Many human genes evolved recently

From New Scientist 07 March 2006

Human genes involved in metabolism, skin pigmentation, brain function and reproduction have evolved in response to recent environmental changes, according to a new study of natural selection in the human genome. Researchers at the University of Chicago, US, developed a statistical test to find genomic regions that evolution has favoured over the last 15,000 years or so – when modern humans dealt with the end of the last ice age, the beginning of agriculture, and increased population densities. Many of the 700 genes the researchers identified – especially those involved in smelling, fertility, and reproduction – are also suspected of having undergone natural selection during the divergence of humans and chimpanzees millions of years ago. But some of the newly identified genes fall into categories not previously known to be targets of selection in the human lineage, such as those involved in metabolism of carbohydrates and fatty acids.

“It’s reasonable to suspect that a lot of these are adaptations in response to new diets and agriculture,” says team member Jonathan Pritchard. For example, gene variants that improve the digestion of lactose have become more common, presumably since the domestication of cattle provided a ready source of milk. And in some Europeans, genes giving a lighter skin have increased in frequency, as populations have moved north to regions where there is less sunlight to generate vitamin D. The researchers analysed the genomes of 209 people from Nigeria, East Asia, and Europe. They found widespread signals of recent selection in all three populations. Only one-fifth of the 700 genetic regions identified were shared between at least two of the groups – the rest were unique to single populations. That supports the idea that the adaptations are recent, Pritchard explains.

The statistical test is a “powerful way of looking for selection in the genome”, says Michael Hammer of the University of Arizona in Tuscon, US. It looks for certain patterns of DNA – called linkage disequilibrium – that show a gene variant is young. It then identifies those that appear at high frequencies, which suggest they have been selected for. Definitive proof that the gene variants are being favoured in the human genome will require detailed analysis of the changes they cause in proteins and how this affects fitness. But Hammer says “they’ve given us a huge list of candidates".

Nonetheless, there are likely to be many more, says Peter Andolfatto of the University of California, San Diego, US: “The genes being mapped here at best probably account for only a small fraction of the targets of recent selection in the human genome.” Identifying the gene variants that are under selection may one day help medicine, Pritchard adds. That is because individuals with a newly evolved gene variant may be better adapted for modern human conditions and less susceptible to certain diseases. Understanding the differences could help guide future therapies.