Is the latest claim regarding “chimp-human” inbreeding a bunch of hooey?

Yes, but not necessarily because it is wrong.

ResearchBlogging.orgSome time ago researchers proposed that the modern DNA signal indicated that chimps and humans continued to interbreed long after they split in evolutionary time. A new study refutes this, and as the author states, this new study is more correct because it “simpler and hence more likely”.


Let us begin at the beginning: There was no interbreeding between chimps and humans. not even “early humans,” because they did not exist yet, so when Elie Dolgin (or a Nature editor? Henry?) makes this statement: “A genetic analysis has called into question the controversial claim that early humans and chimpanzees interbred before splitting into separate species.” [source] they are walking on thin ground riddled with falsehoods.

When the study’s author states that this is more likely to be true because it is the simpler explanation, it makes me laugh. This, of course, is the invocation of the parsimony principle, which many people believe states that the simplest answer is the most likely to be true. But of course, biological systems, events, and scenarios tend to be complex, so this is not a valid guideline. Rather, explanations that are unnecessarily complex tend to have more wrong things in them.

Anyway, getting back to the main point of the study, which in the recently released nature piece is all mucked up with iffy reporting and strange logic by the scientists …

In 2006, David Reich and his colleagues at the Broad Institute in Cambridge, Massachusetts, compared the genomes of humans, chimps and three other primate species, and found that the separation of ancient humans from our closest cousins was more complex than a clean break. The time from the beginning to the completion of human-chimp divergence ranged over more than four million years across different parts of the genome, and the X chromosome seemed youngest of all,…

Which, honestly, nobody believed.

Now, we have this: Other researchers …

…have reanalysed the data and suggest that species differences in the levels of female promiscuity can account for the chromosomal inconsistency. The original hypothesis is “way more of a headache for evolutionary biologists”, says Yi. The data “can also be explained very well by well-established ideas in molecular evolution”.

Here’s my thinking on this: The issue comes down to the historical ‘behavior’ of genes on autosomes vs. sex chromosomes. Using this as data is potentially dangerous. If we had an adequate convincing not-out-of-your-ass theory for how sex chromosomes come to be, then maybe we could use this information, but at the moment we are using a system that is not even close to well understood.


Presgraves, D., & Yi, S. (2009). Doubts about complex speciation between humans and chimpanzees Trends in Ecology & Evolution DOI: 10.1016/j.tree.2009.04.007

Dolgin, Elie (2009). Human-chimp interbreeding challenged Nature News

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0 thoughts on “Is the latest claim regarding “chimp-human” inbreeding a bunch of hooey?

  1. These data, however, can provide useful insight into figuring out how autosomes and sex-chromosomes are distributed within populations. I think there was a paper on PLoS Genetics last year sometime regarding a model for x-chromosome distribution and polygyny, but it took into account many other factors such as gender specific migration (read selective gene flow) and gender specific bottlenecks (possibly read “the rape of the Sabine women?”). I wish I could find it right off, I’ll have to search and get back with you.

  2. Personally, I don’t believe that many incidents of speciation followed the “simple” model. Rather, IMO, most followed something like the case with gray (timber) wolf, the red/eastern wolf, and the coyote. Here, the (admittedly debated) evidence suggests some level of gene flow between the red (and, perhaps, eastern) wolf and the coyote on the one hand, and the red (and, perhaps, eastern) wolf and the gray wolf on the other. Thus, while there (probably) was no significant direct gene flow between the gray wolf and the coyote, an allele that showed up in gray wolves in Europe (for instance) could make its way into the coyote population via red/eastern wolves (as well as the Bering Strait during freeze).

    Once the red/eastern wolves became (or become) extinct, gray wolves and coyotes can be clearly defined as separate species, but as long as those populations of red/eastern wolves served as a bridge for gene flow, gray wolves and coyotes potentially could exchange genes.

    I see no reason that such a situation couldn’t have remained stable for several million years given a lack of sudden major climate changes such as we’ve seen during the Pleistocene. IMO we should expect such scenarios in the majority of cases of speciation, at least in mammals. Note that this is different from the separation, evolve apart, the hybridize scenario being discussed here, but equally consistent with genes of radically different separation ages, as well as the fact that with many genes the human and gorilla versions are clearly more closely related than either is to the chimpanzee/bonobo version. (Something you didn’t mention, Greg, but IIRC is discussed by at least some of the papers involved in the controversy.)

    I’ll predict, based on this, that a similar complexity of divergence ages will be found as the various “species” of gibbon are subjected to similar analyses (if ever). (They’ve seen more divergence in chromosomal structure, but IMO that’s a matter of degree, and doesn’t invalidate the primary thesis: in most speciation, there is a significant period of both reduced (but not eliminated) gene flow and eco-morphological divergence.)

    As for the definition of “species”, that’s a semantic jungle I’m not going to get lost in today.

  3. Wasn’t the original split between two different species of ape? One species would lead to the genera Australopithecine and Homo, while the other would lead to the genus Pan. So one can’t really talk about early chimp/early human interbreeding.

  4. the controversial claim that early humans and chimpanzees interbred before splitting into separate species.

    This isn’t so much a statement “riddled with falsehoods” as a statement that is simply internally contradictory. You really don’t need any background in biology to recognize that, just a course in basic logic would allow a person to recognize that there can’t be “inter” action of any sort when there is only a single something. I hope that was just a careless statement that no editor caught, and not an accurate description of any claim that was ever made?

  5. Well, it wouldn’t “lead” to anything, modern chimpanzees can be traced back to that population and modern humans can trace back to another, but you’ve hit the nail on the head, these were two (maybe more) populations which were interbreeding; they weren’t very different, but the populations did diverge…

    As for the definition of “species,” it really depends upon the context in which you’re using it…

  6. I’ve seen the suggestion that a speciation process could be reticulated, rather than a clean divergence up two phyletic branches. I can well picture this happening. There is also introgression, the movement of genes from one species to another through hybridization events after speciation has occurred. If one reads early humans and chimps to mean both early humans and early chimps, then the statement is understandable to me, at least.

    The how and why of sex chromosomes in interesting. I think they are rare in teleost fishes. There is a Cyprinodontid, Megupsilon aporus (monospecific genus) whose males have a very large Y chromosome. The same is true of a Rivulid, Gnatholebias hoigne;, but its sister species, G. zonatus, has a different number of chromosomes and no noticable Y. The two Gnatholebias species are morphologically very similar, but do not hybridize.

  7. Hey Greg, has anyone actually tried an experiment to see if say human sperm can signal chimp eggs to enter or whether they can go through some cell divisions?

    I’d like to get some idea what the degree of zygotic barriers is. I mean is it total: as in no way it’s getting in. Or is it something like maybe 3 times out of a hundred it gets in.

    Or maybe totally post-zygotic like it gets in but it just doesn’t develop.

    I mean that would be the test. I sure it would cause a media firestorm if they found out but I remember I once heard it said “if you’re not doing a cross, it’s not genetics”

    Haha, I have some dream that after I’m done with undergraduate and grad school that I could like find a suitcase of money and have a mad science lab and actually do these questionable experiments. LOL

  8. A paleontologist has said he was at a party where too much was imbibed. A female chimp was impregnated with human sperm. She appeared to become pregnant. In the sober light of day, the perps decided to dose her with hormones, and the pregnancy, if any, terminated. I was told this by a friend who was told it by the paleontologist. No reason to doubt the story.

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