Imagine reaching old age or suffering a fatal injury, then hitting a biological reset button that transforms you backward into a blank slate, ready to start life over again, indefinitely. This is not science fiction. It is the real life cycle of Turritopsis dohrnii, the immortal jellyfish, the only known animal that can completely reverse its life cycle over and over.

This episode profiles a creature the size of a pencil eraser that defies the one-way clock of mortality. We explore the cellular mechanics behind its immortality, why the oceans aren't overrun with them, how they quietly colonized the globe, and what they might teach human aging research.

• When stressed by starvation, temperature, or injury, the jellyfish melts into a cyst-like blob and rebuilds itself through transdifferentiation, where specialized cells forget thei ...