Who was the first to observe “animalcules” under the microscope?

The first person to observe “animalcules” under the microscope was Antonie van Leeuwenhoek. In the late 17th century, he used his handcrafted microscopes to discover and describe microscopic organisms, which he referred to as “animalcules.” His observations were among the earliest recorded discoveries of microorganisms.

Leeuwenhoek is universally acknowledged as the father of microbiology. He discovered both protists and bacteria. More than being the first to see this unimagined world of ‘animalcules’, he was the first even to think of looking—certainly, the first with the power to see.

A. Antonie van Leeuwenhoek