?It?s Blurring the Distinction Between Humans and the African Apes.?
31 May, 2007 08:00 pm
Upright walking may have begun in the trees. This hypothesis is the result of a study, based on the observation of orangutans, published today in Science. Robin Crompton, the lead author, answers Scitizen?s question.
First of all, orangutans are the only great apes, which are still living in the original great apesí environment which is the peripheral canopy of tropical rainforest. In lots of respects, theyíre the best model for what the ancestor of the great apes was like. The orangutan uses bipedalism, precisely hand-assisted bipedalism, specifically when itís accessing up right fruit in the periphery of the rainforest canopy. Itís quite surprisingly using bipedalism to move on fine supports or very bending supports and, when it does so, it actually uses straight limb postures. That is suggesting that upright bipedalism with straight leg bipedalism, could well have evolved in the ancestor of great apes as an adaptation to locomotion and feeding in the peripheral canopy.
You propose a new evolutionary scenarioÖ
The previous view is that the common ancestor of humans and chimpanzees and gorillas were to be a knuckle-walker, that is moving on the ground supporting its weight on its knuckles. Iíve always been unhappy with this view because it doesnít really explain how moving in a posture which involves highly flexed or bent postures on the high land could be pre-adapted for upright walking. Itís also part of that traditional view that humans are to become bipedal walkers when they move out of forest into savannah. We know now from the fossil record that until about 2 million years ago most homininesí fossils or relatives of humans are associated actually with tropical wood landed environments, not with open country at all. Itís only when they come down to their own genus homo that there is an association with open country. The appearance of bipedal human relatives would be land or forest environments. This is suggested by fossils like Orrorin, the Millennium Man. It suggested to several people before, that we might need to think about an arboreal origin for bipedality, but the problem being to see what the adaptive value could be of being bipedal in what is an unstable and irregular environment. What weíve done is actually to provide what the selective force, the selective value of bipedalism is in an arboreal environment, moving in the trees.
Your study suggests that you can't rely on bipedalism to tell whether you're
looking at a human or other ape ancestor. Can you elaborate on this?
That certainly goes for animals of the channel of Orrorin. I wouldnít say thatís the truth for Australopithecus that are so committed to being bipedal and show some clear adaptation by being bipedal on the ground. But certainly when we go back near the channel of the genetic separation of humans from chimpanzees and gorillas, itís now very difficult to say. Iíve got this feature in the fossil, which is an adaptation for bipedity therefore Iím looking at hominine. Thatís certainly the case for Orrorin. We donít really know on what side of the boundary it lays, or whether it could be a known ancestor to humans as well as to chimpanzees, for example. Itís sort of blurring the distinction between ourselves and the African apes.
Would you say this is a landmark in the field of evolution?
I think itís certainly important because itís a paradigm shift. We really have to look at bipedalism again in a very different light after this work. Lots of people have a very different view of where humans belong in terms of human adaptation. Humans then are rather fairly conservative and have not innovated so much in terms of being bipedal. Theyíre really retraining what is part of the common ancestral heritage.
What is the next step?
We need to look in more detail at the mechanics of moving on bending support. We think that by keeping the limbs straight, orangutans may be getting some energy back from the substrate. We need also to test that in forest environments. We did actually have another paper which was out a couple of weeks ago in the Biology Letters in which we showed that orangutans moving in a forest from tree to tree were actually swayed in the direction they want to go. They certainly do and get quite a lot of energy back from the support. In that case we need to show and test bipedalism on fine supports as well. Thatís the next step.
Crompton et al., Science, 1 June 2007
Robin Crompton is a professor at the University of Liverpool, UK.
Interview by Clementine Fullias