Mountain Babes

Natural History, April, 1999 by Mark Twain

Mark Twain remarked that even though history never repeats itself, it does tend to rhyme, So does evolution.

Selection for different genes and combinations of genes must have occurred in the two adventurous peoples who colonized the Tibetan plateau and the Andean altiplano. Although the Andeans are less well adapted, both groups have acquired the ability to live under these extreme conditions. Scientists have yet to track down any specific genetic changes that have contributed to their adaptation. But it is almost certainly only a matter of time before some are found.

Perhaps the most telling indication of true genetic differences between the two populations comes from an examination of the weight of their babies at birth. Virtually everywhere in the world, high altitude has a strong negative effect on birth weight, even when babies are carried to term. The effect is detectable even when the differences in elevation are relatively small. On average, for every thousand meters in elevation, the birth weight of full-term babies decreases by about one hundred grams.

This pattern turns out to hold true for the Andean Indians, whose babies weigh about four hundred grams less than babies born at sea level. (For the metrically challenged, this translates into a difference of a pound or so.) Such a low birth weight must contribute to the high infant mortality. Babies of acclimatized parents weigh a little more than unacclimatized babies born at the same elevation, but the difference is not much.

This pattern does not hold true for Tibetan babies, however. A notable exception to the worldwide trend, their birth weight at term is the same as that of babies born at sea level. These high birth weights are not the result of unique conditions on the Tibetan plateau. Chinese mothers who live in Lhasa throughout their pregnancies obey the low birth weight/high elevation law: they have smaller babies than do Chinese mothers who go through their pregnancies at sea level.

How do Tibetan mothers manage to nourish their babies so efficiently at such heights? To find out, Lorna Moore, of the University of Colorado, and her coworkers carried out a number of important experiments on Tibetan volunteers. Using as controls some Han Chinese who had spent long periods of time living in Lhasa, they found that Tibetan women excel at supplying oxygen to their babies in utero.

In most people, low levels of oxygen cause the pulmonary arteries, which supply blood to the lungs, to constrict. This turns out to be a reflex, an echo of the moment of birth. As a fetus grows in the womb, its pulmonary arteries are small, and little blood flows to the lungs; but at birth, oxygen rushes into the lungs and the pulmonary arteries expand. When an adult's pulmonary arteries contract, it is a physiological memory of fetal life. Moore and her coworkers found that Tibetans lose that dangerous reflex soon after birth.

Tibetan women also have unusually large uterine arteries. These two traits combined mean that they retain high rates of blood flow under hypoxic conditions.

Tibetan babies themselves exhibit other unusual adaptations. Moore found that shortly after birth, the babies had 10 percent more oxygen in their arterial blood than did babies born to Han Chinese parents who had lived at the same elevation, and that this difference persisted and actually grew more pronounced during the first weeks of life.