Microbes (microscopic organisms), according to fossil evidence, have been present on Earth for some 3.5 billion years. Indeed, all the more advanced life forms, which are of more recent origin, are considered to have evolved from early microbes. However, microbes are too small to be seen with the naked eye. Therefore, they were never seen nor known until the late 1600s. The following video gives a brief history of the discoverer, Antony von Leeuwenhoek, the crude self-made microscope he used and what he saw.
Further discussion is warranted concerning how technology and scientific discovery go hand in hand.