Bugs hold clues to human origins

Written byTia Ghose

| 3 min read

An unlikely source has provided the answer to a long-standing question over how the geographically isolated Pacific Islands became populated: bacteria. By analyzing both genetic variations in human gut bacteria and linguistic evidence, scientists found that people migrated to the Pacific Islands approximately 5,000 years ago from Taiwan, two papers in this week's Science report. "This is the first paper where bacteria were specifically used for human migration patterns," says Mark Achtman, a population geneticist at the University College Cork, in Ireland. Combined, the studies support the "pulse-pause" theory of Pacific migration, in which agricultural peoples from Taiwan rapidly spread southeast, settling at different locations and growing in size. With new technological and cultural innovations, some of these settlers pushed further, through modern-day Philippines, Borneo and New Guinea, and, the study shows, eventually reaching the farthest Polynesian islands. A competing theory proposed that people gradually spread to the distant Pacific Islands from the Wallace Islands, between Borneo and New Guinea, in response to flooding after the last ice age.

Click to enlarge. Map of migration patterns
based on bacterial genetic analysis

To uncover the origins of the Pacific people, Achtman and his colleagues measured genetic variation in Helicobacter pylori, a common gut microbe that traveled with humans when they first left Africa more than 60,000 years ago. linkurl:Previous work;https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15051885 used genetic analysis of H. pylori to distinguish between Buddhist and Muslim populations in India, but this is the first to answer such a broad question about human origins, he told The Scientist. The group compared related sets of single nucleotide polymorphisms, called haplotypes, for 212 different bacterial populations collected from stomach biopsies of Taiwanese and Australian aboriginals, highlanders from New Guinea, and Melanesians and Polynesians from New Caledonia. They compared these with 99 existing samples from Europeans and 222 from earlier studies in Asia and the Pacific. They found that the haplotypes from people's guts in Polynesia and Melanesia--islands stretching from New Caledonia all the way to Samoa--were genetically similar to the samples from aboriginal people in Taiwan. What's more, the Taiwanese bacteria had more genetic diversity than other populations. Because genetic mutations accumulate over time, and because migrating populations can be assumed to be a smaller subset of the population in the place from which they originate, the researchers concluded that people from the south Pacific originally came from Taiwan. "The unifying concept we are providing is one continuum from Taiwan out to Polynesia," Achtman said. "I am not too surprised by the information present in the bacteria, as they contain considerably more sequence diversity than in typical human data," said Garrett Hellenthal, a statistician at Oxford University. That is key to detecting migration patterns, he said. In a separate study, linkurl:Russell Gray;https://www.psych.auckland.ac.nz/people/Gray/Russell.htm of the University of Auckland in New Zealand and his colleagues conducted a phylogenetic analysis of 400 languages from Southeast Asia, Micronesia, Melanesia, and Polynesia, and found similar results. Like older techniques, their method reconstructed a family tree of languages, but the group used "the sorts of methods that biochemists and molecular geneticists have used" to build genetic phylogenies, said Colin Renfrew an archaeologist at the University of Cambridge who reviewed both papers for Science. Renfrew has collaborated with linguists and geneticists to untangle the roots of Indo-European languages. The group used a computer algorithm to compare more than 34,440 cognate pairs, or words with a likely common root, across all the languages to generate "a forest" of thousands of likely family trees, Gray said. By using archaeological evidence such as dates from ancient texts, the team was able to estimate rates of language change, and use these to extrapolate more precise dates for earlier branches in the tree, he said. Image courtesy AAAS/Science
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