The two BsaI sites flank the S1 region and S1/S2 junction exactly as in prior published chimera work (e.g. WIV1 in Hu et al. 2017), an arrangement that enables efficient RBD substitution and furin-site insertion - matching a deliberate reverse-genetics design rather than an incidental natural map.
Reasoning
Beyond being merely anomalous, the specific placement of SARS-CoV-2’s Type IIS sites is functionally convenient for the experiments coronavirus researchers run: two BsaI sites bracketing the S1 domain/RBD let a researcher excise and replace the receptor-binding domain, or insert a furin cleavage site at the S1/S2 boundary, in a single cloning step without reassembling the whole backbone. This matches published designs - Hu et al. 2017 introduced two BsaI sites into WIV1 precisely to swap spike genes. The correspondence between SARS-CoV-2’s site arrangement and a known engineering design pattern targeting the spike is argued to be additional evidence of purposeful construction.
Original
The two BsaI sites flank the S1 region and S1/S2 junction exactly as in prior published chimera work (e.g. WIV1 in Hu et al. 2017), an arrangement that enables efficient RBD substitution and furin-site insertion - matching a deliberate reverse-genetics design rather than an incidental natural map.
Validity verdict — corrected (checked)
Reconstruction. Premises: two BsaI sites bracket the S1/RBD domain and S1/S2 junction; this arrangement lets a researcher swap the RBD or insert a furin site in a single cloning step; Hu et al. 2017 introduced BsaI sites into WIV1 for exactly this purpose. Load-bearing step: because the placement is convenient for spike-manipulation experiments and matches a published engineering design, it is evidence of purposeful construction “rather than an incidental natural map.”
Evaluation. The evidential half of the step survives — a placement matching a known deliberate design does raise P(engineered) somewhat. But the as-stated comparative claim (“matches a deliberate design rather than an incidental natural map,” i.e. favours design over nature) meets a surviving undercutting defeater that does not deny any premise: the S1/RBD boundary is a natural functional and recombination-breakpoint module in sarbecoviruses, so both engineers and natural recombination “target” this same boundary. The “convenience” is also defined post hoc — sites flanking any domain of interest can be called convenient — so resemblance to Hu 2017’s chosen positions is largely resemblance to the natural modular boundary that Hu 2017 also respected. Granting the premises, the arrangement is therefore equally compatible with an incidental natural map, and the comparative “design rather than natural” does not go through. A weaker conclusion is immune to the defeater: the map is consistent with a deliberate design (weak evidence for it) without favouring design over nature. Corrected to that form. Not rejected, because the possibility/consistency claim genuinely holds; not approved, because the “rather than incidental natural” overclaims past what the premises license. Checked via the reconstruction above.