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As I have said before, I don’t know enough paleontology, so I’m just speculating now and would love to be corrected by someone with more knowledge in the field…

One thing that bothers me with the whole Ardipithecus brouhaha is its age and its relationship with chimps.  Simply because I cannot completely reconcile it with the genetic estimates of speciation with chimps.

Estimating speciation times

First some observations on species divergence, genetic distance and fossils… If we consider even the simplest scenario for speciation — an allopatric speciation (and more complex scenarios will just exaggerate the problem — a population is split into two that eventually evolve into separate species.

Due to the population genomics in the population before the split, individuals in the two resulting populations will have different degrees of relatedness within and between the populations.  On an individuals level, eventually everyone within a population will be more related than between populations, but because of recombination the relatedness between the populations — and eventually species — will vary along the genome and the average sequence divergence will be greater than the divergence between the populations / species.

If we date the speciation by considering genomic differences, we are thus overestimating the time back to the ancestor.

If we consider morphological changes between the two populations — as we do if we consider fossils — those will necessarily have evolved after the split.  At least if we are looking at morphological traits that clearly separates the two populations and not just variation within a single species.

So for fossil evidence we will get an underestimate of the speciation time.

(Please ignore that the skull in the illustration is a gorilla, it was what I had lying around when I made the figure, but my focus is on chimps so that is the speciation I am illustrating)

Now, human-chimp sequence divergence looks like it is about 6 million years ago, but remember that this is sequence divergence and not species divergence.  The speciation event must be more recent.

Through statistical models of population genetics we can estimate the speciation time from the sequence divergence time and if we do, we get estimates of the human chimp species divergence closer to 4-5 million years ago (Dutheil et al. 2009; Hobolth et al. 2007; Patterson et al. 2006).

This puts Ardi, at 4.4 million years ago, smack on top of the speciation event!

Humans, chimps and Ardipithecus

Now I’m not saying that I trust the genetics more than the paleontology.  I understand it a whole lot better, but that is a different issue.

One thing that is clear, though, is that if the genetic estimates are true, then Ardipithecus cannot have evolved a whole lot away from the common ancestor of man and chimp.  It actually could be the last common ancestor, just judging from the time estimates.

If we believe that Ardi shows us that many of the traits we believed were derived traits on the human line are actually ancestral traits that changed on the chimp line instead, then we could put Ardi close to the human-chimp split and reconcile the genetic and paleontological estimates for the time of the split.

If it has evolved significantly towards humans it is back to the drawing board for the genetic models.

Which is it?

I really look forward to see what John Hawks has to say about it when he gets to it in his Ardipethicus FAQ…

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