“ So clearly , something happen nature , ” Andersen says. “ I thought important little clue.It shows ’ mystery tighter binding human [ protein ] pangolins , too. ” The sugar-attachment sites another clue virus natural , Andersen says.The sugars create “ mucin shield ” protects virus immune system attack.But lab tissue culture dishes ’ immune systems , making unlikely adaptation would arise growing virus lab. “ That sort explained away tissue-culture hypothesis , ” says.Sign Up For Latest Science News Headlines summaries latest Science News articles , delivered inbox Client key * E-mail Address * Go Thank signing ! There problem signing up.Similarity SARS-CoV-2 bat pangolin viruses best evidence virus natural , Hodcroft says. “ This another animal spillover humans , ” says .