Finding the Source

The sickle-cell mutation, HbS, is found on five main haploptypes, and for a long time there has been argument as to whether this mutation originated once, or multiple times. A new paper indicates this it happened only once, in Africa, about 7300 years ago, even though you can now find versions in places as far away as India. There’s a recombination hotspot near the gene which apparently shuffled the haplotypes and made it seem as if the mutation had occurred repeatedly.

I’m not sure if people have talked about this (someone may have, it being a much-discussed topic) but there was always a reason to suspect a single origin: it never happened in Southeast Asia or Melanesia, even though there is plenty of malaria in the region, and lots of local genetic defenses like Hemoglobin E in Thailand and Melanesian ovalocytosis. But no sickle-cell. I can think of no case in which the same malaria-protective mutation is seen in Africa or is environs and Southeast Asia or Melanesia. Different mutations of the same gene, sometimes, but never the same mutation.

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20 Responses to Finding the Source

  1. dearieme says:

    This paper’s all very well but they’ve not told us the name of the chap that the mutation first occurred in. Poor show.

  2. ehrgeiz0 says:

    A biologist colleague of mine worked for the now-defunct Montreal Sickle Cell Society. When I asked her many years ago if sickle-cell trait (I’m talking about Hemoglobin S, not the other milder variants) protects against malaria, I was surprised when she said no.

    I would have thought that the abnormal red blood cells would make it harder for the parasite to complete it’s life cycle, and there are other traits that can also provide protection against malaria (like the immune-boosting TNF), so I’m not sure what the deal is.

    IN have read, however, that beta thalassemia trait can arise through spontaneous mutation.

    • gcochran9 says:

      Of course it does. Your colleague is profoundly mistaken. It doesn’t make you bulletproof, but it increases fitness in a highly malarial region by about 15%, nothing to sneeze at. In such places you end up with about 1% of the population carrying two copies of HbS: they don’t live long without extensive medical care.

      Anything can arise through spontaneous mutations, but the chance of that for any given mutation is very low. Somewhat lower than we used to think, since the real mutation is lower than we then thought ( by a factor of about 2.5)

    • Jim says:

      Pages 241-242 of Mathews-Holde Biochemistry, 2nd Ed. discusses the protective effect of the sickle-cell trait against malaria. Obviously it’s not a great solution to the problem of malaria.

  3. dearieme says:

    “I can think of no case in which the same malaria-protective mutation is seen in Africa or its environs and Southeast Asia or Melanesia. Different mutations of the same gene, sometimes, but never the same mutation.” Does that imply anything more general?

    • gcochran9 says:

      Mostly that the chance of having any given favorable mutation occur and survive ( and have happened long enough ago to have become common by the present) is pretty low. With growing populations (agriculture) more favorable mutations are happening, but in most cases there hasn’t been enough time for them to become widespread.

  4. G.M. says:

    Surely parsimony suggests a single origin. If we are finding evidence for multiple origins of all kinds of mutations (especially non-trivial ones, if that matters), it is really profound, & would lead us to reconsider our probablistic understanding of evolution, adjust the odd parameter to Drake’s Equation, etc.

  5. TWS says:

    My brother used to drink gin and tonics saying it helped his malaria. I figured he just lined his gin.

  6. Smithie says:

    Wow! I’m amazed that the estimate is not older. Hominins have been around for a while. Malaria is likely much older. It’s just a single letter change, and blood groups are really old. From agriculture? I’m shocked.

  7. Broseph Walsh says:

    Was this involved in the Bantu expansion at all? Resistance against malaria would have been an enormous leg up on the compitition. Though the time frame doesn’t seem correct to me. The expansion is dated to around three thousand years ago.

    • Steve Sailer says:

      When did agriculture get started in sub-Saharan Africa?

      • Broseph Walsh says:

        Ethiopia and Sudan which had easier contact with Egypt happened earliest, I don’t believe they lagged to far behind. While it only came to the rest along with the Bantu expansion as far as I know.

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