The man in the picture is considered the "Father of Microbiology." He helped to discover and sketch microorganisms. When he turned his microscope on beer, he saw some of the most useful microorganisms in the world — but he failed to recognize them.
This man above is Anton van Leeuwenhoek, and he's wearing an absolutely bitchin' coat because he was a draper by trade. In fact, he draped so successfully that he managed to indulge his hobbies as he got older, one of which was lens making. Anton spent his days making powerful microscopes and sketching the objects he put in front of them. He discovered many things, the most interesting of which were animalcules, things that looked like tiny little animals. His sketches and descriptions, as well as his microscopes, jumpstarted the field of microbiology.
It wasn't long before he turned his lens on beer in the process of brewing. It was 1680 when he first trained his lens on a droplet of beer. At the time, no one knew what it was that made hops, barley, and water turn into beer. Although they knew of yeast as a cloudy substance that appeared in beer after it spent some time fermenting, they were entirely ignorant of what it did; to the point where there were laws against using anything except barley, hops, and water in the beer-making process. Naturally, as soon as Anton looked at brewing beer he saw little circular blobs. He saw the way they aggregated into larger groups. He saw the way that they produced bubbles of what he thought was "air," and floated to the surface.
Despite his obsession with microorganisms, he utterly failed to recognize them as life. These blobs, he believed, had come loose from flour. They aggregated into groups of six as part of a chemical process. Anton was fascinated by these groups of flour globs. He modeled them in wax, because he wanted to figure out the ways six globs could stick together while all being visible from above. This is his sketch of his models.
It took another 150 years before Charles Canard-Latour figured out that the "air" was carbon dioxide and the sextets of blobs hadn't aggregated together, they'd grown. Archaeologists believe that beer was probably first brewed around 3000 BC. That means that we used an organism for nearly 5,000 years before we realized it even existed.
Top Image: RKDimages, Art-work number 16081, Second Image: Public Domain.