ADAM WAS THE WORLD’S FIRSTBORN

Midrash Rabbah - Numbers 4:8

TAKE THE LEVITES, etc. (Numbers 3:45). Our Rabbis have said: Why did the Holy One, blessed be He, order the firstborn Israelites to be redeemed by means of the Levites? Because originally, before the tribe of Levi arose, the firstborn performed the sacrificial service … There is proof that the firstborn offered the sacrifices before the tribe of Levi took office. Go back to the beginning of the creation of the world.

Adam was the world's firstborn. When he offered his sacrifice, as it says: And it pleased the Lord better than a bullock that hath horns and hoofs (Psalm 69:31) - he donned high priestly garments; as it says: And the Lord God made for Adam and for his wife garments of skins, and clothed them (Genesis 3:21). They were robes of honour which subsequent firstborn used.