Ancient urbanization and tuberculosis resistance.
Tuberculosis is a bit of a pet topic of mine, so this article from the March 2011 edition of Evolution caught my eye:
“Ancient urbanization predicts genetic resistance to tuberculosis.”
Barnes, I., Duda A., Pybus, O. & Thomas, M.
It is logical that an increase in urbanization and consequent close quarters association between large numbers of people would increase both the incidence and prevalence of various transmissible diseases. As described by Diamond in “Guns, Germs & Steel”, this leads to the tendency of a given urbanized population to develop greater rates of resistance to the pathogens responsible for these disease states. The article above explores this assumption by measuring the frequency of an allele (SLC11A1 1729 + 55del4) related to resistance to infection by Mycobacterium tuberculosis, the aetiological agent of tuberculosis, against the estimated length of time a given population has been urbanized. In reinforcing my initial statement, the results showed a strong positive correlation between how long ago a population shifted toward urban living and rates of this TB resistance related allele. So, as would be expected, more ancient population centres in the middle east for example showed relatively high frequencies of this allele when compared to southern African or Scandinavian populations.
The correlation between resistance related alleles and urbanization could be used for other, more novel, studies. For example, frequency testing for this allele or others associated with resistance to diseases endemic to urban centres, could be used in something of a ‘forensic’ manner to detect descendants of population centres that have undergone collapse i.e. the now dispersed members of ancient African or Mesoamerican populations.
On a side note; I found it a little unusual that this study also measured TB resistance against the time at which cattle were domesticated. As mentioned in a previous post (https://confusedious.wordpress.com/2011/04/19/tuberculosis-an-inverse-zoonosis/) it is highly unlikely that humans acquired this infection from cattle, the chromosomal data just doesn’t make sense in that context. Nor does the presence of TB in the New World make sense if cattle were the vector. With these things in mind I really hope they were simply speaking of M. bovis infection (as opposed to M. tuberculosis infection) only in terms of how it could produce a similar selection pressure for the given resistance allele. Even in this case I hypothesise that the role of M. bovis would relatively minor. Hopefully this particular myth of tuberculosis (of the human strain) as a zoonosis of bovine origins will someday find its rightful place in the dustbin.