We all agree that we are taught fundamental moral issues(don't do this/that, this/that is bad etc) when we are a kid (and as we grow up) from parents, teachers, religion, basically from the environment. But how do we (or they, the people who taught us) know/realize that some moral stance are right while others are wrong ?
I think the first stage of moral development is the above but whether we believe them and hence apply them is determined by the agent/subject (individual). This is where I am hypothesizing that morality is linked with biology because whether we apply a moral stance or not is based on the biological outcome (real or realized/simulated). For example, a person A does not slap/club another person B (in a normal everyday scenario) because A knows that if he was at the receiving end himself he would feel physical pain, bruise/wound, tears etc… which are biological manifestations of the body. Though the example is simple minded (and hence could be more sophisticated), it tells us that the main incentive for taking a particular moral stance is biological mechanisms (both physiological and its psychological manifestations such as emotions etc). Therefore, though there may be some exceptions (individuals), in a random population of a particular society the majority would also base their moral standing on this biological incentive and hence these would form the moral ground/code of the society.
On conclusion, I think the first stage of moral development from environment provides a road map and selection of which road to take depends on biological incentives (just as given the choice most people take the asphalt road over the road with gravels, assuming both are of same distance not short-cuts, and you are not in a hurry ;-)).