Cities have apparently influenced human evolution, some scientists now find.
The findings all begin with how cities help spread disease by bringing people closer together, where they can infect one another.
Now researchers have concluded that by making their residents more vulnerable to disease, cities increased the chance that, over time, the survivors' descendants could resist infections.
Evolutionary biologist Ian Barnes at the University of London and his colleagues focused on a genetic variant. This variant, with the alphabet-soup name of SLC11A1 1729+55del4, is associated with natural resistance to germs that dwell within cells, such as tuberculosis and leprosy.
They analyzed DNA samples from 17 modern populations that had occupied their cities for various lengths of time. The cities ranged from Çatalhöyük in Turkey, settled in roughly 6000 B.C., to Juba in Sudan, settled in the 20th century.
The scientists said they discovered a highly significant link between the occurrence of this genetic variant and the duration of urban settlement.
People of a long-populated urban area often proved to be better adapted to resisting these types of infections. For instance, people in areas settled for more than 5,200 years, such as Susa in Iran, were almost certain to have this variant, while in cities settled for only a few hundred years, such as Yakutsk in Siberia, only 70 percent to 80 percent of the population would have it.
"The research shows evolution happening," Barnes told LiveScience. "Urbanization has had an impact on the human genome."
A possible limitation of the study is that in most regions, populations often get replaced frequently throughout history. This could mean the populations that Barnes and his colleagues analyzed did not undergo the centuries or millennia of natural selection the researchers assumed they did to pass down the germ-resistant genetic variant.
Another concern could be that tuberculosis might only constitute a weak threat toward a population, meaning there might not be all that much pressure for natural selection to choose such a variant. "However, none of these invalidate the result; they just make it less likely you will be able to find the association if it's there," Barnes said.
He added, "We did."
Barnes acknowledged that other researchers might have problems with the dates of urban settlement used in his team's study. The approximate dates for the founding of major settlements were pinned down from archaeological and historical records. "We understand that these can never be totally accurate, but they were the best estimates we could make," he said. "However, that still doesn't invalidate the result, and we'd love to open the discussion up with archaeologists and historians about how to take the work further."
This research is a good example of how one might look for adaptations that occurred in response to cities, said paleoanthropologist John Hawks at the University of Wisconsin, who did not take part in this research.
"There are lists of other things one might want to look at," Hawks said. "In addition to disease, one might consider possible behavioral changes — what kinds of changes in genes, if any, might have supported cultural changes brought on by cities?"
The scientists detailed their findings online Sept. 14 in the journal Evolution.