Human Migration and Parasites

http://www.nytimes.com/2007/03/13/science/13para.html?n=Top/News/Science/Topi...
 

Epic of Human Migration Is Carved in Parasites’ DNA

The New York Times

By NICHOLAS WADE

Published: March 13, 2007

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.

Another faithful member of the human road show is Helicobacter pylori, a bacterium that inhabits half the stomachs in the world. It is a usually well-behaved guest, but gives its hosts ulcers when it acts up. Its pattern of geographic distribution matches that of its host’s migrations, Dr. Mark Achtman of the Max Planck Institute in Berlin and colleagues reported in the journal Nature last month.

There are five ancestral populations of H. pylori — two in Africa, two in Europe and one in East Asia. But all had a common origin, Dr. Achtman said, in a bacterium that started to spread out from East Africa 58,000 years ago, give or take 3,000 years. This is the same time period in which modern humans are thought to have begun their migration out of Africa. The match in dates “implies that H. pylori was present in Africa before the migrations, suggesting that Africa is the source of both H. pylori and humans,” Dr. Achtman and colleagues conclude evenhandedly.

H. pylori seems to be transmitted within families but the exact route — perhaps vomit — is unclear. “It’s amazing that any microbe’s geographical distribution would parallel that of humans as well as pylori’s does,” Dr. Achtman said. “You think of microbes as being easily transmissible and they are carried all over the world on ships and planes, yet some have not lost these signals of ancient migrations.”

DNA analysis has also shed light on the origin of the tapeworm, one of the 400 or so nonmicrobial parasites that regard the human body as home. The lifecycle of the tapeworm Taenia asiatica alternates between people and pigs, an animal that the religious authorities of both Judaism and Islam agree is unclean. It would be of interest to know just when these filthy animals infected people with their parasites. But the answer is not quite what had been expected.

Eric P. Hoberg, of the U.S.D.A.’s Agricultural Research Service in Beltsville, Md., concluded in 2001 that people contracted tapeworms millions of years ago in Africa, long before the emergence of agriculture and domestic animals. It was humans who infected pigs with tapeworms, not the other way around, Dr. Hoberg and colleagues reported. Indeed, people infected pigs not only with Taenia asiatica but also with a second species of tapeworm, Taenia solium, which humans seem to have acquired either by eating each other or by eating dogs.

If pigs had a religion, it is pretty easy to guess which species they would designate as unclean.