Final answer:
Antony van Leeuwenhoek observed the movements of single-celled organisms and discovered bacteria and protozoa using his microscope in the 1600s.
Step-by-step explanation:
The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of single-celled organisms, which he collectively termed "animalcules." He also discovered bacteria and protozoa.