Mammals, including humans, have natural lifespans that have remained about the same through each species’ existence. What are these figures, and why do they vary so greatly across species?

Question: How does the human lifespan compare to our closest primate relatives? 

Tyler Volk: Our closest primate relatives are the chimpanzees and gorillas, with the chimpanzees being closer to us genetically. They’re natural life spans are approximately half of ours. And that’s of some interest because the chimpanzees are a bit smaller than use in body mass and the gorillas are larger than us in body mass. We can’t know for sure the lifespan of the last common ancestor of humans and chimpanzees, but there would be some – you might guess that it would be half of what our current lifespan is. 

Question: This is the natural lifespan? 

Tyler Volk: Let’s make a distinction that we’re not talking about the infants dying – small children dying of diseases or death by predator, but the natural lifespan in normal, almost perfect circumstances. So looks throughout history, there’s always been people who have made it to age 80 or so, or longer, even though the average life expectance at birth, given all factors, given diseases, predators, and then the diseases of aging and senescence, even those have been more prevalent, the natural lifespan has been approximately the same as it is today. The maximum – the natural maximum lifespan, which is different from life expectancy at birth which can vary even in different countries today. Russia right now has a relatively low life expectancy, people in Africa have a relatively low life expectancy, Japan has the highest life expectancy of any nation now. So, there’s variations, but take those individuals into relatively equal healthy environments and they’re all going to live close to the same age. 

Question: And this natural life expectancy has not gone up? 

Tyler Volk: Right. The natural life expectancy has not gone up very much. However, since the diseases, some of the diseases that modern medicine is tackling, such as heart disease and cancer, become more and more the diseases that we are dying from in elderly age, we can be expected to live longer without trying to genetically go in and manipulate our metabolisms in some ways. There’s a lot of work being done on what is called caloric restriction. 

There’s a lot of research being done on what is called caloric restriction. Animals that are given reduced calorie diets, and yet have the essential nutrients that they need, live longer. We haven’t been able to do the experiments on human beings yet. There are people out there attempting to do this by themselves. You can get books and join organizations to try to enhance or help you – how you can make recipes that satisfy your hunger and have caloric restriction. 

I’m saying this to show that there are probably going to be ways that science is going to understand this natural demise, this metabolic demise of our repair mechanisms that set up our natural lifespan that has kept it pretty constant for a long time. And we’re probably going to bring that forward. We’re going to live longer lives I really think. I don’t know if it’s right around the corner, that’s hard to judge. You’ve had people on your show that are saying what they think it’s going to be. But just from reading the literature, it’s clear that these kinds of advances are going to happen. 

Question: Why do lifespans vary across species? 

Tyler Volk: I find it really fascinating to consider why certain species of mammals live longer than other species of mammals. For example, we live longer than dogs, dogs live longer than mice. Often there’s a tendency, or trend, that the large creatures live longer. But you might try to say, from an evolutionary viewpoint, that it would serve creatures well to live for a long time. They can reproduce more, let’s say if they live for longer, and therefore can pass on their genes for even a longer period of time. But we know that there’s a large variation in when creatures senesce and what the average lifespan is. And it turns out, there’s two ways of looking at this. One is to go down deep into the organism and ask, why are the cellular repair mechanisms breaking down when they do; a couple years in the case of some small mammals. For us it’s many decades, 70, 80, 90 years. But the other way to look at it is that these cellular repair mechanisms themselves must be subject to evolution. We know that these repair mechanisms vary among creatures. 

It’s been shown that birds, for example, have better cellular repair mechanisms than mammals do. The current reasoning has to do with the ecological niche that a certain creature lives within as a member of its species. And if that niche allows the possibility for many of the individuals to live long lives, then it has been worthwhile for the evolutionary process to build in better repair mechanisms for its cells to allow it to live longer. So, if I go back to the example of birds, the phrase that’s sometimes used in the technical literature for birds and why the birds live so much longer than the mammals of the same body mass is the phrase, “fly now, die later.” And the idea being that birds in the trees and in flying have very good predator escape mechanisms that make it worthwhile for the birds to have cellular repair mechanisms inside their bodies that allow this longevity for a certain body mass to occur. 

And what I find fascinating here is that there is a tuning of the creature’s niche, or environmental lifestyle and the possibilities that that lifestyle has for longevity and the very internal, deep internal, cellular repair mechanisms. The enzyme repair mechanisms issues about oxidative stress that either facilitate that longevity or cut the life short. One particular example I think is very telling is the case of the several species of the Pacific Salmon that live in the Northwest United States, Canada, and in Alaska. These salmon are born in upstream fresh water streams, or rivers. Very quickly, they go down to the saltwater oceans so they have a transition from fresh water to saltwater ocean. They live in the ocean for typically two to three years, depending on the species. They find their way back through a process somewhat mysterious, but maybe having to do with the water chemistry of their birth stream. They find they’re way back to their birth stream and at that point, the males and the females undergo some physiological changes. Their bodies turn more red, they bolt out – the males, the jaws get very bulked out. And if you look at what’s happening hormonally in them, it’s like they’re on an incredible dose of steroids. They’re revved up for this upstream swim that we see dramatic pictures of where they’re swimming these rapids and can they hop this dam or not, or do they have ladders to go up. 

They go upstream and the males and females mate. The females lay the eggs in these little depressions in the sediments, and then the males and females all die. They do not go back downstream to say, live another year and come back upstream. And you might think that what’s happening is a real waste of these salmon. It’s been shown that they add some nutrients to the stream water, but they’re not dying to add nutrients to the upstream waters, they’re dying because what’s happened to their bodies has put such stress on their bodies that they’ve done this incredible swim and put all their efforts into getting to a place to mate and into mating. And this is a wonderful example of how it’s important for these organisms to remain healthy up until sex, up until successful sex and reproduction and then it’s possible to die. Senecessence is, it’s possible just after sex. If there were senecessence before sex, that creature is out of the evolutionary game, obviously. But you could have death following right on the heels of sex and in the case of the salmon, this has occurred in evolution. So, we can see here that there is a tuning between death and when senecessence that happens and the ecological, or environmental, circumstances that have resulted in various adaptations in the case of Pacific Salmon. A really dramatic case.