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Showing posts with label hands. Show all posts
Showing posts with label hands. Show all posts

Monday, June 16, 2008

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 (Backwell and d'Errico, in press). 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 (Gonzalez-Jose et al. 2008) 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 (Lockwood et al. 2007). 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 (Moyà-Solà et al. 2008). 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 (Wood and Collard 1999). 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