© AMNH/E. Grosch

Eighty percent of all known bioluminescent groups inhabit the world's oceans. At certain depths, nearly all organisms glow. On land, things are quite different. There are bioluminescent fungi and insects, but no flowering plants, birds, reptiles, amphibians, or mammals that glow.

How did bioluminescence evolve? Scientists are still working on this question. But one thing is clear: bioluminescence has evolved independently many times—at least 50, and probably many more. Such widespread occurrence tells scientists that the trait provides an important edge to organisms that have it.