The Demographic Theory of Aging
Summary of Definitions
Life - A thing is alive (an organism) if it is an arrangement of particles that requires resources to maintain its arrangement.
Exponential growth - A population experiences exponential growth if its rate of change is proportional to its current size.
Replicative life - Replicative life refers to organisms that make copies of themselves.
Aging - Aging is the increased probability of death of an organism from internal causes.
This theory explains the following better than other theories:
Lifespan extension of hormesis (calorie restriction and exercise) and heterochronic parabiosis
Nonaging of plants
Sex differences in lifespan
Sudden death after reproduction in many organisms
Small variation and large malleability of lifespans in short-lived organisms
Evolutionary preference for exclusive sexual reproduction and sleep
Evolutionarily conserved longevity pathways
Claim: Aging is a mechanism to mitigate exponential population growth.
The universe is made up of particles that behave according to fixed laws. Certain arrangements of particles, called replicators, developed the ability to make copies of themselves. By definition, the copies of a replicator increase exponentially as long as sufficient resources are available. If the replicator also requires resources to maintain itself - that is, if the replicator is a living organism - then replication poses an inherent problem. A population of replicators will inevitably increase exponentially until all available resources are used up. It is likely that the population will then all die at once from a lack of resources. This is the fundamental problem of replicative life, and many such replicator extinctions have likely occurred.
The replicative life that has survived to the present has many built in mechanisms to mitigate exponential growth. Life on earth today primarily relies on predator-prey dynamics, exclusive sexual reproduction, and aging to mitigate exponential population growth. Aging is the increased probability of death of an organism from internal causes. Aging is the result of self-destruct programs built into organisms.
Claim: Organisms have many pathways of aging.
Just like there are numerous pathways to maintain a cell’s genomic integrity, there are numerous pathways of aging. If there was just a single pathway causing aging, then mutations in this pathway would be selected for, and exponential population growth would not be mitigated. If there are multiple pathways simultaneously increasing the probability of death of an organism, however, then evolution would have negligible means of selecting for longevity.
Implications: Only intervening in all aging pathways will lead to considerable lifespan extension. There must also be numerous aging clocks in the body for the same reason.
Claim: Aging pathways are likely closely linked to pathways vital for maintaining life.
If aging pathways were closely tied with pathways vital for maintaining replicative life, then alterations are less likely to be selected for. For example, reproductive pathways are closely linked to the rate of aging across many forms of life. This could also explain the differences in lifespan between males and females, which has been observed in most animals.
Claim: The body knows how old it is. There is some mechanism by which the body synchronizes all of its aging clocks.
Each aging pathway is governed by an aging clock. Sleep might be the mechanism by which the clocks are synchronized. If they were not synchronized, they could not effectively function as a single aging clock.
Claim: Plants do not age because exponential growth is not a fundamental problem for them.
Plants do not need to age since they obtain the resources needed to maintain themselves from the sun. Solar energy is practically infinite from the perspective of life on earth.
Implications: We can study plants to determine which damage accumulations are associated with aging.