[metalspot]

> the evidence existed but was lost

if the virus crossed over from animals to humans then it must be able to cross back over. viruses evolve quickly, but animals don't, and we have plenty of samples of the original virus. all you have to do is find the right species and infect them with a sample of the virus and show that they can spread it to prove natural origin.

the only scenario where your theory could be true is if the origin species suddenly went extinct after starting the pandemic, which is very improbable.