For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
The rainbow was made a sign that the world will no longer be destroyed by a flood: Noah and his sons were the first to see it. Circumcision was made a sign between Abraham and God: Abraham was the first to be circumcised. Sabbath was made a sign between God and Israel: the Israelites were the first to observe it. Jacob, Isaac, Abraham, Job, or Noah did not observe it. If there were already others observing it, it cannot make Israel a unique group of people.