Source
Kay, Spang, Szöllősi, Pisani, Williams & Donoghue (2026), Nature 650: 129–140, LIT-1566, read in full in NOTE-1234 from the PubMed Central copy of the version of record. The sections “Establishing the eukaryogenesis timeline”, “Dating mitochondrial endosymbiosis”, the four sections on cell systems, “Testing hypotheses of eukaryogenesis”, the Methods, and the captions of Figures 1–5.
What was actually shown
- A dated species tree. 62 concatenated markers and 18 fossil calibrations give nFECA 3.05–2.79 Ga, mFECA 2.37–2.13 Ga and LECA 1.80–1.67 Ga.
- Dated duplications. 135 gene trees (95 archaeal, 40 bacterial) were dated with priors taken from the species tree. Duplications in Asgard-origin genes were constrained between nFECA and LECA, and those in alphaproteobacterial-origin genes between mFECA and LECA.
- The marker. Alphaproteobacterial-origin duplications begin within about 200 Myr of mFECA. Among the earliest are import-system components (TIM14/PAM16, 2.24–1.99 Ga; TIM44, 2.22–2.00 Ga) and paralogues that now act outside the mitochondrion. The authors align endosymbiosis to this onset, about 2.2 Ga, on the ground that duplicate fixation is far more frequent in eukaryotes than in prokaryotes.
- The order. “85% of the pre-LECA duplicates of archaeal origin were estimated to have been fixed prior to mitochondrial endosymbiosis.” The oldest are in trafficking between Golgi, ER and plasma membrane (TRAPPC3, 3.00–2.54 Ga), in γ-tubulin (2.84–2.41 Ga), in ARP3 (2.90–2.47 Ga) and in RNA polymerase subunits (2.91–2.34 Ga). 16 of 19 duplications in non-alphaproteobacterial bacterial genes come after the marker.
- What is left open. Endolysosomal duplications date with the earliest alphaproteobacterial ones, and “our results do not allow us to discriminate between the origin of phagocytosis and the timing of mitochondrial endosymbiosis”.
What could have failed: archaeal-origin duplications could have clustered after the onset of alphaproteobacterial ones, as a mitochondria-early scenario predicts. Their median ages did not. The priors allow archaeal duplications to be older than any alphaproteobacterial one, so they make the result possible but do not force it.
What this does not say
- Not that the host had these features before the mitochondrion. A duplication date is “a maximum estimate for the timing of origin” of a paralogue’s function. The function may have come later. Many intervals (ARP2 2.72–2.06, α/β-tubulin 2.47–1.92, nuclear targeting 2.86–2.08 Ga) overlap the 2.2 Ga marker.
- Not that the mitochondrion entered at 2.2 Ga. That date is the onset of symbiont-gene duplication. Equating the two is argued, not tested.
- Not about the nucleus as a compartment. Its defining eukaryote-specific genes, such as nuclear pores, cannot be dated by this method. What is dated is nuclear targeting of archaeal paralogues and RNA polymerase diversification. The conclusion says “a nucleus (or protonucleus), or at the least … the physical separation of translation and transcription”.
- Not that phagocytosis brought in the mitochondrion, or that it did not.
- Not when non-alphaproteobacterial genes were acquired. Their duplications are dated; their arrival is not. The argument against serial endosymbiosis rests on duplication timing.
- Not about dependence. The host’s elaboration is its own gene duplication. Nothing here says when host and symbiont became dependent on each other, or reproduced together.
Connections
- QUESTION-277. This bears on whether eukaryogenesis was “one two-partner merger or a sequence of associations”. It favours one endosymbiotic partner, after a long host-only phase, with other bacterial genes arriving by transfer. It dates G (the host’s duplications) and gives an early date for the import machinery, the start of C on the symbiont side. It says nothing about D, R or S.
- LIT-1595 (THEORY-1102). Pittis & Gabaldón reach the same mitochondrion-late order from branch lengths. They infer a host already chimaeric with non-alphaproteobacterial genes before the mitochondrion. This paper dates most non-alphaproteobacterial duplications after it. Duplication is not acquisition, so the two can both be right. The comparison is mine.
- LIT-1591. The cultured Asgard isolate is internally simple. This paper says so itself and does not take it as a contradiction: a modern relative is not nFECA’s lineage, and the paper “does not imply that a host cell of crown eukaryote size and system-complexity existed before mitochondrial endosymbiosis”.
- LIT-1597. The hydrogen hypothesis is the paper’s example of a mitochondria-early model, and is rejected.
Note of 2026-10-30: promoted, and the wider thesis filed beside it
The twenty-second status review promoted this reading under ADR-065: it reports what Kay and colleagues’ dating finds and how the authors read it, and NOTE-1234 read the paper in full. Whether the host’s elaboration predates the mitochondrion is THEORY-1203’s question, Proposed, which keeps this entry’s old promote_when.