People in Tibet and the Andes have lived in thin air for thousands of years. These two races have evolved different ways of coping with altitude -- showing, says Cynthia Beall, of Case Western Reserve University, Cleveland, Ohio, that "human biology is capable of more than one response to this extreme stress".

If a lowlander ventures above 3,000 metres -- where they face a permanent shortage of oxygen -- their breathing rate rises. If they stay for a few months they make more haemoglobin, the protein that carries oxygen in the blood.

People who live in the mountains for several years, or their descendents, carry on making more haemoglobin, but their breathing rate returns to almost its original level. Andeans have reacted in this way.

Tibetans, on the other hand, have a high breathing rate (similar to the short-term response of lowlanders) but a lower haemoglobin level. A variety of evidence in recent years has shown that haemoglobin is not as important in acclimatization as was once thought.

Both high-altitude peoples seem equally well adapted: their abilities to do physical work, and the weights of their newborn babies, are roughly equal. "No one is better or worse, they're just different," says Bell.

The genetic bases and cellular mechanisms underlying the different adaptations are not known, although they are probably complex. Understanding them might help predict susceptibility to altitude sickness in lowlanders, and also shed light on the ramifications of oxygen deficiency (hypoxia), a feature of many medical conditions, including heart disease.

Bell presented her work at the annual meeting of the American Association for the Advancement of Science in San Francisco.