Ask the man on the street about evolution, and assuming he doesn’t connect it to Pokemon, he’ll probably identify it as something that happened long ago. But creation is never finished, either underground where tectonic plates grind against one another, producing mountains, or above where endless forms most beautiful prowling around continue to change. We know this well from medicine, because the attempt to conquer a given disease is often frustrated by the sheer pace at which a given bacteria population can adapt. But what if some illnesses continue to be pervasive because it’s beneficial to us? Such is the argument advanced by Sharon Moalem in this, one of the most interesting biology books I’ve read in a while.
How can being susceptible to a disease help us? Diseases are often debilitating, sometimes confining the affected to bed – not exactly a place to take one’s stand in the eternal struggle for existence. But suppose a trait that warped cells ever so slightly – a bad thing, on the face of it — had the effect of preventing an invasive parasite from being able to use those cells, damning it to a death as soon as it had gotten a look around your circulatory system? So it is that sickle cell anemia, which only occurs when two people with those warped cells have a baby, persists in Africa and other places where malaria is common. More people survive malarial attacks than die from it because they’re in possession of those slightly warped cells. (Sickle-cell anemia results when two people with the affected cells have a child, and their child’s cells are so altered they slow the flow of blood.) Another sickness, in which cells horde iron to the point of poisoning their own bodies, is a similar adaptation against malarial infections….but unlike with sickle-cell anemia, those with hemochromatosis can find relief from their internal oxidation by donating blood. These genes persist because, given the odds, they’re more likely to help persons carrying them than to hurt them.
After exploring other cases like this, including a speculative argument that the European propensity for diabetes is an adaptation to the northern climes during the last glacial period, Moahem shifts an even more fascinating topic: methlyation, or the processed by which traits expressed by your genes can be turned off and on, or otherwise modulated, because of factors in the environment, both prenatal and postnatal. We encounter mammals who give birth to different colored offspring depending on how much light the mother is exposed to — allowing her to bear white babies in winter, when snow is on the ground, and brown ones during the summer. Human mothers’ environments also change them: when on a starvation diet, or when eating mostly nutrient-poor junk food, they give birth to small babies that grow up with horders’ metabolisms. Why this has happened is fairly easy to guess: children born in times of famine need to hold on to every scrap of spare glucose they can. Towards the end, Moalem shifts a little off topic to examine other environmental effects on our genes and their expressions, sharing the argument of some that human beings have been partially shaped by a maritime environment, driving our hairlessness and bipidalism.
Survival of the Sickest has been on my to-read list for many years now, and I’m extremely glad to have finally sat down and taken it on. It’s in the same vein as Randolph Nesse’s Good Reasons for Bad Feelings and Why We Get Sick, the former of which I plan on reading before too long.
Health Week will wrap up with Spark: The Revolutionary New Science of Exercise and the Brain, on Monday.