Published online 21 September 2010 | Nature | doi:10.1038/news.2010.483

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Evolution in the urban jungle

Tuberculosis resistance is most prevalent in city slickers.

A child walks next to an open sewer in the Ajegunle slum. Lagos, NigeriaUrban populations can evolve resistance to certain diseases over time.Jacob Silberberg/Panos

Survival of the fittest is more than just an apt metaphor for city living. Humans may have evolved disease resistance in response to urbanization.

An immune-system gene that protects against pathogens such as tuberculosis (TB) and leprosy is more prevalent in contemporary cultures with long histories of urbanization than in those where cities appeared more recently, finds a study published in the journal Evolution1.

"There are pros and cons to starting to move into cities, and one of the downsides is you actually get higher disease rates — at least for the short term," says Ian Barnes, a geneticist at Royal Holloway University of London, who argues that those disease rates were high enough to lead to natural selection for resistance.

High population densities and regional trade help pathogens to shuttle between hosts and populations, and poor sanitation, standing water and domesticated animals all provide ample breeding grounds, he says.

To test whether this combination could propel natural selection on disease resistance, Barnes and his team compared the urbanization histories of 17 African, Asian and European populations with the prevalence of an allele of a gene linked to protection against TB, called SLC11A1, in the same populations.

They relied on archaeological evidence for the earliest urban settlements in each area, indicative of hundreds to thousands of people living stably in one place. These settlements appeared beginning from around 6000 BC for Anatolian Turks to the early twentieth centuries for the southern Sudanese and the Sami — a culture native to northern Scandinavia.

Members of such newly urbanized populations proved significantly less likely to possess a TB-protective version of SLC11A1 than populations from the Middle East, India and parts of Europe, where the protective allele is found in nearly everyone and cities have been around for millennia.

Going with the flow

However, natural selection isn't the only explanation for such a pattern. Genes and ideas tend to flow together, so interbreeding and not evolution might instead explain why the TB-resistance allele is extremely prevalent in Italians and Greeks, for instance. Yet, even after accounting for shared ancestry, Barnes and his team still found a strong link between TB resistance and urbanization.

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Pardis Sabeti, a geneticist at Harvard University in Cambridge, Massachusetts, agrees that urbanization has probably influenced natural selection in humans. Her own lab recently scoured the human genome for natural selection within the past 30,000 years and identified numerous genes involved in fending off tuberculosis and leprosy.

If city life did influence the evolution of pathogen resistance in humans, ancient DNA samples could capture that process in action, says Mark Thomas, an evolutionary biologist at University College London, who collaborated with Barnes on the paper. Thomas and Barnes and their colleagues plan to look for signs of natural selection in other alleles of SLC11A1, as well as in other genes linked to infectious diseases. 

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