IIRC, it starts with Noah landing on the Ararat mountains somewhere in nowadays Turkey. Some 300 years later we hear that Shem runs a Yeshiva in Eretz Israel together with his grand-grandson Ever and Terah lives in Ur Kasdim in Iraq.
Because the culture was originally tribal, I presume they should have stayed together, especially if we accept the doctrine of their righteousness, after all, they studied the Torah!
So who was the exception - was it Shem and Ever who deviated from the tribe and came to the Promised Land, or the whole tribe came here and another descendant left Eretz Israel for Mesopotamia?
PS: This question doesn't focus on why (like "why-didnt-our-forefathers-team-up-with-shem-and-ever") but only seeks historical consistency - who went where and when.