Researchers at Charles University discovered a living, previously unknown single-celled protist, Solarion arienae, in an old marine culture. Genome analysis places it outside known eukaryotic groups and groups it with other enigmatic protists into a new supergroup, Disparia. Remarkably, its mitochondria retain a bacterial secA gene fragment lost from most other eukaryotes, offering a rare window into early eukaryotic evolution. The species is being described as a living fossil that highlights how much microbial diversity remains undiscovered.
Accidental Discovery of a 'Living Fossil' Microbe Could Reshape the Eukaryotic Tree of Life
Researchers at Charles University discovered a living, previously unknown single-celled protist, Solarion arienae, in an old marine culture. Genome analysis places it outside known eukaryotic groups and groups it with other enigmatic protists into a new supergroup, Disparia. Remarkably, its mitochondria retain a bacterial secA gene fragment lost from most other eukaryotes, offering a rare window into early eukaryotic evolution. The species is being described as a living fossil that highlights how much microbial diversity remains undiscovered.
04:48 PM, 11/21/2025Science
Researchers at Charles University in the Czech Republic have uncovered a previously unknown, living single-celled organism in an old marine culture — a rare and striking find that may change how scientists view early eukaryotic evolution.
How it was found
Most of the larger organisms in the years-old sample had perished, but one tiny, unusual protist persisted. With ray-like projections radiating from a central body, the organism looks sunlike, and the team named it Solarion arienae. The discovery and genomic analysis were reported in Nature.
Genomic surprises
Sequencing the genome revealed that S. arienae does not fit within any previously recognized eukaryotic lineages. Instead, it groups with several enigmatic protists to form a distinct new supergroup the authors call Disparia. Searches of environmental DNA databases suggest S. arienae is paradoxically both rare and widespread — detected sporadically across marine sediment samples but usually at very low abundance, consistent with a quiet benthic lifestyle.
Mitochondrial relics and deep time
Most strikingly, the mitochondria of S. arienae retain a fragment of the bacterial gene secA, part of a protein-translocation apparatus inherited from the prokaryotic ancestor of mitochondria. That sequence has been lost from nearly all other eukaryotic lineages, so its presence in S. arienae offers a rare molecular window into early stages of the mitochondrial integration that helped define eukaryotic life.
Why this matters
This organism functions as a kind of living fossil: it preserves genetic features that most modern eukaryotes lack, allowing researchers to infer steps in the ancient endosymbiotic partnership that produced mitochondria. The finding underscores how much microbial diversity remains undocumented and how re-examining existing samples can yield major evolutionary discoveries.
“Solarion is a remarkable reminder of how little we still know about the diversity of microbial life,” said Ivan Čepička and Marek Valt. “The discovery of such an evolutionarily deep lineage — essentially a living fossil — shows that key parts of the eukaryotic story remain hidden in places we rarely explore.”
Follow-up studies of S. arienae and its relatives in Disparia will help clarify early eukaryotic branching and the timeline of mitochondrial genome reduction. For now, this tiny sun-shaped protist has already expanded the map of life's deep branches and offered an unexpected peek into cellular evolution.