Many changes harm virus natural selection retains something useful.Mutation process SARS1 spike protein gradually switched preferred target cells bats civets , humans.Mutation seems less likely way SARS2 ’ furin cleavage site generated , even though ’ completely ruled out.The site ’ four amino acid units together , right place S1/S2 junction.Mutation random process triggered copying errors ( new viral genomes generated ) chemical decay genomic units.So typically affects single amino acids different spots protein chain.A string amino acids like furin cleavage site much likely acquired together quite different process known recombination.Recombination inadvertent swapping genomic material occurs two viruses happen invade cell , progeny assembled bits pieces RNA belonging .