What will constitute proof here?

It is possible that the animal which infected the first human is a different species from the animals which brought the virus to Wuhan initially. Especially as the different animals were kept in close quarters together, and we know of at least two separate jumps to humans, easily there could have been animal-to-animal jumps.

@DerekPederson Hard to say how the lab leak conversation will resolve. There's a low probability case where China did find the animal in 2020 and never reported it. Then this would resolve when that information comesto light. We could also get soms information from interviews with wildlife vendors.

Otherwise, it might be a process of exclusion by trial infecting various animals, finding out which can get covid, and excluding others. Maybe we could also get viral positives or antibody positives from animals on farms, to help guide the search. DNA sequencing of raccoon dogs, and other animals, across China can also help determine where each of the 2019 animals came from.

It's also possible, of course, that this market will never resolve because the answer can't be determined conclusively.