Some Random Thoughts About Australopithecus sediba

I am going to be very busy today so I won’t get an in depth post up on Australopithecus sediba until tomorrow. In the meantime three items jumped out at me so I thought I would, briefly, mention them.

First off:

The nearly vertical mandibular symphysis presents a weak lateral tubercle, resulting in a slight mental trigone, and a weak mandibular incurvation results in a slight mentum osseum.

In other words it has a chin! Something early Homo and Neandertals don’t have. This is very perplexing – but maybe I’m over-interpreting this sentence.

But there is no way to over-interpret this:

Thus, although the dentition and postcranial skeleton are at odds in the degree of apparent size differences, the overall level of dimorphism, if these sex attributions are correct, appears slight in the Malapa hominins and was probably similar to that evinced by modern humans.

If you have been following paleoanthropology for the last couple of years you may be able to spot the problem. KNM-ER 42700 is a female skull, attributed to Homo erectus, dating to 1.55 MYA. It is incredibly small and suggests that sexual dimorphism in H. erectus was much greater than previously thought. BSN49/P27 is a pelvis, also attributed to H. erectus, that dates to 0.9-1.4 MYA and stature estimates indicate that it, also, belonged to a small member of H. erectus. If Berger et al are correct then sexual dimorphism decreased up until Au. sediba increased until H. erectus then decreased and some later date – and note the above two specimens aren’t that much more recent than Au. sediba.

Then there are the teeth. Much has been made of the fact that the teeth are more or less similar in size to H. habilis and H. erectus but in the figures provided in the paper they are also similar in size to Au. afarensis. Which similarity is more important?

Update: Okay I’m home and have discovered something else that raises some red flags – or at least makes me say WTF? From Scientific American’s blog:

When asked during a press teleconference whether he had found any tools at the site, Berger said he had not commenced formal excavation of the site [bold mine - afarensis] and so did not want to talk about artifactual remains.

5 Responses

  1. It may be that the chin arose early in our history, then was lost for awhile before being re-expressed.

    Where the sexual dimorphy is concerned, sounds to me like Ms. KNM-ER 42700 was more likely a juvenile, or on the low end for female specimens of that species from that area.

  2. Only Homo sapiens has a chin. On the sexual dimorphism issue, there are three female fossils – all adults – that indicate that it was greater in H. erectus than previously thought..

  3. With respect to sexual dimorphism in Homo erectus, Chris Ruff has argued (with much data) that the new Gona pelvis from Ethiopia is very unlikely to be from that species and probably belongsinstead to Paranthropus. Placing it in erectus stretches sexual dimorphism and intra-female variation beyond plausible limits.
    With only 2 published individuals to go by, one of which is a juvenile, assessing the degree of sexual dimorphism in Au. sediba is fanciful and premature. Both of them are small-bodied and could easily be females, and thereby provide no insight into sexual differences at all.

  4. No need for red flags. The statement makes perfect sense if you know the condition of hominid sites in the South African cave sites. They’ve all been quarried before by lime miners back in the 1800s. Including Malapa. The lime miners didn’t want the breccia with the fossils just the pure carbonate of cave formations but they often broke through breccia deposits to get the latter. Fortunately they didn’t do so much damage at Malapa, so most of the breccia accumulation is intact. All of the fossils in the paper come purely from chunks of breccia in the lime miners discard pile. However they match in situ bones that can be observed sticking out of the undisturbed breccia (including more hominid bones) so we can be absolutely sure where they came from.

  5. Siamangs can have slight chins.

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