Unsettling lights can still be seen, illuminating the darkness, in the planet’s darkest regions, where the Sun never shines.

This is the amazing ability known as bioluminescence, which has independently developed at least 94 times in the history of life on Earth. The ability of bioluminescent animals to control chemical reactions to produce their own glow is a capability that these diverse creatures use for a variety of functions.

Scientists have now tracked the evolutionary history of this instrument back to its earliest known source, a class of corals known as Octocorallia that lived 540 million years ago in the Cambrian ocean’s depths. The tiny deep-ocean crustacean that held the title before it died 267 million years ago is more than twice as old as this one.

“We wanted to figure out the timing of the origin of bioluminescence, and octocorals are one of the oldest groups of animals on the planet known to bioluminesce,” explains lead author and marine biologist Danielle DeLeo of Florida International University and the Smithsonian Institution.

“So, the question was when did they develop this ability?”

The octocorals are an amazing group of animals. They are composed of polyps that group together to form colonies, just like other corals, and they frequently reside on framework built of their calcified secretions. On the other hand, octocorals—named for the eight-fold symmetry of the polyps—have softer skeletons than their more inflexible cousins.

Since they only glow when disturbed, the reason behind the bioluminescence that some of them are known to exhibit is a bit of a mystery. According to scientists, it could be a lure for larger fish that consume coral or a way to draw in predators who would otherwise miss the smaller fish.

DeLeo and her colleagues reasoned that corals, which are among the planet’s oldest species, and octocorals, which are known to shine, could be the best places to search for the earliest examples of bioluminescence. Furthermore, the foundation has already been established; in 2022, a comprehensive family tree comprising 185 octocoral species was produced using genetic data analysis.

Subsequently, marine biologists Andrea Quattrini of the Smithsonian Institution and Manabu Bessho-Uehara of Nagoya University in Japan undertook a study to identify and trace the lineages of known species of bioluminescent octocorals.

Their explorations of the seafloor revealed hitherto undiscovered bioluminescence in five different species of octocoral; this finding could be useful for their next research, known as ancestral state reconstruction.

“If we know these species of octocorals living today are bioluminescent, we can use statistics to infer whether their ancestors were highly probable to be bioluminescent or not,” Quattrini says.

“The more living species with the shared trait, the higher the probability that as you move back in time that those ancestors likely had that trait as well.”

The group used a variety of statistical techniques, and each produced a similar conclusion: 540 million years ago is when bioluminescence first appeared in the common ancestor of all octocorals. Multicellular life was still in its infancy at this time, but the Cambrian seas was shared by marine creatures possessing eyes that could detect light.

Simultaneous emergence of bioluminescence points to the possibility of some sort of interspecies interaction, which may help explain why the ability emerged.

However, there’s still a very important question. Why do relatively few of the thousands of species of living octocorals now possess bioluminescence if their common ancestor did? How did they lose it, and why? The next stage could provide greater insight into the peculiar ecology of the Cambrian ocean.