microwear

Dietary divergence of robust australopithecines

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I’m writing a review of the “robust” australopithecines, and I’m reminded of how drastically our understanding of these hominins has changed in just the past decade. Functional interpretations of the skull initially led to the common wisdom that these animals ate lots of hard foods, and had the jaws and teeth to cash the checks written by their diets.

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Comparison of a “gracile” (left) and “robust” (right) Australopithecus face, from Robinson (1954).

While anatomy provides evidence of what an animal could have been eating, there is more direct evidence of what animals actually did eat. Microscopic wear on teeth reflects what kinds of things made their way into an animal’s mouth, presumably as food, and so provide a rough idea of what kinds of foods an animal ate in the days before it died. Microwear studies of A. robustus from South Africa had confirmed previous wisdom: larger pits and more wear complexity in A. robustus than in the earlier, “gracile” A. africanus suggested more hard objects in the robust diet (e.g., Scott et al., 2005). A big shock came a mere 8 years ago with microwear data for the East African “hyper robust” A. boisei: molars had many parallel scratches and practically no pitting, suggesting of a highly vegetative diet (Ungar et al. 2008).

robust microwear

Microwear in A. boisei (blue) and A. robustus (red). Although they overlap mostly for anisotropy (y-axis), they are completely distinct for complexity (x-axis). Data from Grine et al. (2012) and skull diagrams from Kimbel et al. (2004).

Stable carbon isotope analysis, which assesses what kinds of plant-stuffs were prominent in the diet when skeletal tissues (e.g. teeth) formed, further showed that the two classically “robust” hominins (and the older, less known A. aethiopicus) ate different foods. Whereas A. robustus had the carbon isotope signature of an ecological generalist, A. boisei had values very similar to gelada monkeys who eat a ton of grass/sedge. GRASS!

robust isotopes

Stable carbon isotope data for robust australopithecines. Data from Cerling et al. (2013) and skull diagrams from Kimbel et al. (2004). Note again the complete distinction between A. robustus (red) and A. boisei (blue).

ResearchBlogging.orgWhile microwear and isotopes don’t tell us exactly what extinct animals ate, they nevertheless are much more precise than functional anatomy and help narrow down what these animals ate and how they used their environments. This highlights the importance of using multiple lines of evidence (anatomical, microscopic, chemical) to understand life and ecology of our ancient relatives.

REFERENCES

Cerling TE, Manthi FK, Mbua EN, Leakey LN, Leakey MG, Leakey RE, Brown FH, Grine FE, Hart JA, Kaleme P, Roche H, Uno KT, & Wood BA (2013). Stable isotope-based diet reconstructions of Turkana Basin hominins. Proceedings of the National Academy of Sciences, 110 (26), 10501-6 PMID: 23733966

Grine FE, Sponheimer M, Ungar PS, Lee-Thorp J, & Teaford MF (2012). Dental microwear and stable isotopes inform the paleoecology of extinct hominins. American Journal of Physical Anthropology, 148 (2), 285-317 PMID: 22610903

Kimbel WH, Rak Y, & Johanson DC (2004). The Skull of Australopithecus afarensis. Oxford University Press.

Robinson, J. (1954). Prehominid Dentition and Hominid Evolution Evolution, 8 (4) DOI: 10.2307/2405779

Ungar PS, Grine FE, & Teaford MF (2008). Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PloS One, 3 (4) PMID: 18446200

Dmanisi Homo erectus: I’ll have what she’s having

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Speaking of diet in fossil humans … Herman Pontzer and buddies just published a brief analysis of fine-scale tooth wear in the Dmanisi Homo erectus specimens.

Source: http://bit.ly/uD1LWo

Teeth are useful as hell in life. Humans’ teeth are critical not only for eating, sporting a sexy smile, and biting people (right), but also for speech and song (“f,” “th” and “v” sounds). Some parents even harvest their childrens’ exfoliated baby teeth. The things we do with teeth.

Teeth are also really useful for studying long-dead people and animals – teeth may preserve pretty well for millions of years, they can be used to estimate an individual’s age-at-death, and their shape and composition can be used to learn about diet. In a vile act of revenge, the food that sustains us also scrawls its Nom Hancock into the surfaces of our teeth. So, scientists can study the microscopic marks (= “microwear”) on tooth surfaces to see what kinds of foods were eaten shortly before death. Peter Ungar, an author of the current paper, has done a lot of work here, and his website is worth checking out if you’re interested in learning more. Microwear can’t really tell you exactly what an animal was eating, but can tell you whether the animal mostly ate grasses, leaves, hard objects like nuts, and so forth.

So Pontzer and colleagues (in press) examined the microwear on some of the lower molars of the youngest members of the nearly 1.8 million year old (Ferring et al. 2011) Homo erectus group from Dmanisi in the Republic of Georgia. To the left is a picture of the jaws, from the paper (from another paper. How meta of me). The microwear patterns of these badass early humans fit cozily within the variation exhibited by other Homo erectus specimens.

Microwear in Homo erectus is pretty variable, but still rather distinct from other fossil groups like robust Australopithecus, and a little less distinct from their putative ancestor H. habilis. This suggests that something special about Homo erectus was the species’ great dietary breadth – Homo erectus‘ key to colonial and evolutionary success might not have been the adoption of a key dietary resource, but rather the ability to utilize a wide range of food resources. Atkins diet be damned. What’s neat is that the Dmanisi hominids, though kind of primitive (Australopithecus-like) in terms of brain size and some aspects of skull shape, nevertheless demonstrated key behaviors of H. erectus, namely colonization (Dmanisi is the oldest reliably-dated hominid site outside Africa), and dietary flexibility. This really suggests the success of our ancestors was due to some behavioral innovation, aside from advances in stone tool technology.

Source: http://bit.ly/vCTfeR

Now, these Dmanisi H. erectus kids’ teeth wore like other H. erectus, and it would be reasonable to infer that this is because they ate similar foods. This makes it all the more mysterious that the other Dmanisi jaws, from older adults, have teeth completely worn to shit (sorry to swear). D3444/3900 (left) are the cranium/mandible of an individual who was missing all their teeth, except maybe a lower canine – the earliest example of edentulism in the human fossil record (Lordkipanidze et al. 2005). D2600 (below) is a very large mandible with teeth so worn that the pearly-white first-molar crowns were gone and the internal pulp cavity (and nerve) were exposed. (Interestingly, D2600 is so large that some researchers initially argued it represented a different species from the other jaws – yet Adam Van Arsdale presented evidence that this extreme tooth wear may actually be responsible for making jaws relatively taller in early humans).

Source: http://bit.ly/u6bk6h
So what’s curious is why the older Dmanisi hominids should show such an extreme amount of tooth wear compared to other H. erectus, but microwear on the young suggests their diet was the same (that is, just as diverse in texture) as others in the species. Was Dmanisi-level tooth wear (and tooth loss) comparable to other H. erectus, and we just happen not to have found them at other sites? (KNM-ER 730 from Kenya is the next-most worn early Homo that next comes to mind) Is there another aspect of diet we don’t know about, that caused the Dmanisi teeth to wear especially quickly? Or were these early Homo from Dmanisi actually living longer than other H. erectus? I suspect the second is more likely, but that’s a hypothesis that remains to be tested.
ResearchBlogging.org
Read more, dammit!
Ferring, R., Oms, O., Agusti, J., Berna, F., Nioradze, M., Shelia, T., Tappen, M., Vekua, A., Zhvania, D., & Lordkipanidze, D. (2011). From the Cover: Earliest human occupations at Dmanisi (Georgian Caucasus) dated to 1.85-1.78 Ma Proceedings of the National Academy of Sciences, 108 (26), 10432-10436 DOI: 10.1073/pnas.1106638108
Lordkipanidze, D., Vekua, A., Ferring, R., Rightmire, G., Agusti, J., Kiladze, G., Mouskhelishvili, A., Nioradze, M., de León, M., Tappen, M., & Zollikofer, C. (2005). Anthropology:  The earliest toothless hominin skull Nature, 434 (7034), 717-718 DOI: 10.1038/434717b
Pontzer H, Scott JR, Lordkipanidze D, Ungar PS. In press. Dental microwear texture analysis and diet in the Dmanisi hominins, Journal of Human Evolution (2011). DOI:10.1016/j.jhevol.2011.08.006

What the hell was Australopithecus boisei doing?

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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.
A face not even a mother could love, so now they’re extinct (from McCollum 1999 Fig. 1). Note the very tall face, flaring cheeks, and massive lower jaw which would have facilitated wicked-pisser chewing power.
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?
ResearchBlogging.org
A horse-ish, human-ish hominid? Australopithecus boisei, rest in peace. 2.1 – 1.4 mya.
References
Cerling TE, Mbua E, Kirera FM, Manthi FK, Grine FE, Leakey MG, Sponheimer M, & Uno KT (2011). Diet of Paranthropus boisei in the early Pleistocene of East Africa. Proceedings of the National Academy of Sciences of the United States of America PMID: 21536914
McCollum, M. (1999). The Robust Australopithecine Face: A Morphogenetic Perspective Science, 284 (5412), 301-305 DOI: 10.1126/science.284.5412.301
Ungar PS, Grine FE, & Teaford MF (2008). Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PloS one, 3 (4) PMID: 18446200
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

New twist from teeth

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Peter Ungar, Fred Grine and Mark Teaford recently reported in PLoS ONE on their results of studying the microwear on Australopithecus boisei molars. Their study showed that the microwear differs from that of A. robustus, arguably boisei‘s South African counterpart, and from A. africanus. Here’s the abstract:

The Plio-Pleistocene hominin Paranthropus boisei had enormous, flat, thickly enameled cheek teeth, a robust cranium and mandible, and inferred massive, powerful chewing muscles. This specialized morphology, which earned P. boisei the nickname “Nutcracker Man”, suggests that this hominin could have consumed very mechanically challenging foods. It has been recently argued, however, that specialized hominin morphology may indicate adaptations for the consumption of occasional fallback foods rather than preferred resources. Dental microwear offers a potential means by which to test this hypothesis in that it reflects actual use rather than genetic adaptation. High microwear surface texture complexity and anisotropy in extant primates can be associated with the consumption of exceptionally hard and tough foods respectively. Here we present the first quantitative analysis of dental microwear for P. boisei. Seven specimens examined preserved unobscured antemortem molar microwear. These all show relatively low complexity and anisotropy values. This suggests that none of the individuals consumed especially hard or tough foods in the days before they died. The apparent discrepancy between microwear and functional anatomy is consistent with the idea that P. boisei presents a hominin example of Liem’s Paradox, wherein a highly derived morphology need not reflect a specialized diet.



Note that they refer to boisei and robustus as “Paranthropus,” whereas I (and others) refer to them as Australopithecus. A. boisei and robustus are two “robust” australopithecines, described as such because their skulls and teeth suggest these guys were adapted for prolonged, powerful bouts of mastication (it means chewing, get your mind out of the gutter). Some people argue that these two taxa form a monophyletic group; that is, they share a last common ancestor that is not shared by any other taxon. If this is the case, the generic distinction (Paranthropus) can be made, separating them from the other australopithecines. Though I tend to lump groups, I really think that these taxa do not form a monophyletic group, that they have different ancestors (that their superficially similar masticatory apparati were independently evolved), and that they should stay in the genus Australopithecus. Right now, this issue (wherein I am very interested) has yet to be resolved.


Anywho, what’s important here is that the two robust austrlopithecines differ in their microwear patterns, which suggests that the two subsisted on different diets. Similarly, Wood and Constantino (2007) report that the stable carbon isotope signal from boisei (yet unpublished, but communicated to them personally by Matt Sponheimer) is different from the A. robustus and africanus. Together, these two data indicate that the robust australopitheciens (not to speak about A. aethiopicus…) were quite different in their diets (and possibly lifestyles?). Interestingly, A. robustus‘s molar microwear and stable isotope signals are very similar to that of A. africanus, who was present in the same regions as robustus but a bit earlier in time. This bolsters the scenario in which A. robustus is evolved from A. africanus, or something like it. Could this suggest also that A. boisei is not descendant from A. africanus? Or, is it simply that there were different foods available in the Plio-Pleistocene of South and East Africa?


Another important note that the authors bring up is the fact that of the seven specimens examined, none appeared to have eaten tough or hard foods that might necessitate the use of their (we assume) powerful masticatory muscles. Now why the hell would they have such a derived face, jaws and teeth if they were not eating things that would have required such an apparatus? One proposed scenario about the “hyper-robust” masticatory apparatus of A. boisei is that it is an adaptation for only the toughest of times, when survival might have hinged upon the ability to process and ingest the lowest quality (and hardest to eat) foods. Ungar et al.’s data suggest that this may well be the case, that the powerful masticatory apparatus came in handy only very rarely, and so the dietary signal from microwear reflects what these critters usually ate (and preferred to eat).


If this is really the case, then it might suggest that the robust face of boisei was almost completely genetically acquired, that epigenetic factors did not contribute greatly to produce boisei‘s face. This could be important for teasing out criteria (i.e. skeletal, craniofacial traits) useful in phylogenetic reconstruction. For example, it could be that certain robust features of boisei‘s face indicate a shared genetic ancestry, whereas those of robustus were more epigenetic in nature, acquired over a lifetime of experiencing high chewing forces. Contrariwise, these traits might be the result of these two taxa’s shared ancestry.


Either way, this paper presents interesting new information about the most bizarre hominin evolutionary dead-end, the robust australopithecines.



References

Ungar PS, Grine FE, and Teaford MF. 2008. Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4):e2044.

Wood B, and Constantino P. 2007. Paranthropus boisei: Fifty years of evidence and analysis. American Journal of Physical Anthropology 134(S45):106-132.