![]() Here, we set out to challenge the outside-in perspective. ![]() Thus, existing theories for the origin of eukaryotes share the assumption that the nucleus is a novel structure formed within the boundaries of an existing, and largely unaltered, plasma membrane - they are outside-in models. Likewise, the most widely favored models for the origins of the nucleus assume that it was formed within a prokaryotic cell as the result of invaginations of the plasma membrane - whether by phagocytosis of an endosymbiont that corresponds to the nuclear compartment or by the internalization of membranes that became organized around the chromatin (reviewed in and discussed further below). Existing models for the origin of eukaryotes generally agree that proto-mitochondria entered the cell via phagocytosis. It is now established beyond reasonable doubt that mitochondria are derived from endosymbiotic α-proteobacteria. The key events in the evolution of eukaryotes were the acquisition of the nucleus, the endomembrane system, and mitochondria. Nevertheless, despite being recognized as the single most profound evolutionary transition in cellular organization, the origins of the eukaryotic cell remain poorly understood. The sophisticated cellular compartmentalization and the symbiotic association with mitochondria are thought to have enabled eukaryotes to adopt new ecological roles and provided a precursor to numerous successful origins of multicellularity. The emergence of the eukaryotic cell with its nucleus, endomembrane system, and membrane-bound organelles represented a quantum leap in complexity beyond anything seen in prokaryotes. It also helps to explain a number of previously enigmatic features of cell biology, including the autonomy of nuclei in syncytia and the subcellular localization of protein N-glycosylation, and makes many predictions, including a novel mechanism of interphase nuclear pore insertion. The inside-out theory is consistent with diverse kinds of data and provides an alternative framework by which to explore and understand the dynamic organization of modern eukaryotic cells. Further bleb-fusion steps yielded a continuous plasma membrane, which served to isolate the endoplasmic reticulum from the environment. ![]() The cytoplasm was then formed through the expansion of blebs around proto-mitochondria, with continuous spaces between the blebs giving rise to the endoplasmic reticulum, which later evolved into the eukaryotic secretory system. These blebs functioned to facilitate material exchange with ectosymbiotic proto-mitochondria. We propose that an ancestral prokaryotic cell, homologous to the modern-day nucleus, extruded membrane-bound blebs beyond its cell wall. Resultsĭrawing on diverse aspects of cell biology and phylogenetic data, we invert the traditional interpretation of eukaryotic cell evolution. Models have always assumed that the nucleus and endomembrane system evolved within the cytoplasm of a prokaryotic cell. Although the origin of the eukaryotic cell has long been recognized as the single most profound change in cellular organization during the evolution of life on earth, this transition remains poorly understood.
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