Proarytmi

Proarytmi is a neologism used in some discussions of cellular evolution to describe a hypothetical transitional stage between prokaryotic and eukaryotic cellular organization. It is not a widely adopted term in mainstream biology and does not denote a formally recognized lineage, mechanism, or process. In proposed usage, proarytmi would refer to a stage in which eukaryote-like complexity begins to appear within a largely prokaryotic cell, rather than representing a fully developed eukaryotic cell.

Proposed features include partial compartmentalization of the cytoplasm, emergence of primitive membrane-associated domains, rudimentary cytoskeletal elements, and enduring partnerships with other microorganisms that resemble early endosymbiotic events. The term is intentionally broad to encompass a range of speculative scenarios; accordingly, exact criteria vary among authors.

The term has appeared only sporadically in theoretical discussions and in some popular-science writing. It has not gained traction in peer-reviewed research, and many biologists view proarytmi as a speculative placeholder rather than a defined hypothesis with testable predictions.

Critics argue that the ambiguity of the term hampers clear discussion of the origin of eukaryotes and that existing models, such as endosymbiotic theory and archaeal-eukaryotic continuity, provide more concrete frameworks. Supporters counter that it can encourage exploration of intermediate cellular states, though they emphasize the need for explicit definitions and evidence.