Virus-First and Fusion-Driven Divergence: Testing a Three-Lineage Hypothesis of Early Life Against Palaeontological and Molecular Evidence

We test a hypothesis on the early evolution of life: primordial RNA assemblages diverged into three forms — (i) assemblages lacking the capacity for fusion and engulfment, which remained non-cellular and persist today as viruses; and (ii–iii) assemblages with an innate capacity for fusion and engulfment, here called "plant-ancestor" and "animalancestor" assemblages, which through repeated fusion produced unicellular plants and unicellular animals. The unicellular-animal line is then proposed to have split, one branch freezing its structure and ceasing to fuse (becoming permanent bacteria), the other continuing to engulf, fuse and accept variation, giving rise to multicellular life. We integrate the geological and palaeontological record (oldest microbial, eukaryotic and multicellular fossils), molecular clocks and phylogenomics (LUCA, LECA, Asgard archaea, plastid endosymbiosis), virology (viral hallmark genes, giant viruses, domestication of endogenous retroviruses) and RNA-world experiments, and examine each sub-hypothesis in turn. We find that the central insight — that the capacity for fusion and engulfment is the principal axis along which complexity diverged — is strongly supported. The proposed lineage order, however, conflicts with the evidence: bacteria and archaea predate eukaryotes by billions of years and cannot descend from unicellular animals; plants and animals diverged after the origin of the eukaryotic cell rather than at a pre-cellular stage; and viruses are polyphyletic rather than a single unchanged lineage. We therefore propose a revised Fusion–Stasis Bifurcation model and list falsifiable predictions.