When Europeans began transporting goods and people across the Atlantic in the 15th century, there was a microscopic stowaway - a yeast - that found its way to the caves and monasteries of Bavaria. Scientists believe that yeast may have been transported from a distant shore on either a piece of wood or in the stomach of a fruit fly.

Brewmeisters back then used to store their products in caves and monastery cellars and the new yeast fused with its domestic, yet distant relative, which was used for millennia to make leavened bread and ferment wine and ale.

As a result of that merger, the lager was formed - a clear, cold-fermented beer first brewed by 15th century Bavarians and what is today among the most popular alcoholic beverage in the world, scientists say.

Both scientists and brewers knew for long time that the yeast that so happened to give beer the ability to ferment at cold temperatures was a hybrid. The knew it was the Saccharomyces cerevisiae, the yeast used to make leavened bread and ferment wine and ale. But the yeast remained a mystery  to scientists who were unable to find it among the 1,000 or so species of yeast known to science, according to a press release.

But an international team of researchers believes it has identified the wild yeast that has traveled more than 7,000 miles to make a near perfect microbial match that today underpins the $250 billion a year lager beer industry. The researchers came from Portugal, Argentina and the United States.

It proved to be distinct from every known wild species of yeast, but was 99.5 percent identical to the non-ale yeast portion of the lager genome, says Chris Todd Hittinger, a University of Wisconsin-Madison genetics professor and a co-author of the study.

The study is published in the Aug. 22 Proceedings of the National Academy of Sciences. It describes the discovery of a wild yeast in the beech forests of Patagonia, the alpine region at the tip of South America, and solves the age-old mystery of the origin of the yeast that made cold-temperature fermentation and lager beer possible.

People have been hunting for this thing for decades, Hittinger says in a news release. And now we've found it. It is clearly the missing species. The only thing we can't say is if it also exists elsewhere (in the wild) and hasn't been found.

The team's search was an exhaustive global search led by the New University of Lisbon's José Paulo Sampaio and Paula Gonçalves. The Portuguese team went through European yeast collections, combed the scientific literature and gathered new yeasts from European environments. Their efforts yielded no candidate species of European origin, the release said.

The search then expanded to other parts of the world. Finally, there was some hope when collaborator Diego Libkind of the Institute for Biodiversity and Environment Research (CONICET) in Bariloche, Argentina, found in galls that infect beech trees, a candidate species whose genetic material closely match the missing half of the lager yeast.

Beech galls are very rich in simple sugars, Hittinger notes. It's a sugar rich habitat that yeast seem to love.

Libkind says the yeast is so active in the galls that they spontaneously ferment.

When overmature, they fall all together to the (forest) floor where they often form a thick carpet that has an intense ethanol odor, most probably due to the hard work of our new Saccharomyces eubayanus, he says.

The new yeast was then sent off to the University of Colorado School of Medicine, where a team including Hittinger, Jim Dover and Mark Johnston sequenced its genome.

It proved to be distinct from every known wild species of yeast, but was 99.5 percent identical to the non-ale yeast portion of the lager genome, says Hittinger, who is now an assistant professor of genetics at UW-Madison.

The Colorado team also identified genetic mutations in the lager yeast hybrid distinctive from the genome of the wild lager yeast. Those changes, which takes place in a brewing environment where evolution can be amped up by the abundance of yeast, accumulated since those first immigrant yeasts formed a hybrid with their ale cousins 500 years ago.

They've refined the lager yeast's ability to metabolize sugar and malt and to produce sulfites, transforming an organism that evolved on beech trees into a lean, mean beer-making machine, researchers say.

Our discovery suggests that hybridization instantaneously formed an imperfect 'proto-lager' yeast that was more cold-tolerant than ale yeast and ideal for the cool Bavarian lagering process, Hittingersays . After adding some new variation for brewers to exploit, its sugar metabolism probably became more like ale yeast and better at producing beer.