Thursday, October 13, 2011

Frame-Shift Mutation Linked to Sympatric Speciation in Early Homo?

According to (new?) research from the University of California, San Diego School of Medicine, a mutation creating a slightly different sugar molecule may have caused a reproductive barrier between those that had it and those that didn't in our human ancestors, possibly influencing the split between early Homo and australopithecines. Stephanie Pappas of Live Science writes:
The mutation tweaked one type of sugar molecule, Neu5Gc, produced by early hominids, the first great apes. About 2 million or 3 million years ago, just as human ancestors Homo ergaster and Homo erectus emerged in Africa, a genetic mutation halted the production of this molecule, and the prehuman immune system began to recognize it as a threat. As a result, researchers find, some hominids would no longer have been able to mate and produce offspring with other populations, potentially driving early humans apart from other apes."Over time, this incompatibility would reduce and the eliminate individuals with Neu5Gc," study researcher Pascal Gagneux of the University of California, San Diego, said in a statement.
If the date is closer to three million years, it would have considerable implications for the emergence of Homo. There are many theories about where early Homo came from and what led to the transition from Australopithecus to what became the earliest members of our line. This will shed some light on these ideas and, hopefully, spur more research in this area.

Interestingly, this research is not as new as the story makes it out to be. There was a paper written by A. Varki in 2001 that dealt directly with this data. It can be found here. Varki, however, did not make the connexion with immune suppression response but focused, rather, on how the mutation affected brain evolution.

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  1. Jim, thanks for the heads-up on this. The paper is online (prepublication) at PNAS and is open access.

  2. Hi,
    I think this is a better link directly to the paper at PNAS.

    I was interested in the news blurb because I wondered how they came up with the time of origin for the gene loss. As I suspected this was accomplished through means of the molecular clock analysis. I'm fine with molecular clock in general but the article provides no error bars and just takes the 3.2 and 2.9 million dates and states that there were only 0.3 million years for this fixation to occur. Again, sounds reasonable given their simulations but I'm not convinced the molecular clock is strong enough evidence of point of origin for these genes to warrant the confidence in the conclusions that is exhibited in these science-daily type blurbs. I looked up the 2006 paper that details the estimation of divergence times and it more thorough that I had expected but the total haplotype diversity was very low in their region and a finding another person with another unique mutation could have significantly impacted their results. You may or may not be able to access the article here:

    In 2006 the authors would not have had access to the Neanderthal genome but I suspect this portion of the genome is now available and a reassessment of the dating incorporating that sequence could be very interesting.

  3. Joel, thanks for the link to the 2006 paper. I do have access to it and will look it up. I like the idea that the divergence tracks to between 2 to 3 million years because it answers a bunch of questions but obviously more needs to be done. Using the molecular clock analysis is good but lends a slight air of circularity to the reasoning process.