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.
Gerbault’s team developed a computer model demonstrating that, in order for the adaptation to persist, lactose-tolerant northern Europeans would need to have 1.8 percent more children. In other words, milk drinkers would need to be more successful in reproducing—and this is indeed what is observed there. In southern Europe, which gets much more sunlight, there is no such advantage, and more people are lactose intolerant. The computer model shows that this difference can’t be explained merely by the north’s isolation from the south: The differences in lactose tolerance in these populations are too great. In Africa, by contrast, lactose tolerance is evenly distributed. It’s likely that the lactase gene evolved there simply because it was an easy way to extract more protein from early milk-producing domesticated animals. Thus, two distinct advantages (enhanced calcium absorption in northern Europe and better access to protein in Africa) seem to have promoted the emergence of milk drinking in adult humans.
But Yoder’s post is just the starting point for the online discussion about the evolution of lactation. A search for “milk” on ResearchBlogging.org finds many more posts about milk-related research.
For example, this post by Todd Oakley, a blogging evolutionary biologist at the University of California–Santa Barbara, discusses new work by Danielle Lemay and colleagues that investigates the cow genome for evidence of how milk production evolved. Oakley walks through the evidence that, far from appearing like a bolt out of the blue, lactation probably evolved gradually, from incremental changes to existing structures, such as sweat glands. Another quality explanatory post comes from an anonymous blogger at “Mad Scientist Junior,” who uncovers a recent study in Nature Medicine which claims that, in addition to all its other benefits, breast milk may also function as an allergy preventative.
This is just scratching the surface—there are dozens of additional thoughtful posts explaining groundbreaking related research. And each of them, both in their content and in the online commentary they stimulate, contributes to an extended dialog about a body of work that is itself continually evolving.
Because of its interactive and adaptable nature, the blogosphere might be the best way to learn about and discuss new scientific developments, sometimes from the scientists who are making the discoveries, but always from someone who’s intensely interested in the research and its implications. Come join the conversation.
Originally published August 12, 2009