I read it in Nicholas Wade's article: https://nicholaswade.medium.com/origin-of-covid-following-th... (long read).

Some more reading: https://www.news-medical.net/news/20210217/The-origin-of-SAR...

All these lines of evidence and reasoning show that the acquisition of the polybasic furin cleavage site by SARS-CoV-2 is a “missing link” in our understanding of its evolutionary history, that can only be addressed through the discovery of new viruses.”