Living light has evolved dozens of times, and once guided a lost pilot home
Low on options after his instruments failed, future Apollo 13 astronaut Jim Lovell switched off his cockpit lights and spotted the glowing wake of his aircraft carrier, then followed it down to a safe landing. The glow came from sea creatures making their own light, a trick life has invented dozens of separate times.
Bioluminescence is light made by a living thing through a chemical reaction. Usually an enzyme, a luciferase, oxidises a fuel molecule, a luciferin, producing an excited compound that gives off light as it settles. The fuels hardly vary across life; one called coelenterazine turns up in 11 animal phyla, sometimes simply eaten. The enzymes, however, differ enormously. Some animals make light themselves, while others house glowing bacteria, for instance from the genus Vibrio. Fireflies, certain fungi and dinoflagellates glow too, but the habit is most widespread in the sea.
People noticed early. Aristotle and Pliny the Elder recorded damp wood that glowed, and Robert Boyle later showed that the light required oxygen. Before safety lamps, British and European miners used dried fish skins as dim lamps to avoid igniting firedamp with candles. Darwin described a night sea in which the bow wave looked like liquid phosphorus, though he wrongly blamed atmospheric electricity. In the late nineteenth century Raphaël Dubois, studying click beetles and a boring clam, disproved the old phosphorus theory and named luciferin.
Estimates of how often it evolved keep climbing: at least 40 times by one count, and at least 94 by a newer one, starting with octocorals some 540 million years ago. About 1,500 fish species glow, the ability arising independently at least 27 times among them, while sharks evolved it only once. Why it began is still debated. Howard Seliger proposed in 1993 that luciferases started as oxygen-handling enzymes, while Rees and colleagues argue luciferins first protected cells from reactive oxygen and took on a new role as animals moved into darker water.
Animals use their light to hide, by matching the glow from above so they cast no silhouette, to lure prey, and to court mates. Militaries paid attention: during the First World War a German U-boat gave itself away by its shining wake and was destroyed, and the US Navy later studied the effect during the Cold War. Laboratories now borrow luciferase systems for genetic engineering and medical research.
Source: Bioluminescence
