Got Milk? The Many Evolutions of Lactose Tolerance:
[Via Evo.Sphere]
by Dan Graur
The vast majority of adults in the world are unable to digest lactose, the principal sugar of milk. Such people cannot drink milk as adults without exhibiting symptoms of lactose intolerance, such as nausea, cramps, bloating, flatulence, and diarrhea. The proximate reason for adult lactose intolerance is the switching off of the lactase enzyme in the gut either after weaning or at around 2 years of age. Throughout most of human evolution the only source of milk was the mother, thus, the switching off of the enzyme is usually explained by the fact that it is no longer needed. This ultimate explanation is wrong, because evolution promotes neither efficiency nor thrift. The only currency in evolution is fitness, i.e., the ability to reproduce, and there are numerous examples of waste (95% of our genome, for instance) that seem not to affect fitness. So what may be the evolutionary advantage of not being able to drink milk?
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This is a very nice discussion of the multiple genetic mutations that can be found in the different human populations that developed lactose tolerance. Our ability to use the milk of other mammals has conferred tremendous evolutionary advantages.
