Spring forward, but don’t touch your Molecular Clock

Today we switch out of Daylight Savings time, so don’t forget to set your clocks ahead one hour (though most phones and computers do that automatically). But make sure the only clock you change is the one by your bed whose volume knob fell off and that wakes you up every morning to the classic “Kiss on My List,” such that you wake up every morning to greet the new worst day of your life.

New dates have been published for TM 266, the Sahelanthropus tchadensis cranium (aka “Toumai”). The Sahelanthropus material was originally dated to 7 and 6 Ma, based on comparison of the TM 266 fauna to that of the Kenyan sites of Lothagam and Lukeino, respectively (1). The new dates are more “absolute,” based on atmospheric beryllium-10 (10Be), and give a date of 7.2-6.8 Ma (2). I don’t know the history or accuracy of cosmogenic beryllium dating (I’m more familiar with our friends Potassium and Argon), but the technique appears fairly widely used, and the fact that the radiometric date corroborates the faunal estimate suggests TM 266 is truly about 7 Ma.

Which is all well and good. I can accept that Sahelanthropus is about 7 Ma, but I’m less comfortable with the idea that ‘the earliest hominind is 7 Ma’ (cf. refs 1, 2). Here’s the final paragraph of the new paper’s discussion:

The radiochronological data concerning Sahelanthropus tchadensis . . . 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 S[.] 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 [quoting ref 3].” (2, p. 3230-3231)

Is a recalibration of the molecular clock really necessary? The “molecular clock” is a way of figuring out when lineages (i.e. species) diverged. Assuming a constant rate of neutral mutation, the genetic differences between two lineages (say, humans and mice) can be compared to the fossil dates of their divergence. This allows one to determine the (again, assumed to be) constant rate of genetic change between lineages. Because fossils (arguably) don’t give us a good picture of the human-chimp divergence, the molecular clock is used to determine when humans and chimps split, or when the hominins first emerged.

But these molecular estimates have a large range. Moreover, estimates haven’t been as early as 7 Ma in a while. The most recent ones I’ve read put our divergence at about 6.3-5.4 Ma (3) and as late as 4.1 (4). I have no problem with these dates, although the latter date is a bit troubling, as we’ve got (arguably) unequivocal bipeds (read, “hominins”) at 4.2 Ma with Australopithecus anamensis. But recalibration of the divergence date rests on the claim that Sahelanthropus was a biped (again, read “hominin”). Though apparently many in paleoanthropology accept TM 266 as a basal hominin, the case is not terribly strong that it’s anything other than a Late Miocene ape (read “probably not a hominin”) (5). So recalibrating the clock based on TM 266 at this point would be hasty, if not simply incorrect. Moreover, as John Hawks pointed out, pushing the human-chimp divergence back to 7 Ma would put the divergence of the human and orangutan lineages in the Oligocene (~33 Ma), which is absolutely unsupported by the fossil record (see ref. 4).

The point: Having current absolute dates for hominin fossils is always good. But recalibration of the molecular clock based on TM 266, at least at this juncture, is not only unnecessary but also is most probably incorrect.

Oh, the paper also dated the Au. bahrelgazali site (KT 12) to 3.5 Ma. But, then, we already know there’s all sorts of crap running around–we’ve got bipedal hominins, after all–in Africa at 3.5 Ma.

1. Brunet M et al. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418: 145-151.
2. Lebatard et al. 2008. Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad. Proc Nat Acad Sci 105: 3226-3231.
3. Patterson N et al. 2006. Genetic evidence for complex speciation of humans and chimpanzees. Nature 441: 1103-1108.
4. Hobolth A et al. 2007. Genomic relationships and speciation times of human, chimpanzee, and gorilla inferred from a coalescent hidden Markov model. PLoS Genet 3: 294-304.
5. Wolpoff M et al. 2006. An ape or the ape: Is the Toumai cranium TM 266 a hominid? Paleoanthropol 4: 36-50.