Viruses pose a threat to organisms across the tree of life, from bacteria, to fungi, archaea and eukaryotes like plants and animals. To understand how animals first developed the tools to fight them off, researchers have looked at one of the most ancient surviving branches of the animal family tree: the humble sea anemone.
Sea anemones are marine invertebrates of the order Actiniaria.
They are related to corals, hydroids and jellyfish, which together sit within the ancient phylum Cnidaria — a lineage that diverged from other animals more than 600 million years ago in the Precambrian eon. Because that split happened so early, cnidarians offer scientists a rare window into what antiviral defence looked like near the very root of the animal kingdom.
A previously unknown protein discovered in the sea anemone Nematostella vectensis is now offering exactly that window — and the picture it reveals is stranger than expected. Researchers identified the CARD Inhibitor Binding protein, or CARDIB, which is remarkably similar in structure to the Mitochondrial Antiviral Signalling protein, or MAVS. In humans and other vertebrates, MAVS plays a major role in mediating the innate immune response during infection with RNA viruses — the innate immune system being the body’s first line of defence against invading pathogens, activated within minutes of infection, long before more specialised immune responses kick in.
Given how closely CARDIB resembled MAVS, the research team expected it to behave the same way.
Everything about CARDIB suggested it should function like MAVS,” says Yehu Moran, head of the Department of Ecology, Evolution and Behaviour at the Hebrew University of Jerusalem in Israel and senior author of the study, published in Nature Ecology & Evolution. “Instead, we discovered that it does the exact opposite. Rather than activating antiviral defences, CARDIB normally suppresses them.
That reversal became even more puzzling once the team tested what happened without it. Using CRISPR gene editing, researchers removed the CARDIB gene from sea anemones and exposed them to viral threats. Rather than becoming better protected, removing a suppressor might suggest — the anemones became far more vulnerable to infection.
The findings suggest that a single antiviral strategy did not persist uniformly throughout the evolution of animals. Instead, different lineages appear to have evolved distinct molecular solutions to the same basic problem of fighting off viral infection. “Humans and sea anemones both need protection from viruses, but this work shows that evolution can organise those defences in fundamentally different ways,” says Moran.
What This Could Mean for Human Health
For non-scientists, the practical question is why a discovery in a sea anemone would matter for human medicine.
The answer lies in MAVS itself — the human protein CARDIB resembles. MAVS isn’t just a laboratory curiosity; it’s already an active target in human drug development, precisely because getting its activity level right is a delicate balancing act.
Too little MAVS activity leaves the body vulnerable to viral infection, since it’s a critical trigger for the interferon response — the signal that tells cells to fight back against an invading virus.
But too much MAVS activity has the opposite problem: it’s been linked to autoimmune and inflammatory conditions in humans, including lupus, certain kidney diseases, and cardiovascular disease, where an overactive antiviral alarm system starts damaging the body’s own tissue instead of just fighting infection.
Researchers are already developing both MAVS activators, to boost antiviral defence, and MAVS inhibitors, to calm an overactive immune response, for different diseases.
That’s what makes the CARDIB finding scientifically useful, not just curious. It shows that nature, independently and hundreds of millions of years ago, arrived at a similar solution: a molecular brake that must be present and functioning correctly for the antiviral response to work properly, not simply removed to maximise defence.
That counterintuitive result mirrors exactly the challenge human drug developers face with MAVS-targeted therapies — dial immune activity up too far and you risk autoimmune disease; dial it down too far and you leave the door open to infection.
For preventative medicine, the implication is less about a new drug arriving soon, and more about a design principle: effective antiviral therapies and vaccines likely need to work with the immune system’s built-in checks and balances, rather than simply switching defences to maximum. For pharmacology, CARDIB adds a second, evolutionarily distant example of how a brake protein can be essential rather than purely limiting — potentially opening a new avenue for researchers hunting for safer ways to fine-tune, rather than simply amplify, the human antiviral response.
Source: Study published in Nature Ecology & Evolution, DOI: 10.1038/s41559-026-03112-3.
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