I was debating whether or not to post this over at Primatology.net because it involves orangutans Spread Eagle Orangutanand the research of them. But, I decided not to because the topic, bipedalism, is almost uniquely human in the mammalian world. We are exclusively bipedal, except for that brief period of our lives where we crawl. Bipedialism, as you may have guessed, is the mode of locomotion in which an organism walks upright on two legs.

I once took a Physical Anthropology class titled, ‘Anthropology of Movement’ where we studied how humans and their ancestors most likely moved around. We analyzed fossils, studied gait, and compared anatomical features to other apes and animals. After that class, I understood that the predominant theory on the origins of human bipedalism, based upon the observations from the fossil record, is that human ancestors developed the ability to walk upright after leaving the treetops to live on the African savanna. Thus, we see evidence that bipedalism first appeared in the hominin lineage of primates, of which we humans are classified as a part of.

New research just published in Science is contending this hypothesis. Here is a link to the paper, “Origin of Human Bipedalism As an Adaptation for Locomotion on Flexible Branches.”

Researchers from the Universities of Birmingham and Liverpool studied how Sumatran orangutans of Gunung Leuser National Park in Indonesia moved about the canopy of the rain forest. Here’s a brief summary of what they observed,

“On sturdier branches the orangutans use all four limbs. But on thinner branches in search of fruit, the apes move on two legs and use their arms for balance.”

The authors are using this to argue that human bipedalism is “less of an exploitation of a locomotor behavior retained from the common great ape ancestor.” This behavior implies that prehistoric apes may have developed upright walking while still living in the trees—well before human ancestors ever descended to the ground. A big challenge, if you ask me. The authors hypothesize that upright bipedalism in human ancestors was most likely an adaptation to moving and feeding on ripe fruit in the peripheries of trees. In co-author Robin Crompton’s own words,

“When they are on the very fine stuff, they are using bipedalism… It shows that bipedalism can be adaptive in the trees. People have suspected that it evolved in the trees, but no one has been able to see a sensible reason why it should happen.”

Crompton has been pursuing this question for a while now. I’ve scavenged up a 2004 press release from his university where he discussed what type of gait can be analyzed from Lucy’s, the australopithecine, fossil remains. In this new paper, Crompton et al. have almost conveniently shifted from analytical to observational methods, of a perfect model organism that lives in trees.

I personally don’t buy it as a 100% replaceable theory to what we know from the fossil record right now. In their abstract, the authors say that, “some recent paleontological evidence suggests that adaptations for bipedalism arose in an arboreal context.” What now? Just because a population of orangutans resort to tight rope walking doesn’t mean our ancestors did. What if this behavior, this adaptation, arose in the orangutan lineage independently of our own? …A type of convergent evolution. It is possible. Also, what about orangutans walking upright on the ground? There’s a big distinction between walking along some branches and shifting to exclusive terrestrial movement.

Brian Richmond, biological anthropologist, at George Washington University in Washington, D.C., seems to also share my reasoning. He 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…. 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. It doesn’t show us the origins of our own human bipedalism.”

This research is very interesting. There’s no doubt about that. What I have doubts about, and what I don’t quite get, is how can the authors explain orangutan bipedal tendencies to human bipedal dependencies?

Our bodies, humans that is, are made to be bipedal. Our vertebrae curve and become more dense lower in our bodies to deal with distributing our weight. Likewise, the pelvic girdle distributes weight even more down on two robust, long, bones called the femur. We sacrifice flexibility in our lower limbs for the added strength and stability that is needed to be bipedal.

In orangutans, the design and function of the pelvis does not resemble something that’s made to bear weight. Instead, the orangutan pelvis is flexible. Orangutans can practically rotate their legs 360 degrees, like we do in our arms.

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