The maximum lifespan of an organism is a
constitutional feature of speciation to polygenic
controls and to environmental influences.
Various theories have been put forward to
explain the phenomenon of aging, however, no
single theory has yet accounted for all
phenotypes, though many have attempted to
explain at least some of the major and most
frequent aging phenomena. Thus, an analysis
of molecular, cellular, and systemic events may
reveal a productive path for understanding the
biology and pathology of aging, including
higher longevity in certain groups of animals.
Molecular theories propose that the lifespan
of any species is governed by the genes
interacting with the environmental factors.
Genetic information is stored in the genes
(nucleotide sequence of DNA), is transcribed
to RNA, and is subsequently translated into
proteins. These proteins, either structural or
functional, govern the form and function of
organisms. Aging may result from changes in
DNA template activity, which regulates the
formation of the final cellular products. The
molecular theories of aging include codon
restriction, somatic mutation, error theory, and
gene regulation theory. It also covers
antagonistic pleiotropy, dysdifferentiation, and

soma disposal hypothesis.
Cellular theories relate to changes that occur
in structural and functional elements of cells
with the passage of time. These theories
include wear and tear, age-pigments, freeradicals,
cross-linking, membrane alterations.
Major systemic theory includes the
neuroendocrine and immunological theories.
These theories view that the overall
performance of an animal is closely related to
the efficacy of a variety of control mechanisms
that regulate the interaction between different
organs and tissues. The effectiveness of
homeostatic adjustments declines with aging
and leads to consequent failure of adaptive
mechanisms, aging, and death. Adaptation to
external and/ or internal stress depends on
the control mechanisms orchestrated by the
combined interplay of the nervous and
endocrine systems. The complete surveillance
to foreign invaders is provided for by the
concerted efforts of the immune system. The
failure of this mechanism leads to the reduced
adaptability of an organism to the external
pathogenic challenges in old age. Healthy
aging and maximum longevity may thus
depend on the genetic makeup of an organism
under the strict control of nature and nurture.