September 25, 2009
When Humans First Got Milk
Have you ever stopped to think about how strange it is that we drink the breast milk of another species?
And no, I’m not going all PETA on you. I grew up down the street from a dairy farm in Vermont, and drank a glass of fresh cow’s milk every morning at my mother’s insistence. My morning ritual has since turned to coffee instead, but I still eat plenty of cheese, yogurt and ice cream, so I’m not knocking dairy. I’m just intrigued by the idea that early humans had to figure this out at some point; it requires a leap of logic (as well as an evolutionary adaptation to produce lactase, the enzyme needed to digest lactose).
Maybe it was lightbulb-over-the-head moment for a hungry shepherd or cowherd: Hey guys, we know that human breastmilk is edible, and these sheep, goats and cattle we own are edible. They also make breastmilk… so could we eat that, too?
Of course, we’ll never know for sure what happened, but we know a lot more than we used to. Recently, researchers used computer modeling to figure out when and where the trait of “lactase persistence” probably evolved. They traced it back to the Neolithic cultures of about 7,500 years ago in Central Europe and the Balkans, where it seems to have co-developed at the same time as dairy farming, not surprisingly. What is surprising is that it started so far south; as this press release notes, that negates a popular assumption about why the trait developed. If not in response to low vitamin D levels in sun-starved northern climes, why would the human body bother to develop lactose tolerance? (We addressed that question in a previous post, so I won’t get into it here, although I’d love to hear other perspectives.)
This map, recently published in the PLoS journal of Computational Biology, is a cool way of illustrating the point, complete with a diagram of a lactose molecule and photos of pottery from the Linearbandkeramik culture of the time:
The ability to digest lactose developed first in the regions shown in white, then expanded outward. Map courtesy PLoS.
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Did we really need an evolutionary adaptation to produce lactase? Human milk contains more lactose than cow and goat milk, for example, so it seems that we would have had lactase already to digest mother’s milk.
Great question, Rosalyn! You’re right, human breast milk has lots of lactose, and babies are born with the ability to digest it. But after weaning, and as we age, our ability to produce the necessary digestive enzyme (lactase) decreases or even disappears entirely.
So I should have specified that the evolutionary adaptation was in order to KEEP producing lactase as adults (that’s why scientists use the term “lactase persistence”).
Thanks for asking!
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