It refers to this 2015 paper "The genome sequence of Saccharomyces eubayanus and the domestication of larger-brewing yeasts" as appeared last August in the Journal of Molecular Biology and Evolution.
There's two basic styles of beer: Ales and Lagers. Ale ferments around 65 (18 C) degrees and reaches final gravity usually in about two weeks. Saccharomyces cerevisiae is the original Near Eastern cereal yeast for bread and beer.
Lagers are 'lagered' at a temperature close to 40 (4 C) degrees and since the yeast work slower lagering, finishing can take some time. This yeast is Saccharomyces pastorianus, for a few years now recognized as a hybrid. It produces a crisper, cleaner beer. This accounts for about 95%~ of the world's crappy, mass-produced beers.
According to Baker et al's molecular clock estimate, the hybridization between Saccharomyces cerevisiae and Saccharomyces eubayanus creating Saccharomyces pastorianus (lager yeast) occurred about 500 years ago.
This timeframe might sound rather convenient given European contact with the Americas, but eubayanus has been found in Tibet, Mongolia and China within the last several years as well. Lagering is also described as early as the 1300's so contact with the Americas would be too late to explain the birth of lager yeast. Admittedly though, we're dealing with two secondary evidences 1) monks and 2) molecular clocks.
|Hopefully Making Beer|
Because hybridization requires the presence of both components at the same place from the beginning, we can go ahead a cross Europe off the list of likely points of origin. The Baker paper doesn't give a whole lot of wiggle room either, even if their estimate is too young. So when and where did lager yeast develop?
The obvious location would be 1) an environment where S. eubayanus was native 2) that was also being settled by folks who cultivated cereals with naturally occurring s. cerevisiae.
Kind of sounds like the North Pontic-Caspian region, maybe Northwestern China. The next question is age, not that yeast floating in the air require an archaeological context. Whenever this question is sufficiently resolved, it may tells us a little more about prehistoric brewing.
It seems like the Corded Ware people, as one example, had beer year round to place in burials. That means they either brewed year round and were able to control the primary fermentation temperature or were able to store large amounts of beer in barrels. Or again...they were able to ferment in the lager range.
See also - German Beer Institute