Human Chimp Interbreeding Challenged

As this fascinating Nature piece explains, in 2006 David Reich and colleagues from the Broad Institute in Cambridge, MA compared the genomes of humans, chimps and three other primates and concluded that the divergence of human and chimp ancestors could not have been a clean break but was a messy business involving more than 4 million years and two splits - an initial divide followed by a long period of interbreeding, and then a final separation in which only the young X chromosome was retained. It was the apparent youth of the X chromosome, compared with all the non-sex chromosomes, that demanded this explanation.

Now, however, Soojin Yi and colleagues, from the Georgia Institute of Technology, have challenged an interpretation which has always proved difficult to swallow for the genomics community at large. No need to involve complex speciation, they argue, because the data can be explained by a well-known difference in female promiscuity ranging from high in chimps, through intermediate in humans, to low in gorilla. High female promiscuity leads to relatively large testes and sperm counts. This means, says Yi, more rounds of cell division making all that sperm - in chimps - which increases the mutation load on chimp sperm - more mutations in males than females. This male-biased mutation rate will favour non-sex chromosomes, the mutation rate in the X will be lower, and, since the molecular clock of evolution is calculated in mutation over time, the X will therefore appear to be younger - when, in fact, it is not. Reich challenges back but, at least, as Nick Barton suggests, we now have an exciting alternative explanation for the chimp-human divergence which can be tested. Watch this space!