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City dwellers evolved to fend off TB

UCL (UK) — Over time, populations in areas with long urban histories have developed genetic resistance to diseases like tuberculosis and leprosy.

That finding comes from a new study reported in the journal Evolution.

In ancient cities, poor sanitation and high population densities would have provided an ideal breeding ground for the spread of disease. Natural selection should mean that humans would have developed resistance to disease in long-standing urbanized populations over time.

However, this association has been very difficult to assess—especially in prehistory.

Now, scientists from University College London (UCL) and Royal Holloway have tested the theory by analyzing DNA samples from 17 different human populations living across Europe, Asia, and Africa. In addition, they searched archaeological and historical literature to find the oldest records of the first city or urban settlement in these regions.

By comparing rates of genetic disease resistance with urban history, they showed that past exposure to pathogens led to disease resistance spreading through populations, with our ancestors passing their resistance to their descendants.

“The results show that the protective variant is found in nearly everyone from the Middle East to India and in parts of Europe where cities have been around for thousands of years” says Mark Thomas a professor from the Department of Genetics, Evolution and Environment at UCL.

“The method we have employed here makes novel use of historical and archaeological data, as a means to explain the distribution and frequency of a genetic variant, and to identify a source of natural selection,” says Ian Barnes, from Royal Holloway.

“This seems to be an elegant example of evolution in action. It flags up the importance of a very recent aspect of our evolution as a species, the development of cities as a selective force. It could also help to explain some of the differences we observe in disease resistance around the world,” adds Barnes.

“Population density seems to play an important role in shaping so many aspects of our species”, continues Thomas. “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.”

The research was funded by the Natural Environment Research Council and the Arts and Humanities Research Council (Centre for the Evolution of Cultural Diversity).

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