On this day in 1676, Antonie van Leeuwenhoek applied his hobby of making microscopes from his own handmade lenses to observe some water running off a roof during a heavy rainstorm.
He finds that it contains, in his words, "very little animalcules." The life he has found in the runoff water is not present in pure rainwater. This was a fundamental discovery, for it showed that the bacteria and one-celled animals did not fall from the sky. Link