Several theories have been proposed to explain why we age. In general the same cause has been considered for all organisms although aging is not identical in the different organisms. Some theories suggested a single cause that would trigger a cascade of events leading to the extinction of the organism. Other theories suggested the depletion of a potential, a programmed type evolution, genes differentially expressed through natural selection, or wear and tear through an accumulation of deleterious molecules. There is a discussion going on concerning if there is an insurmountable barrier to the human life span or if it is possible to overcome the actual limit. I think that to find the cause of aging one has to consider the first step for survival in the biosphere - the capacity to meet the energy requirements. Life is dependent on the utilization and transduction of energy and thus has to follow thermodynamic rules. The second law of thermodynamics states that all systems spontaneously change in such a way as to decrease their capacity for subsequent change. A system driven by the utilization of energy, which is in a permanent restructuring has to follow the second law with entropy increasing accordingly.
Keywords: stress, genes, asymmetric cell division, molecular conformation, thermodynamics.