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Thursday, July 5, 2001
Humans, evolve you must
By ROWAN HOOPER
Us lot, contemporary humans in a postindustrial society, we've got a welfare system, social security and even, in some countries, free health care. Premature babies survive, the wounded get better, the hungry get fed. We're shielded from the blind hand of natural selection, aren't we?
Biologists readily agree that natural selection in our cave-dwelling past shaped the animal that we are now. We evolved in prehistoric times and still carry evolutionary leftovers from those times. This accounts for some of the puzzling things about humans compared to other animals: our long maturation period and our small litter size, for example. What's more controversial is whether selection is still continuing. Have our genes not changed since we lived in Stone Age conditions, or is natural selection continuing on the current human population in the modern environment, with modern human culture?
It's continuing, according to new evidence from an international team of scientists. An epic three-year analysis of census data from women enrolled at the Australian Twin Registry suggests that, in the population studied, women are having their first child at earlier ages. What's startling for those who thought we were removed from the animal world is that the research shows that the move to an earlier age of reproduction is an inherited evolutionary change.
Despite the influence of social factors such as religion and education as well as a welfare "safety net," we are evolving -- undergoing selected, heritable genetic changes.
One of the authors of the study, Ian Owens, of the department of biology at Imperial College, London, described the significance of the research: "The most important finding is that changes in society, such as freely available birth control for women and eradication of several important childhood diseases, which have taken place in the last 20 to 30 years, will probably lead to genetic changes in humans through evolution. We can say this because the sorts of factors we found that are associated with reproduction now aren't the same ones that people found in preindustrial populations."
The researchers were interested in the timing of three key life events in humans: the age at which women started menstruation, the age at which they had their first baby and the age at which menopause set in. To do this, they used the registry data to look at the effects of social and historical factors on the number and timing of children born to twins.
The Australian Twin Registry, maintained by Nick Martin at the Queensland Institute for Medical Research in Brisbane, was set up in 1978 to study inherited diseases. But when Owens, an evolutionary biologist, heard about it, he realized it could also be used to look at changes in human evolutionary behavior. The "classical twin method" of analysis compares data from twin sisters and unrelated women, and determines how human traits are determined. Answers range from "wholly environmental" to "wholly genetic."
More than 30,000 pairs of twins are enrolled in the voluntary registry. Over 15 years, three questionnaires asked the women to provide information on the number, sex, and dates of birth and death of their children as well as the timing of the three key "life-history" events.
The work, published in the journal Evolution, differs from previous studies, which were based on data from preindustrialized populations. In past cases, the age of the onset of menstruation was the best indicator of the number of children a woman would go on to produce. In the new work, the age at which women had their first baby was found to be the best predictor. Therefore, because women having babies early are having more of them, genes influencing an early age of reproduction will become more common.
To estimate the influence of social and cultural effects, information on each woman's religious affiliation and level of education was also collected. Religious categories were: none, non-Catholic Christian (Protestant, Orthodox, Evangelical or Fundamentalist), Catholic Christian or other (non-Christian). The level of education was represented in six categories, corresponding to the time spent in education and the level of qualification obtained.
Perhaps not surprisingly, Catholic women had 20 percent higher reproductive success than those of other religions. University-educated women had 35 percent less reproductive success than those who left school as early as possible.
"I was staggered by the results we got," said Owens. "I wasn't expecting anything to come out of it. I thought, 'Let's just run with the analysis.' "
Though it is intuitively understandable that religion and education should have such effects, what astounded the scientists was the finding that genetic changes more strongly influenced the three traits studied.
"Even after we controlled for these social factors," said Owens, "there was still lots of genetically heritable variation in the three life-history traits. This is a really unexpected finding."
This means that although education and religion had statistically significant effects on a woman's reproductive success, a much stronger influence was due to genetic factors.
Evolution is not confined to bacteria and viruses, nor is it a thing of the ancient past, readable only in the fossil record. At the risk of sounding like Yoda, it is everywhere and affects us all.
E-mail Rowan Hooper at email@example.com