Technically, since atheists don't believe in G-d, a god or gods, for us, it ALL comes from people. It's the religious teachings, laws of man, what our parents and various others teach us. I find it a bit frightening to think that the only reason a person may follow any moral code is because of their religion, but I don't really think that's possible.

As for Ed T's comment "Are you suggesting that animals ( using the training you mentioned with your greyhound) would have a moral code of their own without human intervention?" of course they do! Any animal that lives with others has to live by rules. With people, a dog learns not to pee in the house or steal food from the table. Without people around, I doubt a dog would get away with stealing food from an alpha dog, or staring at him or growling at him. Learning and following rules are parts of a society, whether it's a society of people, dogs, or ants.