Newly Discovered Organism Could Represent a Whole New Branch in The Tree of Life

25 Nov. 2025, By J. COCKERILL

https://www.sciencealert.com/newly-discovered-organism-could-represent-a-whole-new-branch-in-the-tree-of-life

Illustration of a cross-section of a Solarion cell.

  (Charles University)

It's not every day that biologists announce an entirely new branch of life, and this one has been hiding under their noses for years.

It was discovered hiding in a lab sample of marine ciliates scientists had been tending to since collecting them from Croatian waters in 2011. But it wasn't until the ciliates suddenly died that this new, tiny creature, which scientists have named Solarion arienae, came into view.

"This organism allows us to look into a very ancient chapter of cellular evolution that we previously could reconstruct only indirectly," say protistologists Ivan Čepička and Marek Valt, from Charles University in the Czech Republic, lead authors of the study.

"The cells of Solarion are tiny and only slightly motile, and we overlooked them in the ciliate culture for several years… Given that we missed Solarion even in our long-term laboratory culture, it would probably go unnoticed in natural samples," the research team writes.

The microorganism's fascinating, sun-like form is rivaled only by what lies within.

The discovery of Solarion arienae has prompted biologists to announce an entirely new kingdom and phylum in the tree of life.

 (Valt et al., Nature, 2025)



The single-celled eukaryote has a membrane-bound nucleus full of DNA, just like each of our own cells, and, also like our cells, it has mitochondria – 'the powerhouse of the cell', where fat and carbohydrate molecules are converted into chemical energy.

The team demonstrated that Solarion does not fall within any of our existing categories for eukaryotes, and so this lonely little sun is sharing a newly-defined phylum with another bizarre protist, Meteora sporadica, which is further nested within a newly-established kingdom shared only with the distantly-related protists Provora and Hemimastigophora.

https://www.youtube.com/watch?v=3hX3KExHaiA&t=1s

But its mitochondria are distinctly different from any others scientists have seen before. They still contain the genetic breadcrumbs of what may once have been an entirely separate being.

Mitochondria, scientists think, were once an organism all of their own, an ancient bacterium. But at some point in the evolutionary history of life on Earth, they took up residence inside the single-celled body of another organism.

We know this because of the remaining genetic code stored inside all mitochondria, which all come from the same ancient phylum.

Over time, these two parts became so inextricably linked that the line between self and other dissolved entirely. Inside most of your cells, there are still mitochondria, complete with their own set of (much abbreviated) DNA, without which you would not survive.

In most eukaryotes – animals, plants, fungi, seaweeds, and a plethora of single-celled friends – the mitochondria contain scant evidence of their primordial independence. But Solarion still carries within its microscopic body a genetic memento from that long-forgotten time: the gene secA, which was once part of the proto-mitochondria's molecular toolkit, involved in getting proteins across its membrane when it lived independently.

This is great news for the endosymbiont theory of mitochondrial origin. It's direct evidence of the life mitochondria led before they were fully integrated into the eukaryotic cell, giving us unprecedented insight into how the last common ancestors of eukaryotes may have related to each other before becoming one.

"Solarion is a remarkable reminder of how little we still know about the diversity of microbial life," Čepička and Valt say.

"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."

The Life of Earth

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