But a few years ago, this picture changed: evidence from toothwear and the chemical composition of teeth suggested A. boisei was actually eating grass or sedges (see the referred post or a nice recent review by Julia Lee-Thorp for more info). Such a diet is totally at odds with what people had hypothesized based on the size of the chewing muscles and teeth.
|Colobus molars (image: http://bit.ly/xefm6t)|
|KNM-ER 15930 (Leakey & Walker 1988, Figure 8)|
But, it occurred to me, maybe high-crowned, shearing molars simply were not an 'option' in the evolution of Australopithecus boisei. Natural selection is a powerful force of evolution, but it is limited because it can work only with existing variation: it does the best it can with what it's got. The earliest surefire hominids*, Australopithecus anamensis and afarensis, certainly did not have 'cresty' molars with pointy cusps, and neither did many late Miocene apes, for that matter. Rather, the ancestors of A. boisei had fairly low bulbous molar cusps, and that's some serious evolutionary baggage for a hominid hoping to corner the grass and sedge market.
So we can draw up the following hypothesis for the evolution of A. boisei: as the early members of the species moved into a niche of eating grass/sedges, rather than evolve cresty teeth, they increased the size and enamel thickness of their ancestors' molars to better-withstand their diet. Perhaps this was the 'easiest' solution to adapting teeth to a crappy diet (maybe some developmental constraint?). Or perhaps there's another, yet unidentified food responsible for the species' curiously high-C4 diet ... who knows? Nota bene: this isn't necessarily what I think happened, it's just a hypothesis consistent with current evidence about A. boisei's anatomy and diet.
If Life on Earth has taught us anything, it's that there are many ways to do the same thing. What's more, evolution is highly constrained by pre-existing biology and historical circumstance. Australopithecus boisei may have been 'a victim of its times,' forced into an herbivorous niche for which it was ill-equipped.
Leakey RE, & Walker A (1988). New Australopithecus boisei specimens from east and west Lake Turkana, Kenya. American Journal of Physical Anthropology, 76 (1), 1-24 PMID: 3136654
Lee-Thorp, J. (2011). The demise of "Nutcracker Man" Proceedings of the National Academy of Sciences, 108 (23), 9319-9320 DOI: 10.1073/pnas.1105808108
* I only mention australopithecines because I'm still on the fence about the hominid status of Ardipithecus, and not convinced by Orrorin or Sahelanthropus.