Prior to the 18th century, yeast was the unknown quantity in beer production. Yeast goes unmentioned in the Reinheitsgebot, the 1516 Bavarian 'beer purity law' that specified only water, hops and barley as 'approved' ingredients for beer (although the rule did not really extend over a terribly wide area—its influence is often overstated). At that point, yeast was simply not a recognized part of the brewing process. Brewers were aware that something 'magical' happened somewhere along the line, turning their watery mixture into a pleasant alcoholic beverage, at least when all went well.
While spontaneous fermentation was often successful in some regions (indeed, Belgian lambics are still produced using this method, and it is again gaining in popularity on a small scale elsewhere), the less-fortunate brewer had to hope things would turn out as they wished—but spoiled beer caused by a lack of good hygiene or 'rogue' yeasts was not uncommon.
As brewing became a more industrial endeavor, spoiled beer likewise became a larger issue, and some of the most well-known names in scientific history helped to solve the problem.
Antonie van Leeuwenhoek, the first master of the microscope, first identified yeast (although he did not realize it was a living organism), and an awareness that it was somehow involved in the fermentation process began to take hold throughout the later 18th and early 19th century.
Things finally became clearer when Louis Pasteur was able to demonstrate that without living yeast multiplying and thriving, fermentation would not take place—he published his initial findings in 1857. He offered his knowledge to the French brewing industry, suggesting that impurities in yeast could be removed through a number of purification methods, although they were not wholly successful. Enter Emil Hansen, a chemist at Carlsberg.
In 1883, Hansen was able to isolate and clone the yeast (ideally) used to produce Carlsberg's lager. The rest, as they say, is history. The yeast is known as Saccharomyces carlsbergensis or Saccharomyces pastorianus, depending on which scientist one is looking to honor, and many of today's modern lager yeasts are close of (or descendants of) Hansen's yeasts. In that regard, brewers worldwide still owe Carlsberg a debt of thanks: their scientific approach heralded the beginning of what we would recognize as modern yeast production and maintenance for brewers large and small (and their Jacobsen brand's beers are a far cry from their rather bland flagship lager).
Brewing Up The PastWhile today's breweries oversee their own yeast production, there is much interest in using 'old' yeasts when they are discovered. Flag Porter is made in England using yeast cultivated from an 1825 shipwreck; it was quietly alive in its original bottles as it lay in the English Channel for more than a century and a half.
In America, the aim of Fossil Fuels Brewing is to go well beyond that—they are taking the Jurassic Park approach, with 45 million year old yeast preserved in amber. Finnish brewers are looking to revive their own 19th century shipwrecked beer as well.
Yeast may have kept itself hidden from brewers for the larger part of its history, but its longevity in some instances means that modern drinkers can enjoy something potentially quite close to what their forebears drank—or that those more ancient strains can be used to create something entirely new. Without a little science and some bad luck for long-dead sailors, beer culture (in every sense of the word) would be much less rich.