Natural Science Forum / Biology / Paleontology / June 2007

http://www.eurekalert.org/pub_releases/2007-05/aaft-lft052407.php

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.
These findings appear in the 1 June 2007 issue of the journal Science,
published by AAAS, the nonprofit science society.

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.