Earlier quoted context omitted.
It isn't unusual at all. Many coronaviruses including MERS have a furin cleavage site and they're common and likely evolved independently multiple times, showing that this acquisition of an FCS is common: https://www.sciencedirect.com/science/article/pii/S187350612... The FCS in SARS-CoV-2 is NSPRRAR, we now know of sarbecoviruses like like the BANAL viruses in Laos which have NSPAAR, some other viruses like RmYN02 h…
You're answering why furin cleavage sites in general are not unusual. Not why this particular furin cleavage site is unusual from a natural evolutionary view. It's if you line up the related bat coronaviruses sequences they are all much the same in that region apart from Sars-Cov-2 which seems to have a clean insert of a PRRA sequence. Hence the "bothered by the furin site and has a hard time (to) explain that as an…
That is from:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7211627/#mmc1
Specifically the lower bit of figure 2:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7211627/figure/...
If RmYN02 picks up the QTQT sequence from recombination with other bat coronaviruses (or RmYN02 is a deletion away from a common progenitor we haven't found yet -- and the QTQT deletion is seen in SARS-CoV-2 variants in humans) then it is one insertion away from PRAAR which would be an active FCS, which is then one substitution away from the PRRAR SARS-CoV-2.