Genesis 3:15: A Look at the Effect of this Curse on Humans
By M J Hood - 10 Apr 2004
Early Christians and Jews tended to have a vegetarian diet because animals were expensive to keep. The ones you had you didn't eat, you used for sources of food, such as milk or eggs etc. That's why to the ancients sacrificing an animal was a really big deal, and to sacrifice the very best even more so. It was a sacrifice for the family to give up one of its animals. Also, there are prohibitions about which animals may be eaten and how.
In the 613 Mitzvot of Judaism, it specifically addresses how animals will be treated even to stating that your animals will eat before you do, you will not eat until they have been fed. When humankind was young, it felt a greater kinship to the animals.