Endosymbiosis

Endosymbiosis is a biological relationship in which one organism lives inside another. In the context of eukaryotic evolution, the endosymbiotic theory posits that mitochondria and plastids originated as independent bacteria that were engulfed by an ancestral host cell and later became permanent, integrative components of the cell.

Mitochondria are widely accepted to descend from an alpha-proteobacterium, while plastids (including chloroplasts) trace their origin to cyanobacteria through primary endosymbiosis. Support comes from multiple lines of evidence: double membranes surrounding these organelles; their own circular DNA and ribosomes similar to bacterial counterparts; similarities in gene content and organization; and phylogenetic analyses linking organelle genes to bacterial lineages. The transfer of many organellar genes to the host nucleus, known as endosymbiotic gene transfer, further confirms deep integration.

Endosymbiosis occurred in several stages. Primary endosymbiosis gave rise to mitochondria and plastids in the lineage leading to plants, algae, and many other groups. Secondary endosymbiosis describes the capture of a photosynthetic eukaryote by a non-photosynthetic host, leading to plastids surrounded by additional membranes; this explains plastids in groups such as certain algae and protists, sometimes accompanied by a nucleomorph. Tertiary endosymbiosis occurred in a few dinoflagellates, where plastids from another algal lineage are retained within additional membranes.

Together, endosymbiosis explains how eukaryotic cells acquired complex energy-generating and photosynthetic capabilities, making mitochondria and plastids essential components of most eukaryotes. Ongoing comparative genomics and cellular studies continue to refine the timeline and diversity of endosymbiotic events.