Close to Homo? – The announcement of Australopithecus sediba

A diagram of how the skeletons of Australopithecus sediba came to be preserved in the Malapa cave deposit. From Dirks et al, 2010.

A little less than two million years ago, in what is now South Africa, a torrential downpour washed the bodies of two humans into the deep recesses of a cave. Just how their remains came to be in the cave in the first place is a mystery. Perhaps they fell in through the gaping hole in the cave roof just as hyenas, saber-toothed cats, horses, and other animals had, but, however the humans entered the cave, their bones ultimately came to rest in a natural bowl carved into the rock. This mode of preservation would keep their remains in good condition until their discovery in 2008, and today in the journal Science a team of researchers has described them as the latest addition to our family tree, Australopithecus sediba.

As is so often the case with the announcement of a new species of fossil human, what everyone wants to know about Australopithecus sediba is whether it was one of our ancestors. This may be asking the wrong question. Around the two million year mark there were many species of humans living at the same time, including the early representatives of our own genus, and to understand where the new hominin fits in among our assorted relatives we have to look at how it compares with its contemporaries.

The skeletons of Australopithecus sediba. The more complete skeleton of the adolescent male (MH1) is on the left, and the less complete adult (MH2) is on the right. From Berger et al, 2010.

Fortunately, the scientists behind the new study had a lot of material to work with. The more complete skeleton, that of a 12-13 year old male, is represented by a nearly complete skull, several limb bones, a partial pelvis, and a few other assorted fragments. The second skeleton, which the authors interpret as an adult female, preserves almost the entire right arm (from fingers to shoulderblade), several jaw fragments, and a few other bits and pieces. There are still some missing pieces, particularly the ribs and parts of the spine, but altogether the collection of bones has provided us with a good picture of what Australopithecus sediba looked like, and it shares a close resemblance with another hominin.

Described in 1925, Australopithecus africanus was the first fossil hominin known from South Africa (though it took nearly two decades before the majority of physical anthropologists took it seriously as an early human), and the newly-described hominin is very similar to it. Even though A. africanus was a bit older, A. sediba shares the same body plan; a lower body well-adapted to walking upright and an upper-body which still retained some traits related to an arboreal lifestyle (such as long arms). The face of A. sediba is quite like that of A. africanus, as well, although the newly described species has slightly smaller teeth and cheeks that do not flare out as much as in its predecessor.

Three views of the skull of the adolescent A. sediba (MH1). From Berger et al, 2010.

The similarities between the two hominins and the existence of A. sediba in South Africa shortly after A. africanus disappears from the fossil record has led the authors to suggest that A. sediba is the descendant of A. africanus, but it is the relevance of A. sediba to another transition that has been kicking up so much attention. According to the authors, A. sediba shares some peculiar features of the hip with Homo erectus to the exclusion of other early members of our genus such as Homo habilis and Homo rudolfensis, meaning that those latter hominins are really australopithecines and not early members of Homo at all. Consequently, Australopithecus sediba would either be ancestral to Homo or close to the ancestry of our genus on the basis of its similarities to Homo erectus, but is this interpretation correct?

One alternative possibility, which the authors do no consider, is that the specialized, Homo-like characteristics interpreted from the partial hip of Australopithecus sediba are convergences and not signals of a true ancestral relationship. Since many of the characteristics they cite are related to more efficient walking, it is entirely possible that the hips and lower limbs of Australopithecus sediba were adapted in a manner similar to Homo erectus due to a shared lifestyle in more open, grassland habitats. Given that there were multiple species of human walking around similar habitats around 2 million years ago I would not find it surprising if they showed convergences in the parts of their skeleton related to locomotion.

Then there is the contradictory argument the authors make in an attempt to undermine the relevance of Homo habilis to the evolution of other Homo species. Early on in the description they cite the 2007 Nature studythe 2007 Nature studythe 2007 Nature study which found that Homo habilis and Homo erectus overlapped in time for about half a million years on the basis of fossils found at Ileret, Kenya. The describers of A. sediba take this to mean that Homo habilis could not have been ancestral to Homo erectus, yet such a relationship cannot be so easily ruled out. While the idea that the entire species of Homo habilis gradually transformed into Homo erectus in a linear fashion has been refuted by the recent find, the pattern seen between the two species is in accord with punctuated equilibirum. In this case, Homo erectus would have originated relatively quickly from an ancestral population of Homo habilis while the rest of the Homo habilis populations underwent little change (or were in “stasis”) until the time of their extinction.

What does this have to do with A. sediba? Well, the two skeletons at the Malapa site are between 1.98-1.75 million years old, and the oldest known Homo erectus remains are also known from around that time (with the earliest putative Homo habilis being a bit older, and the oldest definitive Homo habilis being about the same age). Clearly A. sediba roughly overlaps with the earliest known members of our genus, and, following their argument about Homo habilis, this would make the ancestral status of A. sediba to Homo questionable. Interestingly, however, the authors argue for a “punk eek” model for their own fossils, suggesting that A. sediba had probably originated much earlier and occupied a greater geographic range than illustrated by the first specimens. In this scenario A. sediba would have split with A. africanus sometime before 2.4 million years ago and the earliest Homo (or the ancestor to the earliest Homo) then split from A. sediba around 2 million years ago or so. The argument they use to propose the ancestral status of A. sediba could just as easily be used to retain the hypothesized relationship between Homo habilis and Homo erectus, but instead the authors seem to engage in a bit of (as George Carlin might have put it) “Your stuff is shit and my shit is stuff” argumentation instead.

The upshot of all this is that Australopithecus sediba may not be as close to the ancestry of Homo as the authors propose. Figuring that out, though, will depend upon how we define the earliest members of our genus and extensive comparison between the new fossils and previously-discovered specimens. Even so, I am hoping that the discovery of Australopithecus sediba will help paleoanthropologists crack some of the mysteries surrounding other bones found in the caves of South Africa. There are a number of controversial specimens which have alternately been attributed to australopithecines and early Homo, and the discovery of Australopithecus sediba provides another reference point by which to compare these fossils. There is still much to discover, and as paleoanthropologists continue their research we will gain a clearer picture of Australopithecus sediba in its evolutionary context.

Lee R. Berger, Darryl J. de Ruiter, Steven E. Churchill, Peter Schmid, Kristian J. Carlson, Paul H. G. M. Dirks, Job M. Kibii1 (2010). Australopithecus sediba: A New
Species of Homo-Like Australopith
from South Africa Science, 328, 195-204 : 10.1126/science.1184944
Paul H. G. M. Dirks, Job M. Kibii, Brian F. Kuhn, Christine Steininger,, Steven E. Churchill, Jan D. Kramers, Robyn Pickering, Daniel L. Farber,, & Anne-Sophie Mériaux, Andy I. R. Herries, Geoffrey C. P. King, Lee R. Berger (2010). Geological Setting and Age
of Australopithecus sediba from
Southern Africa Science, 328, 205-208 : 10.1126/science.1184950