What Did Australopithecus africanus Eat?

Hyenas are amazing animals. It takes a single hyena less than two minutes to consume an entire Thompson’s gazelle. A pack of 21 hyenas was able to polish off a 220 kg zebra and a 150 kg foal in about 30 minutes. An extinct species of borophagine dog (Borophagus) was probably able to accomplish similar feats as well. Borophagines, being descended from canids, retained post-carnassial molars. This pushed the carnassials forward in the jaw and they are located in the region of maximum bite force production and is what allowed them to crack bones efficiently using their carnassials (the carnassials are composed of a blade-like upper fourth premolar and a somewhat blade-like lower first molar). In hyenas, however, post-carnassial molars underwent a reduction and hyenas crack bones between their upper and lower third premolars not with the carnassials, which are behind the region of maximum bite.

I bring this up because there is an interesting article in PNAS that looks at the feeding biomechanics of Australopithecus africanus. The article looked at facial morphology that previous research indicated might be a result of premolar loading. The research used finite element analysis in combination with experimental data and comparative morphology to model the strain during simulated bites in the long tailed (or crab eating) macaque and A. africanus (for A. africanus they used a composite of Sts 5 and Sts 52). Bites with molars alone, molars and premolars, and premolars alone were simulated. Overall, results indicated a higher amount of strain in the facial skeleton of A. africanus particularly on the anterior pillars and at the root of the zygoma. Additionally, while both experienced high amounts of strain during premolar biting, in A. africanus this waas dominated by compression while in macaques the strain was a mixture of compression and tension. Significantly, the browridges experienced little strain.
This leads the researchers to suggest that these features of A. africanus were an evolutionary response to eating based on premolar loading – specifically on eating seeds and nuts:

The processing of some large, hard food items may have entailed combining premolar biting with hand-assisted manipulation, as has been observed in extant primates in the wild … and during our in vivo experiments in the laboratory. Premolar-focused biting might also be associated with the ingestion of large, displacement-limited foods. One might hypothesize that the facial features examined here evolved in response to the frequent consumption of such items, but this hypothesis requires that skeletal adaptations to consuming displacement limited foods evolved in australopiths without attendant dental adaptations. Thus, biomechanical considerations do not exclude the possibility that australopiths had a high volume diet or that they consumed smaller and/or displacement-limited items … Indeed, some aspects of australopith craniodental form may be adaptations to such diets … However, these diets do not provide the best explanation for the evolution of craniofacial features functionally related to premolar loading. Thus, large, stress-limited objects are likely to have been a selectively important component of a diet that may have otherwise been quite varied.

Note: The information on carnivores came from the following paper:
Van Valkenburgh 2007 Deja Vu: the evolution of feeding morphologies in the Carnivora. Integrative and Comparitive Biology 47(1):147-163

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7 Responses

  1. Nice post, but I have to say that reading that first paragraph I thought the topic was What Ate Australopithecus africanus.

  2. Its not surprising that gracile and robust australopithecines would eat the most abundant food items that were found in their environment (fruits and their nutty cores). And they were certainly not the first thick enameled hominoids to exist in that environment.
    Marcel F. Williams

  3. Are you familiar with the theories of Aaron G. Filler MD, PhD, as documented in The Upright Ape: A New Origin of the Species? According to his model, the common ancestor of all the great apes (including homo) had a roughly Australopithecine form.
    What would this mean in terms of these dental features? Could the main line leading to the great apes have had such a diet, with the three offshoot lineages (oran utan, gorilla, and chimpanzee) independently reaching their current state? Filler shows that the “knuckle walking” adaptations of these lineages are different enough to have probably been independent, also the changes to the hip that restrict twisting during brachiation. Could the same be true here?

  4. AK – Yes, I reviewed Filler’s book. The Strait article was more about diet and reconciling morphological and isotopic data.

  5. I realize that. However, the dietary implications have their own implications, different whether you assume that Australopithecus was derived from a brachiating-adapted ancestor (as I’m sure Strait et al. do) or was the original line. I don’t know enough about great ape tooth structure to even guess whether it’s plausible that the three lineages independently developed their current structure from the Australopithecene condition described here.
    Assuming they did, however, it raises some interesting dietary issues. For instance, I’ve seen speculations that Australopithecus developed from a chimpoid that moved into more grassy areas and shifted to a diet containing more tubers (e.g. The rise of the hominids as an adaptive shift in fallback foods: Plant underground storage organs (USOs) and australopith origins by Greg Ladena, and Richard Wrangham). If Australopithecus came first, perhaps it had lived in such areas all along and the brachiating lineages developed from marginal sub-populations that moved into deep forest areas with closed canopies.
    IMO Australopithecus was best adapted for life in very rough brushland, such as existed in sub-Saharan Africa prior to it’s being covered by Kalahari sands (couldn’t find a reliable link). Unlike the present, the terrain had considerable relief, so the same sort of vegetation that exists today (including downwood) would have combined with the landscape to provide something like a “super jungle-jim” running from ground level up to perhaps 10 meters. Movement would have required a combination of bipedal and four-handed scrambling with some brachiation and vertical climbing. AFAIK this sort of terrain would have been well supplied with tubers.
    If this landscape lasted from, say, 25 MYA to the late Pleistocene (see e.g. Holocene aeolian activity in the southwestern Kalahari Desert, southern Africa: significance and relationships to late-Pleistocene dune-building events by David S.G. Thomas, Stephen Stokes, Paul A. Shaw), it could have supported a large, stable population of Australopithecenes during this time.

  6. Strait, of course, has written quite a bit on bipedalism and knuckle-walking. None of which have anything to do with this paper. At best one could argue that this paper supports the old idea that A. africanus was the ancestor of the robust group. Strait et al, in this new paper, are claiming that they have solved the C4 conundrum that Laden and Wrangham, Thorpe and Sponheimer, Unger, Teaford and Grine, and others have looked at. It was a very narrowly focused paper.

  7. I must admit I don’t understand:

    This leads the researchers to suggest that these features of A. africanus were an evolutionary response to eating based on premolar loading – specifically on eating seeds and nuts:

    AFAIK grass seeds, including C4 seeds, tend to be too small to be included in the group mentioned, while no nuts are C4. Or does this food group include the underground storage organs of C4 sedges?
    Unfortunately I can’t access the original article, so I’m probably missing one or more links in the chain of logic here.

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