Despite Long Lives, Humans Age Like Other Primates

Some scientists believe humans age more slowly than other primates, judging by the exceptionally long human lifespan. But a study in the journal Science suggests humans age just like baboons and chimps. Study author Susan Alberts discusses what the study implies about human longevity.

JOE PALCA, host:

This is SCIENCE FRIDAY. I'm Joe Palca. Ira Flatow is away.

It's not unusual for humans to live into their 70s and 80s, sometimes even to 100. But look at the lifespan of our primate relatives in Africa, gorillas, for example, and you'll see much shorter lifespans. Chances are good that a gorilla won't make it past 40. The same goes for chimps.

The fact that we live so long has led some researchers to think that our clocks must tick more slowly, that we age at a more leisurely pace. But a study out this week in the journal Science suggests that isn't true. We humans like to think we're exceptional, and we certainly are in some ways, but this appears not to be one of them.

Joining me now to talk more about this is one of the authors of the study. Susan Alberts is a professor of biology at Duke University. She's also associate director for science at the National Evolutionary Synthesis Center in Durham, North Carolina. She joins us by phone from way out in the bush in the Amboseli National Park in Kenya. Welcome to SCIENCE FRIDAY, Dr. Alberts.

Dr. SUSAN ALBERTS (Duke University): Well, thank you. It's nice to be with you.

FLATOW: And if you'd like to talk with us about this topic, our number is 800-989-8255. That's 800-989-TALK. And if you're on Twitter, you can tweet us, @scifri. And if you want more information about what we're talking about this hour, go to sciencefriday.com.

So Susan Alberts, when I say way out in the bush, I guess I really mean way out in the bush, right?

Dr. ALBERTS: Yes. I'm in our research camp in the Amboseli National Park, sitting in my tent, and it's 11 o'clock at night here.

PALCA: Wow, thats - it's just - that technology alone is pretty remarkable. But tell us a little bit about this study. What were you looking at? What were you trying to compare in terms of age length or longevity?

Dr. ALBERTS: Well, the reason that we wanted to do this was because it's long been suspected that humans were pretty unique in their aging patterns. That's a natural thing to suppose when you compare humans to almost any other animal that we sort of have ready access to because we obviously live so much longer than most of them.

But what we had never really had available was a reasonably large set of other species of animals that are pretty closely related to humans, like primates are. And so our dataset really offered an opportunity to do that.

Dr. ALBERTS: We particularly looked at two different ways of measuring aging, or I should say two different measures that contribute to aging. One is what we call the initial mortality rate. It's the rate at which animals are dying when they enter adulthood, that initial mortality rate at adulthood, which varies. In some species, it's higher; in some species, it's lower. But it sort of sets the pace of aging, if you will.

And then the other thing we looked at was the rate at which the probability of dying increases as a function of age. So those are the two metrics we looked at.

PALCA: Well, do - I mean, I presume that all primates don't enter adulthood at the same relative age. Or do they?

Dr. ALBERTS: That's right. No, no, they don't. Every species has its own life-history pattern, and every species reaches adulthood at a different age. Nonetheless, the rate of mortality at the onset of adulthood is one of the metrics that sort of sets the pattern of mortality throughout the lifespan.

PALCA: And so...

Dr. ALBERTS: It's considered an important component of aging.

PALCA: Huh. So where do these statistics come from? I mean, you say you have these datasets. How have they been collected?

Dr. ALBERTS: Well, so that's one of the really exciting and special things about our dataset. We had seven different groups of researchers come together for this study, each of whom have been watching primates in the wild for at least 25 years.

That literally means day in, day out, somebody's in the field watching every animal in the study population and collecting data on how long they lived.

And a few years ago, several of us got together and asked the question: Gosh, can we get together and start comparing these data across these very diverse primates - two South American species, one Madagascan species and several African species - and really look at comparative mortality patterns in a way that had never been done before.

PALCA: Right. And, I mean, so how - the differences, then, are not explained by some sort of internal clock ticking at a different rate, or at least that's what your data seems to be showing.

Dr. ALBERTS: Well, I don't know if I'm going to give an opinion about the internal clock. I think that what our data show is that there is a lot of similarity in those two basic aging metrics across all these different primates. And humans are not outliers.

In fact, if you look at human females, they pretty much cluster right in there with a couple of other species of primates, and it's hard to differentiate, statistically, the human data from the data on other primates.

If you look at human males, human males do age more slowly than other primates, which is interesting because if we just consider humans, we think of males as aging more rapidly than women, and we think of men as living shorter lives than women.

But if you compare men to other male primates, human males are actually aging more slowly than other male primates. But in spite of that difference, it's not an enormous difference between humans and other primates, as some researchers have suspected it would be.

It's a small difference. It's a significant difference, but it's a small difference in men. And in women, it's actually not statistically significantly different in either metric.

PALCA: So any ideas of why this sexual difference here?

Dr. ALBERTS: Why the sexual difference between men and women?

PALCA: Yeah.

Dr. ALBERTS: Well, so in the overall primate dataset, we found pretty clear evidence that males live shorter lives than females. Males age more rapidly than females. And this was not surprising. It's been known from a number of other species in the last few years, actually. People have started to compile these kinds of data on other wild animals.

And there's pretty good evidence that in species that are monogamous, where males don't compete intensively for access to each mating opportunity, male and female mortality patterns are more similar to each other than they are in species where males compete intensively for access to reproductive opportunities, in polygynous species.

PALCA: Wait, wait. Say that word again. What word was that, polygynous?

Dr. ALBERTS: Polygynous, that means - think of the term polygamous. It'll be familiar to you: mating with multiple mates.

PALCA: Got it.

Dr. ALBERTS: And humans are, you know, a lot closer to being monogamous than most of the other primates we looked at. And that may explain why human males age more slowly than males of other primates.

PALCA: There are so many issues there that I am not even going to speculate on in the slightest.

(Soundbite of laughter)

PALCA: So why don't we take a call? Why don't we take a call and go to Glen(ph) in Columbus, Ohio. Glen, welcome to SCIENCE FRIDAY.

GLEN (Caller): Hi, I've never accepted the idea that a long time ago, people were considered old when they hit 35 because of some kind of statistical average. I think a lot of people died from a lot of different reasons, but they didn't age any differently than they do now. They just got disease, or they died in childbirth or childhood diseases, and that throws off the statistics. And I had a follow-up question.

PALCA: OK. Well, what do you think? I mean, we are sort of talking, Dr. Alberts, about the difference between lifespan and aging, I guess.

Dr. ALBERTS: Well, yeah, I'm actually going to respond to Glen's question, which I think is a really interesting and important one, by saying that it's a really hotly debated question whether or not prehistoric humans had lifespans that look like our lifespans.

And many people say not possible, they must have died at a much younger age. It's unthinkable that we had anybody living to the age of 80, say, in prehistoric times.

And the fact is that the data are not in yet on that. I think that eventually we will be able to have large enough sample sizes of skeletons that we can age, that we can estimate the ages at death for, to get a really good answer to that question.

But at the moment, it's pretty speculative, much as it's been speculative that humans age differently than other primates until this dataset became available.

GLEN: Is there anything - we hear in the news that girls in developed countries are reaching puberty at an earlier age. Is that going to have some influence on their overall lifespan?

Dr. ALBERTS: Probably not. I'm not going to speculate about that. My guess would be not because it's almost certainly an environmental and not an evolutionary phenomenon. And if there were an effect on lifespan, it would have to be something like reaching menarche earlier imposes costs. Reaching maturity earlier, reaching puberty earlier, imposes costs and, say, shortens your lifespan in the future.

And in the West, we live in such a rich environment. It's really possible that reaching puberty earlier just means that you live in such a fabulous habitat that you'll also live longer.

It's - we're probably not going to see an effect on lifespan, but I wouldn't want to be quoted on that 100 years from now.

PALCA: OK, we'll destroy the tapes.

(Soundbite of laughter)

Dr. ALBERTS: OK.

PALCA: Glen, thanks very much for the call. Let's go - we have time quickly for one more call. Lawrence(ph), I'm sorry, Matt(ph) in Lawrence, Kansas, I presume. Welcome to SCIENCE FRIDAY.

MATT (Caller): Yes, hi. I had a question: Are primate telomeres the same length, and, you know...

PALCA: Are they shortening? These are - the telomeres are this thing at the end of chromosomes that get shorter as cells age, right?

MATT: Correct.

PALCA: OK, so Dr. Alberts, have you looked at that at all?

Dr. ALBERTS: Well, what a great question. In order to look at telomere length, you have to have really high-quality DNA from a fairly large number of individuals. And there aren't - I think it's fair to say - aren't very many wild populations where that's been possible yet.

It's very hard to get high-quality DNA from wild primates because they don't give up their DNA that easily. And it's pretty challenging to get those kinds of samples.

However, what evidence there is suggests pretty clearly that primates, like all animals, as far as we know, have the same problem with telomeres that humans have, which is that they shorten with age.

PALCA: It's a fascinating field, and I thank you so much for staying up and -OK, sorry, we just - we have time I guess for one more quick question. Why are these - I mean, you mentioned that DNA was hard to get a hold of. Why so? I mean, you don't need very much to be able to do some sampling.

Dr. ALBERTS: Well, that's true, but you have to get tissue from the animals to do it, and many primates are endangered, which means that it's very difficult to get permission to dart them or immobilize them in order to get tissue. Even if you can get permission, it could be logistically challenging to dart an animal that lives in the trees.

Increasingly, actually, a lot of primate biologists are using fecal samples, dung samples, to get low-quality DNA that you can nonetheless do a lot of work with if you try really hard.

PALCA: OK, well, I'm sorry. After asking you another question, I have to cut you off, I'm afraid that's all the time.

Dr. ALBERTS: That's OK.

PALCA: Susan Alberts is a professor of biology at Duke University. We'll be right back.

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