I think that the majority of ethic laws that are followed in most societies result via the divine command theory of ethics, meaning a higher being commanded people to act the way they act. Most cultures have a major religion that has a moral code associated with it. These moral codes have general similarities, such as don't kill and don't steal. Over time these groups of morals have been refined and combined and the similarities are part of our general ethic code of today. .
Ethics are nothing more that ideas. These ideas are useless everyone in a society agrees to adhere to the ideas. These ideas had to be though up by someone at some point in time. At the creation of these ideas, it would be possible for the person who created these ideas to choose whatever direction they would want to go, such as claiming polygamy as acceptable. The fact that most major religions have similar codes of ethics would suggest either proof for the divine command theory or humans have an instinctual nature towards ethics and morals. As there is no concrete evidence towards a higher being, humans must have some pre-disposition to ethics. Ethical theories are then useful to explain the differences in peoples views on ethics, but no ethical theories are necessary for people to understand what is right or wrong. .
Ethics are a necessity for a society to operate. Laws in our society tend to stem from moral codes used by a society. With a lack of morals and ethics, we end up in society without laws. We have seen through history that anarchy, a society with out laws, is doomed to failure. This leads me to believe that without morals and ethics, our society would be unable to exist. This doesn't mean that ethics are necessary for humans to exist, but for humans to live and work in the society we have created, morals and ethics are necessary.
