Next time you're enjoying a cool pint of lager at your local pub or restaurant, take a moment to think about yeast, and how it gave us the lighter, smoother beer so many of us prefer.

The earliest yeasts known to man have been used for millennia to make ales, bread and wine, but lager beer—and the yeast used to create it—dates to just 500 years ago.

In Bavaria in the 15th century, brewers there began to notice that beers stored in caves over the winter were continuing to ferment.

The result was lighter and smoother beer dubbed lagers (after largern, the German word for "to store") that were immensely popular in the 19th and 20th centuries and became the dominant preference in the U.S.

Now, researchers at the University of Wisconsin-Madison say DNA sequencing has allowed them to identify the genetic signatures of lager yeasts and have discovered that modern domesticated lager yeast is a hybrid of two different yeast species: Saccharromyces cerevisiae, the original ale-making variety, and Saccharromyces eubayanus.

They were also able to determine that the two major yeast lineages used to make lager beer, Frohberg and Saaz, originated separately from each other rather than sharing a single precursor, as some researchers had suggested.

Genetic studies suggest at least two distinct hybridization events between nearly identical strains of S. eubayanus, with relatively more diverse ale strains of S. cerevisiae, says University of Wisconsin-Madison evolutionary geneticist Chris Todd Hittinger.

"Lager yeasts did not just originate once," he says. "This unlikely marriage between two species, genetically as different from one another as humans and birds, happened at least twice."

Likely different at the beginning, they were probably changed in predictable ways as brewers slowly fixed and domesticated the yeasts, he says.

That doesn't mean all modern lager beer yeasts are the same, he adds.

In the strains of yeast utilized in industrial-scale brewing today, "there's a lot of diversity that's been left on the table," he notes. "It raises the question: In the entire population, are there additional variants that might be useful? Is it an accident of history what gets hybridized?"

Researchers looking for modern examples of the original S. eubayanus have found plenty of it in the Southern Hemisphere but only a few samples in the Northern Hemisphere, including strains in Wisconsin and in China.

It still hasn't ever been found in Europe, and scientists can't really explain how it might have made its way there and ended up as a component of lager yeast.

"The biogeography is still very much a mystery," Hittinger says.

The study has been published in the journal Molecular Biology and Evolution.