There are any number of animal species which have moral codes too, and most of the individuals can be seen to conform to those codes pretty consistently, while those who don't get into trouble with the rank and file. Observe wolves, elephants, herd animals, etc., for evidence of this. Crows, who have a complex community life, have even been know to hold what certainly appear to be "trials" over an individual crow who has violated the code. They all discuss it for awhile. Then, if he's found guilty, they kill him (or her as the case may be).
As I said before, actions which are plainly constructive are generally seen as good by most if not all community members, while actions that are plainly destructive are seen as bad by pretty well everyone except the perpetrator.
Thus, our ideas about ethics are not entirely arbitrary by any means.
Most people are inclined to be fairly constructive most of the time, unless they are suffering great fear, and upon that is based our considerable ability to cooperate and form healthy societies...as do wolves, crows, and elephants.