Humans have a maximum lifespan of about 120 years, but everyone who's gotten close to that point had multiple disabilities such as blindness or deafness. The pessimists among us often point out that natural wear and tear on the body is responsible for the maximum lifespan of a human; that our cells become damaged over time and cease to divide (senescence) or divide too much (cancer). Yet other animals like the Galapagos tortoise can tool around the beach for almost 200 years, staying pretty much intact. In fact, giant tortoises don't show any signs of aging that scientists can detect. Why can the sea turtle keep its body together but we can't? Or, to give another perspective on it, why do we humans get to enjoy 70-80 years of life while most dogs live only 15 and most insects live only a few days? How is this determined?

Unsurprisingly it comes back to that old schemer: Evolution. The lifespan of any organism is fundamentally tied to the act of self-replication. That is, after all, the main point of being alive, and that, unfortunately, is why we have to die. But why is self-replication the cause of our limited lifespan? To put it simply, it's because living a long time (i.e. longer than 60 years for humans) has no benefit for the survival of one's offspring. This is true for humans but especially true of lower organisms such insects.

Life found it more expedient to make new versions of itself rather than to try to keep older versions, an idea that has been named of the "Somatic Theory of Aging". On the other hand, for sea turtles, there is apparently some sort of evolutionary benefit to having them mosey about the beach 200 years, or else their lifespan would be much shorter. Why did evolution cause their bodies to age so slowly, if at all, while ours give up after 100 years? Scientists are now investigating this question, and at the same time learning how can we tweak evolutionarily-determined checkpoints to our favor.

There are two main strategies of self-replication: one is to quickly reach maturity as soon as possible, produce as many offspring as possible, and then die soon afterwards. Many of these offspring will die before reaching maturity but the sheer numbers will assure that some make it to an age where they, in turn, will reproduce and then quickly die. There's no evolutionary need to keep the organism alive after they have reproduced, since they play no part in the actual upbringing of their young. Thus genes that would have brought about longevity in lower organisms usually get lost during successive generations.

Scientists once thought that when parents competed for resources with their young the young would be less successful and so longevity would actually be a detriment to a species' success. However, this idea has been debunked: when flies with longevity genes are mixed with normal life flies in environments with abundant food, the longevity flies are completely gone after a few generations. In this experiment there is no competition for food, so longevity genes must be inhibiting the flies' reproductive success in other ways. There seems to be something going on with evolutionary fitness that negates longevity. But what? Why do genes that allow quick maturity and reproduction also bring about faster mortality?

This concept is called antagonistic pleiotropy. It refers to genes or traits that benefit an organism during its reproductive prime but go on to cause deterioration once the organism is no longer making offspring.

Antagonistic pleiotropy also is seen in organisms employing the second strategy of self-replication, where an individual does not produce many offspring but sees to it that a majority of these offspring will make it to adulthood. Many mammals, including humans follow this method, where parental longevity is essential for offspring survival to adulthood. These organisms evolved to have longer life in order to ensure its reproductive success. So, even though humans lose the ability to reproduce (at least females), they ensure their offspring's success by nurturing. Other long-lived animals that are less involved with the rearing of offspring, such as the giant Galapagos tortoise, do not show any loss of reproductive capability and in fact become more successful with age.

One possible reason that animals such as giant tortoises can live for so long is that they experience very little stress Thus, the answer to the problem of aging may have a lot do with stress.

Stress is a ubiquitous part of our environment; all organisms have developed ways to combat different kinds of stressors. Stressors are basically anything that interferes with an organism's normal functioning. One of the most basic stresses is starvation, but other types of stress involve temperature extremes, or exposure to predators or, in the case of higher organisms, mental stress such as anxiety. For instance, scientists have found that mothers with chronically ill children, who are not sick themselves but under constant stress, have immune cells which show increased aging. Likewise, another kind of stress, poor regulation of blood sugar, as seen in diabetes, can also cause premature aging which can manifest as blindness or dementia.

Organisms need to have ways of dealing with stress, which usually represent a short-term problem to the organism's survival, but often these stress strategies cause wear and tear that act in long-term ways. The energy-making components of the cell, mitochondria, make byproducts, called reactive oxygen species, which can damage DNA and proteins. When a cell is stressed, it often needs more energy to combat stress, and this in turn causes mitochondria to produce more reactive oxygen species. The more stress, the more damage is done to the cell, thus causing detrimental aging. Long-lived species, that often have slower metabolisms than short-lived species, show better control of reactive oxygen species production. Food restriction, which slows metabolism, can increase lifespan in a variety of animals, such as worms, mice, and possibly humans. In fact, food restriction seems to be the most consistently effective way of lengthening lifespan.

Unfortunately, living in a state of semi-starvation doesn't make the prospect of a long life very worthwhile. And, indeed, recent research has shown that being somewhat overweight after age 70 increases lifespan more than being normal or underweight. In any case, scientists are currently studying ways to get the health benefits of food restriction without actually having to restrict one's caloric intake.

So, as of yet, there isn't much we can do about our evolutionarily engineered lifespan. One comforting thought is that our deaths are necessary for the well being of our offspring. As the poet Stanley Kunitz, who died earlier this month, once said, "Death is absolutely essential for the survival of life itself on the planet. It would become full of old wrecks, dominating the population."

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