Bacteria clue to migration of humans

 

RESEARCHERS have a new ally in tracing the migration routes followed by early humans – a common stomach bacterium. It helps explain how groups were related as our ancient ancestors spread steadily across the globe.

Discovering the routes taken by early humans, who left Africa tens of thousands of years ago, is a difficult process. Now the bacterium Helicobacter pylori is helping to confirm theories and garner new insights, according to Mark Achtman, professor of microbiology at University College Cork, formerly of the Max Planck Institute for Infection Biology in Berlin. Achtman is also a Science Foundation Ireland principal investigator.The latest discovery, published only last week, explains how Australia and many of the Pacific islands were settled by migrating populations.

“These findings confirm and extend thinking based on a combination of archaeology, linguistics and human genetics, and describe two independent major migrations into the Pacific,” Achtman says.

More than half of the world’s inhabitants carry the H pylori bacterium. It is an unwelcome guest given that it can help trigger stomach ulcers and stomach cancer, but it is now also an invaluable source of information on the movement of humans across the globe.

“The original discovery was by a Dutch group who found some [genetic] differences between H pylori samples taken from China and Holland,” Achtman says. That was in 1997.

Achtman became interested in the findings and wanted to know more. He began studying different populations of H pylori and was the first to demonstrate that human migration could be tracked closely by looking at the form of the bacterium living in the modern inhabitants of a region.

“We looked at that in some detail in a publication in 1998,” he says. “It was so clear. There was an Asian group of bacteria, a Euro/American group of bacteria and, using a single African sample, there seemed to be an African group of bacteria.”

That finding was in itself highly unexpected. H pylori populations didn’t merge to produce a common form; they remained geographically distinct, travelling along with their human hosts as they migrated away from earlier home territories. “Most bacteria don’t do that,” Achtman says.

There were five known populations of H pylori until Achtman and his group, including researchers in Cork and Berlin, announced a sixth H pylori population. Details of his discovery were published last Friday in the prestigious journal, Science.

Migrating humans reached Australia, Tasmania and New Guinea as far back as 31,000 to 37,000 years ago, travelling there from Asia. They carried with them an H pylori population called hpSahul. Analysis of more samples from the region indicated, however, that there was a much more recent migration event which seems to have originated in Taiwan and then moved on to Melanesia and Polynesia. This was based on the discovery of a sub-population of the bacterium hpEastAsia known as hspMaori. These two forms remained geographically distinct from one another.

“It is astonishing that hpSahul and hspMaori do not exist side by side in any of the samples,” Achtman says. There are several reasons why this might be so; for example, the migrating newcomers might have had little contact with the established locals.

It could be that one of the bacterial forms can suppress the other, but there is also the possibility that the two H pylori forms do share a geographical area that has not yet been identified, he added.

For this reason Achtman is attempting to organise funding for a follow-on study, looking for H pylori samples recovered from other Pacific islands. “What do we find if we look at new locations in the Pacific?” he asks.

The question takes on added interest given that archaeologists believe they have located evidence of human habitation that looks much older than the known Polynesian migration. Further study might help identify yet another wave of migration into the region.

Achtman’s approach has proved useful in revealing migration patterns from the distant past. Work published two years ago showed details of migration out of Africa dating back 60,000 years. It is also prompting the study of other bacterial forms that might help study migration.

“TB also has bio-geographical differences, but no one is yet trying to use this to study human migration,” he says.

Some groups are beginning to look at bacteria found in the pigs and rats that migrated along with the human populations. Even head and body lice may provide evidence in these studies.

H pylori so far retains pride of place, however. “As far as I can tell, it is the best of them,” Achtman says.