- The waste theory considers death as the final consequence of biochemical decay. From the first moment that an animal begins to breath, its cells act as miniature biological converters that turn oxygen and other substances into chemical products that are consumed to keep the organism alive. That conversion process generates a certain amount of biological waste which slowly accumulates in the body. When the amount of chemical waste surpasses the body's ability to deal with it, the animal dies.
- The exhaustion hypothesis regards death as the natural depletion of the body's capacity to replace its own cells. While an animal is alive, its cells are continuously dying and being replaced by new cells, which are almost identical to the ones that have died. According to this theory, cells can only reproduce themselves a limited number of times without losing important genetic information. This limitation is what determines the maximum lifespan of each species, which in the case of human beings is estimated to be around 120 years.
[Image by Mike Boehmer under Creative Commons Attribution License. See the license terms under http://creativecommons.org/licenses/by/3.0/us]