The genetic similarity between one variety of common garden snails found in Ireland and the Pyrenees, in Southern Europe, may be explained by human migrations over 8,000 years ago, says a new study published in PLoS ONE. 

This is not the only animal to hitchhike on humans on their way to Ireland, as "other species may show a similar pattern, including the strawberry tree, the Kerry slug, and the Pyrenean glass snail", explains Dr Angus Davison, an evolutionary geneticist from Nottingham University, UK, and leading author in the study.

This migratory phenomenon may in fact be so widespread it is sometimes referred to as a Lusitanian shared history, "describing a collection of plants and animals in Ireland that seem to have an origin in Lusitania, the old name for the Roman Empire in Iberia" continues Dr Davison.

In the new study, Dr Davison's team analysed DNA from 880 animals from over 100 distinct populations of snails and identified seven different varieties across Europe. Of those, one variety of the banded wood snail (Cepaea nemoralis), was found almost exclusively in Ireland and Southern France. Despite the massive distance between these areas, these snails shared many genes that were not seen in other parts of Europe.

The best explanation for this distribution involves transport by humans during their migrations in the Mesolithic era, more than 8,000 years ago. "Alongside the circumstantial evidence of harvesting and eating snails, human transport is probably the most likely explanation, because it is the simplest", said Dr Davison. The Garonne River, located along the Pyrenees, provided the means to connect the Mediterranean to the Atlantic, and the presence of these snails in Ireland could be a long lasting consequence of this ancient transport route.

"This seems quite plausible" agrees Dr Valerio Ketmaier, a population geneticist from Rome University. In fact, " this is not the first study to document human-mediated transport of land snails, even across vast distances," he adds. There are, for instance, examples in the Alpine region that can be traced back to postglacial re-colonization of Northern Europe or Neolithic migrations from the Mediterranean.

Alternative scenarios seem unlikely. In theory, snails could have been transported to Ireland on the wings of a bird, but it would either have to involve several individuals landing in the same place, or a large mature adult containing fertilised eggs. Gradual colonisation and travel across the sea attached to a log are hypothesis that also don't fit well with the results.

When asked about future plans to develop this work, Dr Davison says he plans to focus on overcoming the limitations in the current study. "We used only a single marker - mitochondrial DNA, but in the future, we would like to use further genetic markers to gain a more detailed picture of the colonisation" he says.