If I cut my arm off, I will end up with a permanent stump that’s covered in scar tissue. By contrast, if a newt or salamander loses its leg, it will grow a new one. The wound will close and, over time, it will create new bones, muscles, nerves and skin.
Healing powers of this kind were first discovered in 1740, when Abraham Trembley discovered that a green pond animal could regenerate its tentacle-crowned head if it was amputated. He called it Hydra after the head-renewing monster from Greek mythology. Since then, scientists have discovered more animals with regenerative powers. Lizards restore lost tails. Starfish grow dismembered arms back. Some flatworms can rebuild their entire bodies from a single cell.