Adam and Eve were vegetarians. When God placed Adam in the Garden of Eden, the Torah tells us that God said: “Of every tree of the garden you may freely eat…” (Genesis 2:16). While God had granted Adam dominion over all creatures, only the plants were marked for human consumption.
Vegetarianism, in fact, was the only diet of humankind until after the great flood. Following their departure from the ark, Noah and his family are informed by God that “Every creature that lives shall be yours to eat; like the green herb, I have given you everything. Only flesh with the life thereof, which is the blood thereof, you shall not eat” (Genesis 9:3-4). Thus there was only one caveat to their completely carte-blanche diet…one could not eat an animal while it was still alive (including cutting off a limb from a live animal).
Why did God allow omnivorism after the flood? The exact reason is unknown. Some say that it was related to the fact that Noah saved the animals from destruction and was therefore entitled to benefit from them. Others note that the spiritual greatness of humankind in general had diminished over time, and since humans were now less spiritual and more physical, they needed the extra nourishment provided by meat. In fact, the Malbim (Rabbi Meir Leibush, Eastern Europe, 1809-1879) suggests that the physical nature of both humankind and the world in general was altered after the flood. Not only did produce no longer supply the same level of nutrients, humans themselves were now weaker physically.
While God initially allowed all creatures to be used for food, after He selected the Children of Israel to be a “Holy Nation,” He commanded them to limit their intake of meat/poultry/fish to kosher species, which were deemed spiritually beneficial, or at least not spiritually polluting.