When modern humans, who evolved in Africa, left their homeland some 60,000 years ago and made their way to the far corners of the world, they took along a stomach bacterium, Helicobacter pylori.

The bacterium is today found in about half the world’s population, producing inflammation of the stomach and, in some cases, stomach or peptic ulcers. But only in less than one per cent of people does the infection lead to stomach cancer, a leading cause of cancer-related deaths worldwide.

Moreover, not all populations are equally susceptible to developing stomach cancer in the face of H. pylori infections. In Africa, for instance, infection with the bacterium is widespread but stomach cancer levels are very low, a phenomenon that has come to be called the ‘African enigma.’

A group of scientists, after examining the interplay of human ancestry with those of the H. pylori they carried, has found that coevolution of the two over long periods of time reduced the risk of gastric disease.

Their research was published this week in the Proceedings of the National Academy of Sciences (PNAS).

The scientists looked at two towns in the South American country of Colombia. Both towns had virtually identical prevalence of H. pylori infections but dramatically different rates of gastric cancer.

The town of Tuquerres in the Andean mountains has a cancer rate that is about 25 times that of Tumaco only 200 km away on the coast.

Spanish settlers who colonised South America introduced their genes as well as the H. pylori strains they carried into the native Amerindian people living there. The settlers also brought slaves over from Africa, adding more ingredients to the human and bacterial genetic mix.

In order to decipher such complicated ancestry, the scientists relied on the genetic patterns found in the genomes of the 242 individuals from the two towns who participated in the study.

The ancestry of the H. pylori those people harboured was established in a similar manner.

The mountain town of Tuquerres, it turned out, was mostly Amerindian. The coastal town of Tumaco, on the other hand, had largely African ancestry. Their H. pylori had African, European and East Asian roots.

“We found that the people living on the coast who were largely of African descent had mild stomach lesions if they had an H. pylori infection that had a largely African ancestry,” said Barbara Schneider, a professor at Vanderbilt University Medical Center in the U.S. and a senior author of the paper.

“But the Amerindian people in the mountains, if they had a strain that had a large amount of African ancestry in it (it didn't even have to be a majority), they had worse lesions,” she observed in an email.

African H. pylori ancestry was relatively benign in humans of African ancestry but was deleterious in individuals with substantial Amerindian ancestry, the scientists pointed out in the PNAS paper.

“We conclude that coevolutionary relationships are important determinants of gastric disease risk and that the historical colonisation of the Americas continues to influence health in modern American populations.”

The genes that protected individuals with African ancestry were not yet known and was the subject of ongoing research, noted Dr. Schneider in her email.

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