Aging Theory — Human Life Span and Programmed Aging Theory
Aging theories cover the biochemical, genetic, and physiological properties of a typical organism, and the way these properties change with time. Genetic theories deal with speculations regarding the identity of aging genes, accumulation of errors in the genetic machinery, programmed senescence, and telomeres. Biochemical theories are concerned with energy metabolism, generation of free radicals, the rate of living, and the health of mitochondria. While Physiological theories deal about entirely with the endocrine system and the function of hormones in determining the rate of cellular senescence.
Are we programmed to get old? If we are, is it like the program that guides our development from a single fertilized egg to a multicellular organisms? Or is aging the unfortunate side effect of adaptations that make it possible for us to have and protect our offspring? Many gerontologists believe that aging is a matter of evolutionary neglect, rather than design.
However human life span evolved, it is clear that our genes have the final say in how long we are going to be on the stage. Even though flies and humans are constructed from the same kinds of cells (eukaryotes), one animal lives two weeks, the other 80 years. If those eukaryotes had remained free-living, as their protozoan ancestors have done, they would live for millions of years.
The genes in a multicellular organisms appear to be regulating cell life span for the good of the cell community as a whole. The size of the community, the animal’s intelligence, the number of offspring, and the pressure the animal experiences from its predators, are all taken into account. The final cell life span seems to be a balance of all these forces and, given these forces, may be the best deal the organism can hope for. There would be no point to nature’s producing a fruit fly that could live a thousand years, because their predators eat them all in a matter of days. Scientists might try producing a fly that could live that long, but what in the world would an animal with that level of intelligence do for all that time? This is not just a whimsical point. There is a very strong correlation between longevity and the weight of the brain: Smart animals live longer than dumb animals.
The goal of gerontologists is to try to get a better understanding of the covenant between the genes, the organism, and the environment. Whether intended by evolution or not, many genes are directly responsible for an animal’s life span. These genes may be exerting their effects through inappropriate behavior (that is, they are turning on or off at the wrong time) or through a mutation that eventually damages the protein product.
Damage at the gene level re-invokes the error catastrophe theory, but many experiments have failed to establish a role for genetic (or somatic) mutations in cell senescence. This is because the cell can detect and repair DNA damage as easily as it deals with errors in translation, and those repair systems remain intact long after the animal shows visible signs of age.
The inappropriate expression of certain genes as a major cause of aging is only now being addressed in a comprehensive way. With the genome project now complete, it will soon be possible to screen for the expression of all human genes, in every tissue and organ of the body. When this job is complete (and it will be as big a job as the genome project itself) we will finally have an idea of which genes are responsible for human life span.