News on the Sickle Cell Mutation

The New York Times science writer Carl Zimmer recently had a story on new research that has traced back the genetic source of the sickle cell gene. Scientists had thought it arose in five different places, but the new research makes it clear it happened only once, about 7,300 years ago, and spread from there.

The mutation gives some level of immunity to malaria, and when only one copy is present in a person, that's a good thing. People with that mutation are more likely to survive childhood and reproduce in areas where malaria is common, so the gene spread over millennia.

The problem arises when two people, each with one sickle cell gene, have children together: the children have a one in four chance of getting two copies, which results in sickle cell anemia. Before modern medicine, life expectancy for those kids was five years.

All of that is well known, and the new piece in Zimmer's article is that the mutation spread from just one person in one place, rather than arising multiple times. Which makes a lot more sense, really: mutations are random, so why would this bit of randomness have happened five times? Possible, of course, but Occam's Razor and all that.

The reason I'm writing, though, isn't because of the basic findings of the research — revelatory as they are — but because of one paragraph in Zimmer's write-up that made me question both his grounding in history and his understanding of how evolution works:

Some West Africans captured in the slave trade brought the sickle cell mutation to the Americas. But in places like the United States, where malaria was uncommon or nonexistent, the mutation offered less of an evolutionary advantage. As a result, African-Americans have a lower rate of sickle cell anemia than Africans today.

First of all, part of the reason Africans were stolen and enslaved to work in the U.S. south (and South America) is specifically because there was malaria there, according to Charles Mann's book 1493, and adult West Africans were known to be less susceptible to the disease.

Second, the reasons modern-day African-Americans have a lower rate of sickle cell anemia than modern Africans is because most African-Americans are descended from African women who were raped by white men, and so most folks have significant portions of their DNA from Europeans. (Watch any episode of Henry Louis Gates's Finding Your Roots for confirmation of that.) That decreases the chances that the mutation would be in the gene pool. While malaria was not common north of the Mason-Dixon line, that would not decrease the likelihood of the mutation in the relatively short, couple-of-centuries time scale we're talking about.