Chullin 100: Which Came First, the Mitzvah or the Practice?

The rabbis question which animals, kosher or non-kosher animals, are forbidden when it comes to the sciatic nerve.  Bereishit (32:33) teaches that Jacob limped back after his fight with the angel, and the Torah said "Therefore the children of Israel eat not the sinew of the thigh-vein which is upon the spoon of the thigh, unto this day; because he touched the spoon of Jacob's thigh, even int he sinew of the thigh-vein."

Our new Mishna teaches that only kosher animals apply.  Rabbi Yehuda argues that Jacob's children were allowed to eat ritually impure animals before the Torah was given but they did not eat the sciatic nerve.  The Mishna responds that the mitzvah prohibiting the sciatic nerve was given at Mount Sinai and then it was placed in context in Bereishit.  This does not address the question of whether or not Jacob's children kept the tradition before receiving Torah.

Steinsaltz explains the general principal regarding Biblical mitzvot: they require the Jewish People because they were given as part of the Torah and not because they may have been part of earlier Biblical narrative.  The example provided by Rambam is that of brit mila, circumcision, is kept because the Torah commands us to circumcize our children like Abraham was circumsized, not because Avraham was commanded to circumcize himself and members of the family.

Even in antiquity we see our rabbis grappling with the questions that arise around time: how could one thing have happened before it was written?  Which rules did we begin to follow at which times?  How can we use logic to better understand creation, time, the will of G-d, and Halacha?