Originally Posted by
Sherkeu
I'm not sure why we do not include humans as intermediary hosts on the path- the path back to humans. We do know now we can pass it to cats, dogs, and cattle. Maybe more. Probably more. And then the process could reverse. There is a team of researchers looking into that now.
Covid-19 enters humans cells by binding to ACE2, but it doesn't bind efficiently to the bat version of ACE2 so it would have a lot of difficulty infecting bats. This effectively rules out the possibility that it's binding mechanism evolved in bats. Conversely the Covid's closest know relative, RaTG13 can't bind efficiently to human ACE2 so neither have much chance to jump directly between species.
Another important factor is that the changes that allow Covid to bind to human ACE2 appear to come from a Pangolin virus Pangolin-CoV-2019 via a recombination event. This event could have occurred in some animal other than a Pangolin, but it must have involved exposure to a Pangolin.
The scenario where the recombination happened in yet another species would require 2 different viruses to jump species and infect the same individual which is less likely than one virus jumping to a Pangolin then recombining with a virus that commonly infects Pangolins.