From Greek mythology to Turritopsis jellyfish: an investigation into immortality.

The voyage has been smooth so far, with fair winds filling up your sails and allowing you to move quickly despite the marshland. The weather has been relatively calm, with not a grey cloud in sight. Too calm, perhaps.

Suddenly, you see a disturbance in the waves. It starts as a small ripple, but turns into an undulating bump that rises higher and higher — until a fearsome head pops out of the water. Your vessel wobbles violently as the monster’s snakelike body thrashes around beneath the water’s surface. Two more heads emerge beside the first. A hunting pack? But no — all the heads are connected to the same body!

Up rises the head, ready to strike anyone who dares resist it. But no, you think: two can play that game.

You pull out your sword and drive a vicious blow to the nearest snarling face. Adjusting your angle, you hack at it, again and again, until the head rolls off. Without wasting time, you swing around and attack the second head. Then you move on to —

Wait, something’s wrong.

The first head you chopped off? There are two more heads sprouting in its place.

If you peek carefully into a tropical freshwater pool, you may find tiny transparent tubular creatures, clinging to the rocks and water weeds within. No more than a centimetre in length, their body expands and contracts like jelly, with a sticky ‘foot’ at one end for clinging on tight. At the other end is a wide round mouth, surrounded by delicate tentacles. Woe betides the unfortunate insect that comes across these tentacles — for they will immediately take aim and fire a thin thread of neurotoxins.

The Hydra is a fearsome monster that lurks in the lakes of ancient Greek mythology. Cut off the head from the mythical hydra, and two more grow in its place.

The Hydra is also a small freshwater hydrozoan found across a lot of the world, from Alaska to the Amazon rainforest and from Siberia to Madagascar. And it has a regenerative power of its own. Chop the tiny Hydra into two, and both pieces will grow back: the head will grow a new foot and the foot will grow a new head. If you chop it into several slices, then each middle slice will grow both a foot and a head. Blend it in a mixer, clump the cells together, and they’ll eventually re-form themselves into a new Hydra.

The Hydra’s talents don’t end there. Even when not chopped up, the Hydra doesn’t seem to age. In 1998, biologist Daniel Martinez published a study describing how he and his colleagues found no signs of ageing in the mature Hydra specimens they’d been studying for the past four years. The study was repeated, for eight years this time, and the results were the same: no ageing. Martinez had originally set out to show the creature could age, but his own study proved him wrong.

Does that mean Hydra don’t die at all? Not really. In fact, 167 of them die every year, on average, no matter their age.

Martinez believes that “an individual Hydra can live forever under the right circumstances”, but not all circumstances are ideal. The animals in the experiment were given a private dish to live in, with fresh food and water in abundance. In the wild, the situation isn’t always so rosy. Just like the mythical Hydra, which was eventually slain by Heracles, biological Hydra too sometimes meet their match.

What keeps the Hydra cells immortal? One possible answer lies in stem cells.

All biological beings are formed from cells, but stem cells are special. They have no properties of their own, but can be programmed to take on the role of whatever cell type is necessary — a kind of “raw material” cell, so to speak.

Humans have stem cells too. Before you were born, your embryonic stem cells formed the inner cell mass of your early embryo. They were generic cells — nothing — and that’s what allowed them to take on whatever role was needed — anything. Over time, as they specialised, they lost the ability to change their properties. On the positive side, that’s how they ended up forming the person they are now: you.

Adult stem cells aren’t as versatile as embryonic ones, but they can take on any of the roles required in the place they’re found. Endothelial stem cells, for example, grow in your bone marrow and can specialise into any of the different cells that line your blood vessels. Similarly, groups of stem cells grow in places like inside your breasts, on the lining of your nose, behind the strands of your growing hair, and even within your brain.

As you get older, your stem cells will begin to lose their special ability. They won’t be able to replace dysfunctional cells as easily as before. Except, that is, if you’re a Hydra.

Hydra stem cells remain potent forever, leading some to dub them as the “eternal embryo”. Naturally, people are very interested in the Hydra: if they can uncover how the stem cells keep going, perhaps they can use the same technology to become immortal themselves.

As a human, why can’t you regenerate damaged body parts? Actually, you can, in a way. When Prometheus leaked the gift of fire to humans, his punishment was to be nailed to a mountain while an eagle ate up his liver every day…only to have the liver grow back at night!

That’s a legend, of course, but there’s a grain of truth in it. If your liver gets damaged — by too much drink, for example — then, given a chance, it can regrow itself. Also, be careful about whom you give the power of fire to, although if it isn’t too bad your skin can regrow and recover from burns. If you lose a fingernail, your body can grow it back; when you break a bone, your body generates fresh bone tissue to mend the fracture.

But if we can get this far, why not take it further? Your tiny embryonic self grew your arms once, so why not do it again? One possible explanation is that these stem cells need to be kept under control. If their powers aren’t limited, they could grow even when they aren’t supposed to — and then you have what is known as cancer.

Wouldn’t it be great if we could activate these cells only when needed, without fear of cancer? That’s where regenerative medicine comes in. Regenerative medicine is a new, experimental, field of science that aims to stimulate stem cells — either on their own or with the help of donated tissue from someone else — and get them to grow in and fill the gap.

Plants can regrow pretty much any part of theirs that you chop off, and they do it automatically. Interestingly, plants get cancer too, but theirs is contained and therefore not so deadly. The simpler a life-form is, the less chance there is of something going wrong — which allows them to do some pretty funky stuff.

The jellyfish writhes, trying to move as far away as possible from the snapping jaws of the Loggerhead turtle. One more snap will be the end of this jellyfish. Summoning every last particle of energy, it propels forward into the seaweed bush.

Zooming past the first set of plants, the jellyfish burrows deeper and deeper into the gloom, hoping to escape the life-threatening predator. Finding a little spot tucked beneath the soft, swaying leaves, it stops at last. And realises that there is yet another problem.

Nearly four of its tentacles are gone. Snapped up by the turtle no doubt. What remains is just ⅔ of what a jellyfish needs, to swim and to feed. It had managed to make it this far through pure reserve energy, but its strength is fast depleting. Since its tentacles help catch phytoplankton (the jellyfish’s primary food), with lesser tentacles, there is less chance of catching it. There is only one option left to survive.

A few days later, the jellyfish starts to disintegrate. First, its remaining tentacles retract, then its body starts to shrink, and finally sinks to the ocean floor to become a polyp colony. That’s right, the single injured jellyfish creates a whole colony of polyps that eventually buds and releases medusae that are genetically identical to the original one!

In 1991 a NASA mission launched off with 2,478 jellyfish on board. They were to take part in an experiment on the jellyfish’s relationship to gravity and to see if it was similar to that of humans. More than that, they wanted to see what happened to jellyfish who were born in space, because that would give us a clue to how space-born humans would behave.

As things turned out, the Earth-born jellyfish went through hormonal changes and had a hard time trying to adjust to the different environment, because they could not figure out which way was up. The space-born ones, on the other hand, were bewildered by gravity when they returned to Earth, and ended up being more passive and inert than normal. This is not necessarily a bad thing, though, because it means they could adapt from Earth-born jellyfish to space-born jellyfish in just a single generation!

The fact that NASA used jellyfish shows something deeper: Though they’re so different from humans, there are some ways in which jellyfish, humans, and other creatures are very much the same. Which brings us back to the question: can jellyfish teach us something about immortality, as well as about gravity?

Humans may be able to regenerate tissue, and Hydra regenerate their whole bodies, but jellyfish take it a step further. Turritopsis dohrnii– also known as the immortal jellyfish–can transform from an adult back to a baby, not just once but as many times as needed!

Part of this is because jellyfish don’t have brains. Not just that, they also don’t have bones, eyes, or a heart — bringing down the complexity of the jellyfish body, and making self-repair and self-regeneration easier. It’s as if evolution got inspired by Marie Kondo and decided to come up with a minimalist creature that “just works”.

Jellyfish don’t have a very complex digestive system: just one multipurpose hole which serves as a mouth, a garbage chute, and a vent to shoot out jets of water to propel the creature forward. What digestion happens, happens incredibly quickly, so the jellyfish can stay light and floaty.

Not having a brain means delegating power to various parts of the body. Take stinging, for instance. The jellyfish sting is one of the fastest processes in biology, with small sub-cellular harpoons shooting out at speeds high enough to punch a crustacean shell or even kill you with their poison. But a jellyfish never decides to sting you. Stinging is just something that automatically happens when a tentacle comes in contact with something. Even a dead jellyfish tentacle could sting you. So could a half-digested jellyfish inside that squid that you decided to eat.

The immortal jellyfish was discovered in the Mediterranean Sea in 1883, but its special ability was found only in the 1990s. Most of the time, it reproduces asexually and dies the “normal” way, but when its life is threatened — by starvation or physical damage — then its body will revert to its polyp stage: that is, the earliest stage of its life. All of the Turritopsis’ existing cells transform into a younger state–an adult cell, which is specialized for a particular tissue, can become an entirely different type of specialised cell–and sink into the ocean floor to start their lives all over again.

Of course, this is not something humans can do. Regenerating something as complex as a skeleton and digestive tract, let alone a brain, isn’t an easy proposition. If humans want to get into rebirth, reincarnation, and past lives, they’d have to try it some other way.

Even the immortal jellyfish doesn’t have a choice about dying. While it can regenerate, it doesn’t have a choice to do so whenever it wants to. Only when its life is being threatened will it automatically go back to its polyp state. And it could still die from disease or from being eaten by a predator.

And then there’s the ultimate question: once a jellyfish has become young and grown-up again, is it still the same jellyfish?