It may have come from a far less harmful virus that has been around in us for a while. Sort of like flu. Some who had it may have had a mild flu or just a cough, then that was it.
But as it was transmitted between people it would have mutated. Viruses mutate under circumstances of transmission, it’s part of the reason why they are so hard to develop a vaccine for.
Therefore it’s also possible that many people have had the precursor virus and their bodies thus have some immunity to that, and by extension, SARS-COV-2. And it would explain the unpredictable way many people seem asymptomatic.
Because the “pre-SARS-COV-2” virus would probably fool our current tests as well, and appear like SARS-COV-2, and thus be classed as an “asymptomatic case” when in fact it’s either the precursor, or a person who had the precursor and then caught SARS-COV-2 but were resistant or immune to it.
The worst take-home from this is that with a population heading for nine billion, it’s pretty much inevitable that further transmission mutations of other viruses will result in new strains that are as harmful as SARS-COV-2 has been – or worse…