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The elusive ancestor of the yeast species used in modern lager beer has been found in Europe for the first time. The discovery of the species living in Irish soil suggests the yeast was present in Europe the switch from ale to lager-style brews that occurred at the end of the Middle Ages.

The first beers made in Europe were ales and stouts, rather than today’s more popular lager-style brews. Ales rely on fermentation from a yeast called Saccharomyces cerevisiae – often called brewer’s yeast – which is still used in modern ales, stouts and bread. But when European beer makers were required to shift from brewing in warmer months to cooler months to limit bacteria growth, the yeast species in their brews incidentally changed to those that could withstand the cold.

“We know that there was a shift in the yeast species that was carrying out the fermentation,” says at University College Dublin in Ireland. “Instead of Saccharomyces cerevisiae, it was a new organism that we call Saccharomyces pastorianus,” the same yeast used in lagers today.

Genetic sequencing in the 1980s led researchers to discover that this lager-producing yeast has two ancestral species: S. cerevisiae and Saccharomyces eubayanus. The latter was first detected in the Patagonian Andes in 2011 and since then has been found in North America, China and New Zealand. But the species had never been found in Europe until Butler and her students sampled soils in the wooded area of their university campus.

When the researchers sequenced the genomes of yeasts in their soil samples, they were surprised to find that two samples taken 17 metres apart contained strains of the lager yeast parent they had been searching for, S. eubayanus. Butler says one reason they were able to find the yeast could be because others had focused their search in warmer climates or near historic breweries, unlike her team.

“It really did feel like Europe was somewhat of a missing link where it seems like [S. eubayanus] should be there,” says at Stanford University in California, who was not involved in the work. “So, this paper is pretty exciting.”

One reason S. eubayanus may have taken over is because of the species’ ability to thrive at lagers’ low brewing temperature of around 10°C (50°F). That same trait may have helped the yeast survive the chilly Irish climate.

Next, Butler and her team hope their discovery will lead to a new brew with the ancient yeast. “We would like to try and make a beer,” she says. “We’re actually looking to see if we can get a commercial partner interested.”