From Ouroboros: "The free radical theory of aging (FRTA) was first advanced by Denham Harman more than 50 years ago. The theory proceeds logically from a small number of straightforward assumptions, based on observations from radiation biology ... Over a half-century, the FRTA has evolved substantially (eventually focusing on the mitochondria as a major source of the initially postulated endogenous radicals), and has lately been the subject of several reviews evaluating its explanatory power and extent of current acceptance. ... The basic chemical process underlying aging was first advanced [in] 1954: the reaction of active free radicals, normally produced in the organisms, with cellular constituents initiates the changes associated with aging. The involvement of free radicals in aging is related to their key role in the origin and evolution of life. The initial low acceptance of the FRTA by the scientific community, its slow growth, manifested by meetings and occasional papers based on the theory, prompted this account of the intermittent growth of acceptance of the theory over the past nearly 55 years."

Link: http://ouroboros.wordpress.com/2009/05/28/the-free-radical-theory-of-aging-a-retrospective-by-its-creator/