This could happen naturally with slow changes of one in ten thousand sequences each year. This appeared all at once. This gives support to the belief that the Wuhan lab took the bat virus and attached the SARS receptor.
However, this could happen with the mechanism of antigenic shift. While influenza viruses change all the time due to antigenic drift, antigenic shift happens less frequently.
Antigenic shift is the process by which two or more different strains of a virus, or strain of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is often applied specifically to influenza, as that is the best-known example, but the process is also known to occur with other viruses, such as visna virus in sheep. Antigenic shift is a specific case of reassortment or viral shift that confers a phenotypic change.