As promised, malaria resistance in baboons

Last week I started to mention a recent paper in Nature on the evolution of malaria resistance in baboons, but then went out and partied instead. Not wanting to be a bastard, I’d better make good. While I’ll try to pull a good lesson from this, be warned that I’m about to discuss a topic about which I am no expert.

Malaria sucks, you don’t want to get it. There are anti-malaria medications out there, but I understand that they can make you insane, or at least have crazy dreams. Fortunately for millions of humans, there is a genetic basis for malaria resistance, so they don’t have to buy the anti-malaria crazy pills. Now, the paper tells me that a polymorphism in part of the FY gene turns the gene off in red blood cells, and that individuals with this variant are then strongly protected from malaria. No Lariam for these folks.

Jenny Tung and colleagues analyzed the homologous region of the FY gene in almost 200 yellow baboons (Papio cynocephalus) from Kenya, as well as tested these baboons for Hepatocystis parasites–relatives of Plasmodium vivax, which don’t cause malaria in baboons, but does really suck for them. And wouldn’t you know it–this same region on the baboon FY gene is also associated with Hepatocystis infection, where individuals with certain genetic variants have a lower susceptability to infection!

Now, the underlying genetic architecture and subsequent mechanisms of infection resistance are not exactly the same. But here’s the take home message from the paper:

“These results suggest that the genetic basis of phenotypic variation in different primate species can exhibit a remarkable degree of parallelism. In this case, not only are the similarities present at the molecular level . . . but they also extend to the mechanism that links molecular and phenotypic variation”

In other words, closely related species are equiped with very similar (or often the same) genetic or developmental “hardware,” and so evolution can cause them to come up with similar solutions to the same problem. In this case, there’s a similar genetic basis underlying infection resistance in humans and baboons. But I think this is a lesson that can be extended to, or at least kept in mind when considering, phenotypic evolution generally.

I’ve always (well, for the past three and a half years since I’ve been studying physical anthropology) thought that such a situation might characterize the “robust” australopithecines of East and South Africa. It is possible that these groups are not each others’ closest relatives, but that they evolved many craniodental characters in parallel, in response to selection for a heavy-chewing diet. This becomes even more plausible if it should turn out that many of these cranial and dental features are morphologically integrated–something I’m working on at the moment (if anyone reads this and scoops me, you will pay).

So, interesting paper. Reference

Tung, J. et al. Evolution of a malaria resistance gene in wild primates. Nature, in press.

Advertisements

Zach, get over the robust australopithecines

Let’s return to my favorite group (though I think they’re not terribly closely related) in the human fossil record, the robust australopithecines. They have popped fairly frequently in the news this year, most recently regarding their possible use of bone tools <!–[if supportFields]> ADDIN EN.CITE Backwell21221217Backwell, Lucindad’Errico, FrancescoEarly hominid bone tools from Drimolen, South AfricaJournal of Archaeological ScienceJournal of Archaeological ScienceIn Press, Accepted Manuscripthttp://www.sciencedirect.com/science/article/B6WH8-4SPYKKN-2/1/388e9a02101a532d68bfcccbe79ba29b <![endif]–>(Backwell and d’Errico, in press)<!–[if supportFields]><![endif]–>. This group is funny. They first appeared probably some time around 2.7 million years ago, in the controversial form of Australopithecus aethiopicus. No one knows where exactly this enigmatic group came from, save for that the only known, fairly complete cranium (KNM WT 17000) has many primitive features, and is largely similar to the earlier A. afarensis. Around 2.3 million years ago or so, A. aethiopicus appears to be ‘replaced’ by A. boisei, whose face is less protruding, has smaller anterior teeth, and has a derived P3 morphology. This is all in East Africa, mind you. Then, some time probably around 2 million years ago, or a little later, a robust form (A. robustus) appears in South Africa, where erstwhile the only hominin was A. africanus (some argue that there are 2 species in the A. africanus hypodigm). Personally, A. robustus looks like a more ‘robust’ A. africanus: larger posterior teeth, more anteriorly placed cheeks—but there is much overlap in many of traits between these two taxa. And much to the chagrin of many cladists, the South and East African robusts appear fairly different morphologically; a recent study <!–[if supportFields]> ADDIN EN.CITE Gonzalez-Jose200821021017Gonzalez-Jose, RolandoEscapa, IgnacioNeves, Walter A.Cuneo, RubenPucciarelli, Hector M.Cladistic analysis of continuous modularized traits provides phylogenetic signals in Homo evolutionNatureNature775-77845371962008Nature Publishing Grouphttp://dx.doi.org/10.1038/nature06891http://www.nature.com/nature/journal/v453/n7196/suppinfo/nature06891_S1.html <![endif]–>(Gonzalez-Jose et al. 2008)<!–[if supportFields]><![endif]–> that examined hominoids morphometrically (that is, in terms of aspects of cranial shape) found the two robust taxa to be distinct (but that’s a topic for another post . . .). And all the while these buggers lived right alongside Homo, our ancestors! That’s some effed up stuff.

Now for the recent paper. Blackwell and d’Errico (in press) report on an assemblage of bone tools from the site of Drimolen in S. Africa (~2-1.5 million years ago). Drimolen is very near Swartkrans and Kromdraai, two other cave sites with a wealth of A. robustus, and to a lesser extent Homo, material has been recovered. First, how do they know these bones were tools? The tools were compared to other bones, known to have been worn (down) by other processes, like gnawing or carnivore chewing, and the tools appear quite different from these. Also, experimental studies of actually using bones as digging tools (to dig up underground tubers and especially to dig into termite mounds) have produced the same kind of wear as the fossil bone tools. Finally, many bone tools are known from the site of Swartkrans, which as I mentioned above is also laden with A. robustus remains. So it appears pretty likely that A. robustus (or Homo) was using bone tools to dig for foodstuffs around Drimolen and other cave sites in the early Pleistocene. *Note: our friend and colleague Julie Lesnik is currently finishing up her fieldwork, examining bone tools from Swartkrans, and observing chimpanzees digging for termites in Senegal; perhaps we could get her to write a good post on the topic.

Second, how do they know who used these tools? This an excellent question, which similarly plagues the postcranial material from these S. African sites (and even E. African sites, cf. the OH 7 hand and the OH 8 foot, again another topic for another post on another day . . .). The basic argument, similar to that for the postcrania, is that where these bone tools are found, the hominin assemblage is dominated by A. robustus remains; well, at least there are more A. robustus teeth and cranial material relative to comparable Homo fossils. So it’s guilt by association for the bone tools and postcrania here. Of course the only real way to test the hypothesis that most S. African postcrania and bone tools are to be associated with A. robustus and not Homo would be to find a complete skeleton of either, and to find bone tools in strong association (i.e. almost still in the clutches) of a certain taxon. Another argument used in support of robustus as the bone tool user is the fact that stone tools—the hallmark of early humankind?—are relatively absent at these sites. So it’s probable, but certainly not unequivocally proven, that A. robustus was the user of these stone tools (note that the authors never make any claim of such proof).

The authors use the bone tool assemblages from Swartkrans and Drimolen to infer ‘cultural’ behavior patterns for A. robustus. A recent paper claimed that since larger (male) A. robustus cranial specimens tended to be older than smaller ones, that this was evidence for extended male growth in the taxon <!–[if supportFields]> ADDIN EN.CITE Lockwood20079917Lockwood, Charles A.Menter, Colin G.Moggi-Cecchi, JacopoKeyser, Andre W.Extended male growth in a fossil hominin speciesScienceScience1443-1446318Australopithecus robustusdelayed maturation200730 November 2007doi:10.1126/science.1149211<![endif]–>(Lockwood et al. 2007)<!–[if supportFields]><![endif]–>. This, coupled with the pronounced sexual dimorphism in this taxon (and all hominins until only fairly recently), suggested Lockwood et al. that A. robustus had a social structure similar to that of gorillas, in which one or two males associate with a number of females and their offspring (a simpler answer is that selection favored larger males, and so they were living longer, but Mary and I had trouble trying to demonstrate this). So Blackwell and d’Errico figure that this evidence, along with data from chimpanzee termite-foraging behavior, suggests that females were the predominant practitioners of termiting using bone tools.

Why do I think this is interesting? I think this shows a potentially very important ecological divergence between the robust australopithecine lineages on the on hand, and between A. robustus and Homo on the other. Could the disparate toolkits of these hominins have played an important part in the evolution of these lineages? It is debated whether robust australopithecine hands were physically capable of actually making stone tools (which is quite difficult). Part of this stems from the fact that it’s hard to find a complete hand (damned beetles…) and then to attribute it to a specific taxon. For instance, the OH7 partial hand, which was supposed to be part of the H. habilis holotype, was recently found to be most similar morphologically to A. robustus from S. Africa, and functionally not adapted to tool-making <!–[if supportFields]> ADDIN EN.CITE Moyà-Solà200826126117Moyà-Solà, S.Köhler, M.Alba, D. M.Almécija, S.Taxonomic Attribution of the Olduvai Hominid 7 Manual Remains and the Functional Interpretation of Hand Morphology in Robust AustralopithecinesFolia PrimatologicaFolia Primatologica215-2507942008http://www.karger.com/DOI/10.1159/000113458 <![endif]–>(Moyà-Solà et al. 2008)<!–[if supportFields]><![endif]–>. Of course, this relies on assumptions about whether hand material from Swartkrans represents A. robustus or Homo. Nevertheless, it may well be that part of the adaptive divergence of A. robustus and Homo was the former’s use of bone tools to dig for termites, while the latter was able to manipulate stone to exploit higher quality resources. Cool.

Also significantly, I think this points to more evidence against uniting A. robustus and boisei into the genus Paranthropus. Yes, they both had large posterior teeth (though boisei’s were generally larger) and faces built to house large chewing muscles. But other than that they appear pretty different. As noted above, a recent study looking at modularized traits in crania found that the two robust groups were not monophyletic, which is a criterion when making taxonomic decisions (Gonzalez-Jose et al. 2008). A. robustus has striking affinity with the earlier A. africanus, while A. boisei has striking affinity, to the exclusion of A. robustus, with A. aethiopicus, suggesting that A. robustus and boisei didn’t share a common ancestor. It has also been argued that sharing an ecological space is another criterion for generic membership <!–[if supportFields]> ADDIN EN.CITE Wood199926226217Bernard WoodMark CollardDepartment of Anthropology, George Washington University, 2110 G Street NW, Washington, DC 20052, USA; Human Origins Program, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA.; Department of Anthropology, University College London, Gower Street, London WC1E 6BT, UK.The changing face of genus HomoEvolutionary AnthropologyEvol AnthropolEvolutionary Anthropology195-2078619991520-6505http://dx.doi.org/10.1002/(SICI)1520-6505(1999)8:63.0.CO;2-2 10.1002/(SICI)1520-6505(1999)8:63.0.CO;2-2<![endif]–>(Wood and Collard 1999)<!–[if supportFields]><![endif]–>. But isotopic and dental microwear evidence show A. robustus and boisei to have been distinct. Also, (shame on me) I’m not sure the extent to which A. boisei is associated with bone tools, but to my knowledge it is not. So here we have morphological, geological, temporal, and ecological discontinuity between the two main ‘robust’ taxa. That pretty much sinks Paranthropus for me. I don’t know why I am so adamant about these taxa’s paraphyly, but I am. I feel that taxonomy should reflect important biological and phylogenetic reality. And it seems to me that because there isn’t compelling evidence that A. robustus and boisei share the same ancestor, or interbred, that they shouldn’t be taxonomically separated from the other australopithecines. And that’s my two cents.

References

Backwell L, d’Errico F Early hominid bone tools from Drimolen, South Africa. Journal of Archaeological Science In Press, Accepted Manuscript

Gonzalez-Jose R, Escapa I, Neves WA, Cuneo R, Pucciarelli HM (2008) Cladistic analysis of continuous modularized traits provides phylogenetic signals in Homo evolution. Nature 453(7196):775-778

Lockwood CA, Menter CG, Moggi-Cecchi J, Keyser AW (2007) Extended male growth in a fossil hominin species. Science 318:1443-1446

Moyà-Solà S, Köhler M, Alba DM, Almécija S (2008) Taxonomic Attribution of the Olduvai Hominid 7 Manual Remains and the Functional Interpretation of Hand Morphology in Robust Australopithecines. Folia Primatologica 79(4):215-250

Wood B, Collard M (1999) The changing face of genus Homo. Evolutionary Anthropology 8(6):195-207<!–[if supportFields]><![endif]–>

New twist from teeth

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.