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Two important points were left out of the article, however. First, there is the possibility that some of these genes only became disadvantages very recently. Milk consumption among adults, for example, was probably unheard-of until a few thousand years ago. Perhaps there simply hasn't been time to weed out a gene that only represents a minor survival disadvantage.

Second is statistics. A new allele that represents even a huge survival disadvantage (Hemophilia, for example) might gain a foothold in the genome--especially if it's recessive--simply by virtue of the fact that it didn't happen to kill its host. Alleles get weeded out when their hosts die or fail to breed, but if they themselves don't tip the balance, they have the same shot at survival as any other allele.

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.

I doubt if the full-blown version of these have much benefit, but they seem to have multiple causes, and a "mild" dose could be beneficial.

There is an epidemic of autism in silicon valley, suggestion that autism has varying degrees of severity, and in its milder forms is just extreme nerdiness.

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?

We evolved as hunter-gatherers--I doubt our forebears chased antelope merely to milk them.

And modern adults would probably be better off if they didn't drink milk--unless they got some exercise too, like making believe they're chasing antelope.

Why are 3 percent of American children struggling in school, distracted by attention deficit hyperactivity disorder?

Probably because they'd rather be running through forests and practicing their aim with bow and spear and having fun. A better question would be why is modern learning not much fun?

Why does one in every 28 people of European descent carry the gene for cystic fibrosis?

I dunno.

Might be like the gene for sickle cell anemia: Having one gene for sickle cell does not give you sickle cell, but it does protect you from malaria.

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.

It was theorized long before this study that ADD/ADHD is a legacy from our hunter-gatherer ancestors. I had not heard of the data that American Indians have the highest rate of ADD, but it certainly fits the model: Native Americans were among the penultimate hunter-gatherers.

The flip side of this is to imagine what it must be like, if you have a novelty-seeking disposition and are easily distracted, to have to get up every morning in the same place, and spend dawn to dusk tilling the soil, sowing seeds, pulling weeds, and harvesting crops. People with this gene were never suited for that kind of existence.

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.

The evidence in the article suggests that Peltonnen''s theory is certainly plausible, but her statement should properly have been...

"It's an interesting example of the possible interplay between environment and genetics."
Or, "This may be an interesting example of the interplay between environment and genetics."

Is asking for a little reserve against incautious statements too much to ask of scientists? Here's another example...

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 (emphasis mine) just 10,000 to 40,000 years ago.

Jenny, this goes to the presumption of spontaneity (and hence, random causation) of mutations that I posed HERE. While that's again plausible, it's by no means demonstrated.
Also, any discussion with genetic clock calculations, even one as loose as 75% of the range of time suggested into the past, is highly speculative. The notion of genetic clocks, assuming some constant rate of mutation, is another that's not at all demonstrated. How are genetic clock theories reconciled with punctuated equlibrium, or tendencies of families to evolutionary stasis or evolutionary dynamism?
Or for that matter, how do we reconcile a regularly ticking genetic clock with spontaneity and random causation?
I'm not questioning the bulk of the information in the article or importance of this research, simply making an observation about areas where the discussion of science gets sloppy.

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.

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.

I drink a lot of milk. So either I'm of European descent, or I'm rather horrible to be around. (Could be both, I guess.)