* Robert Hooke (1665): He first observed and named "cells" while looking at a thin slice of cork under a microscope. He didn't realize these were the fundamental units of life, but his observations laid the groundwork.
* Anton van Leeuwenhoek (1674): He was the first to observe living cells, including bacteria and protozoa, using a more powerful microscope.
* Matthias Schleiden (1838): He concluded that all plants are made of cells.
* Theodor Schwann (1839): He extended Schleiden's work by stating that all animals are also made of cells.
* Rudolf Virchow (1855): He contributed the key principle that "all cells arise from pre-existing cells."
Therefore, while Hooke's initial observations were crucial, it was the combined work of Schleiden, Schwann, and Virchow that solidified the cell theory.
