27 October 2010

The Genetics of Body Odor And Earwax

Body odor and earwax type are two of the traits that tend to distinguish Asians from non-Asians. In particular, Koreans are less prone to B.O. and have drier ear wax than any other population. Given that my name is half-Korean (as are my children), this is naturally a topic of interest. Everything you could possibly want to know about the genetics of these traits is found in a blog post by Razib Khan.

He notes research hypothesizing a link between high latitudes and the dry earwax gene, and also research that suggests that the dry earwax gene, something that would seem to have little selective impact, may be linked to the same gene that regulates body odor. Low body odor might conceivably confer the 1% per generation selective advantage that would appear to be necessary to account for the current mix of those genes over the 50,000 years the distinction between Asia and the rest of the world is appeared to have evolved.

"Recent" population migrations could also help explain the current mix.

While modern humans have been present in Europe for 50,000 years, more or less, most have roots in the warmer far Southern Europe or the Near East in the last 8,000 years, and the links to Southern Europe and the Near East are even younger as one moves North in Europe. Thus, current Europeans have faced cold weather selection pressures for less long than their current domiciles would imply.

The demographic history of Asia, in a nutshell, is one of Northeast Asian population migrating South and pushing South and West, with existing populations forced to migrate away from these wave of expansion. Japan and Korea have strong and relatively recent (i.e. within the last 4000 years) genetic ties to the vincinity of Manchuria and the region to the North of there. Mongolian and Turkish speaking populations of Central Asia arrived in Central Asia from roughly the same region mostly in the last 2000 years or so (before then, Central Asia had stronger ancestry links to Europe). The ancestors of many Southeast Asian arrived their from Southern China when Han Chinese from the North squeezed the out of their homelands. Thus, many East Asians have demographic histories from further North (or in the case of some Han Chinese, from higher elevations towards the Tibetan Plateu), than their current homes.

One can also imagine body odor reaching a tipping point in the sexual selection process. When lots of people have body odor, low body odor may be unimportant in choosing a spouse. But, as body odor becomes more rare, people may be more prone to notice it and avoid choosing a smelly spouse. Alternately, one can imagine low body odor being considered a positive social class indicator in time periods when expanding populations from low body odor Northeast Asia were conquering populations where people tended to have more body odor.

UPDATE:

This post is noted are responded to in this post which notes that with regard to selective advantage that:

People can interpret results however they want, it’s a free country. In fact I do so all the time. But I want to enter into the record that I’m skeptical of this particular model of negative selection against stinkiness.


FWIW, I'm not particularly wedding to any particular hypothesis, but can't find any that are more plausible either.

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