How long are we supposed to live for?

If our evolutionary purpose is simply to have children, then what's the point of being old?
23 May 2017





Is there an ideal biological age? In other words, how long are we “supposed” to live for?


Chris put this question to Andrew Holding from Cambridge University.

Chris - Andrew, an interesting question here from John who asks, ‘How long are we “supposed” to live for?’

Andrew - Well, thank you for that really cheery question. This is actually really interesting though. You can take the very simple idea that your only job as a living creature is to have babies, and then they go on and have more babies. If you take that approach, thirty is probably reaching a ripe old age because by that time you’ve had several babies. The mortality rate for having children is quite high, certainly for women over men, and your job is done.

That isn’t actually what seems to have happened in humans. At some point, we suddenly realised it was actually quite useful to keep old people around to look after the kids when the fit people went off hunting. So we’ve ended up where you find a point somewhere around Neanderthal to Homo Sapiens where the life expectancy starts going up. And, of course, we don’t know if the life expectancy goes up because of social change or a physiological change but we do know it suddenly became very advantageous to keep the old people around to look after your children.

Chris - Jess?

Jess - Is there something about the number of heartbeats that you have as a mammal? Humming birds with crazy numbers of heartbeats a minute have something like a much shorter life.

Andrew - The only thing I’d be careful about is humming birds aren’t obviously a mammal. But, if you look at mammals, like a hamster versus an elephant, you’re absolutely right and we are massively off that. It’s like a line between length of life versus heartbeat rate and humans are off it. They’re completely away from the line…

Chris - I think the point that Jess is eluding to is if you have a higher metabolic rate because you are small, because a hamster or a mouse has to burn a lot of energy to stay warm because it’s losing huge amounts of heat through this enormous body air relative to a tiny core. Whereas, we’re a bit bigger so we don’t have that constraint but yet, if you look at how long we live, it’s out of step with how long we should live and so people are, you're saying, suggesting there must be some sociological reason to keep people in the population? Could it not just be though that the world we grew up in was just so awful when we evolved as a species that we had to breed ourselves to be genetically superfit in order to compete with that environment, and it just means that that gives us this added longevity?

Andrew - Where it gets complicated is the fact that you can’t pass genes on after you have children. But in a way you because if your elder generation is helping, by proxy your offspring are going to survive even though they’re a generation delayed. That’s where it gets interesting because most animals don’t do that, you don’t see a multigeneration survival. And it also means that offspring of a different line survive and that’s when we get these really odd social effects, really hench benefiting a species which you wouldn’t other wise expect to see.

Chris - And we don’t see this, or do we see this in other animals? If we look at whales because they are very long-lived and elephants as well. Is there a similar sort of grandmother effect there?

Andrew - We don’t see this deviation from the heartbeats in anything other than humans as far as I’m aware.

Chris - Even though there are these long-lived animals that live in a big group?

Andrew - No. There’s no evidence for it.


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