Evolution of aging

There are two question about lifespan often heard in freshman biology class. Why even among relatively similar species, such as mammals, the rates of aging differ almost a hundred times? Also, why are short-lived species far more common that long-lived ones? After all, basic principles of life are the same for most organisms. All species use the same information carriers (DNA and RNA), and similar building blocks for their cells (amino acids, proteins, lipids, etc.). Why all this disparity and asymmetry when it comes to longevity?

Although Darwin's evolution theory is often treated as old news, it remains one of the best tools for understanding the origin of differences among species. Only those species survive who best adapt to the environment by developing advantageous traits. Long life is obviously a great bonus for an individual, but in many cases it does not provide any competitive advantage to the species, and may actually be a disadvantage. Imagine a situation when the environment constantly changes, at least a little. To remain fit and competitive, species would also have to change. Living organisms evolve through spontaneous mutations, which are random changes in the genetic blueprint (DNA). Most mutations are harmful or even lethal, but a few provide an advantage. In the situation of changing environment, a faster rate of reproduction is an advantage because there is a greater chance to produce a mutant offspring better fit for the new conditions. The lucky offspring with beneficial mutation mutant offspring will survive and reproduce better than its peers.

Therefore, in most situations, the evolution favors high reproduction rate and short lifespan because they accelerate the rate of adaptation. Indeed, long-lived organisms not only tend to be slower in reaching reproductive stage of the life cycle but they also tend to survive long after they have lost the ability to reproduce, competing for resources with their reproductively active peers. Yet another reason why a longer life span may be an evolutionary disadvantage is that super-efficient self-repair mechanisms required for longer life consume the organism's resources, which could otherwise be used for reproduction.

Despite all these possible evolutionary drawbacks of longer lifespan, long-living species do sometimes evolve and humans are a prime example. There seem to be several evolutionary reasons why humans live longer than other mammals. An advanced brain gives us an enormous evolutionary advantage, but full development of such a complex brain in an individual takes a long time. It is, therefore, an advantage if parents can be around for many years to care for and teach their children. The payback, in evolutionary terms, exceeds the cost because an offspring with a powerful brain has superior survival capacity. Another reason for our longevity is that we can compensate for adverse changes in the environment by creating artificial habitats, devising new methods of food production and otherwise tailor outside world to our needs. As opposed to essentially all other species on Earth, humans did not have to undergo further evolutionary change to survive in changing environment. They simply created an environment of their own and, therefore, could afford and even benefit from a relatively slow reproduction and long life. The downside of this situation is the lack of evolutionary pressure for humans to evolve further -- towards an even longer life span. Fortunately, that powerful brain of ours can help us take matters in our own hands by devising ways to slow down the mechanism of aging beyond what nature has allotted.