New Data on Human/Chimp Split

The New York Times is reporting on new data concerning the date of the evolutionary split between the human and chimapnzee lineages:

Scientists are re-evaluating a pivotal event in human evolution, the split between the human and chimpanzee lineages, in light of a startling new comparison of the human and chimp genomes.

The new analysis, by David Reich, Nick Patterson and colleagues at the Broad Institute in Cambridge, Mass., sets up a serious conflict between the date of the split as indicated by fossil skulls and the much younger date implied by genetic analysis.

The conflict can be explained, Dr. Reich’s team suggests in an article to be published Thursday in the journal Nature, if there were in fact two splits between the human and chimp lineages. In that scenario, the first split would have been followed by a hybridization of the two populations.

Hybrid populations often go extinct because the males are sterile, so the hybrid females in this population may have mated with male chimps to produce viable offspring. The human lineage finally re-emerged from this hybrid population, Dr. Reich suggests, explaining the younger genetic dates, while the very early fossils with humanlike features may come from the earlier period before the hybridization.

We’ll have to wait until the article’s publication, and the ensuing reaction from other experts, to make a final decision about this. But since I suspect the ID folks are going to jump on this as an example of how evolutionists cling to favored hypotheses even in the face of contrary evidence, allow me to make a preemptive point.

The ID folks will try to argue that since there is an apprent conflict between the paleontological and genetic evidence in this case, the best conclusion is that both fields of study are unreliable in informing us about the common ancestry of modern organisms.

The reality, however, is that fossils and genes simply provide different sorts of evidence. Fossils are snapshots in deep time. They tell us that creatures possessing certain morphological features actually existed at a certain time. They allow us to make statements like, “Phenotypic trait X is at least Y years old.” Thus, the oldest hominid skulls tell us that by a certain time in history, what we nowadays regard as a human-like skull had come into existence.

Genetic comparisons tell us something different. By sequencing and comparing the genomes of many different organisms, we can determine the relative relatedness of the species in question (as in, species X and species Y shared a common ancestor more recently than either did with Z). But they can also provide a good estimate on the length of time that has elapsed since two lineages (the one leading to humans and the one leading to chimps, in this case) last shared genetic material among themselves.

So how do we explain the disagreement between the fossils and the genes? Well, this disagreement means simply that there were creatures with recognizably hominid skulls prior to the time when the human and chimp lineages became genetically distinct. Surprising, perhaps, but not implausible.

It also tells us something about the tree metaphor for evolution. The typical evolutionary tree diagram presents nice, clean points that represent a splitting of one ancestral species into two daughter species. In reality, though, speciation is a messy process. We can say that at time X there was one species and that at the later time Y there were two species. But is there a precise moment in time we can point to and say, “Speciation just took place?” I don’t think so.

The article talks about two splits between humans and chimpanzees. That seems like a confusing way of putting it to me. It would be clearer to say something like: Morphologically, the point at which hominid skull features can be distinguished from chimpanzee skull features occurred no later than time X. But the time when the lineage leading to chimps stopped trading genetic material with the lineage leading to humans happened much later.

Or something like that.

Anyway, I’ll have go read the actual article in Nature before coming to any firmer conclusiosn. It’s always hard to judge these things just from a short article in the Times.

Comments

  1. #1 Heathen Dan
    May 18, 2006

    How reliable is genetic dating? I’ve read a few books on evolution when the idea of genetic clocks was still new, and most of the authors are pretty cautious (even skeptical) about its reliability. Has there been major strides in genetic dating and how good is it compared to radiometric dating of fossils?

  2. #2 Anonymous
    May 18, 2006

    You will see this in most cases of speciation. Gradual divergence is expected in cases where there isn’t complete reproductive isolation, such as cases of geographic isolation. Consider two populations of the same species separated geographically where one population moves to an isolated island and the other remains on the continent. Since there is no interaction between the populations, genetic isolation starts at the point of population separation. Now consider populations separated environmentally where occasional interaction is possible. These separated populations will occasionally reproduce, making the genetic divergence toward reproductive isolation much slower.

  3. #3 Syd
    May 18, 2006

    I hope they are going to do a similar comparison study between the bonobo and human DNA, which I expect will show even more recent and more frequent interbreeding … ;)

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