Genetic analysis challenges human-chimp interbreeding

London, August 29 (ANI): A genetic analysis has called into question the controversial claim that early humans and chimpanzees interbred before splitting into separate species.

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, they reported in Nature.

The authors argued that there were in fact two splits - an initial divide, followed by interbreeding, and then final separation in which only a young X chromosome was retained.

Many researchers took issue with this interpretation, arguing that large ancestral population sizes could explain the wide range in genetic divergence times, so there was no need to invoke a complex speciation process.

But these critiques still could not account for the youth of the X chromosome.

Now, according to a report in Nature News, evolutionary geneticist Soojin Yi of the Georgia Institute of Technology in Atlanta, together with Daven Presgraves of the University of Rochester in New York, have reanalyzed the data and suggest that species differences in the levels of female promiscuity can account for the chromosomal inconsistency.

Males competing for mates produce different amounts of sperm depending on the mating habits of the species. Chimps are highly promiscuous, humans less so and gorillas not much at all.

As such, male chimps face the stiffest competition, so they have the highest sperm counts and the largest testes of the three species.

That means that they also undergo more rounds of sperm cell division and make more DNA copying mistakes, leading to higher mutation rates in males than in females.

Reich and others had assumed that all primates had the same mutation bias, but Yi and Presgraves argue that mating relationships should be taken into account.

Because females have two X chromosomes and males have only one, the X spends more of its evolutionary history in females, whereas non-sex chromosomes split their time evenly between each gender.

Thus, a male-biased mutation rate will lead to proportionally fewer genetic changes on the X and will seem to be younger when using a molecular clock, even if all the chromosomes diverged at around the same time, the researchers argue.

Complex speciation is therefore unlikely to be the cause, according to the researchers. (ANI)