Endosymbiosis and the eukaryotic cell

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The origin of the eukaryotic cell was not a gradual improvement of a bacterium but a merger. Mitochondria descend from a free-living alphaproteobacterium taken up by an archaeal host and never digested; chloroplasts descend from a cyanobacterium acquired later, in a separate event, by the lineage that became algae and plants. The evidence is unusually direct for something this old and it is not morphological: both organelles retain their own reduced circular genomes, replicate by division rather than being built by the cell, use bacterial-type ribosomes sensitive to bacterial antibiotics, and are enclosed by a double membrane consistent with engulfment. Their gene sequences place them unambiguously inside bacterial groups on the tree of life. The discovery of the Asgard archaea in the 2010s identified the closest known archaeal relatives of eukaryotes and made the host side of the merger far more concrete. The transition matters here because every organism with genuinely complex multicellularity is a eukaryote — the energetic economics of the mitochondrion appear to be the precondition.

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