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Thursday, August 13, 2009

The History of Lactose Tolerance

Evolution is the most powerful idea in science. Scientific American Editor in Chief John Rennie said that. He's right. We don't have to spend much time thinking about quantum mechanics or relativity. We are, and need to be, obsessed about our bodies, the food we eat, bugs spreading infectious diseases, all the animals in the world, the floral environment, medicine, surgery, everything we do and experience every hour of every day of our lives. The only way that life in all its ramifications can be understood is to study the basics in terms of evolutionary systems. As seminal biologist Theodore Dobzhansky once wrote, "Nothing in Biology Makes Sense Except in the Light of Evolution."

My point of entry into the world of evolutionary history is always lactose tolerance. People who can drink milk as adults display the most recent major mutation in the human body. Scientists are fascinated by the utterly amazing fact that 30% of the world's population shares a mutation that is less than 10,000 years old.

John Rennie and Steve Mirsky devoted part of a podcast to this bit of amazing genetics.

Steve: For example, our ability of, some of us, to digest milk, the lactose in milk as adults, it's a very recent adaptation in evolutionary history.

Rennie: Well, right, because really until we started to develop agriculture, until we started to herd animals and collected milk as a good source of protein, mammals don't continue to breast-feed throughout their lives; so the young have the ability to digest breast-milk and then after they stop drinking it, they stop making that lactase enzyme that allows them to breakdown the lactose sugar in the milk. But we kept drinking milk: We raised cows and milk was a ready source of protein and other nutrients, and we would keep on drinking that throughout our lives. And so evolution started to act on the human populations and in populations that traditionally drank a lot of milk, we have this ability to keep making the lactose throughout our lives, or lactase throughout our lives.

Steve: Let's just explain a little bit mechanistically. I mean, its likely that the mutation that enabled adults to digest lactose cropped up now and again, you know, throughout the history of human evolution; but there was never any selection pressure to keep it around until we had agriculture and were starting to try to use milk as a nutritional source, as a food, as adults.

Rennie: Right.

Steve: At that point in human history, all of a sudden those individuals who happen to have this genetic mutation have a big advantage over their comrades who can't digest the lactose, and so the combination of the environment and that genetic influence makes that genetic construct get selected for and preserved in the population. And all of a sudden, you know, within a couple of thousand years, the majority of Europeans can digest lactose.

Rennie: Right. You know, that's a good point, because it's always important to remember, you know, people always have these sorts of arguments about nature versus nurture and are there genes for various traits; you know, discussions about genes for intelligence and so forth are always notorious about this sort of the thing. But the reality is, you can't really discuss a gene, the idea of a meaning of a gene outside of the environment in which it's going to be expressed. You can't really talk about the meaning that it has, what it will do, whether it has any sort of positive or negative value in that way. Ultimately, you know, we talk about genes as though they are building blocks for some sort of complicated traits, even a trait like, say, being able to drink milk. But of course, the reality is, the molecular biological reality is, that the gene is just a stretch of DNA that happens to make a protein that breaks down a sugar that is in milk. So only under a number of different circumstances in which people happen to have exposure to, they happen to have easy access to, a lot of milk that happens to contain a lot of lactose that they can't digest very easily unless they happen to still make a lot of the lactase enzyme that they all made as children. All of these circumstances come together to make something like that beneficial. Anything that breaks down that set of circumstances, it's just another little stretch of DNA that may not prove its worth and as you said, it vanishes back in as random noise again.

It's exactly this idea that DNA changes just happen, without direction or outside influence or intention or for the good of the body, that seems so threatening. If we are nothing more than a collection of random events how do we claim a purpose for ourselves?

While I understand this fear, I don't share it. Our lives, our actions, our interactions are exactly what we make of them, good or bad, helpful or hurtful, intelligent or ignorant, caring and giving or selfish and self-indulgent. I choose to help, to gain information and to share it. We can all make that choice or a similar one no matter what is happening inside our DNA. Evolution has given us that power of individual choice, a power no other animal or plant can wield. Evolution is not just the most powerful idea in science. It's the power inside all of us.

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