Question: "What made some animals clean and others unclean (Genesis 7)?"
Answer: Noah took two of every kind of animal into the ark, right? Not exactly. The Bible specifically states, “Take with you seven of every kind of clean animal, a male and its mate, and two of every kind of unclean animal, a male and its mate, and also seven of every kind of bird, male and female, to keep their various kinds alive throughout the earth” (Genesis 7:2-3). Clean animals included seven of each animal on the ark. Only unclean animals came in pairs.
Leviticus 11 defines the difference between clean and unclean animals, but Noah lived before the giving of the Law. We are not told how Noah knew which animals were clean and unclean, but he obviously knew the difference. Sacrifices to God were made before the Mosaic Law (Genesis 4:4), which means God had somehow communicated to man what animals were suitable for sacrifice (and, later, for eating).
Leviticus 11 defines which birds, land animals, and sea creatures were clean and unclean. Here are a few of the clean and unclean animals in those lists:
Clean animals: land animals that chew the cud and have a divided hoof, such as cattle, deer, goats, and sheep; seafood with both fins and scales, such as bluegill, grouper, and cod; certain birds, including chickens, doves, and ducks; and even some insects, such as grasshoppers and locusts.
Unclean animals: land animals that either do not chew the cud or do not have a split hoof, such as pigs, dogs, cats, horses, donkeys, and rats; seafood lacking either fins or scales, such as shellfish, lobster, oysters, and catfish; some birds, such as owls, hawks, and vultures; and other animals, such as reptiles and amphibians.
While the New Testament teaches that we are no longer judged regarding what foods we eat (Colossians 2:16), nutritionists have noted that the listings of clean and unclean foods in the Old Testament actually make for a healthy diet. In a time period lacking modern food safety regulations and refrigeration, a diet consisting of clean animals would have protected against many health problems.
Ultimately, God’s distinction between clean and unclean animals was about more than one’s diet. Many of God’s regulations were to remind His people, Israel, to be holy and set apart to worship the one, true God. The original audience of Genesis 7, during Moses’ day, would have associated the reference to clean animals with the animals God had set apart for their diet as well as what were used for sacrifice. It would only make sense to include more clean animals than unclean on the ark. Noah made a sacrifice immediately after the Flood (Genesis 8:20). Since seven of every clean animal had been aboard, the sacrifices would still have left three pairs to begin replenishing the earth.