Rats begin to suffer from old age at 2 years old. They develop cancer, arthritis, and many of the other ailments that humans develop in their later years- some 30 times later.

In 1988 it was discovered that nematode worms could have their life extended through a simple mutation. This mutation called age-l, doubles the normal lifespan of the worm from 22 to 46 days. This has been observed in many forms of life in more recent years- yeast, flies, mice, and fungus- small mutational changes increase the life span substantially.

Along with this increase in life span, the entire aging mechanism slows. The organism does not just live longer- it lives healthily longer. The organisms do not have an extended old age- they have an extended life.

They have a longer juvenile period, a longer period before sexuality, and a longer period before getting the geriatric diseases.

so the questions are- why don't many organisms live a long time, and are there any that do?

The first question- why don't organisms in general live a long time- is easily answerable. If you have a shot at having children at a younger age, in general this will be to a species benefit. If you wait to have children, because you are forced to by slowed aging, then the chances you take a fatal blow from one source or another increases the chances your genes get wiped out.

So in general, there is going to be selective pressure towards having shorter life span, not a longer life span. However, it has been observed in wild populations that in some instances when predation decreases markedly- the life span of organisms can mutate upwards, instead of downwards.

The second question- are there any organisms that live much longer than other organisms?

To find such an organism, it would have to have an extended juvenile period, live much longer than other organisms of a similar size, and reach sexual maturity late. This type of organism would appear immortal to other organisms whose lifespan was much shorter. If one lives two times as long as another- the longer living animal would appear to essential be immortal.

So what qualifies for this? And a related question- what sort of selective pressures would push upwards on age before sexual maturity? What type of organism would benefit its offspring production by waiting?

If we examine the lifespan of different organisms, the answer becomes apparent

Squirrel: 16
Mountain Lion: 20
Tiger: 25
Bear: 32
Hippo: 49
Human: 72

The answer is there is an organism, which to others, appears to be an immortal. The human.

What sort of selective pressures would have pushed humans towards later sexual maturity? The answer- culture.

Culture may have provided a selective pressure where those humans who waited to aggregate property and social status would have a greater chance of having their young live.

Thus, we may have ALREADY had mutational changes which extended our lifespans significantly longer than other organisms. The earlier humans may have had much much shorter lives, not only due to harsh environments, but because their bodies decayed out from underneath them at much younger ages.

Certainly biologists are working hard on figuring out what other mutational changes could influence our life span, but it may be that evolution will provide some very slowly aging individuals who will walk among us, and to our eyes, with their delayed sexual maturity and incredible long life span, will appear as immortals to our eyes. Are they already among us?

Im hoping that, a low intake diet will increase my lifespan. I think its been proven that rats kept on a near starvation diet lived longer than ones that were overfed.
(sorry but dont have references atm)

I also think that over-excercising will also lead to a decreased lifespan.

And todays air/pollution in smog filled cities is working against us too.

A natural-selection type argument for why an organism's sexual reproduction might wait longer than other species could be that a longer childhood allows children more susceptible to illness to be wiped out before reaching fecundity. Organisms that reproduce younger would reproduce genetic vulnerabilities to pathogens, causing them to evolve more slowly than organisms that expose their young to more opportunities to be "selected" for reproduction.

Organisms are actually not homogeneous in their reproduction culture or culture generally. One animal may find itself reproducing frequently from a young age, due to favorable environmental conditions, and selection would only occur later when these conditions turn against the population. Another animal in the same species might find itself in resource-scarce environment such that reproducing less frequently ensures sufficient availability of food. In that case, individuals with higher disease resistance would be more likely to reproduce their genes, since they would survive the relatively long search for a mating partner.

Personally, I think that evolutionary theory is weak in that it presumes all survival-fitness to be the result of genetic code. In actuality, much evolution probably takes place in the micro-organism ecology of digestive systems, so that one animal develops a more effective digestive system and immune system than another, just because of what micro-organisms it consumed, and the cocktail they evolved into in combination with other micro-organisms already present.

Some individual organisms get sick or starve, die of food allergies or indigestion, while others manage to digest almost anything they eat and fail to get sick from it. This need not be the result of the organism's genetic code. It can be the result of environmental coincidences and the evolution of the digestive system ecology.