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.
"If [a genetic feature] is found around the world, genetics tells us that it must be very old," Peltonen says. "Perhaps it was even in the genome of humans before they migrated out of Africa," that is, before modern humans differentiated into today's geographical "races."
So lactose intolerance isn't really a disorder; it's "normal," Peltonen says, and the ability to produce lactase into adulthood is a recent genetic mutation.
But the "mutants" must have enjoyed a survival advantage somewhere, or their ability to digest lactose would never have become as common as it is.
Such lactose "tolerance," it turns out, is most common among people of northern European descent. Seventy-five percent to 80 percent of them have no trouble with dairy products, compared with only 10 percent to 25 percent of African and Asian populations.
The mutation may have been present in a few individuals everywhere. But Peltonen suggests it was in northern climes, with only one harvest a year, where such people would have found a survival advantage in the year-round protein and calories available in the milk of goats, sheep and cows. Over the millennia, that's where the trait would have spread.
"It's an interesting example of the interplay between environment and genetics," she says.
Another discovery reported last month may reveal an interplay of genetics with early human migration.
In the Jan. 8 issue of the Proceedings of the National Academy of Science, a team of American and Chinese scientists reported evidence that a gene strongly associated with both attention deficit hyperactivity disorder (ADHD) and "novelty-seeking" behavior resulted from a spontaneous mutation just 10,000 to 40,000 years ago.
But what really caught the team's eye was that this gene - called the 7R allele - has since spread rapidly among the world's population. It was "positively selective" because it gave its recipients a survival advantage.
But what advantage? The 7R allele is part of the dopamine neurotransmitter system involved in movement, learning and responses to psychological rewards.
Kids with ADHD today have impulsive behavior problems. They can't sit still and have difficulty concentrating. People with the "novelty-seeking" trait are thrill-seekers and risk-takers. They frequently get mixed up with addictive drugs and alcohol.
The answers are still purely speculative. But Moyzis, the California biochemist and a member of the study team, says the timing of the 7R mutation coincided with a list of cultural innovations, and a restless surge of modern humans out of Africa and across the globe, displacing earlier populations.
Perhaps individuals with personality traits such as novelty-seeking, and an uncontrollable tendency to persist in certain actions or activities drove the expansion, the study concludes. Moyzis notes that Native Americans, whose ancestors pushed their wanderings the farthest, also have the highest incidence of ADHD.
"Whether it's good or not to have this gene may depend on the environment one finds oneself in," he says. "If you're in a society that requires you to go out and use your wits; to run around finding animals; if you're dealing with novel situations all the time - it might be good to have this gene."
It may not work so well in today's classrooms, Moyzis says. But if we decide such traits can be assets, he argues, - say in entrepreneurs - "then it might be best, for some kids labeled ADHD, to put them in a different kind of school environment that nurtures that, rather than calls it a disorder."
On the other hand, he cautions, it's also quite possible that it's not the ADHD traits at all that evolution has favored, but rather some other unidentified trait that's also linked to the 7R allele.
Exposing the hidden survival advantages in what appear to be genetic disorders is one of the most intriguing promises of evolutionary genetics.
For example, until recently cystic fibrosis killed most of its victims before they could reproduce. Yet evolution has conserved the CF gene and made CF the most common inherited disease among people of European descent. Why?
Statistically, Peltonen says, 25 percent of the children of two CF carriers will get the disease. But 50 percent will inherit just one copy of the gene. They're spared the disease, and inherit instead a trait that slows the release of salts into the intestine. Scientists believe that may have saved them from severe dehydration and death from the diarrheal diseases that killed seven of 10 newborns in Medieval Europe. So the CF gene prospered.
Some researchers are now investigating whether other common, genetically linked disorders such as autism, depression and schizophrenia may also have been positively selected because they confer hidden survival benefits.
"As long as whatever is being selected for is very positive, individuals are always going to be there that exhibit these negative things," Moyzis says.
Such research is "a new way of thinking about human genetics and biology," he says. "A lot of genetics has been too simplistic. It's going to turn out to be a little more complicated, with interactions between society and the expression of these genes."