Rare microbial relict sheds light on an ancient eukaryotic supergroup

During the last decade, our understanding of eukaryotic evolution has increased immensely. Newly recognized eukaryotic supergroups have been established^1–3, and the majority of enigmatic orphan lineages had their relationships resolved^4–6. Studies on deep-branching unicellular eukaryotes have also played a crucial role in understanding the evolution of mitochondria, the fundamental organelles of the eukaryotic cell derived from an alphaproteobacterium. The retention of the ancestral alphaproteobacterial pathways in some protist lineages reveals that the mitochondrion of the last eukaryotic common ancestor (LECA) was more bacterial-like than previously expected^7,8. Here, we present the discovery of such a novel deep-branching eukaryote, Solarion arienae gen. et sp. nov., an inconspicuous, free-living heterotrophic protist with two morphologically distinct cell types and a novel type of predatory extrusome. We assign Solarion to the new phylum Caelestes. Together with Provora, hemimastigophoreans, and Meteora, they form a new eukaryotic supergroup, Disparia. Moreover, S. arienae possesses intriguing mitochondrial genomic traits, particularly the mitochondrially-encoded SecA gene, a remnant of an ancestral alphaproteobacterial protein secretion pathway, that has been almost entirely lost in extant mitochondria^9,10. The discovery of S. arienae broadens our understanding of early eukaryotic evolution and facilitates the study of proto-mitochondrial metabolic remnants, shedding light on the complexity of ancestral eukaryotic life.