Genetic similarities between banded wood snail fossils found thousands of miles apart in Ireland and the south of France serve as a historic calling card from an ancient human migration that took place nearly 8,000 years ago, according to researchers who believe the snails were carried across the Atlantic Ocean on boats when humans colonized Ireland.
It's unclear whether the snails were stowaways or brought intentionally on to early ships, but it is clear that the same snails are not found in Britain, which suggests humans carried them from the Pyrenean region of France to Ireland.
It is believed that the isle of Britain was formed at the end of the last Ice Age by rising sea levels and a great tsunami, which separated the land from Ireland and the rest of Europe. Britain's isolation also meant that land-dwelling animals could not migrate to the island by their own volition.
"You would think that anything that gets to Ireland would go through Britain, but it has been a longstanding mystery as to why Ireland is so different from Britain. For these snails, at least, the difference may be that they hitched a ride on a passing boat," said Angus Davison of the University of Nottingham.
The revelation that common snails in Ireland are near genetic copies of snails in France provides further evidence that humans brought the snails to Ireland, not stopping in Britain along the way.
"There is a very clear pattern, which is difficult to explain except by involving humans. If the snails naturally colonized Ireland, you would expect to find some of the same genetic type in other areas of Europe, especially Britain," Davison said in a statement. "We just don't find them."
Yet a fossil record shows the snails were present in Ireland for the past 8,000 years, and researchers believe the snails were eaten by people living in the Pyrenees.
"There are records of Mesolithic or Stone Age humans eating snails in the Pyrenees, and perhaps even farming them," Davison said.
"The highways of the past were rivers and the ocean - as the river that flanks the Pyrenees was an ancient trade route to the Atlantic, what we're actually seeing might be the long lasting legacy of snails that hitched a ride, accidentally or perhaps as food, as humans travelled from the South of France to Ireland 8,000 years ago."
Davison added that the origins of the snails in Ireland are consistent with what is known about human colonization of Ireland -- people came from somewhere in Southern Europe, just like the snails.
Dan Bradley, a population geneticist from Trinity College Dublin told BBC News that the results of Davison's study falls in line with other migration theories.
"It's consistent with the idea that almost everything we have in Ireland, that can't swim or fly, was brought here on a boat," he said.
Davison and his colleagues' research is published in the journal PLOS ONE.