Nobody knew cells existed until someone built a better lens. In 1665 the English scientist Robert Hooke used an early compound microscope to look at a thin slice of cork. He saw rows of tiny empty boxes and called them cells, because they reminded him of the small rooms that monks lived in. What Hooke actually saw were the walls left behind by dead plant cells, not living cells.

A few years later, in the 1670s, a Dutch cloth merchant named Antonie van Leeuwenhoek ground his own single-lens microscopes. They were simple, but his lenses were far better than anyone else's, and they let him see living things nobody had seen before: single-celled organisms he called animalcules, and later bacteria. He was not a trained scientist, and other researchers only believed him once they could repeat his observations.

For over a century, progress waited on the technology. Early lenses produced blurred, coloured edges. When improved lenses arrived in the 1830s, two German scientists used them to look at a huge number of samples. In 1838 Matthias Schleiden concluded that all plants are made of cells, and in 1839 Theodor Schwann concluded the same about animals. In 1855 Rudolf Virchow added the last piece: every cell comes from another cell that already existed.

Together those conclusions became the cell theory: all living things are made of one or more cells, the cell is the basic unit of life, and all cells come from existing cells. It was not the work of one person or one moment. It came from better instruments, many observations, and scientists checking and building on each other's work. The electron microscope, developed in the 1930s, later revealed the detailed structures inside cells that a light microscope cannot resolve.