A symbiogenic merger enabled by oxygen initiated the evolution of eukaryotes.

Oxygen-linked electron transport in an endosymbiont is proposed to have enabled symbiogenic merger mechanisms during the FECA-to-LECA transition, shaping early eukaryotic evolution.

• Why it matters: Gene-based reconstructions have entrenched a two-domain view that underestimates endosymbiont contributions, obscuring how key eukaryotic traits emerged from the FECA-to-LECA transition.
• What they did: The work marshals evidence for symbiogenic mechanisms, emphasizing the endosymbiont electron transport chain and acceptor molecular oxygen, especially for reactive oxygen species formation in the host cytoplasm.
• The result: It argues that many eukaryotic properties are better explained by a merger rather than an acquisition, implying rare, coevolution-driven pathways that open new evolutionary possibilities.

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