All this talk about animals eating fermented foods... natural fermentation produces alcohol but in very low amounts. It's not comparable to modern beers.
Modern beers were invented around the year 1800 when refrigeration technology allowed the use of low temperature yeasts (saccharomyces cerevisiae) to produce more alcohol before dying.
Before, the yeast would create alcohol, and once the alcohol concentration was too high, it would kill the yeast, thus stopping the fermentation. With refrigeration, newer types of yeast could be used, and that's how beers went from something like 1.5% to 4% and 5% of modern beers. Yes, back then 4% was considered a high-alcohol beer.