Looks like SARS Cov-1 evolved into SARS Cov 2 all on its own...
SARS-CoV-2 probably arose in the same fashion.
Viruses are incapable of reproducing on their own. Instead they take over the internal mechanisms of a host cell.
It's roughly analogous to a tank bursting through the walls of an automotive factory and forcing the workers to produce tanks instead of cars. The copies of the original tank then go on to attack other factories.
Sometimes two or more closely related viruses will attack the same cell at the same time and the result is a fusion between the two. --A chimera with an affinity for other species too.
In this case, the other species is unfortunately, us.