Disposable soma theory: based on the idea of mortal soma and immortal germ lineage.
Errors and repairs theory of aging: “error” theories propose that aging is caused by different kinds of errors occurring in the body, which accumulate over time and lead to aging and natural death. Errors may happen as a result of environmental damage and/or randomly taking place in the body.
Stochastic theory of aging describes aging as an accumulation of errors in the translation process from genes to proteins. Such errors create defective proteins, which are not functioning properly, resulting in damages, failure of function and death.
Free-radical theory of aging: assuming the aging process as a result of damages caused by the accumulation of free radicals in cells and tissues during life.
Immune system theory of aging: proposing that the immune system is controlling the aging process and its speed. The number of immune cells, crucial for immune response of organisms, decreases and become less functional and beneficial. For example, the thymus begins to shrink from age 20. Lack of immune response allows acceleration of the aging process.
Inflammation theory of aging: explained aging by connecting aging and inflammation.
Maintenance theories of aging: Aging is a deteriorative self-accelerating process with damages and failures on all levels of an organism’s organizational hierarchy. Living organisms have some different physiological counter mechanisms preventing or repairing those damages.
Non-programmed theory of mammal aging proposes that different mammal species have different mechanisms with different capabilities for maintenance and repair. The more efficient and more diverse maintenance mechanisms species have the longer life span they have. Such mechanisms include damage repair of oxidative stress, telomere shortening, etc. Species with short life spans have earlier sexual maturity, and longevity is less important for them, so efficiency of such maintenance mechanisms did not develop high enough. For those species the deterioration process starts at early ages and rapidly progresses.
Neuroendocrine theory of aging: this theory is basically the wear and tear theory focusing on the neuroendocrine system; particularly on the regulatory mechanism network of biochemical signals and feedback, controlling production and hormone release, as well as other vital elements. Wear-and-tear deterioration leads to self-accelerated deregulation of neuroendocrine system organisms and progresses aging.
Order to disorder theory of aging: sexual maturation triggers the process of disordering in living organisms, causing chaos in the order of living organisms’ organization and chain reactions of messy changes. Disorderliness varies from type to type of cells and tissues, from individual single organisms to others, and from species to species, and it explains different rates of deterioration and different aging speeds.
Reproductive exhaustion theory of aging: this theory explains aging and natural death by exhausting organisms during reproductive periods and years, and because it is incapable to continue their functioning.
Reproductive-cell cycle theory: this theory is defining “aging” differently from an acceptable definition as deteriorating the process and decreasing functionality. This theory defines aging as a continuation of changes starting from single cell development into multicellular organism. Multiple chemical reactions are controlling cell development, regulating cell growth, reproduction, aging and death. Reproductive cell cycle theory is suggesting that all cellular changes are controlled by reproductive hormones.
Stem cell theory of aging: proposes aging process as an outcome of decreasing proliferation ability of stem cells, resulting in the incapability to replace damaged, exhausted cells in different tissues and organs, causing functional and structural damage and death.
Waste product accumulation theory of aging: while functioning, cells are producing different types and amounts of waste, including toxins; if this waste is not properly removed from cells, it is accumulating and on a certain level of accumulation preventing their proper functioning and at the end destroying them.
Wear-and-tear theory of aging: functioning cells and organs of living organisms accumulate damages and failures on a cellular level, caused by different external (hostile environment conditions) and internal factors (errors). Even regular use of organs creates wear and tear in their cells, additional external hostility adding and accelerating wear-and-tear formation. Living organisms have maintenance and repair mechanisms to prevent wear-and-tear formation and accumulation, but with age those mechanisms are losing their ability to prevent, remove and compensate damages and failures, and the aging process is accelerating until natural death occurs.
Other aging theories mainly describe and explain different mechanisms of aging; partly useful, they usually miss answering questions as to why and how aging appears and why there are no sufficient mechanisms of repairing aging damages.

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