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Sunday, April 16, 2006

The Lactase Gene: A Precursor to Cultural Evolution?

All mammals produce milk to feed their young, and virtually all mammals use the sugar lactose, found nowhere else on earth, as an energy source for that milk. The young manufacture the enzyme called lactase, which split the lactose into simpler sugars, which can be absorbed by the intestines and used by the body. And all mammals lose their lactase-making ability at about the normal age of weaning, when it presumably would never be needed again.

Humans are mammals, and for most of our existence we functioned just like all the others. Humans naturally stop making lactase at about three years of age, and are lactose intolerant after that.

Except that a few had a mutation to a single gene. This mutation never sent out the signal to stop making the lactase. It was what is called a "neutral" mutation. For most of human history nobody ever saw the effects of this mutation, because no adult human ever drank milk.

But sometime around 10,000 years ago or so, humans started domesticating animals that were milkable. And at some point after that, they started regularly collecting the milk and using it, either to feed motherless children, or to process it into yogurt, kumiss, kefir, and other milk products.

The people who could manufacture lactase as adults suffered from fewer symptoms when they had these dairy products, and were also better able to use the nutrients in the milk. They prospered and spread their mutated genes through the local population. Since these people happened to be the ancestors of today's Europeans, these genes also got spread throughout the entire world, making adult milk-drinking common almost everywhere.

That's the standard story of lactose tolerance. What's new is that some scientists are postulating that the story of this one mutated gene may be something that's been duplicated with other genes without our being aware of it.

A story online from Wired magazine, "Getting Evolution Up to Speed," by Annalee Newitz talks about these notions, especially a study that demonstrates that two key genes connected to brain size are currently under rapid selection in populations throughout the globe. Newitz cites University of Chicago geneticist Bruce Lahn when she writes:

Chicago geneticist Lahn is most intrigued by the possibility that cultural factors are involved in brain evolution. "We think some of these new gene variants may be as young as a few thousand years, a period when human culture was changing dramatically," he said. "Maybe these genes are selected not for hunting but because of organized society." He cautioned that this is just a hypothesis, but "recent cultural evolution and biological evolution may be linked."


Jonathan Pritchard, an evolutionary biologist also at the University of Chicago, has also been looking at the lactase gene in these terms:
"The time scale for a strongly favored mutation to sweep through a population is about 5,000 years," [Pritchard said]. "It's hard to get an exact estimate for rates of change, but we know that the lactase gene is evolving the fastest in humans. It was new 5,000 years ago and now it's in virtually everybody in Europe."

The lactase gene is what allows humans to metabolize dairy products as adults. It's widely believed to have evolved in response to humans' domestication of dairy animals -- individuals who could enhance their diet with dairy products had such a strong survival advantage that the gene spread at the speed of, well, several thousand generations.

A storm of publicity greeted Pritchard's recent paper on signals of selection across the human genome. The response came in large part because Pritchard and his colleagues had found such overwhelming evidence that many human genes are evolving: not just ones that govern the brain, but also ones associated with reproduction, disease resistance and the ability to process certain kinds of foods.

"I think my work is changing people's ideas about evolution, because now natural selection seems to have continued all the way up to the present day," said Pritchard. "There's no reason to think it stops now."


Many other scientists are arguing that human changes to health, medicine, and the environment are overwhelming these natural evolutionary changes, which is why Pritchard and Lahn's notions are so controversial.

Both pathways might come into play in the future.
Lahn is comfortable with this idea. "If there's an evolutionary advantage to be had by using technology, then people will do it," he said. "People are going to start changing the game in evolution in ways Darwin never anticipated."

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