Mortality

Why do people and other living things endure as long as they do? Why aren't we immortal? The answer to the most basic question of why we age is still an unsolved problem in biology, as the late famous biologist Sir Peter Medawar said in 1951. However, scientists are quickly closing in on at least some of the possible reasons why aging occurs. One of the most prominent theories of aging today is known as the free-radical hypothesis. During the process of metabolizing food and water and operating the machinery of life in a toxic world, damaging substances known as free radicals are generated. Although the human body has a highly efficient mechanism to protect itself from these damaging substances, it is not perfect. It is this lack of perfection that leads to accumulated damage to the DNA contained within the nucleus and the mitochondria (energy factories) of most cells. The level of damage moves up the scale of biological organization from DNA to cells, tissues, organs, organ systems, and ultimately to the whole organism—contributing to a degradation in the functioning of biological systems and an increased susceptibility to the diseases now associated with aging. Even though the damage that occurs to DNA is itself repaired with near perfection, it is the lack of perfection that is the basis for the free-radical hypothesis of aging.

There are a number of other prominent theories about the mechanisms of aging. Among them are the wear-and-tear theories and the discovery of an attribute of nuclear DNA known as the telomere. If the human body is viewed as a living machine with pulleys, pumps, levers, and hinges, much like that of a man-made mechanical device, it is evident that such machines cannot be operated indefinitely because of wear and tear. There are changes that occur in most human biological systems with the passage of time, including the loss of bone and muscle mass, increased brittleness of the circulatory system, and a degradation of the immune and reproductive systems.

Telomeres are the end caps of nuclear DNA, and they are known to shorten in length with each cell division. When they become short enough, the cell experiences a phenomenon known as programmed cell death, or apoptosis. An enzyme referred to as telomerase is known to be present in larger quantities in cells that are protected from aging, such as eggs, sperm, and stem cells, but there is no evidence so far to suggest that adding telomerase to other cells in the body would extend length of life. Although some scientists believe that this is one of the major biological mechanisms that contributes to aging, most people tend to die well before telomere shortening poses a serious problem for the whole organism.