January 23, 2009
Parting is such sweet sorrow, as the saying goes. But apparently that sweetness doesn’t include relief from stomach pain, and leaving town doesn’t rid you of your ulcers. But that’s good news for scientists trying to piece together the story of where we all came from.
In a new study in Science magazine, a team of researchers used DNA from ulcer-causing bacteria to trace early human pathways across Asia and into Australia and Polynesia. Their results show two waves of movement from Asia into present-day Indonesia, New Guinea, and Australia some 30,000 years ago, as well as a much more recent wave from Taiwan into the Philippines (5,000 years ago), to the Melanesian islands, and then to New Zealand and the Pacific islands.
The responsible bacteria are called Helicobacter pylori. (After centuries of doctors blaming ulcers on everything from spicy food to chewing gum, two Australians confirmed that ulcers arise from a bacterial infection in our guts. Proving it involved drinking a cupful of infected stomach juices – and won the pair a Nobel prize – but that’s another story.)
Helicobacter pylori are exquisitely adapted to live inside our stomachs (about half the world’s people are infected, though 80 percent never show symptoms). Since the bacteria don’t live outside our bodies, that means two things: first, they go where we go, and second, they evolve as we evolve. That’s pretty useful if you’re studying ancient human migrations, because people today are still carrying around H. pylori strains descended from the bacteria of their ancestors.
The advantage of using H. pylori DNA instead of simply looking at human DNA is that there are fewer strains of it than there are mixtures of human genes, so patterns show up more clearly. When natives of Indonesia develop ulcers, most are suffering from just one single strain of H. pylori – and it’s different from the strain that gives ulcers to mainland Asians, or Australians, or the Maori of New Zealand, all of whom have their own unique strain.
To retrace the steps of early colonizers, researchers looked at how these strains were related to each other, and then connected the most similar ones with lines on a map. Those related strains marked where a people had arrived, paused a while as if on a stepping stone, and then moved on, carrying a slightly altered H. pylori with them. Australian H. pylori is different from New Guinea H. pylori – the two have been separate for some 25,000 years. But those two strains are far more similar to each other than they are to Maori H. pylori of New Zealand. And that, say the researchers, is because the Maori are descended from seafaring Taiwanese tribes who hopscotched to New Zealand from the Philippines just 5,000 years ago, carrying a brand of H. pylori far more closely related to the East Asian variety.
Who knew that the road to civilization could be signposted with stomach bacteria? But don’t let the thought of all this moving around stress you out. Our species has survived countless abrupt moves already. And now you know the stress won’t give you ulcers, either.
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