1. Microorganisms (1674)
Microscope lens grinder Anton Van Leeuwenhoek accidentally discovers microorganisms in a drop of water. Using his own microscopes, he observes sperm, bacteria and red blood cells. His observations lay the foundation for the sciences of bacteriology and microbiology.
2. The Cell Nucleus (1831)
While studying an orchid, botanist Robert Brown identifies a structure within the cells that he terms the "nucleus."
3. Archaea (1977)
Carl Woese discovers bacteria are not the only simple-celled prokaryotes (unicellular organisms without a nucleus) on Earth. Many of the organisms classified in the new kingdom of Archaea are extremophiles. Some live at very high or low temperatures, others in highly saline, acidic or alkaline water. Some have been found in environments like marshland, sewage and soil. Archaea are usually harmless to other organisms and none are known to cause disease.
4. Cell Division (1879)
Walther Flemming carefully observes that animal cells divide in stages and calls the process mitosis. Eduard Strasburger independently identifies a similar process of cellular division in plant cells.
5. Sex Cells (1884)
August Weismann identifies that sex cells must have divided differently to end up with only half of a chromosomal set. This very special division of sex cells is called meiosis. Weismann's experiments with reproduction in jellyfish lead him to the conclusion that variations in offspring result from the union of a substance from the parents. He refers to this substance as "germ plasma."
6. Cell Differentiation (late 19th century)
Several scientists participate in the discovery of cell differentiation, eventually leading to the isolation of human embryonic stem cells. During differentiation, a cell turns into one of the many cell types that make up the body, such as a lung, skin or muscle cell.