Causes of Bipedalism: Efficient Long-Distance Walking

Outline

One view about bipedalism is simply that it is more efficient than quadrupedalism. Although this might seem a little counter-intuitive - if it were true, how come no other animals do it? - it has been shown that at slow speeds human walking is actually more efficient than most quadrupedal animals going at similar speeds. Rodman & McHenry wrote a paper in 1980 suggesting that as this was true, it was all the selection pressure that was necessary. In the paper they implied that even in extant apes bipedal locomotion is no less efficient than quadrupedal locomotion. As they put it "there is no bipedal rubicon that must be crossed." This, however, is hard to accept. Surely, if it were better for chimps to walk bipedally they would do so - one of the problems of this theory is that it would certainly predict that chimpanzees would also be bipedal.

Authors

Rodman & McHenry (1980) 

Location

An open, grassy environment is assumed for this theory to work. It serves as a beautiful example of how the savannah theory of the origins of bipedalism, discredited since the discovery of Lucy, was re-worked into a modified theory that now includes woodland but is still based on the idea walking long-distances over grassland.

Selection Pressure

It is logical that individuals that were able to walk long distances efficiently would have this trait selected for assuming that they did indeed have to walk long distances regularly.

Intermediate Steps

This is perhaps the weakest part of the theory. It relies on the counter-intuitive premise that bipedalism is really no big deal, that it is actually more efficient even for chimpanzees. If this were not true, and it would be surprising if it were, it would make the argument very difficult. Evolution has to work in discrete steps, each one an advantage over the previous form. It has no 'long term goal' and so efficient as full striding bipedalism may be, it is difficult to imagine each intermediate step on the way being of benefit for this too.

Both Sexes?

Both sexes would equally be selected for.

Explains Australopith Anomalies

No. This is another very weak area of this theory. It pretty much relies on the idea that Australopithecus walked in a very human-like way. If it did not, and a large and growing body of evidence suggests that it did not, it would be difficult to imagine how long-distance walking could have been part of the driving force towards full bipedalism.

Why not Pan? Observed in Pan?

Another weak area. If it was so efficient for early hominids why didn't chimpanzees adopt it too. Chimpanzees have been observed walking bipedally, but never very far.

Links with other Theories

This theory is quite compatable with most of the other theories and would complement them very well if seen as providing "the finishing touches" perfecting striding bipedalism. It does not sit very comfortably anywhere, however, as a possible origin to bipedalism.

Popularity

It has become a very popular theory and is listed in 6 out of 8 of my sample textbooks.

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