Non-human animal-to-animal transmission has been observed, like on Danish mink farms. But you can sequence the virus from many human and non-human infections an

Non-human animal-to-animal transmission has been observed, like on Danish mink farms. But you can sequence the virus from many human and non-human infections and build a phylogenetic tree from the sequences, knowing that each variant should have all the mutations of its ancestor plus maybe some new ones. In such a tree, the proximal zoonotic host should have infections with no human-infection ancestor. This was observed for the original SARS (palm civets) and MERS (camels), each within about a year. For SARS-CoV-2, we're still waiting.