of bipedalism as key criteria for distinguishing early human, or "hominin," fossils from those of other apes. But, this distinction is complicated by recent fossil evidence that some early hominins, including Lucy (Australopithecus afarensis), lived in woodland environments, while even earlier forms such as Millennium Man (Orrorin) appear to have lived in the forest canopy and moved on two legs.
"Our findings blur the picture even further," said Robin Crompton of the University of Liverpool in Liverpool, Great Britain, who is one of the study's authors. "If we're right, it means you can't rely on bipedalism to tell whether you're looking at a human or other ape ancestor. It's been getting more and more difficult for us to say what's a human and what's an ape, and our work makes that much more the case."
Crompton and his colleagues, Susannah Thorpe and Roger Holder of the University of Birmingham in Birmingham, Great Britain, came to their conclusions by observing wild orangutans in Sumatra, Indonesia. Orangutans spend almost their whole lives in trees, making them useful models for how our ancestors moved around several million years ago.
To collect the data, Thorpe spent a year living in the Sumatran rainforest and recording virtually every move the orangutans made. Then, she and her colleagues used these observations to test the hypothesis that bipedalism would have benefited tree-dwelling ape ancestors.
Because these ancestors were probably fruit-eaters, as orangutans are, they would have needed a way to navigate the thin, flexible branches at the tree's periphery, where the fruit typically is. Moving on two legs and using their arms primarily for balance, or "hand-assisted bipedalism," may have helped them travel on these branches.The researchers analyzed nearly 3,000 examples of observed orangutan movement, and found that the orangutans were more likely to use hand-assisted bipedalism when they were on the thinnest branches. When bipedal, the animals also tended to grip multiple branches with their long toes.
On medium-sized branches, the orangutans used their arms more to support their weight, changing their moving style to incorporate hanging. They only tended to walk on all fours when navigating the largest branches, the researchers found.
Hand-assisted bipedalism may have offered several advantages that allowed our arboreal ancestors to venture onto thin branches. They could have gripped multiple branches with their toes and distributed their center of gravity more effectively, while keeping one or both of their long arms free to reach for fruits and other supports.
Orangutans also keep their legs straight while standing on bending branches, the authors report. The exact benefit of the straight legs is still unclear, but when humans run on springy surfaces, we also keep our weight-bearing legs relatively straight, so this may have an energy-related advantage.
"Our results suggest that bipedalism is used to navigate the smallest branches where the tastiest fruits are, and also to reach further to help cross gaps between trees," said Thorpe.
The authors propose an evolutionary scenario that begins as other researchers have envisioned. Somewhere toward the end of the Miocene epoch (24 to 5 million years ago), climate in East and Central Africa became alternately wetter and drier, and the rainforest grew increasingly patchy. Apes living in the forest canopy would have begun to encounter gaps between trees that they could not cross at the canopy level.
The Science authors suggest that early human ancestors responded to this by abandoning the high canopy for the forest floor, where they remained bipedal and began eating food from the ground or smaller trees. The ancestors of chimps and gorillas, on the other hand, became more specialized for vertical climbing between the high canopy and the ground and thus developed knuckle-walking for crossing from one tree to another on the ground.
"Our conclusion is that arboreal bipedalism had very strong adaptive benefits. So, we don't need to explain how our ancestors could have gone from being quadrupedal to being bipedal," Thorpe said.
Observations of orangutan movement should be useful for conservation efforts, according to Thorpe. These animals are seriously endangered, primarily due to habitat destruction.
"If you can understand how they cross gaps in the forest, you can learn about effects that living in logged or degraded habitat would have on their locomotion. These could affect energy levels, for example, if they have to go to the ground, which is incredibly risky because the Sumatran tiger is down there licking its lips. The Sumatran orangutan population is predicted to be extinct in the next decade if habitat degradation continues. Our research further highlights the need for protecting these animals," she said.
These findings appear in the 1 June 2007 issue of the journal Science, published by AAAS.
Article: "Origin of Human Bipedalism as an Adaptation for Locomotion on Flexible Branches," by Thorpe et al. This research was supported by The Leverhulme Trust, The Royal Society, The L.S.B. Leakey Foundation and The Natural Environment Research Council.
Note: This story has been adapted from a news release issued by American Association for the Advancement of Science.
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An orangutan.


Scientists from the United Kingdom who spent one year watching orangutans have revealed in a study that humans may have learned to walk while still living in the trees, and that humans may not be that closely related to chimpanzees.

"An increasing number of people have been questioning this old 'up from the apes' idea" of how bipedalism evolved. As the forests became sparse, the strategy of our human ancestors was more or less to abandon the canopies and come down to the ground, where they could use this bipedalism immediately to get around," said one University of Liverpool scientist who headed the study, Robin Crompton.

Museums and schools across the world have been teaching that humans evolved from an animal much like that of a chimpanzee and that humans started to walk along the forest floors, with their arms hanging, and knuckles scraping across the ground. It is also taught that those animals then began to walk upright once they adapted to living on the ground.

Orangutans were observed by researcher Susannah Thorpe of the University of Birmingham in Sumatra, Indonesia for one year. She documented that the orangutans would generally walk on their hands and feet, but when food was at a height that they could not reach, the orangutans would stand on their feet, extend upright, and grab the fruit or food item they want. This movement also allowed them to swing from the branches of trees more efficiently, without having to touch the forest floor.
Frankfurt, Germany. 
"When they move to the skinniest branches, where the tastiest fruit grows, they stand stiffly straight-legged, like a person. It's energetically quite economical for orangutans to feed and move on these bendy branches using bipedalism," said Thorpe who also has over 3,000 different movements on film that show orangutans standing on their own 2 feet, upright, and reaching for objects.

"Our conclusion is that arboreal bipedalism had very strong adaptive benefits. So, we don't need to explain how our ancestors could have gone from being quadrupedal to being bipedal," added Thorpe.

The researchers also compared evidence from the remains of Lucy, past climate conditions on the planet and fossils to the workings of orangutans, and all suggest that humans were living and swinging in the trees for a much longer period of time than previously thought. The study shows that humans may have learned to walk at least 24 million years ago, rather than 6 million years ago.

Some experts disagree with the study.

"The main evidence is that our closest living relatives are not orangutans, they're chimps and gorillas, and since both climb trees and walk on their knuckles, it's most likely our ancestors did that too. One of the only anatomical features we share explicitly with chimps and gorillas is that we only have eight wrists bones, while almost all other primates have nine. In humans, chimps and gorillas, two bones have fused into one to stabilise the wrist, making it stronger for knuckle-walking. It's not a smoking gun, but it's the best evidence we have," said anthropologist at George Washington University, Brian Richmond.