Lee Berger and colleagues report in Science today on 2 incredibly well preserved skeletons – including perhaps the best-preserved hominid skull in South Africa, in some ways as good as or better than Sts 5 (Australopithecus africanus). The specimens come from a site called Malapa in South Africa, dating to around 1.9 – 1.7 million years ago. The authors argue that it is so unique in its features that it warrants a new species – Australopithecus sediba – linking the earlier A. africanus with later Homo habilis. Is it really a new species? In my personal opinion, there’s not much distinguishing it from A. africanus.
The amazingly preserved skull is of a subadult, maybe 11 years old. The highly angled root of the zygomatic, positioned just above the M1 alveolus is classic A. africanus. It really reminds me of Sts 17, or possibly Sts 52 in the lower face. The prognathism is modest and lacks the anterior nasal pillars which tend to be fairly pronounced in A. africanus; in this regard it is quite comparable to specimens like TM 1512. Like Sts 52, it has multiple infraorbital foramina. The cranial capacity is estimated at 420 cubic centimeters (cc), which is pretty small, but within the estimated A. africanus range of variation. The authors say that the relatively low position of the temporal lines, spaced far apart from the sagittal suture, and the fact that the zygomatic arches do not flare to the sides, are features more like Homo than like A. africanus. But the specimen is only 11 years old; while the brain is finished growing, the face and chewing muscles probably aren’t. So if this were a fully adult specimen, I’m sure both of these features would come to look more like A. africanus.
The upper and lower first and second molars increase in size posteriorly, and have a distinct protostylid (enamel shelf along the side of one of the cusps) which has a very high frequency in A. africanus. The upper molars, while not totally complete, preserve something that I’ve noticed and I’m sure is in the literature, that the M1 is fairly small and squared compared to the generally larger and not-quite-as-square M2. In a few words, then, the skull seems to fit comfortably in the range of A. africanus variation.
Perhaps the least A. africanus-like aspect of the skull is the supraorbital torus. The supraorbital, or brow, is generally a modestly expressed in most africanus specimens that preserve it. The Malapa specimen is much more similar, to my eye, to later Homo in its projection and arching over the eyes. What could this mean? Moss and Young’s (1960) functional matrix model of looking at the cranium views the supraorbital as a function of the relative position of the brain to the orbits. Perhaps the spatial relationship between the vault and the face which becomes characteristic of later homo becomes established in earlier in the lineage. Other than the supraorbital, this specimen seems purely A. africanus to me. In all, the contour of the vault may not be too different from younger Dmanisi specimens like D2700 or 2280; that Malapa lacks the occiput gives an artificially short front-to-back look to the specimen. The face, however, is totally A. africanus.
Perhaps one of the most striking images is the lateral view. This photo looks strikingly similar to a subadult chimpanzee, albeit with a taller face and less prognathic snout. Maybe I’ve just seen a subadult ape before that this thing reminds me of.
So, this is an immensely exciting set of fossils. Is there a new species of Australopithecus? I wouldn’t bet my life on it. If you go with the widely held idea, that A. africanus or something like it was ancestral to later A. robustus on the one hand, and our Homo ancestors on the other, this thing would fall on the Homo side of that split. So in this case, since we’re not seeing anagenetic evolution – evolutionary change within a lineage – but rather branching, how do you name this thing? It might be slightly more derived toward a Homo than either its “pure” africanus ancestors and A. robustus evolutionary cousins, does this make it Homo? The issue is that adaptively its morphology doesn’t seem to be different from A. africanus, which would argue against the generic distinction. But if its later ancestors become H. habilis and nothing else, then I suppose this would make it a “chronospecies” of H. habilis. So maybe we should call this thing H. habilis? I think most people would argue with this simply on the brain size issue. And the brain is way smaller than any proclaimed Homo specimen.
Taxonomically, this will be a tough call.
Berger L, de Ruiter DJ, Churchill SE, Schmid P, Carlson KJ, Dirks PHGM, and Kibii JM. 2010. Australopithecus sediba: A New Species of Homo-like Australopith from South Africa. Science 328: 195 – 204
Moss ML and Young R. 1960. A functional approach to craniology. American Journal of Physical Anthropology 18: 281 – 292