So, we tend to think that it’s just common sense that most animals reproduce sexually (actually I think most people would assume that _all_ animals do; in fact, as with most things, there are edge cases), but, well, to an extent that’s because we already know that. The historical view was a bit different: https://en.wikipedia.org/wiki/Spontaneous_generation

This isn’t even the only weird idea that people used to have about reproduction; there’s also stuff like https://en.wikipedia.org/wiki/Barnacle_goose_myth