We take it for granted today, but there was a time when not only did humanity not understand the nature of cells, we didn't even know they existed. That would all start to change when Antonie van Leeuwenhoek, a draper and tradesman, would peek into a drop of water through his superior microscope (not to say anything of what would happen when he peeked into a drop of semen) and discovered a world hitherto unknown, full of microscopic life. The baton would then be taken up by luminaries such as Robert Hooke and things would never be the same again...
Progress, however, was slow... had to be slow. Science had to contend not simply with religious dogma and medieval assumptions but with the lack of a proper cell theory, so before we arrived at a proper and basic understanding of what cells are, where we can find them and where they come from, we sort of had to stumble upon a number of dead ends.
That story, as Adam Rutherford makes clear in the following documentary, is absolutely fascinating
And if you want to be fascinated even further, check out The Invisible World.