Brewer’s yeast has changed many times as agriculture spread & different human cultures emerged.  New forms, as distinctive as species, emerged in association with beer & wine production in different regions.  Some of these yeasts changed further to give a wide variety of bread yeasts.  In a monk’s cave in Germany, the cold-tolerant yeast used to make lager evolved as a hybrid of S. cerevisae & a species that hails from Patagonia. Mysteriously, this occurred more than 100 years before Europeans reached the New World. 

New Scientist, 26 Jan 2013, via Google Groups

You can now add brewer’s yeast to items that managed to traverse the globe before there was human contact.

Rob Dunn reiterates this at Scientific American:

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?

If it was humans who carried the yeast across the Atlantic, that would also suggest that it was a deliberate act. And that means very clever people.