eukaryogenesis

Eukaryogenesis is the evolutionary origin of eukaryotic cells from prokaryotic ancestors. Eukaryotes are characterized by a membrane-bound nucleus, extensive endomembrane system, cytoskeleton, and energy-related organelles such as mitochondria, and in plants and algae chloroplasts.

The central mechanism is endosymbiosis. Mitochondria derive from an aerobic alpha-proteobacterium engulfed by an early archaeal host. Chloroplasts in plants and algae derive from a cyanobacterial endosymbiont acquired later. Evidence: double membranes, circular DNA, ribosomes similar to bacteria; widespread endosymbiotic gene transfer to the host nucleus.

Debate continues about timing and order of events; some researchers emphasize an archaeal host with eukaryote-like features; others stress stepwise endosymbioses, including secondary and tertiary events that spread plastids to diverse lineages.

The study of eukaryogenesis informs understanding of cellular complexity and evolution of metabolism and multicellularity; recent genomic data from Asgard archaea and comparative organelle biology continue to refine the model.