Why do we get old?

(written by Lawrence Krubner, however indented passages are often quotes)

Humans could live 1,200 years with suspended senescence. I was wondering how long humans could live if their senescence could be suspended the way it is suspended in turtles. I stumbled upon this:

“In theory, if mortality rates did not increase as usual during aging, humans would live hundreds of years. I have calculated for humans (Finch 1990 book: Longevity, Senescence, and the Genome) that at mortality rates of 0.05% per yr, as found at age 15 in developed countries, the median lifespan would be about 1,200 years. In natural populations of long-lived animals, mortality rates are rarely less than 1% per yr. For very slowly aging turtles, rockfish, the number beyond 70 is 1-2%. However, there are long-lived trees, like the bristlecone pine at 5,000 years.

This, however, is wrong:

Either negligible senescence is something certain animals developed because it gave them a reproductive advantage – obviously, the ability to keep reproducing indefinitely is a huge boon to a species’s survival chances -

No, that is exactly backwards. We should always assume that a given population is up against the limit of what the local eco-system can support. Individual success is not success for the species.

To make this clear: assume a hypothetical vertebrate species and call it a “zunkzunk”. Let’s suppose zunkzunk’s live on an island, and this island has the resources to support 100,000 zunkzunks. For the species, it doesn’t matter if zunkzunks are short lived or immortal, the population will probably continue to press hard up against the upper limit of 100,000. For the individual, being immortal could be a considerable boon – a single lucky individual might have hundreds of children and therefore their direct descendants might come to represent a substantial percentage of the overall population. But the overall population does not grow just because some individual has been lucky enough to have a lot of children.

But really, things are worse than that. If the population should spike to an unsustainable level of 110,000 then, perhaps the island will be stripped bare of all resources, and mass starvation might ensue, plummeting the population down to 10,000. So, over the long run, the species actually has some evolutionary advantage in moderating population growth.* How might this be done? In one of 2 ways:

1.) the zunkzunks can have lots of children, but also get old quickly and die

2.) the zunkzunks can be immortal, but rarely have children

Both of these tactics can keep the population stable and thus avoid the kind of catastrophic over-population that might result in extinction.

But wait, what happens when an exogenous negative shock occurs? See, the island has a volcano that blows up once every 10,000 years. So now it blows up and spews hot ash all over the island. 90% of every species is killed. Now, which species bounce back the fastest? Look at our 2 options up above. It is clear, yes? “Rarely have children” means a lack of resilience in the face of such shocks. “Have lots of children” suggests a faster bounce back.

The truth is, immortality for the individual will usually lead to extinction for the species. This why immortality is relatively rare in nature. Lots of children, combined with rapid aging, offers both stable populations and fast recoveries after exogenous negative shocks – and thus, this strategy is the one predominates all over the world.

* (For the individual, the advantage of moderating the population is gained by not competing for resources with your own children – thus when rats face population pressures, the mothers eat their newborn children, rather than allow resources to go to waste by going into children who can not be kept alive, given the overpopulation. This is well explained in Richard Dawkins book, The Selfish Gene.)

(I’ve been working a video game about this, where the goal is to help the zunkzunks survive as long as possible, so I’ve had a chance to consider this question for awhile.)