Aug. 22, 2011  -- This is the story of a stowaway -- a microscopic spore, about 500 years ago, that somehow hitched a ride from South America and ended up 7,000 miles away in Bavaria.

That would be the end of it, except that the spore was a species of yeast that happened to like cold weather. And Bavaria, with its cool climate, is the home of lager beer, perhaps the most popular alcoholic beverage in the world today. It would not have been possible without the mysterious arrival of that yeast, which found its way into long-ago breweries.

"Our collaborators sampled strains on five continents, and this was the one clear match," said Chris Todd Hittinger, a genetics professor, now at the University of Wisconsin, who was called in by an international team to sequence the genome -- the DNA -- of the newfound yeast, which they called Saccharomyces eubayanus. "They're extremely prevalent in the beech forests of Patagonia, but we haven't found them elsewhere."

How did they get from South America to Europe? We'll never know, say the researchers, but it may have been carried in a piece of wood, or the gut of a fruit fly, that was inadvertently carried across the Atlantic on an early European sailing ship.

"The possible man-aided migration from South America to Europe [if it really happened] is indeed uncommon, but not unique," said Jose Paula Sampaio, a leader of the study, from the New University of Lisbon in Portugal.

One spore would have been enough. It would have multiplied and combined with other yeasts native to Europe. German emigrants, settling in Milwaukee or St. Louis in the 1800s, would have spread it further. If you've ever had a Miller High Life, or a Heineken, or a Coors Light, those are all lager beers -- part of a $250 billion-a-year industry.

'The Champagne of Bottled Beers'

If not for that long-ago biological accident, people would, of course, still be drinking. Fermentation is part of nature, and other forms of beer date back to the beginning of civilization. But what would people choose? Ale? Merlot? Green tea?

Patrick McGovern, the scientific director of the Biomolecular Archaeology Laboratory at the University of Pennsylvania Museum, has studied and written about the history of drink, and said he found the study very interesting, but was doubtful about the idea of spores making the long trip from South America to northern Europe.

"Perhaps some Patagonian beech was used to make a wine barrel that was then transported to Bavaria, and subsequently inoculated a batch of beer there?" he asked in an email. "Seems unlikely." He speculated there may have been a related species, now gone, lurking in the oak forests of Europe.

Hittinger said he'd anticipated such questions: "Let me hasten to say that we have not ruled out every other possibility," he said. "But we've looked a lot and haven't found any other good matches."

McGovern said he hoped the study would provoke more research. "Despite my reservations, history and archaeology are full of surprises," he said. "Nowhere is this more true than of the seemingly miraculous process of fermentation and the key role of alcohol in human culture and life itself on this planet."

The researchers say their study, published in this week's edition of Proceedings of the National Academy of Sciences, is not some light-hearted paean to the world's barflies. This began as a study of biological diversity.

"All of us are interested in Saccharomyces biodiversity to help answer basic research questions in ecology, metabolism, biodiversity, genetics and biomedical science," said Hittinger in an email to ABC News. "Describing the origin of lager yeast is a happy (and tasty) bonus."