Genesis 3:21 The Lord God made garments of skin for Adam and his wife and clothed them.
The only way to get skin off an animal is to kill it, so this was the first animal sacrifice. Animals were likely tame before the fall, and may have been like pets. Adam and Eve were vegetarians (Genesis 1:29, 9:3), and had never seen an animal die. But naked and ashamed, they needed the animal’s skin to cover their disgrace.
To make his point, God may have made them watch the slaughter of this innocent victim: throat slit, blood spilt, eyes wild, body trembling, life draining, lights out. It was a bloody object lesson to portray the wickedness of sin and the terrible cost involved.
In fact, the Old Testament is filled with the blood of sacrificial animals: sheep, goats, bulls and more. But all the blood of all the animals sacrificed could never take away sins (Hebrews 10:11), says Hebrews. That required nothing less than the shed blood of Jesus Christ—God himself in human flesh. Sin is man taking the place of God; salvation is God taking the place of man.
As Adam and Eve were clothed with the skin of a beast, all who believe in Jesus Christ are clothed with him. [Y]ou who were baptized into Christ have clothed yourselves with Christ (Galatians 3:27), wrote Paul. We have no righteousness of our own, but God has arrayed [us] in a robe of his righteousness (Isaiah 61:10), wrote Isaiah. The first animal sacrifice pointed forward to the sacrifice of Jesus Christ.