Monday September 27 2010
The study does not tell us about modern cities
“People from traditionally urban areas could be genetically better suited to fighting infection,” BBC News reported.
The news is based on a study that looked at how resistance to infectious disease may be linked to urban living in our ancestors. The study's authors described the process as “evolution in action” and the findings could help explain differences in disease resistance seen around the world.
The study analysed DNA from 17 global populations and compared the frequency of a particular gene variant known to protect against infectious diseases, including TB. It found that the protective gene was more common in populations that had been living in large settlements for longer, supporting the theory that urbanisation led to disease resistance. However, this resistance appears to have developed due to natural selection in the face of disease and not because of any particular benefit of city living. While fascinating, the study should be seen as an investigation of our distant past but does not show that people living in urban areas today are better at fighting off infection.
Where did the story come from?
The study was carried out by researchers from the University of London, University College London, the University of Oxford and Uppsala University in Sweden. It was funded by the Natural Environment Research Council and the Arts and Humanities Research Council. The study was published in the peer-reviewed journal Evolution.
The media covered the research well and highlighted the general scientific interest of this story.
What kind of research was this?
While infectious diseases have played a defining role throughout recorded history, the impact of sickness and death from infections before this time remains unknown. It is theorised that the spread of infectious disease in ancient history was linked to increases in population density, movement of diseases through trade and travel routes, and exposure to livestock because of urban settlement. If this were the case, it would also be expected that over generations, exposure to diseases in urban populations should have led to the evolution of greater disease resistance in these people than in those without a history of urban living. The study explored this selection pressure and its relationship to urban living by assessing whether the frequency of the resistance gene was affected by the history of urbanisation.
This cross-sectional prevalence study assessed the frequency of a particular form (allele) of a gene called SLC11A1, which is known to be linked to resistance to TB and leprosy in populations with different urbanisation histories. The purpose of the study was to determine whether living in towns could have affected a population’s resistance to infection.
What did the research involve?
To define the global distribution of the TB-resistance allele, the researchers compared the frequency of the protective genetic variant across 17 different populations with a range of urbanisation histories. For 13 of these populations, the research team analysed samples of DNA, while for the other four they used data from other studies in the field. Their DNA samples were taken from Iranians, Italians, Anatolian Turks, English, Koreans, Indians, Greeks, Japanese, Sichuanese, Ethiopians, Berber, Gambians, Yakuts, Sudanese, Cambodians, Saami and Malawians.
The researchers were interested in whether the frequency of the resistance allele would differ across the population and whether the difference would be related to how long the population had been living in large settlements.
The length of urbanisation was approximated for each population using literature to identify the oldest recorded date of the first city or another significant urban settlement in the region of the sampled population. Much evidence was used to make this decision, including reports of population size or density and settlements described as major towns or cities, for example.
The researchers considered that if there was a link between the presence of the allele and the degree of urbanisation, it might be due to shared histories with nearby populations. Therefore, they considered this as a confounder in their analysis and adjusted for its influence.
What were the basic results?
There was a strong link between the estimated date of urbanisation and the frequency of the SLC11A1 allele, which conferred resistance to infection.
How did the researchers interpret the results?
The researchers say that their results support the interpretation that infectious diseases became more important after the advent of urbanisation and that they highlight the importance of population density in human health and the genetics of human populations. They say that while a number of different infectious diseases may have played a role in the global distribution of this genetic variant, it is likely that TB was the most important one.
Despite some shortcomings, this research is an interesting attempt to understand how ancient interactions between settlement, infectious disease and environmental pressure have contributed to our genetics today. The study found a link between the length of urbanisation of a population and the frequency of a particular protective genetic variant in modern times. The researchers acknowledge that their measure of the urbanisation history of a population may be “an inaccurate measure of the extent of exposure to urbanisation” under certain circumstances.
The findings add to the evidence that infectious diseases were linked with urbanisation and, in turn, resistance to those diseases. The theory is that in urban areas with high levels of infectious disease, people who had genetic variants giving them resistance to infection would be more likely to survive and reproduce. They would, therefore, pass on these variants, which would gradually become more common in the population over the generations.
While the findings contribute to our understanding of how disease may have altered our genetics over successive generations, it cannot tell us whether a rural or urban lifestyle is healthier for the modern individual.