Tooth decay and human migration out of Africa

Nicholas Wade:

A human body is not the individual organism its proud owner may suppose but rather a walking zoo of microbes and parasites, each exploiting a special ecological niche in its comfortable, temperature-controlled conveyance. Some of these fellow travelers live so intimately with their hosts, biologists are finding, that they accompany them not just in space but also in time, passing from generation to generation for thousands of years.

The latest organism to be identified as a longtime member of the human biota club is Streptococcus mutans, the bacterium that causes tooth decay. From samples collected around the world, Dr. Page W. Caufield and colleagues at New York University have found that the bacterium can be assigned by its DNA to several distinct lineages. One is found in Africans, one in Asians and a third in Caucasians (the people of Europe, the Near East and India), his team reported in last month’s Journal of Bacteriology.

The geographical distribution of these lineages reflects the pattern of human migration out of the ancestral homeland in Africa. If the tooth decay bacterium spreads easily from person to person, any geographical pattern would soon be blurred. But Streptococcus mutans is transmitted almost entirely from mother to child, preserving its lineages over thousands of years. The bacteria apparently infect the infant during birth, beginning the work that provides the dentistry profession its livelihood. “We’ve never seen father-to-child transmission,” Dr. Caulfield said. Thanks, Mom.

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