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Thursday, Feb. 10, 2005
DNA 'flip' highlights our ongoing evolution
By ROWAN HOOPER
Stung by the phenomenal success of the "Harry Potter" books, some people like to preach about the infantilization of culture, and some critics worry that adults are wallowing in childhood.
A companion worry is that children are growing up too fast, becoming sexualized at too young an age. With some girls wearing makeup when they're barely past being toddlers, not to mention rising levels of teenage sex and pregnancy, this is an understandable worry.
For good or ill, however, these things are cultural and social aspects of our modern life.
Yet there are other changes, harder to see, that are having a deeper effect on us -- a genetic, naturally selected effect. The evidence comes from Iceland, where researchers at a drug company, deCODE Genetics, were looking at the sequence of chromosome 17 in the hope of finding genes linked to schizophrenia. Instead they found something quite different. They found a long, 900,000-unit stretch of DNA that didn't correspond to the pattern in the standard human genome: The section was inverted, flipped in the opposite direction.
The scientists wondered if the inversion had any effect on the people who carried it. What they discovered, to their surprise, was that women with the flipped region had slightly more children on average than those who didn't have the flip.
Measured from year to year, the number of extra children is modest. Icelandic people with the variant have 3.2 percent more children per generation than those without. But over evolutionary time, 3.2 percent is a huge difference.
All of those extra 3.2 percent carry the inversion, and all of them will have more children, also carrying the sequence. This is how natural selection works.
While we are used to seeing selection in action in lions in the Serengeti and bacteria in Petri dishes, we take for granted that modern humans are cushioned from evolution. But the Icelandic work, published in the February issue of Nature Genetics, suggests that humans are still evolving.
The new study follows work in 2001 by scientists analyzing the patterns of births shown in the Australian Twin Register. They found that both genetic and social factors, such as religion and education, have a profound effect on the timing of human reproduction.
Despite anecdotal evidence to the contrary, the Australian data showed that women are having their first child at earlier ages. Moreover, because the work was done on twins (and social effects could thus be excluded), the scientists (again, to their own surprise) found that the decreasing age of childbirth is a trait that is being fixed by natural selection.
And now the Icelandic study also documents genetic changes currently occurring in humans.
Iceland was settled by the Vikings in the ninth and 10th centuries, and remarkably, its gene pool hasn't changed much since then. To exploit this, deCODE Genetics has compiled a genealogy database starting from the founding of the country more than 1,000 years ago, and covering the entire present-day population.
The researchers, led by Hreinn Stefansson, found that the inversion turns up in as many as 20 percent of Europeans, rarely in Africans and hardly ever in East Asians. It means there is something in the European environment providing an advantage to carriers of the inversion.
Stefansson and colleagues were also able to date the origin of the inverted sequence -- and here they found something else intriguing: The inversion has been around for about 3 million years. But modern humans only came out of Africa some 150,000 years ago. So where did the ancient inversion come from? One possibility that excites anthropologists is that it came as a result of interbreeding with an older form of hominid. Maybe we picked it up through sex with Neanderthals. After all, research last year suggested we picked up a form of head louse through contact with Homo erectus.
While the origin of the inversion remains a mystery, it is not disputed that it has a positive effect on reproduction.
Evolution in humans is ongoing. Other examples include the changes in genes in the immune system. These genes protect us from disease, and they must change to counter pathogens that try to evade the immune system.
A question with broader social consequences is whether these evolutionary changes in human reproduction are reflected in cultural changes in behavior.
One of the most popular girls' toys is currently the Bratz doll, a voluptuous, sexualized doll in makeup and a miniskirt, with long hair and full, open lips. The success of the doll, and its sexualized message for children, has scandalized liberals and the Christian right alike. However, children have always played at being adults, just as critics have always complained about dumbing down. And Freud reckoned childhood was all about sex.
These are just aspects of the cultural soup we live in; we've always been like this. The take-home message from Iceland is that, far from being something that only happens in the wild, natural selection continues to work on modern humans. We are evolving just like everything else in the world -- of course we are, because for all our civilization and culture, we are part of the natural world.
That conclusion, which surprised even the scientists who first realized it, is one that politicians would do well to take on board.
A book of Natural Selections columns translated into Japanese, "Nou to sekkusu no seibutsugaku (Evolution, Sex and the Brain)," has been published by Shinchosha. Rowan Hooper is a biologist at Trinity College, Dublin. He welcomes readers' comments at firstname.lastname@example.org