Immortality

The impossible claim Cut a planarian flatworm into 20 pieces and each piece regrows into a complete animal — eyes, brain, everything. They are functionally immortal. The verified facts The planarian (Dugesia, a freshwater flatworm, ~10–15 mm) is the regeneration champion of the animal kingdom. Cut one into pieces — as long as each fragment contains some stem-cell-rich tissue — and every fragment regrows into a complete worm: head, eyes, brain, mouth, all of it. ...

The impossible claim This animal doesn’t age — scientists watched a group for four years and their death rate never went up. Not even a little. The verified facts The freshwater hydra (Hydra vulgaris) is a tiny (~10 mm) tentacled polyp — and in a landmark 4-year study, Daniel Martínez kept 2,256 hydras and found no increase in mortality with age. While control animals died on schedule, the hydras’ death rate stayed flat. If aging is rising death risk over time, hydras may not age at all. ...

The impossible claim This animal can revert to its juvenile polyp stage when injured, starving, or aging — restarting its entire life cycle, potentially indefinitely. The verified facts Turritopsis dohrnii, the “immortal jellyfish,” is a small (~4.5 mm) hydrozoan. When environmental stress or old age threatens it, it performs transdifferentiation: its adult medusa cells transform directly into polyp cells, collapsing the bell into a cyst that regrows into a new polyp colony. Genetically, it’s the same individual — biologically, it’s a baby again. ...