Earwax: We all have it, sometimes so abundantly that we have to reach for a cotton swab. Yet scientists don't really know its function, although hypotheses range from keeping our ears clean and moist to trapping insects. Moreover, not all humans have the same type of earwax. It's wet, brown and gummy in nearly all people of European and African origin; but more than 80% of East Asians have a dry variety that lacks a waxy substance called cerumen. A team of Japanese researchers has now tackled this sticky subject and identified the gene that determines earwax type.
The team, led by human geneticist Koh-ichiro Yoshiura of Nagasaki University, had previously localized the earwax gene to a region on human chromosome 16. To find the gene itself, Yoshiura and his colleagues looked for genetic differences in this region among 64 Japanese with dry earwax and 54 Japanese with wet earwax. They discovered that earwax type was determined by a change in just one DNA nucleotide in a gene called ABCC11, which was previously found to code for a protein involved in transporting molecules across cell membranes. The findings, reported in the January 29 online edition of Nature Genetics, are consistent with earlier work showing that wet earwax is genetically dominant over dry earwax.
The team then looked at the distribution of this so-called single nucleotide polymorphism (SNP) in 33 populations around the world. The genetics matched the international earwax pattern very well. For example, almost all sub-Saharan Africans had genotypes for wet earwax, while 100% of Han Chinese and Koreans had the genotype for dry earwax. From the geographic pattern, Yoshiura and his coworkers concluded that dry earwax arose among prehistoric populations in northeast Eurasia and then spread to East Asia, possibly because of an evolutionary advantage it provided to its carriers. One idea is that an overall reduction in bodily secretions might have been a benefit in the cold climates of northeast Eurasia.
The team's results are "very convincing," says human geneticist Mark Jobling of the University of Leicester in the United Kingdom. But he questions whether dry earwax necessarily provided a selective advantage. Rather, Jobling says, this characteristic could have arisen by mutation in a small founder population and then simply spread as this population migrated to other parts of the world.