For 5/6 of the history of life, all life was unicellular, and went through the cell cycle on a regular basis. As soon as enough growth had occurred, the cell would begin the "S" phase, eventually leading to mitosis and cell division.
Between 600 and 400 million years ago, some eukaryotes became multicellular (animals, plants, fungi). Most of their cells now divide only a limited number of times before pausing in "G1" and specializing. These cells, all formed by mitosis, are genetic clones of the zygote and of all the stem cells that do not specialize. …
