I already pointed out, about three years ago, that the Sumerians invented the rap battle.
I found the reference: The Method and Message of Jesus’ Teachings (revised edition) by Robert H. Stein (Westminster John Knox Press, 1994), page 13.
Google Books (in a short section on puns): The Method and Message of Jesus' Teachings - Robert H. Stein - Google Books
That contains a reference to note 14, given on page 156, which explains that camels are also unclean: The Method and Message of Jesus' Teachings - Robert H. Stein - Google Books