Charles Funk in his book, “A Hog on Ice,” observes that when someone says a speaker using this expression is referring to someone they wouldn’t recognize. Perhaps they once knew the person, but have now forgotten. He speculates the expression originated over arguments about painters depicting both Adam and Eve with navels while critics insisted they did not have them. 

A Sherlock Holmes fan proposed in 1944 that when Holmes died and went to Heaven he was assigned to solve the mystery of what happened to Adam and Eve. “He alone knew all others from Adam, and could speedily pick out the missing pair, for he alone…knew that they would be the only two without navels.”