The dry form, the researchers say, presumably arose later somewhere in northern Asia, because they detected it almost universally in their tests of northern Han Chinese and Koreans. The dry form becomes less common in southern Asia, probably because the northerners with the dry earwax gene intermarried with southern Asians carrying the default wet earwax gene. The dry form is quite common in Native Americans, confirming other genetic evidence that their ancestors migrated across the Bering straits from Siberia 15,000 years ago.

The Japanese team says that the earwax-affecting gene, known to geneticists as the ATP-binding cassette C11 gene, lies with three other genes in a long stretch of DNA that has very little variation from one person to another. Lack of variation in a sequence of DNA units is often the signature of a new gene so important for survival that it has swept through the population, erasing all the previous variation that had accumulated in the course of evolution.

But earwax seems to have the very humble role of being no more than biological flypaper, serving to prevent dust and insects entering the ear. Since it seems unlikely that having wet or dry earwax could have made much difference to an individual's fitness, the earwax gene may have some other, more important function. Dr. Yoshiura and his colleagues suggest the gene would have been favored because of its role in sweating.

They write that earwax type and armpit odor are correlated, since populations with dry earwax, such as those of East Asia, tend to sweat less and have little or no body odor, whereas the wet earwax populations of Africa and Europe sweat more and so may have greater body odor. Several Asian features, such as small nostrils and the fold of fat above the eyelid, are conjectured to be adaptations to the cold. Less sweating, the Japanese authors suggest, may be another adaptation to the cold climate in which the ancestors of East Asian peoples are thought to have lived.

Myles Axton, the editor of the journal that is publishing the report, said he was not persuaded by the argument that the dry earwax gene had been favored by natural selection. New versions of a gene can also become universal in a population through a random process known as genetic drift. The dry form of the gene could have become universal in the ancestral population of northeast Asia by drift alone, and then spread to other regions of the world by migration, he said.