I think that one strong indicator that the stories of the patriarchs are mythology is that they picture entire ethnic groups as being descended from single ancestral couples. Each of the Twelve Tribes of Israel is descended from one couple each, as are the Levites, the Ammonites, the Moabites, and so forth.
Usually only the male member of each couple is mentioned in any detail, but that's another story.
But when one looks at ethnicities that have emerged in well-documented times, they emerge from full-scale populations, and they usually emerge very gradually. Two good examples can be found in the land of the Bible itself over the last century:
The modern-Israeli ethnicity emerged from generation after generation of Jewish settlers.
The Palestinian Arab ethnicity emerged from the Arabs that were living there, including those who fled the area when the State of Israel became independent.
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