So, when is it really efficient to get up and move around on two feet? I know that’s what you’re thinking this morning — and your answer is probably, if someone would bring me a cup of coffee in bed, well, that might shift the whole equation. But a recent piece by Sylvester and Kramer asks this question of a model for the shift to bipedal locomotion in primates.

As most folks who do research on or teach about human evolution will tell you, we spend a lot of time and energy thinking about bipedalism. Because it emerges earlier in the fossil record than the really large brains of later hominids, bipedalism seems to be a key adaptation, a kind of evolutionary watershed that opened up environmental niches that weren’t available to other primates.

But it’s really hard to figure out when exactly it started or why; theories about the reasons for bipedalism include a wide range of explanations, from avoiding too much contact with the sun in open savanna to walking on branches while supporting the body overhead on other branches, from predator spotting to low-fruit foraging from the ground. While it’s clear bipedalism has created all sorts of opportunities, it’s not clear which one of them was necessarily the decisive one that sealed the deal and made bipedalism work for ancestors to modern humans.

As I’ve argued elsewhere, it’s also very tricky to argue backward from subsequent use to the ‘reasons’ for an evolutionary anatomical change, something that the late Stephen Jay Gould was especially persuasive in arguing.

I’m working on a piece on the significance of overhand throwing in neuroanthropological perspective, and it’s a similar issue. Some primates have shoulders that allow vertical rotation over the plane of the shoulder, which makes it possible to throw; but because this allows us to throw doesn’t mean it evolved for throwing. In fact, it likely shows that some early primate that eventually gave rise to hominids brachiated, or hung from its arms in trees. Being able to throw is sort of like the extra set of knives we got because we purchased right now with our credit cards and were among the first 200 callers. It’s not a new idea; even Nietzsche wrote about it in his own inimitable way.

But bipedalism is a particularly important anatomical change because it seems to fundamentally change a number of things about human functioning; everything from freeing up our hands to use tools and carry things like food and kids, to changing the shape of the birth canal and affecting where we can effectively forage for food. With so many changes rippling on from this fundamental shift in the way that the vertebrate skeleton was oriented and employed, why start walking around in the first place?

In November of last year, the American Journal of Physical Anthropology carried a journal article by Adam D. Sylvester and Patricia A. Kramer, of UT Knoxville and U of Washington respectively, on a model of energy expenditure that they have employed to find the threshold at which, energetically, it’s more efficient to shuffle around from one food source to the next than to keep lifting the body up and down. Presumably, early pre-hominids could have sat up to eat off branches; all primates can do this. How much moving around and sitting or standing up would an animal have to do before it would just, in a manner of speaking, say ‘stuff this, I’m just going to stay on my feet.’ (Of course, we know that this wasn’t conscious, intentional, yada yada…)

As Sylvester and Kramer describe, bipedalism has energetic advantages of quadripedalism, but one of the key problems is that fully-fledged running-across-the-savanna bipedalism could not have sprung instantaneously from a quadripedal animal. That is, while expert bipedalism has advantages, what about shambling, shuffling, or wobbling about — when are they an advantage?

Using a host of different assumptions about the energetic cost of bipedalism (because quadripedal primates are typically not very efficient when they walk bipedally), the pair compared the energy savings of not having to lift the body repeatedly to do whatever the switching-to-bipedal primate was doing on two feet (presumably eating, but perhaps other things, too).

What they found was, as we might expect, the higher the primate had to raise the body, the more energy efficient bipedalism started to look (because it saved the energy used to lift up the torso). Using their model, they looked at chimpanzees: ‘At the extreme, our results suggest that chimpanzees should strongly prefer to shuffle, if already standing bipedally, for distances less than 1 m, and that they should rarely, if ever, use bipedalism for distances greater than 80 m’ (487).

Depending on the relative efficiency of quadripedalism and bipedalism in a species, that maximum starts to drop; using efficiency comparable to macaques, for example, suggests any stroll of close to 9 meters is going to provoke a primate to get down on all fours. Estimates for chimpanzee relative efficiency suggests an upper threshold of around 16 meters before shifting to quadripedal locomotion.

Although Sylvester and Kramer don’t offer any estimates for proto-hominids in the AJPA article, they did talk to Science Daily for the article, Did Walking On Two Feet Begin With A Shuffle?. Although Kramer mostly discusses the direct modeling on chimpanzees, and the production of testable hypotheses on how far chimps are willing to walk bipedal, the piece also includes the suggestion that these energetic limits point to a ‘shuffling’ origin for bipedalism. That is, the first hominids to go bipedal likely did so for very short distances — less than 16 meters or so — and didn’t really walk. In other words, the argument that walking was ‘selected for’ because of its efficiency as a long-range transportation method has to contend with the argument that animals with chimp-like bodies (closer to the proto-hominid body than modern humans, many argue) are just not efficient walkers over any distance. While it may be romantic to think about early hominids running sleekly across the savanna chasing prey, we have to contend with the fact that the first primates to stand and walk were likely just shuffling around, about as far as from the bed to the bathroom.

References

Adam D. Sylvester and Patricia A. Kramer. 2008. Brief Communication: Stand and shuffle: When does it make energetic sense? American Journal of Physical Anthropology 135 (4): 484-488. DOI: 10.1002/ajpa.20752