The Record Book of Longevity
Beneath the surface of the ice-blue Greenland Sea glides a huge shadow. The twenty-foot giant isn’t in a hurry; its top speed is less than two miles per hour.
In Latin it’s called Somniosus microcephalus – ‘the sleepwalker with the tiny brain’. In English, it has a slightly more flattering name: the Greenland shark. As its Latin name suggests, this shark is neither fast nor particularly quick-witted – though despite this, you can find the remains of seals, reindeer and even polar bears in its stomach.
Our mysterious companion takes its time because time is something it has a lot of. When the United States was founded, it was already older than any human has ever been. When the Titanic sank, it was 281 years old. And now, it’s just turned 390. Despite this, researchers estimate that it could have several more years to live.
This is not to say that the Greenland shark has no problems. Its eyes are infected with bioluminescent parasites that are slowly making it blind. And, despite its impressive size, the Greenland shark shares an enemy with all other inedible fish – Icelanders. You see, the flesh of a Greenland shark contains so much of a toxic substance called trimethylamine N-oxide that you get dizzy – ‘shark-drunk’ – from eating it. But, of course, the brave people of Iceland have found a way to do so anyway.
The Greenland shark is exactly the kind of animal that belongs at the top of some kind of list. And that is where we find it. With its impressive lifespan, the Greenland shark is the longest-lived vertebrate ever recorded. Being a vertebrate – an animal with a backbone – it is actually our distant relative. We might not look much alike, but the basic anatomy is recognisable: a heart, a liver, an intestinal system, two kidneys and a brain.
Of course, there is still quite some distance on the evolutionary tree between us and a giant fish. Humans are mammals, and that means we have certain fundamental characteristics that we don’t share with the Greenland shark. In biology, the rule of thumb is that the closer an animal is to us in evolutionary terms, the more we can learn about ourselves from studying it. That means we can learn more from fish than from insects, but also that we can learn less from fish than we can from, for instance, birds and reptiles. Not to mention our closest relatives – other mammals.
Oddly enough, the Greenland shark shares its home with another lifespan record-holder that is a much closer relative of ours. If you’re fortunate in the seas around Greenland, you might encounter the sixty-foot-long bowhead whale. While the surface characteristics of a bowhead whale don’t resemble ours either, its inner wiring is much closer to humans than that of the Greenland shark. Whales have large brains, even for their size, four-chambered hearts like us, lungs and many other common characteristics.
We used to hunt these magnificent animals to use their blubber in oil lamps, but fortunately they are protected today. Only native peoples, such as the Iñupiat people of Alaska, are allowed to continue hunting them – at subsistence levels, as they have always done. Occasionally, after a successful hunt, the Iñupiat will visit local authorities to hand off old harpoon tips recovered from the whales’ blubber. These harpoon tips stem from unsuccessful hunts in the 1800s. Together with molecular methods, they have been used to determine that bowhead whales can live more than 200 years. That’s the longest lifespan recorded for a mammal.
Moving away from humans on the evolutionary tree can reveal some even more impressive lifespans. The best examples come from actual trees, for whom ageing doesn’t really exist – at least, not in the way that we typically understand it. While our own risk of dying increases as we age, trees only get larger, stronger and hardier. That means trees have a decreased risk of dying each year they live. At least up to the point where they get so tall that they get knocked over in a storm. But dying in an accident has nothing to do with ageing.
This means that some trees are really old. One of the oldest single trees, Methuselah, is a 5,000-year-old bristlecone pine in a secret location somewhere in the White Mountains of California. At the time Methuselah sprouted from the soil, the pyramids were still being built in Egypt and the last mammoths roamed Wrangel Island in Siberia.
Yet even Methuselah is a spring chicken compared to the wooden record-holder. In the Utah Fishlake National Forest, roughly 350 miles northeast of Methuselah, is an American aspen named Pando. Pando (Latin for ‘I spread’) is not a single tree, but a kind of superorganism – a giant network of roots filling an area around one eighth the size of Central Park in New York.
Pando is the heaviest organism on the planet and sprouts more than 40,000 individual trees. Most of these trees live between 100 and 130 years, dying off in storms, fires and so on. But Pando continuously sprouts new trees, and the root network superorganism itself is more than 14,000 years old.
Obviously, I can’t write a chapter on exceptionally long-lived organisms without mentioning turtles. One of the oldest turtles ever was the radiated tortoise Tu’i Malila, who lived with the royal family of the tropical island kingdom of Tonga. Tu’i Malila was given as a gift to the King of Tonga by the British explorer James Cook in 1777. When she died in 1965, as a very old lady, she had lived about 188 years. That’s the age record for any turtle whose age we can verify with certainty. However, Tu’i Malila is about to be overtaken by the Seychelles giant tortoise, Jonathan, who lives on the tiny Atlantic island of Saint Helena. Jonathan was hatched around 1832 – before the invention of the postage stamp – and has lived through the reigns of seven British monarchs and the terms of thirty-nine US presidents. By the time you’re reading this, Jonathan might be the new record-holder.
While some organisms can live significantly longer than us, others have different ageing trajectories altogether. That is, ageing happens to some organisms in a completely different way than it does to us.
As humans, we age exponentially; after puberty, our risk of dying doubles approximately every eight years. This happens as our physiology gradually declines, making us ever frailer. Our way of ageing is the most common one and we share it with most of the animals we’re in daily contact with. However, it is by no means the only pattern of ageing in nature.
There’s a particularly weird group of animals that reproduce only once, followed by immediate and rapid ageing. This is called semelparity, and if you like watching nature documentaries, you might recognise it from the life cycle of Pacific salmon.
Pacific salmon hatch in small streams, where the tiny salmon mature in relative safety. Later, they head out to sea, where they stay until eventually becoming sexually mature. At some point, it’s time to make the next generation of Pacific salmon, but unfortunately the salmon only breed in the exact stream in which they hatched themselves. That means the poor fish must swim back inland – sometimes a distance of hundreds of miles – against the current and uphill. It still boggles my mind that any fish is actually able to make it up a waterfall. It’s a wild journey.
Even more unfortunate for salmon is the fact that we are not the only animals aware of how tasty they are. When the salmon start migrating, every single local predator – bears, wolves, eagles, herons and so on – are patiently waiting, ready to feast. To give itself a shot, the Pacific salmon pumps its body full of stress hormones and completely stops eating. Every day and night becomes a tireless battle against Mother Nature herself. Most salmon don’t make it, but the few who do go on to spawn the next generation in the very streams in which their own lives began.
Having achieved this feat, you might think the hardy salmon would have no problems returning to the sea. After all, this trip would be downhill and helped along by the current. But the salmon show no interest in even trying. Once they’ve spawned, they go into terminal decline, like plants withering in an instant. A few days after hiding their fertilised eggs in the sandy riverbed, the entire previous generation is dead.
This kind of bizarre and rather tragic life story is actually more widespread in nature than you might think. Here are some of my other favourite examples:
Conversely, there are also some animals that don’t age at all – at least, not in the way we traditionally define ageing. One such example is the lobster. Just like trees, the king of the crustaceans doesn’t get weaker or less fertile as time passes. Quite the contrary, actually – lobsters grow continuously throughout their lives and get stronger and stronger over time. Of course, this doesn’t mean that they live forever. Nature is cruel, and eventually predators, competitors, diseases or accidents will do the job. If not, the biggest lobsters end up dying from physical problems due to their large size. Old age for a lobster, however, is not at all the gradual decline we know ourselves.
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Nature also hosts organisms that have developed some truly peculiar tricks to prolong life. Some bacteria, for example, can go into a kind of dormant state. When stressed, the bacterium transforms itself into a compact structure resembling a seed. This structure, called an endospore, is resilient to anything nature might expose it to – even extreme heat and ultraviolet radiation. Inside the endospore, the processes normally required to sustain the bacterium are all paused. It’s as if the bacterium isn’t even alive anymore. However, the endospore can still sense its surroundings. When times get better, it can unpack itself and become a fully active bacterium again like nothing ever happened.
Exactly how long bacteria can spend in their dormant state is hard to say. Maybe there’s not really a limit at all. It’s routine practice for scientists to revive endospores that they have found which are over 10,000 years old. In fact, there are reports of endospores being awakened after millions of years of dormancy.
I think, however, that I would give the prize of ‘greatest ageing trick’ to the tiny jellyfish Turritopsis, the namesake of this book. To the untrained eye, Turritopsis seems kind of dull. It’s a tiny jellyfish roughly the size of a fingernail that spends its life drifting around eating plankton.
But treat it right and Turritopsis might reveal its secret.
If the tiny jellyfish is stressed – for example, by hunger or sudden temperature changes in the water – something strange happens: it reverts from its adult form back to something called the polyp stage. This is akin to a butterfly turning back into a caterpillar, or to you having a stressful day at work and deciding to revert back to being a kindergartner again.
When Turritopsis returns to its polyp stage, it is in fact ageing backwards. Afterwards, it can grow up anew with no physiological recollection of having been older. To make this Benjamin Button-esque trick even more impressive, research suggests that Turritopsis can repeat its rejuvenation again and again. Obviously, being a tiny jellyfish in a huge ocean means that Turritopsis doesn’t live forever in the wild. Eventually, something will eat it. However, it’s quite possible that it could live forever in the safety of a laboratory. Turritopsis may well be an example of the holy grail of ageing research – biological immortality.
As it is with all good ideas, though, odds are someone else had it too. While Turritopsis is my favourite example of backwards ageing, nature actually has other examples too, including another ‘immortal’ jellyfish, Hydra, and a primitive flatworm called Planaria. When there is plenty of food, Planaria, like Turritopsis, lives an unimpressive life. But if its food disappears, it reveals a special trick. A starved Planaria will eat itself, starting with the least important parts, and it doesn’t stop until nothing but the nervous system is left. This buys the flatworm time in the hope that conditions improve. When Planaria senses that better times are ahead, it can then rebuild itself and begin its life anew. While other worms of a similar age die, the rejuvenated Planaria will swim around, still full of youthful energy. In fact, the Planaria flatworm is so good at regenerating itself that you can cut it in half and – instead of ending up with two halves of a dead flatworm, you end up with two living worms.
Imagine if we could one day learn how these animals work their magic.
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Bowhead whales live a long time. So do twenty-foot Greenland sharks and large tortoises. Do you detect a pattern? How about if I told you the average mouse has to be lucky to live for two years – even in the protection of captivity?
The secret these long-lived animals share is their size. In general, large animals live longer than small ones. Whales, elephants and humans are long-lived. Most rodents are not.
The evolutionary reason is probably that size protects against predators. When the risk of becoming someone else’s dinner is smaller, there can be an evolutionary pay-off from a slow life course. That is, a life course characterised by maturing slowly, having few offspring that are nurtured for long periods of time and investing in the upkeep of the body. On the other hand, if a species is constantly in danger, it doesn’t make much sense to live for the future. Instead, such a species should mature as quickly as possible, disregard the future for the present and get a ton of offspring in the hope that fate will be kind to at least some of them.
One example that brilliantly illustrates this trade-off is the opossum. Biologist Steven Austad was studying these small marsupials in the Venezuelan rainforest, when he began to wonder why they seemed to age so rapidly. If Austad caught the same opossum twice, there would be visible physical differences, even after just a few months.
Photos of the rainforest might make it look like paradise, but in reality it is more like a tropical nightmare for its inhabitants. Danger lurks behind every tree trunk and the life course of the resident opossums reflects that. The opossums have evolved to focus less on bodily upkeep and instead on the mission of reproducing before something eats them. Conversely, Austad also managed to find a population of opossums living in something that is akin to opossum paradise. On Sapelo Island, off the coast of Georgia in the United States, there are no predators, and the local opossums spend their days lounging in the sun, carefree. This population of opossums have lived in relative protection for thousands of years. And as a result, they have evolved longer lifespans than their mainland cousins – when the likelihood of surviving is higher, there’s a bigger pay-off for focusing on bodily upkeep.
That a relatively safe life allows the evolution of a longer lifespan could also explain our own special status: even though humans are large mammals, we live longer than you’d expect from our size alone. The reason is probably that we’re at the pinnacle of the food chain. Most animals are smart enough to avoid us and you can imagine the ones that weren’t so inclined learned the hard way back in the Stone Age.
Similarly, this hypothesis also explains some of the exceptions to the rule about size and lifespan. Most of the small animals that have managed to buck the trend share a similar adaptation which helps when evading predators: they can fly. For instance, birds live longer than mammals of the same size. And the only flying mammals, bats, live three-and-a-half times longer than other mammals of a similar size.
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Now that I have convinced you that large animals live longer than small ones, which dog breed do you think lives the longest: a Great Dane or a chihuahua? If you’re a dog lover with a preference for larger breeds, you might know that one of the tragedies of this love story is that large dogs don’t live very long. A Great Dane will typically live for around eight years, while small dogs, such as chihuahuas, Jack Russells and Lhasa Apsos, can live more than twice that long. The reason is that while large animal species live longer than small animal species, the opposite is true within each species. That is, small individuals live longer than large individuals. Ponies live longer than horses, for instance, while the species lifespan record-holder for mice is held by something called the Ames dwarf mouse.
In the same way, female mammals almost always live longer than males of the same species. This rule holds whether you’re looking at lions, deer, prairie dogs, chimpanzees, gorillas or humans. But why? One clue is that female mammals are almost always smaller than their male counterparts. Among humans, men’s bodies are fifteen to twenty per cent larger, and on average women live a few years longer. In the few species of mammal where males and females are equally sized, like hyenas, males and females have roughly equal lifespans.
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We’ve yet to meet the animal that life-extension researchers hold dearest of all.
Our anti-ageing all-star originates in East Africa, but is nowhere to be seen in the vast savannah landscape. Dig a few inches underground, though, and this tiny animal can be found scampering away through the miles-long tunnels it’s constructed.
The naked mole-rat, as this creature is called, is not a favourite among scientists due to its looks. Imagine the rat from your worst nightmares, and then keep going. Its skin is bald, pink and wrinkled. Scattered long hairs protrude from its body. Its front teeth, used for digging, are located outside the mouth. And its barely functional eyes are nothing but tiny black dots.
Despite its looks, though, the naked mole-rat has plenty of friends. The creature’s East African tunnel kingdoms are built and maintained by colonies of 20 to 300 members who patrol them in search of enemies and food.
When off-duty, colony members reside at the headquarters, where there are rooms for food storage, sleeping quarters and even toilets. The colony’s headquarters is also the domain of the most special naked mole-rat of all: the queen. You see, a colony of naked mole-rats does not work like a normal herd of mammals. Instead, the small rats are some of the only mammals that are eusocial, the type of social structure we more commonly associate with insects such as ants and bees. The queen is the only naked mole-rat to have cubs, while the rest of the colony consists of temporarily sterile workers and soldiers – except for a few males that the queen has selected as her boytoys.
Researchers of ageing find naked mole-rats so fascinating because they don’t conform to the usual correlation between size and lifespan. An adult naked mole-rat weighs about thirty-five grams, which is not much heavier than a mouse. Despite this, naked mole-rats live well over thirty years, while the species record for mice is about four years.
To understand the importance of all this, imagine the following: you’re a researcher who wants to study ageing. Where do you look for inspiration? An obvious option is to study long-lived animals – maybe you can learn some of their secrets.
You think to yourself: Animals that live a long time . . . whales? Those would be a little hard to keep in the laboratory. Elephants? Same problem. Birds in little cages? Animal torture (besides, they aren’t even mammals). How about the naked mole-rat? Long-lived? Check. Can be kept in a laboratory? Check. A mammal like us? Check. So far, so good.
The next challenge you face is finding something to which you can compare your animal. The obvious choice is to use a short-lived relative. Then you can examine the differences between the two to see if you can explain their dissimilar lifespans. Here, again, it turns out that the naked mole-rat is a perfect choice. The two most-studied laboratory animals – mice and rats – happen to be closely related to the naked mole-rat while having very dissimilar lifespans. So this little creature is perfect for studying ageing.
Researchers around the world have beaten us to it and have been studying naked mole-rats for decades by now. These researchers report that it’s almost impossible to tell the difference between young and old naked mole-rats. One might add that the threshold for looking young is pretty low for a naked mole-rat: you just have to be hairless and wrinkled. Nonetheless, it’s an interesting observation. Not only do scientific tests show that naked mole-rats age slowly— we can also see it.
Naked mole-rat researchers also report that their animals are virtually immune to cancer, even when the researchers try to induce it artificially. Out of the thousands of mole-rats studied, only six tumours have ever been found. That’s particularly remarkable in such a small animal. By comparison, signs of cancer can be found in seventy per cent of all laboratory mice after their deaths. And, in general, it is normal for twenty to fifty per cent of individuals to get cancer in any given species, including our own. In many developed countries, for example, cancer has overtaken cardiovascular diseases as the most prolific killer. And yet somehow, this small, obscure rodent from East Africa has found a way to tame the disease. A miraculous creature indeed, and one that has a central role to play in our unfolding story of ageing.