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Centuries later, once microscopes were around, the bubbles that cause the dough to rise were shown to be produced by microörganisms. Oversimplifying, there are two kinds of them. One kind is lactic-acid bacteria, which eat the sugars present in the flour, excreting carbon dioxide and lactic acid. Bacteria like these are also responsible for milk going sour (hence their name: "lactic" means "milk", "acid" means "sour") and for cabbage turning into sauerkraut. In bread, pockets of the carbon dioxide are trapped in the dough, making the crumb porous and fluffy, while the lactic acid gives the bread its specific tart flavour. And yes, the dough which is left for bacteria to make it sour is called sourdough.
The other kind is a bunch of single-celled fungi found in the froth from beer wort and called yeast. The yeast eats Yeast cells eat sugar, excreting carbon dioxide and ethyl alcohol. Yeast like this is They are also responsible for producing wine, mead and vodka. In bread, the carbon dioxide makes the bread fluffy, while the alcohol mostly evaporates during baking (which means you can't get drunk on bread; tough luck). Bakers, as we've already seen, used to obtain their yeast from breweries and add it to the starter for their dough (not sour at all).
The year 1842 saw the brewing of the world's first lager in Pilsen (now Plzeň, Czech Republic). Lager is a lower-fermentation beer, which means that the yeast, once done, sinks to the bottom of the vat rather than float to surface. Pilsner-type lager caught Europe by the storm, which had the unexpected side-effect of bakers no longer having access to a source of fresh yeast, as there was nothing to collect from the surface of the lager brew. And fresh yeast was indispensable. In the case of sourdough, it's common to leave some of it to kick-start the next batch, with the resulting flavour only getting better every time. This approach doesn't work with yeast, though, as old yeast gives bread a rather unpleasant, stale aroma.