Thursday, March 01, 2007

Milk: a driving force of evolution?

How many of you are lactose-intolerant? If you are, no need to fret - it's the normal mammalian trait to have.

As you know, lactose is the sugar found in milk, and lactase is the enzyme that helps break it down. Mammals obviously have a need for lactose and the production of lactase - after all, one of the defining mammalian characteristics is that moms produce milk as nourishment for their young. If a mammal is born without the ability to produce lactase, chances are they're going to have a hard time surviving. But, eventually, mammalian babies grow up and wean. For all mammals but us, this is the last time they ever drink milk. Thus, most mammals stop making lactase as they get older - what's the point of spending energy to make an enzyme you don't need anymore. Therefore, most adult mammals are lactose-intolerant.

But a curious thing happened during human history - the domestication of cattle, goats, and sheep. Being able to digest milk became beneficial in adults. So, as you could guess, natural selection has pushed adult humans towards lactose-tolerance. A new study has actually determined that the evolution of adult lactase production in humans happened as recently as 8,000 years ago. This is way after the evolution of Homo sapiens. In fact, by this time most of the globe had been settled by modern humans and various ancient cultures were beginning to establish themselves. Thus, it is likely that lactose-tolerance evolved in only some places, or it evolved multiple times independently. Lactose intolerance is least common in people of Northern European descent, while 80% or more of African-Americans, Asian-Americans, and Native Americans are lactose intolerant - these numbers are probably consistent with if, when, and how much these groups' ancestors utilized domesticated milk sources (though I don't know this for sure).

For some of our ancestors, milk has been a strong evolutionary force - it only took 8,000 years to evolve lactose tolerance (yes, only 8,000 years).

Evolution: It Does a Body Good

Ancient Human Timeline

update on 2/2/07: Just found a blog entry by Carl Zimmer discussing this topic as well.

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