I was reading Genesis 7 where God told Noah to take seven of every clean animal and only two of the unclean ones. The Levitical Laws weren’t given until the Israelites were in the Wilderness. How did Noah know which animals were clean and which were not?
It’s true that the Law was organized and written down by Moses during the 40 years the Israelites spent in the wilderness after leaving Egypt. But what Moses did was to formalize a system that had been in effect since man’s expulsion from the garden. That’s how Abel knew to bring a lamb to the Altar and why Cain’s offering of produce he had grown was rejected.
Cain was upset, but God said, “Why are you angry? Why is your face downcast? If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it.” (Genesis 4:6-7)
The fact that Cain knew what was right tells us that after the fall God had instructed Adam and his family on the procedure for handling sin including all the things humans would now have to do to maintain their relationship with God while awaiting the Redeemer who would come to restore them. This would have included the proper offerings, as well as which animals were acceptable and which were not.
Knowing that Noah would need to make several offerings before the first generation of post-flood animals was born, He had Noah take a larger number of those He had declared acceptable to insure that there would be a sufficient number both for sacrifices and for breeding.