Glow-in-the-Dark Animals May Have Been around for 540 Million Years

The more humans have explored the deep oceans, the more examples we’ve found of animals with a seemingly magical talent: bioluminescence, the ability to produce their own light.

Bioluminescence is surprisingly common, with various mechanisms having evolved perhaps around 100 separate times over the course of hundreds of millions of years. New research published in the Proceedings of the Royal Society B traces bioluminescence in the family tree of unusual animals called octocorals and suggests that the phenomenon may have evolved in the sea more than 500 million years ago—thereby making its first known emergence more than twice as old as previously calculated.

For animals, especially those that live deeper in the ocean than sunlight can reach, bioluminescence can make the difference between life and death: for example, it can lure prey and deter predators. Biologists are still working to understand the full scope of the phenomenon’s uses. “We’ve explored so little of our own planet, and there could be so many more organisms down there that are using light in ways we haven’t even begun to understand yet,” says marine biologist Edith Widder, who is CEO and a senior scientist at the nonprofit Ocean Research & Conservation Association. “That’s what intrigues me the most about bioluminescence: how animals use it to survive.”

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But despite its ubiquity today, past bioluminescence is remarkably difficult to study because it rarely leaves a trace in fossils—when fossils even remain. The octocoral varieties called soft corals, for instance, don’t form the massive, rocklike reefs that coral is typically known for. Instead they build colonies by excreting a soft structure embedded with tiny chips of skeletonlike material. This body type means that soft corals leave behind only the tiniest of fossils, a challenge for scientists who try to peer into their histories.

Still, zoologist Andrea Quattrini, curator of corals at the National Museum of Natural History in Washington, D.C., and her colleagues were determined to understand how—­and when—bioluminescence may have developed in octocorals. Quattrini has spent about a decade testing living octocorals collected from the ocean by sequestering the creatures under a blanket or in a dark room and nudging them with a pair of laboratory tweezers, looking for signs of light.

Quattrini’s team mapped such results in an evolutionary tree that shows how different modern octocorals are related to one another, letting the scientists look for patterns in which branches can and can’t create light. By searching for the simplest possible evolutionary story to match these observations, the researchers concluded that bioluminescence most likely evolved just once in these animals. Then they used the rare fossils that have been confidently identified as belonging to specific octocoral types to root the tree in time. The analysis suggests the first known evolution of bioluminescence in a marine environment occurred some 540 million years ago—much longer ago than previous estimates of 267 million years.

The proposed date falls just before or during an event that paleontologists have dubbed the Cambrian explosion, when a burst of biological diversification occurred. It was likely also around that time that animals first moved from the shallow oceans into the depths where sunlight doesn’t penetrate. This timeline for developing bioluminescence makes sense, say Quattrini and Widder, who both note that rudimentary light sensors also developed around this time. In this context, bioluminescence became a communicative tool for corals to use to confuse prey or startle predators, like “a burglar alarm,” Quattrini says.

“I think our study really points to the fact that it’s one of the earliest forms of communication in the oceans—maybe one of the earliest forms of communication on Earth, really,” Quattrini says. “It’s a fascinating form of communication that’s really quite simple at its core.”

Yuichi Oba, a biologist at Chubu University in Japan, who has studied bioluminescence but was not involved in the new research, says he would like to see more caution taken with the determination that oc­­tocoral bioluminescence didn’t arise independently multiple times. If it did, that would make the phenomenon more recent than the new analysis says—perhaps just 400 million to 200 million years old, Oba suggests. Quattrini says the shared mechanism for bioluminescence across octocorals supports the idea of a single evolution.

Quattrini and her colleagues next plan to analyze the gene that builds luciferase, the protein responsible for bioluminescence in octocorals. The same gene shows up in both bioluminescent and nonbioluminescent octocorals, she says, so the researchers want to understand how some of the animals seem to have lost the ability to light up.

This kind of work helps to paint a better picture of what the ecosystems of the ancient Earth—which seem so alien to us today—may have looked like. “Imagine the ocean where coral emit light and carnivorous predators have large eyes in midnight water,” Oba says. “Life is wonderful.”