Symbiogenesis

Symbiogenesis is a concept in evolutionary biology that posits major evolutionary innovations arise from symbiotic relationships between distinct organisms, particularly when one cell is incorporated into another. The most well-supported aspect is endosymbiotic theory: mitochondria are derived from intracellular bacteria, likely alpha-proteobacteria, acquired by an ancestral eukaryote through primary endosymbiosis; plastids (including chloroplasts) originated when a eukaryotic host engulfed a cyanobacterium, also via primary endosymbiosis. Secondary endosymbiosis describes events in which a eukaryote engulfs a photosynthetic eukaryote, leading to additional membranes around plastids.

Historically, ideas about symbiogenesis trace to early 20th-century thinkers such as Konstantin Mereschkowski and later Ivan Wallin. The modern formalization and broader advocacy are associated with Lynn Margulis, whose 1967 paper and subsequent work argued that symbiosis underlies key evolutionary transitions. Over decades, genetic and cellular evidence—circular DNA in mitochondria and plastids, prokaryotic-type ribosomes, double-membrane structures, and phylogenetic analyses—has reinforced the endosymbiotic origin of these organelles and the occurrence of secondary endosymbioses in diverse lineages. Today, mitochondria and plastids are widely accepted as endosymbiotic descendants, while the broader role of symbiosis in evolution remains an active area of research.

Beyond organelle origins, the symbiogenesis concept informs studies of genome evolution, cellular integration, and the ecology of holobionts, highlighting the importance of intimate, cooperative associations in generating evolutionary novelty. It remains a central framework in understanding eukaryotic diversity and the complex interactions between hosts and their microbial partners.