LUCA — the last universal common ancestor

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1. Story
2. Not the first organism, and this is the single most common misreading of the term. LUCA is the most recent population from which every organism now alive descends — bacteria, archaea and eukaryotes alike. Everything that branched off earlier is gone, so LUCA marks the root of the surviving tree rather than the origin of life, and there may have been a long prior history of cellular life that left no descendants. LUCA is reconstructed rather than observed: genes present in both bacteria and archaea today are inferred to have been present in their common ancestor, which yields a picture of a cell with a genetic code, ribosomes, ATP synthase and an anaerobic, probably acetogenic metabolism. A 2024 phylogenomic study placed it around 4.2 billion years ago — startlingly early, within a few hundred million years of the Moon-forming impact — and inferred a genome of roughly 2.5 megabases coding on the order of 2,600 proteins, implying an already complex organism embedded in an ecosystem rather than a lone primitive cell. That date is considerably older than most earlier estimates and is not a settled result.
3. Knowledge
4. LUCA — the last universal common ancestor
5. The Great Oxidation Event
6. Connections
7. Molecular clock dating
8. The Great Oxidation Event
9. The origin of life
10. LUCA is dated to about 4.2 Ga by phylogenomic analysis
11. LUCA — the last universal common ancestor
12. Mass-independent fractionation of sulfur isotopes
13. Banded iron formations
14. The origin of life
15. Stromatolites
16. The Cambrian explosion
17. Endosymbiosis and the eukaryotic cell
18. Mass-independent fractionation of sulfur isotopes
19. Banded iron formations
20. Evidence
21. Supports the c. 4.2 Ga age estimate for LUCA and the inferred genome size and metabolic character. Deliberately cited without volume or page numbers, which were not confirmed. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked.