Bioluminescence first evolved in animals at least 540 million years ago

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

The results, published today, April 23, in the Proceedings of the Royal Society B, push back the previous record for the luminous trait's oldest dated emergence in animals by nearly 300 million years, and could one day help scientists decode why the ability to produce light evolved in the first place.

Bioluminescence -- the ability of living things to produce light via chemical reactions -- has independently evolved at least 94 times in nature and is involved in a huge range of behaviors including camouflage, courtship, communication and hunting. Until now, the earliest dated origin of bioluminescence in animals was thought to be around 267 million years ago in small marine crustaceans called ostracods.

But for a trait that is literally illuminating, bioluminescence's origins have remained shadowy.

"Nobody quite knows why it first evolved in animals," said Andrea Quattrini, the museum's curator of corals and senior author on the study.

But for Quattrini and lead author Danielle DeLeo, a museum research associate and former postdoctoral fellow, to eventually tackle the larger question of why bioluminescence evolved, they needed to know when the ability first appeared in animals.

In search of the trait's earliest origins, the researchers decided to peer back into the evolutionary history of the octocorals, an evolutionarily ancient and frequently bioluminescent group of animals that includes soft corals, sea fans and sea pens. Like hard corals, octocorals are tiny 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 ability to produce light 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 bioluminesce," DeLeo said. "So, the question was when did they develop this ability?"

Not coincidentally, Quattrini and Catherine McFadden with Harvey Mudd College had completed an extremely detailed, well-supported evolutionary tree of the octocorals in 2022. Quattrini and her collaborators created this map of evolutionary relationships, or phylogeny, using genetic data from 185 species of octocorals.

With this evolutionary tree grounded in genetic evidence, DeLeo and Quattrini then situated two octocoral fossils of known ages within the tree according to their physical features. The scientists were able to use the fossils' ages and their respective positions in the octocoral evolutionary tree to date to figure out roughly when octocoral lineages split apart to become two or more branches. Next, the team mapped out the branches of the phylogeny that featured living bioluminescent species.

With the evolutionary tree dated and the branches that contained luminous species labeled, the team then used a series of statistical techniques to perform an analysis called 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 said. "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 researchers used numerous different statistical methods for their ancestral state reconstruction, but all arrived at the same result: Some 540 million years ago, the common ancestor of all octocorals were very likely bioluminescent. That is 273 million years earlier than the glowing ostracod crustaceans that previously held the title of earliest evolution of bioluminescence in animals.

DeLeo and Quattrini said that the octocorals' thousands of living representatives and relatively high incidence of bioluminescence suggests the trait has played a role in the group's evolutionary success. While this further begs the question of what exactly octocorals are using bioluminescence for, the researchers said 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 the common ancestor of all octocorals likely already had the ability to produce its own light, they are interested in a more thorough accounting of which of the group's more than 3,000 living species can still light up and which have lost the trait. This could help zero in on a set of ecological circumstances that correlate with the ability to bioluminesce and potentially illuminate its function.

To this end, DeLeo said she and some of her co-authors are working on creating a genetic test to determine if an octocoral species has functional copies of the genes underlying luciferase, an enzyme involved in bioluminescence. For species of unknown luminosity, such a test would enable researchers to get an answer one way or the other more rapidly and more easily.

Aside from shedding light on the origins of bioluminescence, this study also offers evolutionary context and insight that can inform monitoring and management of these corals today. Octocorals are threatened by climate change and resource-extraction activities, particularly 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 advance and share knowledge of the ocean with the world. DeLeo and Quattrini said there is still much more to learn before scientists can understand why the ability to produce light first evolved, and though their results place its origins deep in evolutionary time, the possibility remains that future studies will discover that bioluminescence is even more ancient.

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Discovery of 500-million-year-old fossil reveals astonishing secrets of tunicate origins

Karma Nanglu says his favorite animal is whichever one he's working on. But his latest subject may hold first place status for a while: a 500-million-year-old fossilfrom the wonderfully weird group of marine invertebrates, the tunicates.

"This animal is as exciting a discovery as some of the stuff I found when hanging off a cliffside of a mountain, or jumping out of a helicopter. It's just as cool," said Nanglu, postdoctoral researcher in the Department of Organismic and Evolutionary Biology at Harvard University.

In a new study in Nature Communications, Nanglu and coauthors describe the new fossil, named Megasiphon thylakos, revealing that ancestral tunicates lived as stationary, filter-feeding adults and likely underwent metamorphosis from a tadpole-like larva.

Tunicates are truly strange creatures that come in all shapes and sizes with a wide variety of lifestyles. An adult tunicate's basic shape is typically barrel-like with two siphons projecting from its body. One of the siphons draws in water with food particles through suction, allowing the animal to feed using an internal basket-like filter device. After the animal feeds, the other siphon expels the water.

There are two main tunicate lineages, ascidiaceans (often called "sea squirts") and appendicularias. Most ascidiaceans begin their lives looking like a tadpole and mobile, then metamorph into a barrel shaped adult with two siphons. They live their adult life attached to the seafloor. In contrast, appendicularians retain the look of a tadpole as they grow to adults and swim freely in the upper waters.

"This idea that they begin as tadpole-looking larva that, when ready to develop, basically headbutts a rock, sticks to it, and begins to metamorphosis by reabsorbing its own tail to transform into this being with two siphons is just awe-inspiring," sais Nanglu.

Interestingly, tunicates are the closest relatives of vertebrates, which includes fish, mammals, and even humans. How this odd-looking creature could be related to vertebrates is hard to imagine were it not for that tadpole beginning. Tunicate's close relationship to vertebrates makes studying them critical for understanding our own evolutionary origins. Unfortunately, it's not easy to do as tunicates are almost completely absent from the entire fossil record, with only a handful of fossils appearing convincingly as members of the group.

With so few fossils, scientists relied mainly on what could be learned from modern tunicate species. Because no one knew the morphology and ecology of the last common ancestor of the tunicates, scientists could only hypothesize that it was either a benthic animal with two siphons, like the ascidiaceans, or a free-swimming animal like the appendicularians.

M. thylakos had all the basic hallmarks of an ascidiacean tunicate, a barrel-shaped body and two prominent siphon-like growths. But the feature that stood out to the team was the dark bands running up and down the fossil's body.

High powered images of M. thylakos allowed the researchers to conduct a side-by-side comparison to a modern ascidiacean. The researchers used dissected sections of the modern tunicate Ciona to identify the nature of Megasiphon's dark bands. The comparisons revealed remarkable similarities between Ciona's muscles, which allow the tunicate to open and close its siphons, and the dark bands observed in the 500-million-year-old fossil.

"Megasiphon's morphology suggests to us that the ancestral lifestyle of tunicates involved a non-moving adult that filter fed with its large siphons," said Nanglu. "It's so rare to find not just a tunicate fossil, but one that provides a unique and unparalleled view into the early evolutionary origins of this enigmatic group."

M. thylakos is the only definitive tunicate fossil with soft tissue preservation that has been discovered to date. It is the oldest of its kind originating from the middle Cambrian Marjum Formation in Utah. The fossil was recognized as a tunicate by co-authors research associate, Rudy Lerosey-Aubril, and Professor Javier Ortega-Hernández (both in the Department of Organismic and Evolutionary Biology) while visiting the Utah Museum of Natural History (UMNH) in 2019.

"The fossil immediately caught our attention," said Ortega-Hernández, "although we mostly work on Cambrian arthropods, such as trilobites and their soft bodied relatives, the close morphological similarity of Megasiphon with modern tunicates was simply too striking to overlook, and we immediately knew that the fossil would have an interesting story to tell."

Fossils from the Marjum Formation date from shortly after the Cambrian Explosion, one of the most significant evolutionary events in Earth's history which occurred approximately 538 million years ago. During this time the most major animal groups appeared in the fossil record for the first time radically changing marine ecosystems. Tunicates, however, are noticeably absent in Cambrian rocks even though they are diverse and abundant in modern oceans.

There are many Cambrian fossil sites with exceptional preservation in the United States, but these are often overlooked compared to those from the Burgess Shale in Canada and Chengjiang in China. "The discovery of Megasiphon perfectly illustrates why Javier and I have been conducting fieldwork in Utah for the last ten years," said Lerosey-Aubril. "The Marjum strata has all of our attention right now as we know that it preserves fossils of animal groups, such as tunicates or comb jellies, that are almost entirely absent from the Cambrian fossil record."

Molecular clock estimates suggest that ascidiaceans originated 450 million years ago. However, at 500 million years old, M. thylakos provides the clearest view into the anatomy of ancient tunicates and their earliest evolutionary history. Significantly, M. thylakos provides evidence that most of the modern body plan of tunicates was already established soon after the Cambrian Explosion.

"Given the exceptional quality of preservation and the age of the fossil, we can actually say quite a bit about the evolutionary history of the tunicates," said Nanglu. "This is an incredible find as we had virtually no conclusive evidence for the ancestral modes of life for this group before this."

After collecting hundreds of new fossils again this spring, the researchers are convinced the Marjum Formation has only started to reveal its secrets.

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