Natural selection is the mechanism of evolution originally proposed by Charles Darwin. This process is driven by mutation and death. Therefore, "individual death" is a critical event for evolution of species. Analogous to the significance of individual death in species evolution, recent biological observations have revealed that "cell death" is an important event for maintenance of individuals and offspring. Cell death is caused not only by injuries and pathological conditions, but also by programed intracellular signals. This type of programed cell death is called "apoptosis." Apoptosis signals are regulated in mitochondria, eukaryotic organelles of symbiotic origin, and play a critical role in survival of individuals by eliminating some cells.