Where lager yeast came from
In summer the Bavarian brewers had trouble. Warm weather encouraged the wrong microbes, and a spoilt batch was a loss. A Bavarian ordinance of 1553 banned brewing between St George's Day in April and Michaelmas at the end of September. Beer for the summer was brewed in spring, stored in cold cellars and cooled with ice cut from lakes and rivers. The Bavarian word for storing is lagern, and it gave lager its name.
Over generations the yeast that survived in those cold cellars changed. It worked slowly at low temperatures, settled at the bottom of the vessel and made a cleaner and crisper beer than ordinary ale yeast. It is now called Saccharomyces pastorianus.
A hybrid with a missing parent
Geneticists found that lager yeast has two sets of genes, one from the ale yeast S. cerevisiae and one from a species that nobody had identified. In 2011 a research team found the second parent in beech forests in Patagonia, in Argentina. It was named S. eubayanus. How it travelled to Bavaria is still debated. The hybridisation probably took place in a brewery, perhaps in the fifteenth century.
The technology caught up in the 1870s, when Carl von Linde developed refrigeration machines with the Munich brewer Gabriel Sedlmayr of Spaten. Brewers no longer depended on the winter, and lager spread across the world in a few decades.