I thought I was done with this miniseries on alcohol, yeast and civilization, but several people have written me asking about one particular mystery of great relevance to their lives, lager beer. And so here you have it, the seventh (see the first, second, third, fourth, fifth and sixth here) in my miniseries on civilization, fungus, and alcohol.

If you have had a beer lately, especially if it was in a can and cost less than two dollars, it was probably a lager. Lager has become almost synonymous with “beer,” yet the story of lager is among the most enigmatic of all the stories of the foods and drinks we ferment. It is just one of the mysteries I have been thinking about lately, the other involves a monkey.

The first mystery—As we moved around the world from where yeast was first relied upon for the production of alcohol, we took particular strains of yeast with us, accidentally, on our fruits, grains and vessels. We did not know we were carrying them and yet we depended on them for our golden drinks and rising foods. These strains evolved so as to be able to better take advantage of the conditions and foods we provided in different regions. In the light of evolution, many of the specific histories of many drinks make sense1. Then there is lager, beer.

The very first beer would have been distinct from modern beers, but relied on yeasts similar to those found in ales and wines, not lagers. Eventually, Germanic and Celtic tribes took it north and west in the first century AD. Some tribes pillaged gold. Leave it to the Germans and Celts to pillage beer and, although they did not know it, yeast. Historically, most beer was made when conditions were relatively warm. The yeast found in most foods and drinks, Saccharomyces cerevisae, likes to live at temperatures between 15° and 24° C2. But while warm brewing conditions are good for beer-making yeasts, they are also good for other less beneficial species of yeasts and bacteria. Many vats of beer were lost to bad yeasts, bacteria, and chance—acts of microbes.

Beginning in the 1400s, Bavarians appear to have taken some of the chance out of the equation through a bit of ecological subterfuge. They carried their fermenting beer to caves in the Bavarian mountains, where beer could be brewed at cooler temperatures and then aged for several weeks. The same caves that once gave shelter to Neanderthals came to protect the yeasts dividing on the bottom of vessels, slowly, in the cold, yeasts producing lagers.

With time, brewing what we now call lagers in cold conditions became the definition of good Bavarian brewing. In 1516 the Bavarian government, troubled by the preponderance of bad beer, instituted the Beer Purity Law, which specified that beer could not be brewed in the summer (and must be brewed only using barley, hops and water). They all but enforced the production of beer under cool conditions.

Storing beer under cool conditions during fermentation is part of what makes a lager a lager rather than an ale, but why do cooler conditions produce a different beer? The magic is in the yeast. When yeast biologists began to consider the story of lagers, they hypothesized that fermentation in caves and other cold realms had favored varieties of S. cerevisae, humanity’s yeast, with greater tolerance to the cold (which in turn produced different flavors, etc…)4.

The yeasts used in lager beers are indeed more tolerant of cold than those involved in making nearly all other yeast-dependent foods and drinks. But lager yeast is not just a strain of S. cerevisae, it is an entirely different species. Breads, sake, wine, warm weather beer, even palm, they all rely on one species, Saccharomyces cerevisae. Not lager.

A new species is discovered in (and making) lager beer—Early genetic studies revealed lager yeast (now called Saccharomyces pastorianus) to be a hybrid between “humanity’s yeast,” Saccharomyces cerevisae, and another species. Lager yeast has two versions of most genes, one like those found in S. cerevisae and one from another species. But what is the other species?

One would be forgiven for imagining such a yeast species would come from Bavaria. The new kink in the narrative is the revelation that the yeast that came together in love and peace with humanity’s yeast to hybridize and produce lager yeast is not from Bavaria or even from Europe, or even for that matter from the Middle East, Africa or Asia. It comes from Patagonia, where it was, until recently, an unnamed species3, 4.

The yeast in lager beer is a hybrid of humanity’s yeast and a newly named species of yeast from cold forests at the bottom of the Earth, now called Saccharomyces eubayanus. This hybridization happened twice. In the first instance, the hybridization gave rise to the yeast now used in Saaz type beers, now produced in the Czech Republic and in Carlsberg breweries in Denmark. In the other, the hybridization gave rise to the Frohberg-type yeast now usually used in the Netherlands and in the other (non-Carlsberg) Danish breweries. Twice, humanities yeast and S. eubayanus came together in the dark of a vat and lager appeared.

The problem with this new Lager beer story, the first mystery, is timing. Lager beer was apparently first brewed in the 1400s. How do you get a yeast from Patagonia into a brewing vat in Bavaria before European ships had gone to Patagonia and back?

The second mystery—The second mystery that has preoccupied me lately is the case of the marsupial monkey. In the same forests in which one of the two ancestors of lager yeast lives, also lives the monito del monte. Monte in Spanish is the untamed woods. A monito is a little mono, a little monkey. But the monito is not a monkey, it is a possum-like marsupial with opposable thumbs and so, where there were no monkeys, it seemed close enough to be called the little monkey, a monito.

The monito of the woods is a very interesting marsupial. Like other marsupials it has a pouch, but in a variety of ways it seems more like the marsupials of Australia than it does like those of the Americas. Even knowing it is a marsupial and not a monkey, in other words, leaves one with questions. Because it seems quite different than all of the other marsupials in South America, the first is where it came from.

Recently, two separate evolutionary studies have found the monito to be most closely related to Australian marsupials (in one case the wild and crazy marsupial mole). How could the monito, a tree-dwelling mammal the size of a large rat, have made it to Patagonia5?

Occam’s whimsical resolution?--So now we have the two mysteries, how a yeast species got from Patagonia to Bavaria and how marsupial got from Australia to Patagonia. Ironically, these stories are connected, at least superficially, and maybe ecologically. The monito consumes fruit on which yeast is likely to grow, perhaps the same yeast that helped to produce lager beer. Assuming that these fruits are sometimes a little fermented, these monitos may have been the first to sample some hint of the taste of what lager beer might become.

Unfortunately, drunk monitos, however adorable, do not explain anything.

Let’s come back to the question of how the monito got to Patagonia, and how a Patagonian yeast got to Bavaria. For the yeast, one possibility is it was actually floating around many places but was outcompeted each time it landed by other yeasts and only in the moment when brewers in Bavaria started putting out their vats in caves did it find a source of food of which it could easily take advantage.

This is more plausible than it seems. Many single-celled species seem to have the ability to ride around the world in the wind and clouds and, in doing so, to be nearly everywhere at least some of the time, even if they do not find success everywhere.

Alternatively, the Patagonian yeast might have been brought back from Patagonia by early explorers, perhaps even in their old beer containers which would have been common on the ships. The problem is the first explorers to Patagonia arrived hundreds of years after the first lagers appear to have been brewed, though maybe those very first lagers were different than the modern ones.

The most plausible explanation for the monito’s modern distribution appears to include a mix of geology and magic. When Antarctica and Australia were still connected, before they broke along their geologic seams and went very different ways, a lineage of marsupials diversified. Most members of the lineage stayed in Australia, but one or more made their way to South America and diversified a little more, with one species, the monito, surviving until today in Patagonia. It all makes sense, I guess. But how did it make its way to the new world? Via some as yet unknown bridge? Floating?

All of these ideas are reasonable starting points for more study of each of these independent mysteries. But conversations with my friend Pajaro Morales offered another possibility. Pajaro studies the monitos, lives in Patagonia, has an office down the hall from the authors of the Patagonian yeast paper, and is a font of ideas. Talking with him led to a single explanation for both mysteries. Maybe the monitos can fly and, when they do, they carry the yeast with them. I wish we could say we were under the influence of lager when we came up with this idea, but alas we were under the influence of nothing more than Occam’s razor.

William Occam—who was exiled to Bavaria, though before the advent of lager—argued that the solution requiring the fewest assumptions tends to be the right one. In fields where general principles hold strong sway Occam’s argument—often called his razor—is both sharp and reasonable. The elegant explanation often wins in population ecology or physiology.

But biogeography—the field charged with telling the stories of the distributions of creatures like yeast and monitos—is a historical science. Occam’s razor, in confronting the strange histories of the monito and the lager, would unite them. It favors the flying monito hypothesis which assumes monitos can and do secretly fly, but nothing else.

This time then, Occam is almost certainly wrong. The beauty of biogeography, like the beauty of history itself rests not in the general stories, but instead in the specific narratives of individual species as they move around the Earth encountering and resolving (or failing to resolve) problems. So, when, in a few weeks, Pajaro and I meet up for the first time in quite a few years in Scotland to enjoy some of the consequences of yeast’s specific history, we will have to come up with some new ideas 6. Though it seems equally fun to keep our eyes on the sky, on the off chance that monitos covered in yeast really know how to fly.

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1-Sicard, D., Legras, J.-L. (2011). Bread, beer and wine: Yeast domestication in the Saccharomyces sensu stricto complex. C. R. Biologies 334 (2011) 229–236. This is a great article. The evolutionary tree in this article could spawn a dozen other essays. Note, for example, that lab strains of yeast seem to be most closely related to palm wine strains.

2-This is why you put yeast into warm water when making bread. Bread yeasts are the descendents of wine yeasts in most of the world, though just which wine your bread’s yeast descends from depends on where you live.

3-Libkind, D. et al. (2011). Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast. PNAS 108: 14539–14544

4-The word for this yeast’s product, lager, in turn, comes from the German for storehouse, the process of making the beer embedded in its name.

5-Palma, R. E., Spotorno, A. E. (2009). Molecular Systematics of Marsupials Based on the rRNA 12S Mitochondrial Gene: The Phylogeny of Didelphimorphia and of the Living Fossil Microbiotheriid Dromiciops gliroides Thomas. Molecular Phylogenetics and Evolution. 13: 525-535.

6-Though Scotland it should be said is a land not of lagers but of ales, which are brewed using a lineage of humanity’s yeast, not lager yeast.

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Images: Safety razor by Hustvedt at Wikimedia Commons; The Brewer, designed and engraved in the Sixteenth Century by J Amman by Mikhail Ryazanov at Wikimedia Commons; Monito del Monte by WolfmanSF at Wikimedia Commons; William of Ockham, from stained glass window at a church in Surrey by Moscarlop at Wikimedia Commons.