Brother, early Genesis is as much truthful history as the rest of the Bible. As for there just being Adam, Eve, Cain and Abel around the time they offered sacrifices to God, who's to say there were only four people on earth? The Bible mentions that Adam had other sons and daughters. Presumably he had many of each, their names just aren't listed as they're not important to the Messiah narrative. And yes, animals would've been kept for their products (not meat, but eggs, milk, cheese, maybe yoghurt) and to provide clothing for humanity at the time. Why do you think there were no predatory animals around at this time? Adam and Eve had rebelled against God quite some time earlier.