Causes of Bipedalism: Efficient Long-Distance Walking with reduced group size

Outline

Very much based upon Rodman & McHenry's efficient long-distance walking hypothesis. It attempts to explain why, if bipedalism was more efficient, didn't chimpanzees become bipedal too.

The idea is based upon the observed fission-fusion behaviour of chimpanzees. They tend to split into smaller groups to go searching for food types that are harder to find and/or better quality. Their theory is that as Africa dried some apes stayed close to trees where they remained essentially chimp-like, whilst others had to travel further and further for food across open areas of grassland. The later group would therefore naturally become bipedal because it was naturally more efficient for them to travel long distances on two legs. 

Authors

Isbell & Young (1996)

Location

This theory is based on the idea of gradually receding woodland, where it is separated by patches of grassland. 

Selection Pressure

Like Rodman & McHenry's theory, it would be 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. The benefit for small groups of hominids having to walk long distances for food also has clear survival benefits for those individuals.

Intermediate Steps

Again, as with Rodman & McHenry, 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?

This is a 'fix' for the inadequacy of Rodman & McHenry's theory.

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

I did not find it mentioned in any of the textbooks, perhaps because it is still relatively new. 

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