why are some of the characters in Cormac McCarthys "The Road" without their thumbs?

The author does not directly tell the reader, but a good arguement can be made for it being some sort of tribal ritual. One good example would be the burned bodies that he and the boy come across that haave missing fingers and tribal markings. Another time the father orders a thief to strip and remove his shoes. They notice that the man has no fingers.