National Geographic and New Scientist are both reporting on a new study, in the journal Science, that provides an interesting angle on the question. The study is the result of over 3,000 observations on orangutan behavior in Sumatra (from New Scientist):
Thorpe spent a year recording orang-utan behaviour in the Gunung Leuser National Park in Sumatra, Indonesia, and from nearly 3000 observations of locomotion, the team concluded that the apes were more likely to walk on two legs – using their hands to guide them – when they are on the thinnest branches, less than 4 centimetres in diameter.
On medium-sized branches – those greater than 4 cm but less than 20 cm diameter – the apes tended to walk bipedally, but used their arms to support their weight by swinging or hanging.
Only on the largest branches, with a diameter greater than 20 cm diameter, did the animals walk on all fours.
This leads researchers to suggest:
“Walking upright and balancing themselves by holding branches with their hands is an effective way of moving on smaller branches,” says Robin Crompton of the University of Liverpool, UK, who was also involved in the study.
“It helps to explain how early human ancestors learnt to walk upright while living in the trees, and how they would have used this way of moving when they left the trees for a life on the ground.”
So, rather than evolving to walk on two feet after scrabbling around on the floor on all fours, the theory suggests our ancestors already had the rudimentary means of walking on two feet before they even left the trees.
When the ancestors of chimps and gorillas left the trees, however, they needed to maintain the ability to climb tree trunks. This need for tree-climbing strength and anatomy guided their evolution at the expense of more efficient terrestrial movement, and therefore led to knuckle-walking, says Crompton.
One interesting thing to note is the differing opinions of the outside experts National Geographic and New Scientist consulted on the story. National Geographic went with Brian Richmond who says:
“This is interesting and a very good study of how orangutans climb and walk in trees, but it’s not a study of our distant ancestors,” he cautioned.
“It tells us something about how primates other than ourselves use bipedalism. That could tell us something about how bipedalism might have been used by our distant ancestors. But it doesn’t tell us what actually happened,” Richmond said.
“It doesn’t show us the origins of our own human bipedalism.”
I kind of agree. Certainly, the research does not prove how bipedalism originated, but it does throw some interesting light on the question and gives us new ideas about what led to bipedalism. In that sense, the study is highly valuable (I’m working on tracking down a copy and will probably have more to say later).
New Scientists went with Chris Stringer and Paul O’Higgins (who was also was consulted by National Geographic):
“Nevertheless, this is the best observational data on the importance of hand-assisted bipedalism to orangs, and its possible implications for the evolution of human bipedalism.”
Since all the sites which have yielded fossil evidence of our earliest ancestors were forested or wooded, rather than open, Stringer says, “arboreal bipedalism is certainly a very plausible mechanism for the origins of walking upright.”
O’Higgins mentioned a point mirrored by Richmond:
“If extended hip and knee bipedalism did indeed arise in the distant past, this makes the task of identifying possible ancestors of the human line much more difficult,” he says.
Richmond said something pretty similar as well, but I don’t think this is a new problem, because most paleoanthropologists would expect that the closer we get to the LCA between us and chimps the harder it will be to tell them apart…
Update 1: Kambiz has an interesting discussion of the subject also.
Filed under: Paleoanthropology