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Why Do We Age? A 46-Species Comparison

Why we age is a tricky evolutionary question. A full set of DNA resides in each of our cells, after all, allowing most of them to replicate again and again and again. Why don’t all tissues regenerate forever? Wouldn’t that be evolutionarily advantageous?

Since the early 1950s, evolutionary biologists have come up with a few explanations, all of which boil down to this: As we get older, our fertility declines and our probability of dying — by bus collision, sword fight, disease, whatever — increases. That combination means that the genetic underpinnings of aging, whatever they are, don’t reveal themselves until after we reproduce. To use the lingo of evolutionary biology, they’re not subject to selective pressure. And that means that senescence, as W.D. Hamilton wrote in 1966, “is an inevitable outcome of evolution.”

Except when it’s not.

Today in Nature, evolutionary biologist Owen Jones and his colleagues have published a first-of-its-kind comparison of the aging patterns of humans and 45 other species. For folks (myself included) who tend to have a people-centric view of biology, the paper is a crazy, fun ride. Sure, some species are like us, with fertility waning and mortality skyrocketing over time. But lots of species show different patterns — bizarrely different. Some organisms are the opposite of humans, becoming more likely to reproduce and less likely to die with each passing year. Others show a spike in both fertility and mortality in old age. Still others show no change in fertility or mortality over their entire lifespan.

That diversity will be surprising to most people who work on human demography. “We’re a bit myopic. We think everything must behave in the same way that we do,” says Jones, an assistant professor of biology at the University of Southern Denmark. “But if you go and speak to someone who works on fish or crocodiles, you’d find that they probably wouldn’t be that surprised.”

What’s most interesting to Jones is not only the great diversity across the tree of life, but the patterns hidden within it. His study found, for example, that most vertebrates show similar patterns, whereas plants are far more variable. “You have to then begin to ask yourself, why are these patterns like they are?” he says. “This article is probably asking more questions than it’s answering.”

This sweeping comparison didn’t require particularly high-tech equipment; it could probably have been done a decade ago, if not before. But nobody had done it. One challenge is that it required a deep dive into the published literature to a) find the raw data on all of these species, and to b) get in touch with the researchers who conducted the field work to see if they’d be willing to share it.

After rounding up all of that data there was then the problem of standardizing it. Mortality and fertility rates of various organisms can differ by orders of magnitude. What’s more, for some species — like the white mangrove, red-legged frog, and hermit crab — this data comes from defined stages of development rather than across the entire lifespan. Jones got around these obstacles by defining “relative mortality” and “relative fertility” numbers for each species, calculated by dividing fertility or mortality rate at a particular age by the average rate across the organism’s entire lifespan. This allows for easy comparison across species, just by looking at the shapes of the curves.

“That’s what’s so disarming about it,” says David Reznick, a distinguished professor of biology at the University of California, Riverside, who was not involved in the new study. “They’ve come up with a way of putting everything on the same scale, so you can perceive patterns that have never been looked at before.”

The study shows, for example, that most mammals and, importantly, the species that scientists tend to use in the laboratory, such as C. elegans and Drosophila, have shapes like ours. But others are weird, at least from a human-centric view. Here’s a sampling:

Red lines show relative mortality and blue lines show relative fertility. Shaded areas show the proportion of individuals still alive at a given age. The box colors indicate the type of species: orange is invertebrates; brown is non-mammalian vertebrates; green is plants. From Jones et al., Nature 2013
Jones et al., Nature 2013

“Some patterns have emerged in this paper that none of us knew were there,” says Reznick, who has studied aging patterns among different populations of guppies. “It’s crazy to think that we’ve been working on aging for so long and something as fundamental as this hasn’t been seen before.”

What the new study didn’t find, notably, is an association between lifespan and aging. It turns out that some species with pronounced aging (meaning those with mortality rates that increase sharply over time) live a long time, whereas others don’t. Same goes for the species that don’t age at all. Oarweed, for example, has a near-constant level of mortality over its life and lives about eight years. In contrast, Hydra, a microscopic freshwater animal, has constant mortality and lives a whopping 1,400 years.

This is a problem for the classical theories of aging that assume that mortality increases with age, notes Alan Cohen, an evolutionary biologist at the University of Sherbrooke in Quebec.

“The traditional idea is that this is what most things do, and that there were a few weird creatures out there that were exceptions,” he says. “But there are actually a lot of exceptions.”

The question that the classical theories try to answer — How could aging evolve? — is no longer the most interesting question, Cohen adds. “What we really need to explain is why some things age and some don’t.”

Cohen is currently collaborating with Jones’s team to formulate a new theory that answers that question. (Stay tuned for more on this! I’ll be digging into all of the various theories over the next couple of months, as I work on a feature story for Mosaic, a new digital publication.)

Given my obsession with people, I asked some of these researchers what the new findings might mean for our understanding of human aging, which most of us would like to avoid. Will studying species that age like we do — or those that live 1,400 years, for that matter — help us defy age-related decline? Would these studies lead to treatments that might, say, double our lifespan?

Cohen politely reminded me that we have already figured out how to extend our lives. The new study, in addition to comparing 46 species, compared trajectories of three groups of humans: hunter-gatherers, those who were born in Sweden in 1881, and modern Japanese women. The differences are stark:

Red lines show relative mortality and blue lines show relative fertility. From Jones et al., Nature 2013

“In industrial societies, we continue on average to add about a year of lifespan every five years,” Cohen says, thanks to advances in public health, nutrition, and medical care. That’s pretty impressive, and likely to continue if we all eat well, exercise, and avoid stress and smoking, he says. “That’s not going to get us living to 200, but it might eventually get us to 110.”

35 thoughts on “Why Do We Age? A 46-Species Comparison

  1. What I find fascinating about beings with a constant mortality rate (such as Oarweed) is that there seems to be some kind of clock running out. If I am understanding the article correctly then these beings live lives during which they don’t age and then, one day, simply drop dead(?)

  2. The second paragraph has a couple phrases that might be misunderstood. While total mortality increases with age in organisms that age, that obviously doesn’t necessarily apply separately to each cause, such as sword fights. (In the wild, deaths due to external causes such as predation often increase with age as animals become weaker.) Also, physiological manifestations of aging (possibly determined by genetics) can show up before maturity; it’s just that natural selection really doesn’t distinguish ages before reproductive maturity: dying one day after birth is the same as dying one day before reproduction, from an evolutionary demographic standpoint.

  3. A reply to Axel Rauschmeyer:
    No, there is no clock postulated here, just a constant probability of dying. It’s like getting struck by lightning: it happens to some percent of individuals, and doesn’t depend on age. Even though the risk of getting struck is small, with enough time, everyone will get struck and everyone will die. That’s what happens in species like Oarweed and Hydra – they don’t age, but they can still die from random things, and the chance of random death doesn’t change as they get older.

  4. I don’t understand the connection with classical theories of aging. The “relative” mortality is not important – the absolute mortality is. It would be exceptionally surprising to find an animal that was highly likely to die an accidental death per unit time – and yet had a long lifespan if left to its own devices. An organism in such a circumstance would have to reach sexual maturity quickly and make the most of it before succumbing to disease, predators, etc. Evolution would have no interest in whether their “lifespan” was 10 or 100 years because they’d never make it.

  5. I don’t think we really figured out how to extend our genetically programmed life spans. People who reached very old age existed at all times. We simply managed to extend the *average* life expectancy by reducing metabolic strain during most of our life time (better medical aid and better nutrition). We therefore enter the part of the curve that gets really steep in better starting condition allowing more of us to get into the extremes in that curve. This does not necessarily mean we ‘extended our life spans’ in the sense of genetically programmed death (if that’s what aging is… it might be. The differences in the species suggest that, I think).

    Also, we managed to keep people alive who are not actively living anymore. It is an ethical question, but are decades in vegetative state really an extension of life expectancy or should we better not count in this ‘suspended animation’?

  6. {Virginia Hughes here. Dr. Reznick wrote to me with an additional point, which I’m copying here with his permission}

    There is one thing that they mention in the paper but gloss over, which is the dramatic differences in the nature of the data. Some results come from the lab and some from nature. Guppies look like humans, which we can only see because of the way they transformed the data, but the guppy data were collected in a fashion that makes them comparable to humans, at least in modern society. They were in a lab, protected from extrinsic sources of mortality like predation or disease. Tortoises are quite different (appearing in the lower right of the figure, rather than the upper left), but the data were collected on a wild population. This means that the data confound intrinsic aging with externally imposed mortality risk. Baby tortoises are edible to many predators, like ravens or coyotes, but adult tortoises are pretty invulnerable. What you see in the figure is a combination of the age-specific reduction in risk of predation confounded with the age-specific increase (maybe?) in mortality risk because of intrinsic factors. It really is important to discriminate among these sources of mortality.

  7. Hello,
    This is why we age.
    Then the Lord God took the man and put him in the garden of Eden to tend and keep it. 16 And the Lord God commanded the man, saying, “Of every tree of the garden you may freely eat; 17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.”

    The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away.

    I know, I know. Most don’t believe this. It’s God’s word and explains why we die.


  8. Owen here, the lead author on the paper. How to deal with mortality from predation in these kind of studies is tricky. If an individual becomes weaker with age, and therefore more vulnerable to predation, should a death from predation be considered to be senescence-related? Or should we ideally exclude individuals suffering death from predation from the data set. What about from parasites, or other pathogens? Where do you stop? These arguments hinge on whether mortality is classified as caused by “extrinsic” or “intrinsic” factors, but this is not black or white and it starts to become a matter of philosophy/semantics in the end.

    In humans “old age” is not a scientifically recognised cause of death since there are usually more proximate causes like heart failure, atherosclerosis, cancer etc. which incidentally affect younger people too. Should we treat predation, parasites etc. differently in non-humans?

    Assuming one *could* identify particlar sources of mortality, I would consider the sources/deaths to be part of senescence if vulnerability to deaths from those sources changed with age. If we include deaths that are *not* senescence-related in an estimate of mortality they will simply cause the entire mortality curve to be shifted up (i.e. adding a constant) without altering the steepness of the curve. I doubt if the qualitative shape of the curve would be affected that much.

  9. Alan Cohen here again, jumping into the debate between David and Owen, both with good points. But if we look at human data, curves are actually surprisingly similar between hunter-gatherers and modern Japanese. Clearly, modern Japanese are not being eaten by jaguars or dying of infectious diseases in large numbers. The key point is that vulnerability increases with age in a similar way, if slightly more sharply in the modern population. The big difference is in mortality before sexual maturity (what Daniel Levitis terms ontogenescence). So the solution to comparing aging across species: start the curves at reproductive maturity, as Owen et al. did.

  10. It astonishes me how people study aging without talking about or researching the pituitary gland. Gland that TELLS our cells to age. If that gland would not force our cells to age, we would be immortal. But oddly… no one makes experiments with this gland. Everyone avoids it. How else do you want to influence the aging? By spa’s and creams? This is stupid. Except maybe there is no interest in our immortality. It is better if we die at a fast rate, so the overpopulation is avoided for the moment? It is stupid, or rather these researches are meant for stupid people who do not pose questions, just act amazed by irrelevant studies.

  11. I am not a biologist, nor for that matter am I a theologian (thank the mercy of god for that one), but my understanding of aging has always been that the older we become, the more genetic errors our cells encounter as they reproduce. Our skin loses elasticity, the walls in the chambers of our hearts become thinner, our lungs lose their ability to regenerate alveoli (I hope I spelled that correctly), and so on. Someone once explained it to me like this; Imagine you are a cell shaped like an apple. Every time you reproduce a small piece of your stem gets broken off. Eventually you have no more stem left, and your ability to reproduce is compromised. We can extend the size of the stem on the apple, but not indefinitely. Eventually we simply run out of stem. I am not sure if this is accurate, or even relevant, but I thought I would add my two cents to this conversation. Aging is fascinating.

  12. Hi. Thanks for this interesting article. I can’t afford the $32 for a pdf (!), so I wonder if anyone can tell me what hunter/gatherer data were used for that part of the research? I was surprised by the lack of an elevated death rate at the beginning of the curve, as infant mortality is notoriously high in most h/g groups (due to both unintentional causes and what might be referred to as “family planning”). Thanks.

  13. I love this question … I can not find an explanation in the physiology and morphology? why we do not continue to grow and why we age? if time is our answer, I guess that’s not a researcher or can not be substantiated scientific, perhaps someone could explain or send a link or reference readings.
    why humans have limits?
    thank you

  14. As I read your article with my arthritis aching in my shoulder, neck, and back, I am wondèring the significance of such a study. Why would I choose to live to 110 years old anyway?

  15. I’m a Univ Bio drop out, I was pretty interested in this and know quite a lot about these subjects. I think the global answer to this question is pretty simple: complex oragnisms compared to simple organisms.

    I just googled the list of longest-living organisms, I already knew quite a few, but wanted to confirm my initial theory and I did.
    The human body is clearly the most complex oganism on the planet. So many things we still don’t understand about it, it’s also fairly new. New things usually are better, just compared computers, each generation is better, same happens with living organisms. In the end we are all machines, the huamn body is the most coplex machine ever invented. It’s very hard to do a constant maintenance on such a machine, aging is simply a fail to so. The body has quite a few protocols to self repair itself, but it can’t repair everything. Medical science started to fill the gap, with more advancements, we also live longer, the more we understand about this complex machine, the longer we live. I’ve heard various estimations from scientists that humans could eventually live up to 200 years easily, as science progresses.

    There have been several studies searching for a gene or genes responsible for aging. I don’t think they will ever find these. Like I said, it’s a simple matter of the body failing to repair itself. The skin is the best example. our skincells die quite fast and they’re replaced very easily. With time however something goes wrong and rimples start to appear. We overuse this organ and it starts failing.
    The body has other organs obviously and many of those don’t fail at all, never as much as skin does. Take the example of the liver or the heart, both these organs cannot fail, if they do we die, it’s that simple. Both these organs play various roles, if they stop working, various needs of our bodies are not met and we die. Our bones also stay quite functional over time, they main function is to keep the whole body’s structure, it also protects various parts of this complex machine.

    Now The entire human body does need maintenance. To anyone who got this far I’ll give 3 life prolonging secrets that have been proven over time, people know about these, but simply ignore these facts:

    -Nutrition: a good nutrition prolonges our lifespan. The most important tool to achieve this is knowledge about what we eat. You don’t have follow an extreme diet to eat healthy, it’s all about knowing what’s good and what not.You’d be surprised to know all the things you can eat with minimal harm. The quantity of food you need also matters a lot, feed the body the amount of calories it needs. If you’re very active, you’ll need more food. Btw being active doesn’t mean work a lot, doing a lot of passive activities, the ONLY real reason to require more food is doing a lot of sports. Various sports require a lot of energy, which is gathered from food. Any other excuses for extra amounts of food are just excuses to get fat.

    -You probably already saw this coming, but sports are the second secret. Working out, keeping the body in shape is not only natural, but it’s fun too. Like I already said several times, the body is a complex machine, it’s composed of various organs, all of them need maintenance. Bones get more dense with intense workouts, weight lifting increases the density of the bones directly, same goes for various sports that involve pressure on the bones. Running puts pressure on the leg bones and various other bones, but bones found in the arms will need lifting something for increase in density. A bigger bone density means it’s more healthy and lasts longer, also obviously breaks less easily. Blood pressure is decreased directly through sports. Various studies have proven this fact: a molecule is released directly after an intense activity improving the blood flow. The hearts benefits greatly from this obviously, meaning that oxigen flow goes better and faster, meaning the muscles get their oxigen faster. Muscles obviously need to be used, that’s the way you do maintenance on them. You’de be surprised until what age you can work out, I’ve seen people in their 80ies running marathons, even mega-marathons, these aren’t exceptions, there are many, it’s a lifestyle open to all. Only a few chose to follow it however, so many fail to see the major benefits from it. I also remember seeing a 94 year old woman doing push ups like crazy, clearly she was very healthy.
    Doing sports improves the entire body, no exceptions, even the skin structure is improved. As you move and use your muscles, the firmness of the skin is improve, a good example are the abs, tight abs are achieved over time, with sports and good nutrition, does somebody who doesn’t work out have a tight belly area? I don’t think so, the complete opposite.

    -I was slowly starting with third and last secret to a long life. The skin surface of the face can also be maintaned smooth, but it’s actually the opposite of the other skin surfaces. You need to move it as less as possible. You ask why and how, I’ll start with the how, slowly getting to the why. How? Keeping relaxed and calm. Feeling fear, anxiety, panic, these are all bad feelings, which are reflected on the body in more ways than you may think. The body needs to be relaxed to work at its best. Consider a relaxed body like a well oiled machine, you don’t see the oil on the machine, but it’s there and it’s doing its work. These negative emmotions are reflected directly on the face. A very nervous person, a very stressed person will have a lot more rimples than a relaxed person. Remaining calm and relaxed can be a choice, a lot of time stress comes only from unnessessary high expectations. Just try to remain calm in all situations. Some people have this as a natural ability, while other can learn this ability. Trust me a 100% on this fact, you can learn how to remain calm in all situations, just use logic and as guide. No situation is helped when you’re stressed in any negative way, remaining calm helps you to make the best decisions and the body also works the best relaxed, think of the compairaison I made with a well oiled machine, it’s exactly the same. A soldier usually goes through a lot of life threatening situations he remains calm and acts using the most logical paths. Staying alive is a direct result of his staying relaxed and handling the situation calmly. Remaining calm leads to a straith path to achieve whatever you’re trying to accomplish. Letting stress overwhelm you just leads you astray and makes the path longer.

  16. In early mans history, some lived to 5,6,7 hundred years old. The rainforest like atmosphere protected us from the suns radiation, which is THE main source of aging. I understand mans desperation to live longer, and a more vibrant life……but when you dig to the very root of this topic, its just another revelation, of mans fear of death.

  17. Why would you want to increase age or be immortal? The earth will die and you will go with it. I think its silly to try and live too long. There are reasons why we don’t.

  18. The longer I live and the more I observe of my surroundings I realize that much of what is said in the bible, (such as the verses quoted above by John) seem to be true. The more humans “experiment” with medicine and technology the more problems we create by disturbing the natural balance of life. Everything we ever needed to survive has been provided by “mother nature” so to speak.And t address the author’s final remarks ofbthe article. Honestly, why would we want to extend the human life span? In my opinion medical advances only prolong the inevitable. For someone diagnosed with cancer for instance, treatments such as, chemo, radiation, and surgery may extend a person’s life for a short period of time but eventually they succumb to the disease. And most often quality of life is not optimal. Also if our species continues to live longer and infant mortality rates decrease we will eventually overpopulate the earth, right? And just a side note, I just read a study that stated that deaths from cancer will increase by 50% over the next 20 years. I don’t remember the source but I’m sure it can be found by googling it.

  19. Hi Sebastien: I liked your comments, makes perfect sense.
    Just to show you excercize is important. The organizer
    for our badminton group and still plays just turned 90 yrs old.
    Plus there are a few in their mid 80’s who play and also
    keep active with other sports.

  20. I would like to know, if a species like the hydra were in a safe predator-free, disease-free environment, and it’s mortality numbers stay the same, how would it ultimately die? Or would it?

  21. From my understsnding of AP Biology (I am a high school student,) lobster can live indefinitely due to their ability to reconstruct telomerase. The enzyme that rebuilds telomeres. Telomeres are the caps at the end of a DNA strand that protect our actual genomic coding from running out and being unable to copy new cells during mitosis; however, many other factors include from over all dangers of the world. The key to understanding scientifical immortality lies within the pituitary gland (alerts cells of aging by sending a chemical release into the blood stream,) telomeres (protectional cap of DNA that cushions actual genomic sequences needed to program correctly,) and stem cell research (for the abilities to regrow any part of the body with correct genomic sequence.) All of these could lead to a better understanding of immortality, buuuuut who knows? Maybe cancer is Babel’s Tower.

  22. Alright, for those of you who believe that God set a limit which we should respect, let me remind you of the following; God created us in his image. If you accept this as fact, then you have to recognize that God created us to aspire toward his divinity. That aspiration should be in all things, including science, biology, and all other forms of knowledge. Our pursuit to understand aging is nothing more than our attempt to live up to God’s expectations for us. He created us to be like him, we should not ignore that gift.
    As for those who ask “Why would we want to live forever?” The answer is simple – time. Time to understand, time to learn, time to improve. Population concerns aside, I think many people realize with the advancement of their age that there simply is not enough time left.

    1. I have two basic questions here.

      1. I have heard people refer to the pituitary gland as being responsible for the aging process. Does anyone wish to elaborate on this, or provide links to any external scientific documentation on this. To what extent (percentage) are we saying that the hormones produced by the pituitary gland are responsible for the initiation of the aging process?

      2. Does telomerase rebuild the Telomere in the cells? If so, why not just keep the levels of telomerase in the body consistent?

  23. I think we are programmed to age to protect ourselves from passing on damaged genes. Species that aren’t programmed to age like we are, probably have more stable reproductive systems. Humans are subject to changes specifically engineered to prevent reproduction, such as wrinkles or growing a belly. Not by design, of course, but because people who reproduce when young give offspring better fitted for survival.

  24. I very much support the research done by SENS. I believe that it’s really important that we as a race are able to take complete control of our own lives, and not be forced to be stuck with what we are given at birth. I think we each individually should be able to choose or at least given the option of immortality.

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