Is there an animal that's biologically immortal?

Every living thing you know follows the same arrow: born, grow, age, die. There’s one exception, and it’s smaller than your fingernail.

The jellyfish Turritopsis dohrnii has a power that sounds like science fiction: when it gets sick, hurt, or its surroundings turn hostile, instead of dying it rewinds its life. Its cells transform and the adult jellyfish turns back into its juvenile stage — a polyp anchored to the seafloor — from which it can grow and begin all over again. It’s as if a wounded butterfly could go back to being a caterpillar.

In theory, it can pull that trick off again and again, with no limit set by aging. That’s why it’s called “biologically immortal”: it doesn’t die of old age.

Careful, though — it’s not indestructible: a predator eats it, an infection kills it, and the story ends there. The astonishing part isn’t that it can’t die, but that old age isn’t on its list of causes of death. That’s why scientists study it: understanding how it reverses its own aging might teach us something about ours.

Death by old age looks like a universal law. A jellyfish the size of a lentil found a way around it.