However, recently I've begun to consider whether it could have been an accidental lab release. So far, I am not convinced, either way.
A hypothetical scenario goes like this: animal specimens are collected and brought into the lab. The typical experiments are performed, such that the various viruses are cultured and isolated and characterized. Careless handling of samples results in transfer to lab personnel. Lab personnel transfers non-lethal version of virus unknowingly to surrounding community, where the virus quickly mutates to the current lethal form.
Unfortunately, this paper relies on, (IMHO), a weak argument here: (bold mine)
Furthermore, a hypothetical generation of SARS-CoV-2 by cell culture or animal passage would have required prior isolation of a progenitor virus with very high genetic similarity, which has not been described. Subsequent generation of a polybasic cleavage site would have then required repeated passage in cell culture or animals with ACE2 receptors similar to those of humans, but such work has also not previously been described.
The scenario above that I proposed doesn't depend upon the publication of characterization of the virus that escapes the lab. I realize that the Nature paper must rely upon the literature to form it's argument, but reality doesn't.
The virus may well have mutated in vitro in the lab, even unnoticed and undetected by the lab, and still managed to escape.
Further, according to job postings that were seen in Nov and Dec 2019 from labs in Wuhan, there was active recruiting to work on characterizing novel bat viruses -- strongly suggesting that there were plenty of uncharacterized viruses in the lab.
Getting back to the knowns: there's a bat virus with high homology to SARS-CoV2, but it's RBD doesn't match. There's a pangolin virus with an RBD that matches, but the rest of the virus doesn't match as well as the bat virus does. Neither of those viruses have the polybasic cleavage site, nor the glycan feature (which presumably mutated within humans).
At this point in the story, it feels like a toss-up.
I think the only conclusion this paper can really make convincingly, is that the virus wasn't deliberately engineered.
But I can still see how it may have come from a lab.
I know zero about zoonotic dynamics, but what is the proposed sequence of events for an assumed wet market transfer? A bat infects a pangolin, which then ends up in a Wuhan market, where it infects humans?
Even if a bat infects a pangolin, the virus still needs to recombine with the pangolin virus to get the correct RBD. Or, do both viruses coexist in the pangolin and both infect the human, where the recombination then occurs? Are bats and pangolins living together naturally, or only in the wet markets?
I can envision all kinds of scenarios, but have no idea how likely or even possible they are.
There's just too much to know, sigh.
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