zlacker

I think we've effectively ruled out lab origin of the original SARS and of MERS-CoV, because we've found proximal animal hosts. In other words, we've found animals that we believe first transmitted those viruses to humans, because (a) those animals are infected with a variant of the virus, and (b) that variant's genetic sequence implies it's an ancestor (or at least not a descendant) of known human variants. Condition (b) is why we believe that the animals first infected humans and not the reverse.

If we ever find the same for SARS-CoV-2, then I believe that pretty confidently excludes origin from lab manipulation (e.g., serial passaging). It would still be possible that the first human infected was on a WIV sampling trip, and not even all that unlikely (since an expert deliberately looking for novel viruses is far more likely to find them than e.g. a merely reckless wildlife trafficker).

If we see evidence in the phylogenetic tree of multiple animal-to-human spillover events--as we do for MERS--then that would pretty clearly exclude any scientific activity as the origin. At the very least, it would imply that even if some hapless grad student did accidentally start the pandemic, if they hadn't then someone else would have soon after. But as you say, so far we have neither.