Bioluminescence first emerged in animals at least 540 million years ago, pushing back the oldest dated example

Bioluminescence first emerged in animals at least 540 million years ago in a group of marine invertebrates called octocorals, according to the results of a new study by scientists at the Smithsonian’s National Museum of Natural History.

The results, published on April 23 in the Proceedings of the Royal Society B: Biological Sciencespushing back the previous record for the oldest appearance of the luminous trait in animals by nearly 300 million years, and could one day help scientists decipher why the ability to produce light arose in the first place.

Bioluminescence – the ability of living things to produce light through chemical reactions – has evolved independently at least 94 times in nature and is involved in a wide range of behaviors, including camouflage, courtship, communication and hunting. Until now, the earliest dated origin of bioluminescence in animals was believed to have occurred about 267 million years ago in small marine crustaceans called ostracods.

But for a property that is literally illuminating, the origins of bioluminescence have remained obscure.

“Nobody knows exactly why it first evolved in animals,” said Andrea Quattrini, the museum’s curator of corals and senior author of the study.

But for Quattrini and lead author Danielle DeLeo, a museum research associate and former postdoctoral researcher, to ultimately answer the bigger question of why bioluminescence evolved, they needed to know when this ability first appeared in animals.

In search of the earliest origins of this feature, the researchers decided to look back into the evolutionary history of octocorals, an evolutionarily ancient and often bioluminescent group of animals that also includes soft corals, sea fans and sea cages.

Like hard corals, octocorals are small colonial polyps that secrete a framework that becomes their refuge, but unlike their stony relatives, that structure is usually soft. Octocorals that glow typically only do so when bumped or otherwise disturbed, leaving the precise function of their light-producing ability a bit mysterious.

“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 produce bioluminescence,” DeLeo said. “So the question was when did they develop this ability?”

It is no coincidence that in 2022 Quattrini and Catherine McFadden of Harvey Mudd College had completed an extremely detailed, well-researched evolutionary tree of the octocorals. Quattrini and her collaborators created this map of evolutionary relationships, or phylogeny, using genetic data from 185 species of octocorals. .

With this evolutionary tree based on genetic evidence, DeLeo and Quattrini then placed two octocoral fossils of known ages on the tree based on their physical characteristics. The scientists were able to use the ages of the fossils and their respective positions in the octocoral evolutionary tree so far to roughly figure out when octocoral lineages split to become two or more branches.

The team then mapped the branches of the phylogeny in which living bioluminescent species occur.

Once the evolutionary tree was dated and the branches containing bioluminescent species were labeled, the team then used a series of statistical techniques to perform an analysis called ancestral state reconstruction.

“If we know that these species of octocorals alive today are bioluminescent, we can use statistics to infer whether it was very likely that their ancestors were bioluminescent or not,” Quattrini said. “The more living species with the shared trait, the greater the chance that if you go back in time, those ancestors probably also had that trait.”

The researchers used numerous different statistical methods to reconstruct their ancestral state, but all arrived at the same result: About 540 million years ago, the common ancestor of all octocorals was most likely bioluminescent. That’s 273 million years earlier than the glowing ostracod crustaceans that previously held the title of the earliest evolution of animal bioluminescence.

DeLeo and Quattrini said the thousands of living representatives of the octocorals and the relatively high incidence of bioluminescence indicate that this trait played a role in the group’s evolutionary success. While this further raises the question of what exactly octocorals use bioluminescence for, the researchers say the fact that it has been retained for so long highlights how important this form of communication has become for their fitness and survival.

Now that the researchers know that the common ancestor of all octocorals likely already had the ability to produce its own light, they are interested in a more thorough look at which of the group’s more than 3,000 living species can still light up and which have this property have lost. . This could help map out a range of ecological conditions that correlate with its ability to bioluminescent and possibly elucidate its function.

To that end, DeLeo said she and some of her co-authors are working to create a genetic test to determine whether an octocoral species has functional copies of the genes underlying luciferase, an enzyme involved in bioluminescence. For species of unknown brightness, such a test would allow researchers to get an answer one way or another more quickly and easily.

In addition to shedding light on the origins of bioluminescence, this study also provides evolutionary context and insight that can inform the monitoring and management of these corals today. Octocorals are threatened by climate change and resource extraction activities, especially fishing, oil and gas extraction and spills, and more recently by marine mineral mining.

This research supports the museum’s Ocean Science Center, which aims to promote and share knowledge about the ocean with the world. DeLeo and Quattrini said there is much more to learn before scientists can understand why the ability to produce light first evolved, and while their results place its origins deep in evolutionary time, the possibility remains that future studies will find that bioluminescence is even older. .

This study includes authors affiliated with Florida International University, the Monterey Bay Aquarium Research Institute, Nagoya University, Harvey Mudd College and University of California, Santa Cruz.