Possessive is also contraction: "x's" <- "xes" <- "x his". No citation, but I read that somewhere. Interesting that it started out so primitive-seeming.
Interestingly, this "his genitive" emerged as a backformation of this genitive ending in the 13th century, not only in English but in a few other Germanic languages as well, before dying out a few hundred years later in some of them.