<P> There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells . </P> <Ul> <Li> 1632--1723: Antonie van Leeuwenhoek teaches himself to make lenses, constructs basic optical microscopes and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth . </Li> <Li> 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early compound microscope . He coins the term cell (from Latin cella, meaning "small room") in his book Micrographia (1665). </Li> <Li> 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory . </Li> <Li> 1855: Rudolf Virchow states that new cells come from pre-existing cells by cell division (omnis cellula ex cellula). </Li> <Li> 1859: The belief that life forms can occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822--1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion). </Li> <Li> 1931: Ernst Ruska builds the first transmission electron microscope (TEM) at the University of Berlin . By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles . </Li> <Li> 1953: Watson and Crick made their first announcement on the double helix structure of DNA on February 28 . </Li> <Li> 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory . </Li> </Ul> <Li> 1632--1723: Antonie van Leeuwenhoek teaches himself to make lenses, constructs basic optical microscopes and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth . </Li> <Li> 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early compound microscope . He coins the term cell (from Latin cella, meaning "small room") in his book Micrographia (1665). </Li>

Cells are basic structural and functional unit of living organisms explain it