Saturday, October 11, 2008
Evidence of Recent Human Evolution
It is commonly said that humans evolved as hunter gatherers on the plains of Africa and that today's humans are not significantly changed from our hunter-gatherer ancestors. The most recent issue of Seed Magazine has an article about recent work that suggests that humans may have evolved quite rapidly and significantly during the historical period.
Forgive me if I screw up the biology, but as I understand it, there are two key points that need to be understood as background:
First, DNA is inherited in blocks, with adjacent DNA sticking together and getting transmitted to the offspring. Such a block of of linked DNA is called a haplotype. Over the generations, mutations and recombinations nibble away at the edges of, and poke holes in, haplotypes. So if we look at two populations, the longer, on average, the haplotypes, the more recent the two populations have been connected (because the more time elapses, the more the haplotypes degrade).
Here is a visual representation of a haplotype:
Second, if there is a variation in a haplotype that is common to many individuals, that would tend to signal that natural selection is operating. In other words, out of 100 haplotypes of a related population, let’s say 90 are the same, and 10 are different, each in a different way. One would suspect that the 10 variations are likely random mutations or recombinations, and there is no reason to think natural selection has been involved. But if the 10 different show not different variations, but the same one, that suggests that the variation must confer some benefit, or it would not be reproducing itself.
What the researchers are looking for, then, are long haploptypes (indicating recency) with unusual variations that are showing up repeatedly in the population. When they find these, they believe they have found evidence of recent evolution. They have found several.
Here is an artist's rendering of the heights to which human evolution might ascend if we continue to evolve as rapidly as the researchers think we have:
There are a couple of problems with this. One, which the article does not mention, but which seems in principle a problem, is that, to the extent the holders of a particular shared long haplotype mostly interbreed, the shared variations could be due to genetic drift rather than selection. Second, they find these gene variants and assume they must be selected for, but they generally have no idea what the particular variant codes for, so a big piece of the puzzle is missing. Third, allowing for recent evolution in human populations suggests that different groups may be more or less genetically fit in certain ways, which is politically and philosophically a position with which most of us are uncomfortable at least. Even so, there does appear to be at least some evidence to support them (the recent evolution of the ability to metabolize lactose, more frequently present in some races than in others, for example).
There’s a lot more detail in the article (Seed Magazine, September/October 2008, p.66). It’s a fascinating magazine, by the way, concerning itself with the intersection between science and culture.