A long chain several improbable steps unlikely ever completed.For lab escape scenario , double CGG codon surprise.The human-preferred codon routinely used labs.So anyone wanted insert furin cleavage site virus ’ genome would synthesize PRRA-making sequence lab would likely use CGG codons so. “ When I first saw furin cleavage site viral sequence , arginine codons , I said wife smoking gun origin virus , ” said David Baltimore , eminent virologist former president CalTech. “ These features make powerful challenge idea natural origin SARS2 , ” said . [ 1 ] A third scenario origin.There ’ variation natural emergence scenario ’ worth considering.This idea SARS2 jumped directly bats humans , without going intermediate host SARS1 MERS did.A leading advocate virologist David Robertson notes SARS2 attack several species besides humans .