Evolution

Fast evolution

Inuvialuit of the Western Arctic, Northwest Territories, Canada (excellent resource!). Photo: Canada Parks & Tourism.

Mitochondria are found inside animal cells. They produce ATP -- a chemical that powers countless body processes -- and heat. Is this heat just a byproduct, or can it be a subject of evolution?

Understanding the when, where and how of this adaptation is a critical issue for studying human migration, and now comes an answer, courtesy of work reported this week in Science by Douglas Wallace, a population geneticist at the University of California at Irvine. With his colleagues, Wallace has surveyed the genetics of a structure called the mitochondrion, which is found in all mammalian cells.

For decades, scientists have known that this bitty organelle creates energy, in the form of adenosine triphosphate (ATP). ATP is used to power almost everything the body does, whether it's remembering your mother's maiden name or rubbing your tummy while wiggling your ear.

In the traditional view, the mitochondria produced heat, but simply as a by-product of ATP production. Now, a global survey of human mitochondria DNA by Wallace and company has revealed characteristic sequences among indigenous people who lived in high latitudes -- close to the poles. Because the pattern of mutations was not random, and because it occurred in genetic sequences that affect the proportions of ATP production versus heat output, the researchers concluded that the mitochondria's DNA had evolved as an adaptation to the cold.

Under the conventional view, which focused on ATP production, the finding would be accidental. But the patterns were anything but accidental, Wallace says. "Up until this article, people had traditionally thought of mitochondria as making ATP to do work. In fact, all warm-blooded animals actually maintain their body temperature from mitochondrial production of heat, and maintaining body temperature has become critical for survival of warm-blooded animals because all their biochemistry is optimized at that body temperature."

Calories count -- but for what?
In evolutionary terms, he says, body warmth is just as important as energy, since a freezing Finlander could become a dead Finlander, just as a lethargic Lithuanian could become a dead Lithuanian. "You have this interesting trade-off in warm-blooded animals: For a certain amount of calories you eat in the diet, you can either use them to keep warm, or to do work. ... If you use more calories to keep warm, you will have fewer calories available to do work."

This Dogon man, in Mali, West Africa, probably has mitochondria that produce more energy, and less heat. Photo: ©David Tenenbaum

The history of migration, increasingly studied with genetic tools, shows that our ancestors left Africa roughly 65,000 years ago, and moved toward colder regions. And as anyone who's ever braved a winter in Poland, say, or Saskatchewan or Tibet can testify, you gotta stay warm. And because people who are frozen stiff make rotten parents, that has evolutionary implications. "The insight we had, and that this paper nails down, is the idea that as people moved to colder climates, what became most lethal to them was the extreme cold," Wallace says.

But here's the rub: To make more heat from each calorie of food, your genetics must change before you freeze to death. But the nuclear DNA found in our chromosomes is so stable, Wallace says, that it could not mutate, or change, fast enough to warm our migrating ancestors.

However, the mitochondrial genes, because of how they are selected by evolution, can mutate roughly 10 to 100 times faster than the better-known DNA in the nucleus, Wallace says. If the mitochondria of a migrating group had enough genetic diversity, a few individuals probably could enough heat to survive the new, cold environment.

Take your evolutionary medicine, please!
The study has implications beyond migration. In terms of the emerging science of evolutionary medicine, it sheds light on the tradeoffs of the never-ending quest for a long, healthy life. At least until the invention of central heating, making more body heat was essential to polar people (who make, on average, 20 percent more heat than tropical people). But making heat shortchanges them of ATP, leaving less energy available for action, and setting the stage for rare energy-deficiency diseases, like the blinding disease Leber's hereditary optic neuropathy.

Seng Soy (left) and Chhorn Chhoy, Cambodian Buddhist monks living near frigid Madison, Wis., embody the larger history of human migration - from the tropics toward the poles. They love central heating -- but how did early humans adapt to the cold? Photo: David Tenenbaum

But while more ATP translates into more energy, which is a good thing for people in warm climates, a byproduct is higher levels of reactive oxygen molecules, and these so-called "free radicals" play a role in causing generalized aging, and neurodegenerative diseases, including Parkinson's and Alzheimer's. On the other hand, people with the energy shortage associated with less ATP may enjoy a longer, healthier life.

As Wallace sees it, mitochondria play a key role in this tradeoff. If you have more energy, you age faster, and could even die in a cold climate. If you have more heat, you can survive the cold north, but you may suffer from energy-deficiency diseases.

You take your choice, and you take your chance. That's the role of the roll of the evolutionary dice.

-- David Tenenbaum

Bibliography
Effects of Purifying and Adaptive Selection on Regional Variation in Human mtDNA," Douglas Wallace et al, Science, 9 Jan. 2004.