Jellyfish and Bioluminescence

Fossils of jellyfish dating to about 505 million years ago have been found, long before dinosaurs. Because jellyfish have soft bodies, they rarely fossilize, so the fossil record is incomplete and cannot give an exact date for their origin. Some living jellyfish are bioluminescent: light is produced by a chemical reaction in the organism. Its role varies by species and context; not all jellyfish glow, and the function is not known for every luminous species.