Causes of Bipedalism: Tool Use / Hunting

Outline

This was the earliest theory of bipedalism which basically assumed that humans must have started walking to free their hands for using tools and hunting. The assumption from Darwin's time had always been that intelligence had come before bipedalism and that it was our increase in brain size that drove our need to use tools and that drove the need to get up on two legs.

It has been largely discredited now as a means for the origin of bipedalism, largely because tool use and hunting have now been shown to have arrived much later than bipedalism itself.

Authors

Most authorities attribute this idea to Charles Darwin although in truth many people must have written about the subject. In (1871) in Descent of Man Darwin wrote "Man alone has become a biped; and we can, I think partly see how he has come to assume his erect attitude, which forms one of his most conspicuous characters. Man could not have attained his present dominant position in the world without the use of his hands..."

Location

Darwin was remarkably accurate in predicting that humans originated in Africa. It should be remembered that his did so with no fossil evidence and purely on the basis of comparative biology. There was no doubt that Darwin assumed that the bipedality had terrestrial origins.

Selection Pressure

In Darwin's day the idea of survival of the fittest was still quite new and it was many years before the concept of the selfish gene. For each of these theories I want to pose the purely Darwinian question "How much more likely to pass on his/her genes would an individual be, who was able to perform this behaviour bipedally, than one that did the same thing on all fours." It's a complex issue, one that involves all sorts of issues, like group selection and sociability but it one I want to try to apply to all the theories.

In this case, assuming the hominid was a hunting animal, an individual that could move bipedally would be better suited to hunting and tool use than one that was unable to do so. A quadrupedal hunter/tool user would be severely disadvantaged but may well be able to survive with help from the group. Assuming that hunting would be a collaborative project, it is not clear that the ability to be bipedal would favour every individual and help them to reach sexual maturity.

Intermediate Steps

In this section I want to ask the question "would this explanation only work in the final, human, form? or would it make sense at intermediate steps too?" 

In this case it seems reasonable to assume that if a chimp-like quadrupedal ape had been able to throw something it would have been better able to do that if it had been more erect. 

Both Sexes?

The 'Man-the-mighty-hunter' theory was very much a male-dominated idea and, as stated above, it is hard to see, assuming hunting was a male activity, what would be in it personally for females and younger members of the group.

Explains Australopith Anomalies

In this section I want to test the complex and odd anomalies of the Australopithecine fossil record with the theory. This is a very controversial and technical topic and one I do not claim to have any expertise. Basically I am assuming here that Australopiths did not walk in a fully erect manner like humans, and the question is would the anomalies found be accounted for by this theory.

In this case, it is doubtful whether the Australopith morphology would lend itself to hunting. It does not appear to be energetically efficient enough to be useful for high speeds or long distances. (Ruff, McHenry & Thackery 1999)

Why not Pan? Observed in Pan?

Another important test of a good theory of bipedalism is a) has it been observed in extant chimpanzees/bonobos and b) does the theory explain why Pan did not also become bipedal.

Although chimps have been observed throwing things I do not believe there is any evidence of them doing so bipedally.

The theory does not explain why chimpanzees did not also attain the intelligence to get up on two legs.

Links with other Theories

One of the most enduring features of this theory is that it tends to fit in with any of the others and so cannot really be totally discarded. As humans have undoubtably been hunters and tool users at stages of their history it would seem reasonable to assume that this behaviour must have played a part at some point.

Popularity

As the oldest theory, it is not surprising that is the one most mentioned in human evolution text books. 7 out of 8 of my sample included it, although most did so only for historic reasons.

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