In 1665, Robert Hooke saw that cork was made up of little cubes that he named cells. Later Anton van Leeuwenhoek made the important connection that cells are living things when he saw through his early microscope smallest one-celled organisms. Later Christian Ehrenberg found thatprotista or bacteria were different kinds of cells. In the late part of the 1800s Martinus Beijerinck showed that there were small particles called viruses. 