  Many theories have been proposed to answer two questions on aging: "Why do we
age?" and "How do we age?" Among them, evolutionary theories are proposed to
interpret the evolutionary advantage of aging, and "saving resources for group
benefit" is thought to be the purpose of aging. However for saving resources, a
more economic strategy should be to make a rapid death to the individuals who
are over the reproduction age rather than to make them aging. Biological
theories are proposed to identify the causes and the biological processes of
aging. However, some theories including cell senescence/telomere theory,
gene-controlling theory, and developmental theory, have unfortunately ignored
the influence of damage on aging. Free-radical theory suggests that free
radicals by causing intrinsic damage are the main cause of aging. However, even
if intracellular free radicals cause injuries, they could be only associated
with some but not all of the aging changes. Damage (fault)-accumulation theory
predicts that faults as intrinsic damage can accumulate and lead to aging.
However, in fact an unrepaired fault could not possibly remain in a living
organism, since it can destroy the integrity of tissue structure and cause
rapid failure of the organism. These traditional theories are all incomplete on
interpreting aging phenomena. Nevertheless, developmental theory and damage
(fault)-accumulation theory are more useful, because they have recognized the
importance of damage and development process in aging. Some physical theories
are useful, because they point out the common characteristics of aging changes,
including loss of complexity, consequence of increase of entropy, and failure
of information-transmission. An advanced theory, which can include all of these
useful ideas in traditional theories, is needed.
