O-6 - Molecular-clock dating places tMRCA in mid-December 2019 and lineage B primary infection around 18 Nov 2019
Dating used recCA rooting (a reconstructed recombinant common ancestor across 15 non-recombinant sarbecovirus regions) as well as unconstrained rooting. Median lineage-B tMRCA 15 Dec, lineage-A tMRCA 20 Dec; lineage B primary case predated lineage A in 64.6% of the posterior, by a median 7 days.
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Why this is evidence
The clock dates lineage A’s and lineage B’s primary infections as two distinct, near-simultaneous events rather than one lineage budding off the other later. H-5 predicts exactly such separately dated primary cases; H-16 predicts a single human MRCA from which both lineages descend, with no second independently dated primary case. The dating is model-laden, so the discrimination is weak, but the members read it differently.