Whoever manages the PNAS web servers should thank Afarensis because he found that paper prior to the March 4th date I threatened to bring down their site on. He’s written up an educational overview of the findings, so has Jason of Hominin Dental Anthropology. I eluted to the significance of this study earlier, but I’ll reiterate it once more. This paper is important because it gives a more reliable antiquity for Sahelanthropus tchadensis. The press release didn’t indicate that date for Australopithecus bahrelghazali was also calculated, so I’m super happy that we got a meal deal out of this paper.
Before I get into the details of the dating technique, let’s answer just the basics on what Sahelanthropus tchadensis and Australopithecus bahrelghazali are and how they integrate into our understandings of early human evolution. The formal description of S. tchadensis was made off of TM-266 fossil which is also known as Toumaï. Toumaï is at the very minimum a highly fragmented ape cranium. According to the original authors, Toumaï has some hominin derived dental traits. Some researchers, such as Milford Wolpoff and John Hawks reanalyzed the data of the dental morphology of Toumaï and concluded that the dentition of Toumaï is like that of earlier Miocene apes. This reanalysis was published in the 2006 paper, “An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid?” Other, such as Yohannes Haile-Selassie, Tim White, and Gen Suwa adopted S. tchandensis as possibly a representative of variation with in a single genus of hominins from the late Miocene. Others that could be within the variation spectrum are fossils represented in the genera Orrorin and Ardipithecus. These three putative hominin genera all come from the late Miocene.
I’ve implied that S. tchadensis is old, Miocene old. But the original publication didn’t come with a really definitive dating technique. Instead the age of the Toumaï was based off of the presence of other taxa that was also recovered from the same horizon. The specific presence of the species Libycosaurus petrochii, an even toed ungulate, that was known to exist around 6 to 7 million years ago indicated the horizon was about that old. This sort of dating is called biochronology, where the relative age fossil bearing units is based on known dates of existence of species represented in the unit. A 2005 paper refined the date to 6.5 to 7.4 million years ago, and folded in more S. tchandensis specimens into the record.
Australopithecus bahrelghazali was also found by Michel Brunet. The paper which announces this find is, “The first australopithecine 2,500 kilometres west of the Rift Valley.” The significance of this finding is clearly stated in the title but because the Brunet et al. have locked the fossils away from others to investigate, criticism has ostracized this finding. Additionally, all the defining features of the teeth and a partial jaw associated with A. bahrelghazali fall within normal variation of A. afarensis. The only thing that definitively is derived, is the mandibular symphysis, which is is more modern in appearance than that of A afarensis. For these reasons, paleoanthropology doesn’t really acknowledge A. bahrelghazali as a new species of australopithecine. One last thing, in some sort of sad historical record of Brunet’s lack of attention to detail, the A. bahrelghazali fossil also did not have a definitive dating technique associated with it. Rather, biochronological dating gave us an estimate that A. bahrelghazali is about 3.5 to 3.0 million years ago.
With this basic overview of S. tchadensis A. bahrelghazali, I think you can get the idea that more accurate dating techniques are mandatory to at least fill in the temporal gaps. At around 7 or 6 million years ago, three genera of hominins paint a much different picture from two genera. Potentially, one of the three genera coulda gave rise to the australopithecine lineage. Or as Yohannes Haile-Selassie suggested, Sahelanthropus, Orrorin, and Ardipithecus coulda all been one representation of a widespread genus of hominin. At 3.5 million years, another diversity similar problem exists. If A. bahrelghazali does really represent a new species of australopithecine it is crucial to pinpoint an accurate time to this fossil because it’s so damn far away from all the other Australopithecus fossils.
About 7 years after Brunet found S. tchadensis and 15 years after he found A. bahrelghazali, it seems like he has finally gotten around to using radiochronology to date the sediments associated with either fossil. PNAS is hosting this paper, open access… which I really recommend since it gives anyone interested free and easy access to download the original research and investigate the results for themselves. If there ever was a complaint that Brunet wasn’t transparent with sharing the A. bahrelghazali fossils, it seems like he’s tried to make up for it by agreeing to release this paper open access. So go downloaded it now before PNAS closes down access, here’s the title and link, “Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad.”
The actual technique deployed is like any other isotope analysis, but there’s a twist. Since the fossiliferous sediments did have a volcanic tuff around, uranium, argon, nor potassium dating could be used. Instead, beryllium isotopes were analyzed. Based upon expontential decay and the observed 10Be/9Be ratio, the authors calculated Sahelanthropus tchadensis to be 7.04±0.18 million years old. If you want a more in depth review of science behind the beryllium isotope, please check out Jason’s blog post. He’s done an excellent job. Oh yeah, how could I forget? The date for Australopithecus bahrelghazali comes out at 3.58±0.27 million years old.
This means Sahelanthropus tchadensis is really that old and that (if A. bahrelghazali is really a novel species) it existed at the same time A. afarensis did. Jason and Afarensis have both expressed that last paragraph of this paper raises concern. The authors attempt to say a lot of things, such as their opinion that the last human-ape lineage diverged around 8 million years ago and how the “plesiomorphic and apomorphic characters Sahelanthropus tchadensis” indicate some early hominin and ape hybridization was occurring. If that’s the case, then to ask again, just how human was Sahelanthropus tchadensis?
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