How an immortal jellyfish could end evolutionary theory as we know it

Turritopsis dohrnii, the ‘immortal jellyfish’. Image by Dr. Karen J. Osborn via Wikimedia (CC0 1.0)

Almost all organisms grow old (i.e. physically deteriorate over time), a process that eventually results in death. Biologists call this age-related decline ‘senescence’. We know only a few exceptions. The ‘immortal’ jellyfish periodically reverts to an earlier stage in its life-cycle, like a butterfly turning back into a caterpillar.