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Why Do Only Some Adults Drink Milk?

The answer is more complicated than you might think.

Lactation is perhaps one of the most curious natural systems ever to have evolved. For hundreds of millions of years, reptiles, fish, and amphibians flourished without milk. Then somehow mammals arose, simultaneously developing the ability not only to produce milk but also to digest it. There’s clearly a benefit: Milk allows a mother to provide nourishment for her offspring after birth without having to leave them unprotected while she gathers food. But how did this complex system originate? In humans, the puzzle becomes even more perplexing: Why do adults in many (but not all) cultures continue to drink the milk from other animals long after they’ve outgrown the need for milk from their mothers? Most other animals cannot even digest milk after infancy (a sensible adaptation to motivate weaning), and it’s clear that up until relatively recently, humans couldn’t either.

In order to digest lactose, a sugar found in milk, the body needs an enzyme, lactase, which most mammals stop producing shortly after weaning. There is now considerable evidence supporting the notion that lactase persistence evolved separately in northern Europeans and Africans. (Most East Asians remain lactose-intolerant.)

Scientists have come up with several rival explanations of how this may have happened, but an intriguing explanation was proposed in July in PLoS ONE by a team led by Pascale Gerbault. Jeremy Yoder, who spends most of his time studying the evolution of Joshua trees and the moths that pollinate them, explains the research in an exceptionally clear post on his blog, Denim and Tweed. The researchers say the lactase gene evolved in Europe because Europeans don’t get enough sun to produce Vitamin D, which in turn is needed for humans to take in calcium. Since lactose also assists in the uptake of calcium, adult milk drinking helped northern Europeans meet that deficiency.

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