A little over 2 million years ago there a major divergence of hominids, leading on the one hand to our earliest ancestors in the genus Homo, and on the other hand to a group of ‘robust’ australopithecines, the latter group a failed evolutionary experiment in being human. In our ancestors, parts of the skull associated with chewing began to get smaller and more delicate, while the robust australopithecines increased the sizes of their crushin’-teeth and chewin’-muscle attachments.

Weirder, there is a South African form (Australopithecus robustus) and an East African form (A. boisei, the figure here looks like it’s based off this species) of robust australopithecine. These two may have inherited their robust adaptations from a common ancestor, or they may be unrelated lineages that evolved these features in parallel. A boisei has been referred to as ‘hyper-robust,’ its face and teeth are generally larger than those of A. robustus.

For a while it’s been supposed that these ‘robust’ chewing adaptations in our weird, extinct evolutionary cousins (every family has those, right?) reflected a diet of hard objects requiring powerful crushing and grinding – things like hard fruits, seeds, Italian bread, etc. But a few years ago Peter Ungar and others (2008) examined the microscopic wear patterns on the surfaces of molar teeth of A. boisei and noted that they lacked the characteristic pits of a hard-object feeder. A. robustus on the other hand does have wear patterns more like an animal that ate hard foods. Why such a difference? Why the hell wasn’t boisei behaving robustly?

Also in 2008 Nikolaas van der Merwe and colleagues analyzed the carbon isotopes preserved in the teeth of A. boisei and some other fossils. Briefly, plants utilize two isotopes of carbon (C12 and C13), but ‘prefer’ the lighter-weight C12. Some groups of plants like grasses have thrived because they’re less picky and can get by just as well with C13. Different kinds of plants, then, incorporate different amounts of these two carbon isotopes into their tissues, then when animals eat it, these isotopes get incorporated into the animal’s developing tissues, including tooth enamel. So by looking at the relative amounts of carbon in teeth, researchers can get a rough idea of whether an animal was eating more of the C13-loving or C13-loathing plants (or the animals eating the plants). van der Merwe and others found A. boisei to have a way higher percentage of the plants that don’t discriminate against C13 as much, possibly things like grass, sedges or terrestrial flowering plants. GRASS?!

Last week, Thure Cerling and colleagues expanded on the earlier study led by van der Merwe, including a larger set of boisei specimens spanning 500 thousand years of the species’ existence. Lo and behold, Cerling and others got similar results: the isotopic signature in A boisei is similar to grass-feeding pigs and horses in its habitat – was the badass “hyper robust” A boisei just a hominid version of a horse? Now, the silica in grass make it extremely wearing on tooth enamel, and while A. boisei had crazy thick molar enamel, I would be a little surprised if the boisei dentition could withstand a lifetime of a grassy diet. Nevertheless, boisei‘s diet clearly differed from robustus, based on both dental wear and carbon isotopes.

This raises interesting questions about the evolution of the robust group. Does their shared ‘robust’ morphology reflect common ancestry, with the subtle differences the result of their divergent diets? Or do the subtle differences indicate that they evolved separately but their diets for whatever reasons resulted in similar mechanical loading on their jaws and faces? It should also be noted that while the dates for South African cave sites are always a bit uncertain, it is possible that A. robustus persisted alongside genus Homo until around 1 million years ago, whereas the fossil record for A. boisei craps out around 1.4 million years ago – was A. boisei too specialized on crappy grass, resulting in its evolutionary demise?

A horse-ish, human-ish hominid? Australopithecus boisei, rest in peace. 2.1 – 1.4 mya.

van der Merwe NJ, Masao FT, & Bamford MK (2008). Isotopic evidence for contrasting diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. South African Journal of Science 104: 153-155