Over the weekend a good friend of mine visited for our college reunion. She's lactose intolerant and likes to enjoy ice cream and other dairy products so she brought lactase pills along with her so she could digest the milk sugar lactose. She was also told by her pediatrician the day after she returned home that she's got to fatten her one year old daughter up. I asked her how she's supposed to do that. Cheese! Makes sense, but she's worried her daughter might be lactose intolerant too. Being fully capable of digesting the stuff myself, the concept of lactose intolerance is not something I spend much time thinking about... until recently.
According to Steve Lewis, a professor of Darwinian medicine, mammals lose the ability to digest milk with age. This might "form a biological mechanism whereby the ties between mother and child are broken so that she can go on and have more offspring and the child can go and have offspring of its own." Symptoms of lactose intolerance include gas, bloating, abdominal cramps and diarrhea result. The NIH estimates that in the U.S. about 15% of people of European descent have it. Half of Latinos, 60-100% of Native Americans, 80% of African Americans, and 90% of Asian Americans are lactose intolerant. The ability to digest milk depends on how heavily a culture depended nutritionally on the milk of their herded animals.
What is really interesting to me from an evolutionary perspective is that lactose intolerance is something all humans should experience yet many don't. Relatively new research has discovered a genetic mutation that gave some adult humans the ability to digest lactose. This occurred sometime in the last 5,000-10,000 years and coincides with the origin of pastoralism in Europe.
This finding poses challenges to evolutionary theories about human behavioral adaptations (such as males' preference for mating with nubile women) that are thought to take hundreds of thousands of years longer to develop in a population. EPs can't go back in time to determine what kind of ecological conditions human ancestors lived in at the time we were evolving these adaptations (the so-called "environment of evolutionary adaptedness"), so most EPs infer what kind of behavioral adaptations would have been most adaptive for our ancestors by examining the ecological conditions of extant human hunter-gatherers. They assume the living conditions are essentially the same and that not enough time has passed for new adaptations to take hold, so if it would be adaptive for hunter-gatherers now it was probably adaptive in the past too. Obviously trouble arises when new evidence suggests that humans can evolve faster than expected. If a gene for making lactase well into adulthood can take hold in the last 10,000 years it is also possible that a gene for constructing hypothalamic-visual cortex connections in the brain that fire maximally for men viewing nubile young women could take hold that fast too. I seriously doubt that there was ever a time when the average man was turned on by the sight of a pre-pubescent or wrinkled old woman. It wouldn't make much sense for successful reproduction.
However, there are other adaptations that are more controversial and could conceivably be a product of relatively recent changes. The female orgasm, the desire for men to have more sex partners than women, sexual jealousy, monogamy, etc.
The discovery that lactose is newly digestible does NOT mean evolutionary psychology is a rotten paradigm, as some would have it (David Buller), but it does mean that care needs to be taken when hypothesizing adaptations that assume nothing has changed within the last 10,000 years.
As for my friend's daughter, she should be able to digest cheese just fine. Lactose intolerance doesn't set in until later in life and is a recessive trait, so if her dad can still digest lactose, she should be able to later in life too.