This will be a bit off from the normal fodder for this blog, but I thought it was interesting.
Lets start with a bit of biology.
Babies naturally produce an enzyme called lactase that allows them to digest the main sugar component of milk – lactose. Many (globally it would be most) people lose that ability near puberty. This leads to lactose intolerance and the GI issues it can cause. Many people of European heritage, however, are lactase persistent and maintain the ability to digest lactose – to the delight of the dairy industry.
The standard story in archaeology about dairy husbandry is that lactase persistence will naturally follow along the path of the spread of dairy farming. After all, it would be a great advantage to be able to consume the additional calories and proteins – as an adult – that milk and other dairy products represent. So those that are lactase persistent would be better able to spread their genes and the trait would come to dominate the population.
And when we look at the spread of dairy animals in the Africa and the Levant, that is pretty much what we see.
And then, there are the Mongols.
According to this article from HeritageDaily, the Mongols have been drinking milk as adults for 3,000 years without gaining a majority of lactase persistence in the population.
So, the question is, why?
Has there been continuous influxes of people that are not lactase persistent – essentially swamping any shifts in allele frequency? Have they come up with some other genetic variant that mutes the normal response to lactose?
Hopefully somebody will look into that.