What is bioluminescence, the origins of this wonderful phenomenon

Bioluminescence was born underwater about 540 million years ago.Octocorals, ancestors of soft corals, first evolved the ability to produce light during the Cambrian period.Ingenious tool for communication, predation and defense.

At every moment, for millions of years, thousands of lights have been lit in the darkness of the abyss. Like fireflies traveling in the sea.Corals, algae, fish, small crustaceans or invisible invertebrates shine as they oscillate in depth or shine among the waves crashing on the shore.

There bioluminescence, the ability of living beings to produce light through chemical reactions, is a complex, yet surprisingly common phenomenon in the animal kingdom, having evolved at least 94 times independently.To date it is found in approximately three thousand species, but already 540 million years ago the octocorals, ancestors of soft corals, gorgonians and sea pens, used them for various functions, including attract prey and communicate in the darkest band of the seas, the so-called one aphotic, that is, where the light of the Sun does not reach.He revealed it a new study conducted by researchers at the Smithsonian National Museum of Natural History in Washington, published in the Proceedings of the royal society B, which dates the birth of bioluminescence to almost the same time twice as remote than estimated so far.In fact, it was thought that the most ancient origin of this biochemical ability dates back to around 267 million years ago, and that it arose in the ostracods, small crustaceans capable of producing blue light, even in a synchronized manner, involving thousands of specimens in unison during the reproductive period.They are the ones who transform tropical beaches into surreal and phosphorescent landscapes, like a spell or a magic game.

Bioluminescence was one of the first forms of communication in the oceans, perhaps one of the first on Earth.

Andrea Quattrini, researcher 

A phenomenon shrouded in mystery

Bioluminescence is the product of chemical reaction between the luciferin protein and the luciferase enzyme which determines the emission of light in living organisms.In the marine environment it occurs through specific organs, i photophores, made up of cells called photocytes.

Producing your own light is functional for hunting, confusing predators and camouflage, to indicate danger, stun prey, look for partners.And in the depths, in a dark world isolated in absolute silence, even a small spark can make the difference between life and death.The structures that allow the light reaction have remained almost identical over time and are present in living groups that are very distant from each other on an evolutionary level, witnesses of a ubiquitous and rather common phenomenon, whose history has long remained shrouded in mystery.The past and the mechanisms that have allowed such a wide diffusion, in fact, are very difficult to study:bioluminescence is a behavior that leaves no traces in fossils and, in the words of the authors of the research, "no one really knows why the phenomenon appeared in the animal world".

To complicate the matter even further, i fossils bioluminescent organisms often don't even exist.THE soft corals, In fact, they do not form coral reefs of calcareous material, but they build colonies of polyps expelling a flexible structure, in which only tiny fragments of skeleton-like matter are embedded.

Genetic analyzes on trees of light

To explain why this biochemical ability evolved, according to experts it was necessary to know when it first appeared on Earth.The researchers therefore decided to delve back into the evolutionary history of octocorals, an ancient group of animals that light up only if disturbed from the outside.This condition has always aroused great interest in the scientific community, so much so in 2022 Andrea Quattrini and his collaborators had carried out in-depth genetic analyses, reconstructing the phylogeny of 185 species.And it was within this tree of life that they identified the branches that contained the luminous species.Then, starting from the two best known and adopting a series of statistical techniques, they managed to place in time the origin of the phenomenon.

The Cambrian explosion

“Our study shows that bioluminescence has existed at least since the Cambrian period, around 540 million years ago, when animals moved for the first time from the surface areas of the oceans to the depths where light does not reach” explained the head of the study .And then everything comes back.That was precisely the era in which the Earth went from being populated by simple, unicellular organisms to an exceptional variety of life forms, when strong increases and sharp decreases in oxygen levels alternated in the atmosphere.Over the course of just over thirteen million years, a rapid diversification of new animal species occurred, known as the "Cambrian explosion" with evolutionary peaks and great extinctions.And, needless to say, it was also the period in which the first rudimentaries were developed light sensors.

In such a great moment evolutionary transition, bioluminescence has become the ingenious communication tool that corals have developed to confuse prey or scare predators.

But there's more.In the study, the researchers highlighted the dependence of light reactions on the presence of oxygen in the water.The findings therefore suggest that bioluminescence evolved as detox strategy to eliminate reactive forms of oxygen from the body, i free radicals, with light production as a secondary by-product.

The ability to produce light is widespread in habitats at least two hundred meters below sea level.We therefore believe that the origin of the behavior may be related to a deep-sea lifestyle.For the bioluminescent taxa that live more on the surface, however, diversification would have occurred in a more recent era.

Andrea Quattrini

Despite the latest news, some questions remain still unanswered.If it was really so advantageous, why did some species lose the ability to light up?And how did they lose it? The research front is still open.Future studies will have to better analyze the gene that codes for the protein luciferase, also present in non-bioluminescent octocorals, to understand the mechanisms that led some groups to lose this surprising talent.Discovering them will turn on a new light in the darkness of knowledge.