The other day, a new friend gave me an amazing gift: kefir that she’s cultured from grains that were given to her by a buddist nun.
These aren’t really grains at all, but a mixture of yeast and bacteria that live symbiotically and break down the lactose in milk. I’ve seen kefir in the grocery store, but never tried it. Honestly, I hadn’t really thought much about kefir at all. But now, I’m intrigued, in large part because of the evolutionary history, the anthropology, and the touching guesture of friendship.
My family name, Hlusko (which I think should be spelled Hluško) is from eastern Europe, just a hop-skip-and-a-jump over the Black Sea from the Caucasus. Since the practice of Kefir fermentation comes from that part of the world, this is sort of like discovering a little piece of my long-lost family heritage. Well, close enough anyway, for me as a third-generation-removed American.
Milk fermentation has been an important part of pastoralist cultures because adult mammals (including humans) can’t typically digest the sugars in milk, i.e., lactose. Babies and young children can digest mama’s milk because they have an active LCT gene that produces lactase in the intestines, a protease that breaks down the lactose protein into more simple sugars as it moves through the digestive system. This gene stops being expressed as you age. Since most mammals only drink milk when they are young and nursing, the loss of lactase production in the body as they become adults is inconsequential. This is the usual condition across mammals. From what I’ve been able to discern from the literature, I think that the LCT gene stops being expressed in humans sometime around age 7.
(This mankes me wonder why we stereotypically give adult cats a big bowl of milk. I’m guessing they don’t really like it, but will have to experiment once I adopt another kitty, but that’s for another day.)
If your body doesn’t produce lactase, bacteria in your gut will break down the lactose you may consume for you. But, these bacteria produce gas as a by-product, which is not such a pleasant thing to have build up inside your body. The bacterial fermentation also leads to an increase in-flow of water into the intestines, which leads to diarrhea.
Over millions of years, there have been mutations in the genetic regulatory elements that turn the LCT gene on and off. Some of these mutations caused the LCT gene to continue being expressed long after childhood. In these people, their bodies could digest lactose long past the age when they’d needed this ability. But for the vast majority of human evolution, this kind of mutation was meaningless because they didn’t consume milk once they’d been weaned.
But about 3,000 years ago, some populations of people started keeping animals such as cows, goats, and camels close to them, and then started to utilize their milk. The people who could easily drink/digest milk as adults had a significant food and liquid resource that wasn’t as easily available to other adults. They ended up healthier, and consequently, with more children surviving over time, and the trait rose in frequency in the population.
These adult milk-digesters are what we call lactase persistent, because they persist in the production of lactase into adulthood. It is a mutant phenotype. Being lactose intolerant isn’t an allergy or being sick at all. That is the typical mammalian condition.
The human cultural practice of using kefir to ferment milk makes the milk digestable for adults who don’t have a mutation that keeps their LCT gene functioning past childhood. Thanks to kefir, milk gets fermented before you drink it, rather than after, which is a much more pleasant experience for everyone invovled. Plus, it opened up another way that we can benefit from domesticated mammals. Given how much pastoralist people rely on their animals as the foundation of their entire economy, they don’t like to slaughter them all that often. Their regular subsistence from the animals usually comes in the form of blood and milk, and fermented milk products have proven to be another incredibly beneficial and important resource that these animals provide.
The selection for lactase persistence in humans happened at least four times, once in Europe and three times in Africa (Tishkoff et al., Nature Genetics 2007 Jan;39(1):31-40), and the timing is remarkably correlated with adoption of pastoralism in those parts of the world. There has been a lot of work done exploring this in even more detail since Sarah Tishkoff’s team first discovered that there were multiple mutations effecting LCT expression, but she has been the leader in exploring genetic variation across SubSaharan Africa. For this reason, she’s one of my intellectual heroes. (Aside from her great science, she’s a really nice person to boot.)
The lactase persistence alleles are a classic example of convergence in humans, convergence that results from cultural shifts away from hunting and gathering towards animal domestication. And while the story is one that I am very, very familiar with, it was wonderful to merge a meangingul gift from a new friend to a cultural connection with my family’s heritage, and, to the evolution of my species. It doesn’t get much better than that.