Eight thousands years ago, people brought snails from Spain to Ireland, suggests a new study, which used DNA analysis to identify a snail species that lives today only in Ireland and the Pyrenees mountains between France and Spain.
In both places, the snails have large, white-lipped shells. And, according to the new work, the two groups also share genetic markers that are extremely rare elsewhere in Europe.
Along with other evidence, the findings offer a new window into ancient human migrations.
"It's interesting to use snail genetics to find out how snails colonize, and it also maybe gives us a little insight into what humans were doing, too," said Angus Davison, an evolutionary geneticist at the University of Nottingham in the United Kingdom.
"One really neat thing about this study is that, if we accept that humans transported snails, it really gives us a unique insight into an individual journey 8,000 years ago, and it gives us evidence of that from a source you might not imagine."
For more than 150 years, biologists have been puzzling over an Irish mystery: A number of wildlife species that live in Ireland are absent from the rest of Britain but are found in Iberia, the peninsula that includes modern-day Spain, Portugal and parts of France.
Research into this so-called "Irish question" has failed to produce a single theory that explains how and when various species covered hundreds of miles from one place to the other.
To see if they could add any new understanding to the Irish question, Davison and colleague Adele Grindon focused on a distinctive-looking snail that had the same one-inch long shells in both locations. According to fossil evidence, the snails first showed up in Ireland about 8,000 years ago. The mollusks had lived in southern Europe for tens of thousands of years before that.
First, the researchers enlisted volunteers to help collect nearly 900 snails from both parts of its range. Then, they extracted mitochondrial DNA, which is passed directly from mother to offspring, and they looked at specific areas of the genome that are known to vary from snail species to snail species.
Tracking snails helps reveal where humans traveled.Tom Chance/Corbis
The genetic material they analyzed was essentially identical between the two regional groups, the researchers report today in the journal PLOS ONE. The team was also surprised to find that they could trace the Irish population of snails directly back to a population in a specific region of the Pyrenees. The species lives nowhere in between.
The findings argue against a gradual move from one place to another, Davison said, and instead suggest that the snails migrated from Spain to Ireland in one step. He thinks it unlikely that birds transported the mollusks, partly because there are no known birds that migrate along that route that would have been large enough to carry the snails.
More likely, Davison suspects, people brought the snails with them as they moved. Ancient people may have intentionally brought the snails as a source of food on the trip. Alternatively, the snails may have hitched a ride in the grassy fodder packed for other animals.
In the early 2000s, some studies proposed a connection between Ireland and the Pyrenees, but those studies were later shown to be too small and flawed to be convincing, said Allan McDevitt, a geneticist at University College Dublin, who specializes on questions about the colonization of Ireland. The new research is far more robust.
"I think this study is important in that it does conclusively show that there is some link with Spain based on this snail," McDevitt said. "It's the most definitive proof yet that this is actually a very real migration that was happening."
Still unclear is how people made the trip. At least some of the journey would have been by sea, as Ireland was separated from the mainland by 15,000 years ago. But evidence of primitive canoe-like boats remains scant. Using new genetic tools to investigate unlikely creatures may be key to uncovering a better understanding of the past.
"Ireland has always been a very controversial topic, both in how people reached it there and how animals reached it there," McDevitt said. "We're finding a lot of things by looking at small animals. They actually do tell us a lot about how humans were moving."