Every other beer on the planet is a lager. And until 2011, nobody knew where the organism that makes it had come from.
A brewer works with yeast much as a shepherd works with dogs: he does not design them, but selects them generation after generation, trusting those that get the job done. Bavarian brewers of the late Middle Ages knew that winter beer, aged in cold Alpine cellars, came out cleaner and more stable than summer beer. They did not suspect that their “winter” yeast was a creature with a double genome, a chimera of two species that had no business being together in nature. One parent was Saccharomyces cerevisiae, ordinary baker’s yeast, a companion of human civilization for thousands of years, which prefers to ferment at 15–25 °C. The other was a cold-loving organism capable of working at 4–9 °C, a range where cerevisiae goes dormant. The hybrid of these two species was named Saccharomyces pastorianus—after Louis Pasteur, though Pasteur himself had no idea that lager yeast was a hybrid.
The problem was that nobody could find the second parent. The cerevisiae genome had been sequenced, and its contribution to the hybrid was obvious. But the other half of the genetic mosaic matched no known wild Saccharomyces species. For decades, microbiologists sifted through candidates: S. bayanus, S. uvarum—close, but not it, like a composite sketch in which you can make out the family resemblance but not the particular face. Lager’s cold-tolerant ancestor remained a ghost in the genome.
In the forests of Patagonia, along the border between Argentina and Chile, southern beeches grow—trees of the genus Nothofagus, relics of the ancient supercontinent Gondwana. Galls sometimes appear on their trunks and branches: spherical growths caused by fungal infections. Inside these growths, in the sugary sap, their own microbial life bubbles away. It was there that mycologist Diego Libkind of Argentina’s National Scientific and Technical Research Council (CONICET) and his colleague Chris Todd Hittinger of the University of Wisconsin–Madison found what beer geneticists around the world had been looking for.
In August 2011, the journal Proceedings of the National Academy of Sciences published a paper describing a new species—Saccharomyces eubayanus. The name literally means “true bayanus,” and there is a quiet scientific irony in it: years of searching among related species, and at last—the real thing. The genome of S. eubayanus was 99.5 % identical to that unidentified “cold” half of S. pastorianus. The composite sketch had found its face. The geography of this discovery still amazes me: the yeast that made Bavarian beer possible was found at the other end of the Earth, in a forest where nobody had ever brewed beer.
It is tempting to imagine a barrel of Patagonian yeast crossing the Atlantic aboard some Spanish galleon and ending up in a Bavarian cellar. A lovely story, but science does not work that way. Libkind and Hittinger’s discovery established a wild relative—the genetic source of the cold-tolerant part of the hybrid—but the route by which S. eubayanus (or its closest Eurasian descendant) ended up in the same vat as cerevisiae remains an open question. Strains related to eubayanus were later found on the Tibetan Plateau and in other regions, so Patagonia is most likely not the species’ only reservoir, but the place where it was first noticed and described.
What we know about the hybridization itself is more historical reconstruction than exact chronicle. According to research published in FEMS Yeast Research, bottom (cold) fermentation had been practiced in northern Bavaria for at least 200 years before the appearance of the strain we now call S. pastorianus. Brewers worked with mixed cultures—the so-called “stellhefen,” literally “sediment yeasts,” which settled at the bottom of the fermentation vat. These cultures were microbial zoos: different Saccharomyces species coexisted, competed, exchanged genes. Hybridization in such conditions was a matter of time, not genius.
One of the most concrete episodes that can be tied to the birth of the modern strain is connected to Bavarian dynastic politics. On October 24, 1602, after the Degenberg line died out, Duke Maximilian I confiscated their property, including yeast from a wheat brewery in Schwarzach. This top-fermenting (ale) yeast—essentially a cerevisiae strain—was moved to the Munich court brewery, Hofbräuhaus. There, sometime between 1602 and 1615, it probably encountered a cold-loving partner already present in Bavarian cellar cultures. The result was a hybrid that inherited from cerevisiae the ability to ferment maltose efficiently, and from eubayanus its tolerance for cold.
The hybrid did not remain a single organism. Over time, lager yeast split into two main lineages, which geneticists call Saaz (or Group I) and Frohberg (or Group II). The difference between them is not cosmetic. Saaz strains are dominated by the eubayanus genome: they have lost a substantial portion of their cerevisiae chromosomes and do better at very low temperatures. Frohberg, by contrast, retained more genetic material from both parents and ferment more vigorously, producing more ethanol. Roughly speaking, Saaz is a minimalist; Frohberg is a workhorse.
These lineages are not the result of two independent hybridizations, though that hypothesis has been discussed. Most likely, they diverged after the cross, through centuries of selection in different breweries. The brewer who took sediment from the best batch and used it to inoculate the next one did not know he was practicing directed evolution. But he was—with the same inevitability as the farmer who, by selecting the largest grains, changes the face of wheat over a few generations.
The spread of these strains across Europe can also be traced. In 1806, Gabriel Sedlmayr the Elder went to work at that same Hofbräuhaus and later acquired the Spaten brewery, through which Bavarian yeast mixtures spread to German breweries. In 1845, Danish brewer J. C. Jacobsen brought Munich stellhefen to Copenhagen, to the Carlsberg brewery. And in 1883, the microbiologist Emil Christian Hansen, who worked there, did what brewers before him had not known how to do: he isolated a pure culture of S. pastorianus from that mash of mixed strains. At the same time, Paul Lindner at the Berlin Institute obtained pure cultures from those same Munich lineages. From that point on, lager ceased to be a lottery and became an industrial technology.
When I think about this whole story, what surprises me is not hybridization itself—crossing between species happens constantly in the microbial world. It is the scale of the consequences. A chance encounter between two single-celled organisms in a Bavarian cellar in the 17th century led to the creation of a beverage category that today accounts for more than half the global beer market. No one planned it. Duke Maximilian confiscated the yeast because he wanted a monopoly on wheat beer. Sedlmayr wanted to brew beer better than his competitors. Jacobsen wanted Danish lager. Hansen wanted to get rid of spoilage. Each was solving a local problem, and none of them suspected they were working with an interspecies hybrid.
The discovery of 2011 did not close every question—it essentially reformulated them. Where exactly in Eurasia did S. eubayanus (or its ancestral population) first enter the brewing environment? Was it carried in along trade routes, or had its European population existed unnoticed all that time? Why is it so difficult to find in Europe if the hybridization happened there in the first place? These questions are still waiting for answers. But the structure of the mystery itself has changed: instead of total fog, there is now a framework on which new data can be hung.
The story of lager is one of unintended selection stretched across centuries, and global biogeography compressed into a single genome. A southern beech in a Patagonian forest, a ducal brewery in Munich, the Carlsberg laboratory in Copenhagen, a sequencer in Wisconsin—points on a map connected by a strand of DNA that nobody laid down on purpose. And somewhere in Europe, perhaps, a wild population of eubayanus still survives, one that simply has not yet been found.