It was actually remarkably stable in the early days suggesting it was used to reproducing in human cells. Or so Professor Petrovsky says https://www.dailymail.co.uk/news/article-8356751/How-COVID-1...
As how that could have happened lab wise here's Daszak saying they routinely infect human cells with coronavirus in the lab https://www.youtube.com/watch?t=1701&v=IdYDL_RK--w&feature=y...
Or maybe it was in humans a bit before it took off. I see Daszak's kind of changed his tune a bit these days to not mention anything like that lab stuff.
The common Flu can infect horses and even chickens. "stability in human cells" means pretty much nothing