Senescence is the combination of processes of deterioration which follow the period of development of an organism. It is the gradual deterioration of function characteristic of most complex lifeforms, arguably found in all biological kingdoms, that on the level of the organism increases mortality after maturation. The process of senescence is incredibly complex, and accompanied with a myriad of chemical and physical reactions. In biological term senescence promotes evolution, and keeps Earth from being too crowded. It is commonly believed that cellular senescence underlies organismal senescence.