City Life - and the Evolution of Immunity

Much has been made about the very strong association between rural living and protection against allergies and asthma (for instance, in this recent study here). However, a new study (available "Early View" from Evolution) claims a strong effect of urbanization on the genetic basis of disease resistance - at least in human pre-history. In particular, the authors find evidence suggesting that the duration of urbanization strongly predicts the frequency of a TB resistance conferring genetic allele among modern human populations of known historical affinity. The allele is non-randomly distributed geographically, but the authors attempt to control for this non-independence by also analyzing their data using a partial Mantel test (a non-parametric multiple matrix regression procedure). In this test, they fit a multiple regression model with independent variables consisting of a matrix containing the differences in urbanization and a matrix containing F_ST values computed among each pair of populations. They found that the urbanization effect was still very significant in this model.

One concern raised and discussed by the authors is that the domestication and utilization of cattle (a proposed disease vector for TB) roughly coincides with the progress of urbanization in the region. They argue that we can reject this model because correlation is weaker than in the urbanization model; however, in my mind this argument falls short of persuasiveness because (as they admit) the history of cattle domestication for many of their populations is poorly known. This type of error would obviously also have the effect of depressing our perceived correlation between cattle domestication and genetic TB resistance.

Nonetheless, this is a very interesting study. If the result holds up to future scrutiny, then this will no doubt have many relevant human health implications and the study should be broadly cited.