It could be that the circumcision tradition began as physical proof that one is Jewish. The Jews believe they are the legitimate family line that goes back to Adam and Eve. If one is not Jewish then one is not from a "naturally" legitimate family. This is why they like to marry within their religion and as a result they have acquired their unique appearance. Also, back in Biblical times when the Hebrews were the dominant religion of the middle east people couldn't be officially married unless they conformed to Jewish requirements. For example, if one was the child of a slave they could never be married, ever. They'd have to be two generations removed from slavery. As a result people used to flee to Cyprus to get married. The problem was though that Cyprus is an island and was a dangerous place to go back then. It is also why Christianity became popular. Christianity didn't care if one was a child of a slave or not. It also didn't care if one was circumcised or not.