h+ Magazine looks back at the origins of the free radical theory of aging: "Harman's work on radiation with experimental animals showed him that the symptoms that appear in the aging process has many things in common with what occurs as the result of massive exposure to radiation - including cancer induction, cell death or necrosis, and tissue changes similar to what is seen in aged animals. He knew from his own research results - and earlier research at other laboratories - that ionizing radiation generated enormous amounts of free radicals, particularly hydroxyl radical and hydrogen radical. ... In 1954, Harman hypothesized that the steady state production of hydroxyl radicals from metabolism and the radicals generated by the copper and iron-containing enzymes found in all cells over the lifetime of an organism was a major contributor to aging - or the actual cause of the aging process itself." Since then, the theory has been refined into the mitochondrial free radical theory of aging, and mitochondria have become a major focus in aging research.

Link: http://www.hplusmagazine.com/articles/forever-young/leaders-modern-gerontology-denham-harman-takes-free-radicals