Final answer:
Anton van Leeuwenhoek was the first person to describe living single cells, using a powerful simple microscope to observe bacteria and protozoa, which he termed 'animalcules.'
Step-by-step explanation:
The first person to describe living single cells was Anton van Leeuwenhoek. He achieved this using a simple microscope that he crafted himself with lenses so powerful that they provided better resolution than other composite microscopes of that era.
This groundbreaking work occurred after Robert Hooke had already used a lens to observe and coin the term "cell" when viewing cork. Yet, it was van Leeuwenhoek who first observed bacteria and protozoa, which he referred to as "animalcules." His discoveries mark significant milestones in the field of microbiology and the eventual development of the cell theory.