Combining bat backbones with heterologous RBDs, inserting furin cleavage sites, and serial-passage adaptation in hACE2 animals were all established, published pre-2019 techniques (WIV/Baric gain-of-function work, the WIV1 eight-chimera panel, published FCS insertion, no-see-um seamless cloning) that leave no trace, so constructing SARS-CoV-2’s genome was technically feasible before the outbreak.
Reasoning
The paper documents two decades of chimeric-coronavirus construction: a 1999 feline/murine RBD swap; 2007 WIV ‘bat-man’ spike chimeras; a 2008 Baric live bat-backbone/SARS-RBD chimera; the 2015 SHC014 and 2016 WIV1 mouse-adapted-SARS-backbone chimeras; and a 2017 WIV panel of eight WIV1-backbone chimeras with transplanted bat RBDs. It also cites published methods for inserting polybasic furin cleavage sites into CoV spikes (increasing pathogenicity) and for serial passage in humanized/hACE2 animals. Because site-directed mutagenesis and seamless (no-see-um) cloning leave no residual scar, an engineered virus can be indistinguishable from a natural one. The upshot: the technical capability to assemble a RaTG13-backbone/pangolin-RBD chimera and to add a furin site existed and was in active use before 2019, so lab construction and serial-passage adaptation are feasible, not exotic, hypotheses. This argument bears on the prior plausibility of the lab-construction and serial-passage hypotheses.
Validity (step 6)
Reconstruction. Premises: each cited technique (heterologous-RBD backbone chimeras, polybasic FCS insertion, serial passage in hACE2 animals, no-see-um seamless cloning) was published/in active use pre-2019 and the seamless-cloning route leaves no residual scar. Load-bearing step: from “each component technique existed and is trace-free” to “assembling a chimeric SARS-CoV-2-like genome (and adding a furin site) was technically feasible before the outbreak.”
Verdict: approved (checked). Conditional on the premises, the feasibility conclusion follows — feasibility of the whole requires only that the component operations be available, which the premises grant; and the trace-free sub-premise licenses the separate “indistinguishable from natural” point riding on O-74. Probed for an undercutting defeater: one might object that possessing the operations does not establish possession of the specific RaTG13-like backbone + pangolin-like RBD starting material, so “feasible” could be inflated to “readily done.” But the statement’s conclusion is only technical feasibility / capability-in-principle, not that it was done, so that objection bears on truth/likelihood (priced at step 8), not on the inference. No defeater breaks the step at the stated modest strength.