New Date on Sahelanthropus tchadensis and Australopithecus bahrelghazali

A new article in PNAS discusses the use of a new dating technique that has been used to come up with a new date for Sahelanthropus tchadensis and a specimen of Australopithecus bahrelghazali. The new method dates Sahelanthropus tchadensis to around 6.8-7.2 MYA, while the Australopithecus bahrelghazali specimen is dated to around 3.58 MYA. I won’t go into much detail on the dating method as it seems to be fairly new and I don’t know that much about it. Outside of that, the authors of the paper draw two conclusions. First, obviously, Australopithecus bahrelghazali and Australopithecus afarensis were roughly contemporaneous. Second, the authors of the paper draw this conclusion concerning Sahelanthropus tchadensis:

The radiochronological data concerning Sahelanthropus tchadensis (Toumai¨, TM 266) reported here is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock. Thus, Sahelanthropus tchadensis testifies that the last divergence between chimps and humans is certainly not much more recent than 8 Ma, which is congruent with Chororapithecus abyssinicus, the new 10-Ma-old Ethiopian paleogorillid … With its mosaic of plesiomorphic and apomorphic characters Sahelanthropus tchadensis, the earliest known hominid …, is probably very close in time to this divergence contrary to the unlikely ”provocative explanation,” which recently suggested a ”possible hybridization in the human-chimp lineage before finally separating less than 6.3 Ma” …

Notice the latter sentence is asserted without any kind of back up evidence to support it. Leaving aside the criticisms leveled by Wolpoff, Hawks, and others, which Brunet has responded to, the idea that humans and chimps formed hybrids until about 6 MYA is one that does impact the question and needs a more serious response. Granted, the paper is more about dating Toumai, but if you are going to address a criticism your response should be more than a blank assertion that the criticism is wrong.
Update 1: Both Kambiz and Jason of Hominin Dental Anthropology have posts on the subject as well.
Lebatard, A., Bourles, D.L., Duringer, P., Jolivet, M., Braucher, R., Carcaillet, J., Schuster, M., Arnaud, N., Monie, P., Lihoreau, F., Likius, A., Mackaye, H.T., Vignaud, P., Brunet, M. (2008). Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0708015105

5 Responses

  1. Is this really all that new?
    Anne G

  2. Yes, the paper was just published in PNAS on February 27.

  3. I agree with Anne. I thought it was originally dated at around 7 million years. I also agree witth Afarensis. The date is in no way incompatible with a “possible hybridization in the human-chimp lineage before finally separating less than 6.3 Ma” if not even more recently. Sahelanthropus could well have contributed genes to chimpanzees.

  4. The original dates for Sahelanthropus tchadensis were based on faunal associations. This new date is based on a technique called 10Be/9Be dating. They could not use argon dating on the relevant tephra. Basically, it confirms the previous dates. I should have been more clear on that…

  5. Thanks for clearing that up.

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