When and why did humans start eating meat?
When the world was first created, mankind was given vegetables to eat. "And God said, "See, I have given you every herb that yields seed which is on the face of all the earth, and every tree whose fruit yields seed; to you it shall be for food" (Genesis 1:27).
About 1700 years later, after the great Flood, God told man to add meat to their diet. "Every moving thing that lives shall be food for you. I have given you all things, even as the green herbs" (Genesis 9:3). The only thing specifically forbidden was blood. "But you shall not eat flesh with its life, that is, its blood" (Genesis 9:4).
We are not told by God why He made the change in the diet.