Formally established by Theodor Schleider and Matthias Schwann in 1839, cell theory (or cell doctrine) is the most basic tenet underlying the field of biology. Modern cell theory states that The cell is the structural and functional unit of all living things. All organisms are composed of one or more cells. New cells come from pre-existing cells; lifeforms today have descended in unbroken continuity from the first primitive cells that arose on earth more than 3.5 billion years ago. Hereditary information passes from parent cell to child cell. All cells have the same basic chemical structure. The fundamental biochemical reactions of life take place within cells.