If Darwin was right, and evolution relentlessly weeds out genetic traits that impede a species' survival, then why are a quarter of adult Europeans and 90 percent of Asians unable to digest milk products - a rich, year-round source of protein and energy?
Why are 3 percent of American children struggling in school, distracted by attention deficit hyperactivity disorder? Why does one in every 28 people of European descent carry the gene for cystic fibrosis?
Scientists don't yet have all the answers. But the recently completed mapping of the human genome, and the decreasing costs of the DNA sequencing technology that made it possible, have energized new research in evolutionary genetics. Scientists are gaining intriguing glimpses into the cold logic of human evolution, and the remarkably complex interplay of genetics and human history.
The new genetic toolkit "puts this whole area of research on a more scientific basis, rather than pure speculation," says biochemist Robert K. Moyzis, of the University of California at Irvine.
Consider lactose intolerance.
In the Jan. 14 issue of Nature Genetics, researchers at the University of California at Los Angeles, and in Finland, reported the discovery of the genetic coding responsible for the inability of most adults to produce lactase, the enzyme needed to digest lactose, which is the primary sugar in milk.
Nearly everyone makes enough lactase in infancy to digest breast milk. But for many, the lactase gene switches off sometime after weaning. From then on, the consumption of milk, ice cream, cheesecake and other dairy products can bring nausea, painful cramps, diarrhea, bloating or gas.
"It's easy to say it's not really a problem, but some people are suffering," says Leena Peltonen, chairwoman of human genetics at UCLA, who led the study. Worse, the symptoms may mimic a serious digestive disease or malignancy.
"That's why people are keen to get the diagnosis," she says. Identification of the gene for lactose intolerance means there will one day be an easy diagnostic blood test.
But Peltonen's team found something even more fascinating.
In blood samples from 196 lactose-intolerant people of African, European and Asian descent, they found the same intolerance gene. Koreans, Finns, African-Americans - everybody had the same coding.
