Has Aging Been Programmed By Evolution?

Credit: Charis Tsevis

Why do so many scientists now believe that aging has been programmed by evolution?

Science usually approaches aging from a mechanical viewpoint, but the evolutionary theory of aging has gained more support as we observe the wide variation in aging between species.

The million dollar question: Why do we age?

Theories like the free radical theory of aging propose that aging is an inevitable process of being alive; that every day on this earth exposes the body to reactive chemicals, and aging is simply the gradual accumulation of damage. While we know the body does indeed accumulate forms of damage, the suggestion that this is inevitable is now being challenged. 

We now know that there are many organisms which buck the aging trend and barely age at all. If aging really was an inevitable, chemical certainty, then how could this be?

There are many roads to success

Many people like to think of evolution as some kind of directed, creative process that makes organisms 'better' as time goes on. This isn't the case. Evolution molds organisms to be more effective in their environment, but it can only adapt to existing challenges. We like to think of humans as a masterpiece of bio-engineering, but we're simply a step on the ladder.

The force of selection as a function of age. Nature Education Fabian and Flatt 2011

Why would life favour death?

Life is frequently a brutal affair, filled with starvation and predation. There are some lucky species that are spared the rat race, but surviving is usually a tough business. 

Has aging always been or was it selected for?

Mammals evolved after reptiles, but many reptiles display minimal signs of aging, whereas all mammals do. If evolution pushes for longer, healthier lives then surely we'd be better, not worse off? Reptiles can breed for longer, replace lost limbs and their teeth are constantly replaced. Mammals have very poor regenerative capacity and become infertile with time. 

Protecting the group, not the individual

One team has proposed that mathematics prove aging is modulated by its environment. When times are tough, an environmental feedback system pushes for shorter lifespans in order to conserve resources. We tend to think of evolution as a 'selfish' process, but the group may take priority. In others words, we die so that our children can live...and so on. We may have safety and abundant food now, but our genes don't know that yet. 

         Speed wins the sprint, but stamina wins the marathon

Enter Antagonistic Pleiotropy - Early Fitness, Later Weakness

Antagonistic pleiotropy sounds complicated, but it really means genes that are both good and bad; they produce a beneficial effect and a negative one. Any mutation that confers an advantage can only be passed on if the animal reaches reproductive age, which means that any genes which have a beneficial effect in youth are favoured, but any genes with delayed benefits would be considered irrelevant. This is the reason terrible genetic diseases like Huntington's have persisted, because they only kick off after reproductive age. Some genes might help you grow faster and stronger initially, but the rush accumulates damage which then cripples you later on. Is it better to rush and excel short term, or invest and excel in the long term? The answer is neither - it depends on your environment. Life works on a strict cost/benefit system, in which energy is highly conserved.  The downsides simply weren't worth the time investing energy in to fix. 

Don't lose hope! Organisms DO evolve longer lives under certain conditions

The evolutionary theory of aging is persuasive because it's backed up by evidence and observation. The theory claims that in a scarce and fiercely competitive environment aging will be favoured, but it also proposes that if predation goes down and food goes up, lifespan is free to rise.

This is exactly what we see in the wild.

Giant tortoises can live to beyond 200 and hail from the Galapagos, where they have no real predator. Similar organisms with plentiful resources and limited pressure show the same patterns. Female octopuses can live almost twice as long when their optic glands are removed and various similar fish species display wildly different lifespans. All of these clues point to the conclusion that aging is at least partially programmed.

We can force flies to evolve longer lifespans

Evolutionary biologist Michael Rose has shown that by breeding fruit flies at a progressively later age, their life expectancy goes up. Just as humans have molded dogs from wolves, creating artificial pressure in which only the old are selected for begins to favour longevity. We can't set up a similar experiment for humans (or we could but it would take hundreds of years), but testing on animals could reveal core genes and mechanisms limiting longevity. When we map and understand these we'll have great targets and even existing solutions to them. 

Has evolution really programmed aging, or just tinkered with it?

This makes aging sound like a walk in the park, but for any complex organism it's a huge challenge to maintain health indefinitely. It takes a lot of energy. Modelling and observation show that longevity isn't an impossible goal, but it's tricky. It's unlikely the entire aging process evolved from scratch, but evolution may have removed protection mechanisms to select for shorter lifespans. Life is capable of overcoming the hurdles, but it needs a motivation to do so, and immortality never held much benefit for the group.

What can we learn from this?

Evidence does suggest aging has been favoured by evolution, but that doesn't necessarily mean aging is entirely manufactured. Does accumulated damage cause aging, or is it the system's failure to deal with that damage that causes aging? In reality it's probably a bit of both. Studying long-lived organisms will only yield so much, and we'll probably have to search deeper if we want to achieve significant life extension. 

Read more at io9