It's difficult to pinpoint a single discoverer of the first single-celled organism. The development of the microscope in the 17th century allowed scientists to observe microscopic life for the first time.

Here are some key figures involved:

* Antonie van Leeuwenhoek (1632-1723): A Dutch microscopist who is often credited with being the first to observe and describe single-celled organisms (which he called "animalcules"). He used his self-made microscopes to examine various samples, including water, saliva, and scrapings from his teeth, where he observed tiny, moving creatures.

* Robert Hooke (1635-1703): An English scientist who was the first to use the term "cell" when describing the structure of cork. He observed cell walls in plant tissue, but he didn't see the living contents within them.

It's important to note that:

* Early microscopes were limited: Early microscopes had limited magnification and clarity, so they didn't provide a complete picture of the inner workings of single-celled organisms.

* Understanding of cellular life evolved over time: It took centuries for scientists to develop a full understanding of the complexity and diversity of single-celled organisms.

Therefore, attributing the "discovery" of the first single-celled organism to one person is inaccurate. It was a gradual process of observation, improvement in technology, and scientific investigation over time.