You may have imagined that all living things must inevitably wear out. Cars and vacuum cleaners in daily use eventually stop working. It is easy to suppose that animals age and die in a similar way. But professor of zoology Steven Austad explains: “living organisms are very different from machines. The most fundamental defining character of living organisms, in fact, may be their ability to repair themselves.”
The way your body repairs itself after an injury is marvelous, but the routine repairs it makes are, in some respects, even more remarkable. Consider your bones, for example. “Seemingly inert when viewed from the outside, bone is a living tissue that ceaselessly destroys and rebuilds itself throughout adult life,” This remodeling essentially replaces the entire skeleton every 10 years.”
Other parts of your body are renewed more often. Some cells in your skin, liver, and intestines may be replaced almost daily. Every second, your body produces about 25 million new cells as replacements. If this did not happen and all the parts of your body were not constantly repaired or replaced, you would grow old during childhood.
The fact that we do not wear out was seen to be even more remarkable when biologists began studying the molecules within living cell. When your cells are regenerated, each new cell must have a copy of your DNA, the molecule that contains much of the information needed to produce your entire body.
Imagine how many times DNA has been reproduced, not just during your lifetime in your own body but since human life began! To understand how amazing this is, consider what would happen if you used a photocopier to copy a document and then used the new copy to make next copy. If you did this repeatedly, the quality of the copies would deteriorate and eventually become unreadable. Happily, the quality of our DNA does not deteriorate or wear out when our cells repeatedly divide.
Why? Because our cells have many ways of repairing DNA copy errors. If that were not true, mankind would long ago have become a pile of dust! Since all the parts of our body –from the major structures to the tiny molecules –are constantly replaced or repaired, wear and tear does not fully explain aging. The body’s numerous systems repair or replace themselves for decades, each in a different way and at a different pace. So, then, who do they all begin to close down about the same time?
Why does a house cat live 20 years, but a similar-size opossum live only 3 years? Why can a bat live 20 or 30 years, but a mouse only 3? Why can a giant tortoise live 150 years, but an elephant only 70? Factors such as diet, body weight, brain size, or rate of living do not explain such diversity of life spans.
Locked within the code of the genetic materials are instructions that specify the age beyond which a species cannot live. Maximum life span is written in the genes. But as the end of that life span approaches, what causes all body functions to start closing down?
There seem to be mysterious signals that simply show up at certain times and tell cells to quit doing their normal adult functions. Genes exist which can tell cells, and indeed entire organisms, to grow old and die.
Our body might be compared to a company that has been doing business successfully for decades. Suddenly the managers stop hiring and training new staff, stop repairing and replacing machines and stop maintaining and rebuilding the premises.
Soon the business will start to deteriorate. But why did all those managers change their successful policies? That question is similar to the one facing biologists who study aging. In aging research, one of the great mysteries is trying to understand why cells stop replicating and start dying.
Aging has been called “the most complex of all biological problems” after decades of effort; scientific research has not revealed the cause of aging, much less found a cure. No currently marketed intervention –none –has yet been proved to slow, stop or reverses human aging. Although sensible diet and exercise may improve your health and lower the risk of your dying prematurely from disease, nothing has been proved to retard aging.