Adapting to the Cold

Every January, we residents of Saskatchewan ask ourselves the same question: why are we here, instead of in the tropics?

There’s a scientific version of that same question: how have humans, who evolved in the tropics, managed to survive in the even-icier-than-Saskatchewan climes of the far north?

The January 9 edition of the scientific journal Science provided part of the answer, in the form of a report of a study conducted by Douglas Wallace, a population geneticist at the University of California at Irvine.

Wallace and his colleagues conducted a global survey of the genetics of mitochondria, tiny structures found in all mammalian cells. The mitochondria create a compound called adenosine triphosphate (ATP) which is what the body uses to power just about everything it does, from thinking to walking.

In the process, mitochondria also produce heat. Traditionally, science has viewed that heat as simply a by-product. Wallace and his team discovered, however, that indigenous peoples who live close to the poles have characteristic mutations in their mitochondrial DNA that cause their mitochondria to produce much more heat than those of us from more temperate climes, thus enabling them to better survive the polar regions’ extreme cold.

Even for those of us who don’t have this mutation, the heat produced by mitochondria is vital, Wallace points out; that’s the heat that enables warm-blooded animals to keep their body temperature constant no matter what the outside temperature is, which is important because all of the body’s biochemistry is optimized for that constant temperature.

Since both ATP and heat are vital mitochondrial products, the body has to strike some kind of balance between them. The more of the calories it takes in as food that it uses to produce heat, the fewer it has available to turn into ATP to power the body’s workings.

The mutations found in indigenous people from the polar regions have swung their bodies toward the heat-production side of this balance. This adaptation has happened amazingly quickly; scientists believe it was only 65,000 years ago that our ancestors left Africa and moved toward colder regions. Natural selection drove the mutation: some of the people who migrated would naturally have had mitochondria that produced more heat than other people, and those people were more likely to survive and produce offspring.

But the adaptation might not have been possible in such a relatively short time if not for the fact that mitochondrial DNA can mutate 10 to 100 times faster than the better-known--and much more stable--DNA in the cell nuclei.

With the mutation, Wallace found, polar people make, on average, 20 percent more heat than tropical people. But that shortchanges them on the ATP side of the equation, leaving less energy available for action and making them more prone to certain rare energy-deficiency diseases.

On the other hand, people in warmer climates may get more energy from their mitochnodria, but a byproduct of that is higher levels of reactive oxygen molecules, a.k.a. free radicals, which play a role in aging and neurodegenerative diseases like Parkinson’s and Alzheimer’s. Having less ATP may actually contribute to a longer, healthier life.

Those of us lacking in mitochondrial heat have to make do with the body’s three main built-in mechanisms for staying warm. The first, piloerection, is pretty useless: it’s your body's attempt to fluff your fur. Since few of us have fur to fluff, we end up looking like a plucked waterfowl--which is why piloerection is better known as "goose bumps."

More effectively, the body closes off blood vessels close to the surface, routing warm blood to the vital inner core, and finally, there’s shivering, involuntary muscle contractions that boost your body's heat production fivefold. (Eventually, however, your muscles burn up all the available fuel or get too cold to contract, the shivering stops, and your body temperature plummets.)

Early humans encountering cold for the first time avoided such unpleasantries by wearing animal furs, which traps air in its thick weave of hairs. Heat from the body, a toasty 37 degrees Celsius, warms this trapped air, creating an insulating layer between skin and cold.

This is still how we keep warm in the cold: even the most high-tech ski jacket still keeps you warm by trapping your own heat against your skin.

And you’ll note that even polar people still wear coats. Extra-hot mitochondria, it seems, can only go so far.

These weekly columns on science appear in the Regina (Saskatchewan) Leader Post and Red Deer (Alberta) Advocate.  They are available for one-time publication or regular syndication to any interested newspapers, magazines or on-line publications.  E-mail me for details.

Posted January 20, 2004