#OTD in 1683.
Antonie van Leeuwenhoek writes a letter to the Royal Society describing "animalcules", later known as protozoa. It gives the first detailed description of protists and bacteria living in a range of environments, sent by the author in a Dutch letter of the 9 Octob. 1676 concerning little animals by him observed in rain-well-sea & snow water; as also in water wherein pepper had lain infused.
https://en.wikipedia.org/wiki/Protozoa
Original paper
https://royalsocietypublishing.org/doi/epdf/10.1098/rstl.1677.0003
