Study: City Life Spreads Disease, But If It Doesn’t Kill You…

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REUTERS/Yiorgos Karahalis

Following the news that the rise of cities contributed to the spread of HIV is a more heartening take on urbanity: a history of city living may have helped some populations develop resistance to tuberculosis (TB).

A new study published in Evolution this month found that modern-day residents of ancient cities (Rome and Damascus, for example) are more likely than others to have a genetic variant that lowers the risk of TB infection, the highly contagious bacterial respiratory disease that afflicts 11 million people worldwide each year.

Researchers from University College London (UCL) analyzed human DNA samples from the current populations of 17 different cities across Africa, Asia and Europe. They matched these samples to historical records of those cities’ first settlements. (More from Megacities)

What they found was that the genetic variant was almost universally present in populations throughout the Middle East and India, and was also found in some of the older European settlements.

“Population density seems to play an important role in shaping so many aspects of our species,” said Mark Thomas a professor in the department of genetics, evolution and environment at UCL in a statement. “It was a vital factor in our species maintaining the complex skills and culture that distinguish us from other primates. It drove many of the genetic differences we see today between different populations from around the world. And now, it seems, it also influenced how infectious diseases spread in the past and how we evolved to resist those diseases.” (More from Study: Can We Tell Our Genes to Make Us Fat?)

The authors of the study expressed excitement about the merging of scientific analysis and historical records, but this is also a potential limitation. Scientific and historical conclusions are different in nature and difficult to merge — that is, you can’t necessarily extrapolate science from history. So it might be hasty to suggest that population density was the central contributing factor to the development of the anti-TB genetic mutation.

For one thing, the highest rates of the anti-TB variant occurred in Middle Eastern populations and in areas that were involved in prominent Arabic invasions — for example, Spain (711 to 1492) and India (in the 12th century) — and not in other areas that are home to ancient cities but were not subject to Arab invasion (China). So who’s to say that immunity isn’t a result of Arabic ancestry, rather than city life? Or that some other historical factor triangulated with urban centers to suggest this result?

UPDATE: We got in touch with the study’s lead author, Dr. Mark Thomas to inquire about the possibility of a triangulation. “There are of course alternative explanations for the patterns we see – this will always be the case with allele frequency distributions in space,” he said.  But he cited little evidence of a genetic impact from Arab migration in Europe and said that the study controlled for “demographic episodes.”

“In our study we had a clear prior hypothesis with a robust biological basis – which is why we think the strong correlations we report with urbanization are indicative of an evolutionary response to crowded living.”

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