How long ago did people know that the bodies of living things are made of cells?

The first person to know that all organisms are composed of the smallest elements – cells, was Robert Hooke. Studying a cut of a tree under a special magnifying device, which was improved by him, this English scientist discovered that the cut is a huge number of tiny honeycombs, which he later called cells. This discovery took place in the second half of the seventeenth century and this was the beginning of a new science – cytology. Previously, this was impossible because there were no powerful microscopes to see cells.