A common argument among theists and atheists centres around the nature of morality. We can all claim to have an idea of what is moral, but we often fail to agree why we think it is so. Many theists would argue that we are bound to objective moral standards which are immutable, regardless of perspective, opinion, or circumstance. An atheist may argue that our sense of morality is grounded in little more than this.
We could argue that determining which party is correct, rests on identifying and understanding the origins of our morality. If we are to assume that we were the creation of a god, it would be plausible to accept that this god has set the rules by which we should live. If this is the case, these rules are not subject to be changed as we see fit but are inescapable regardless of perspective, opinion, or circumstance. Does the fact that we often disagree on what is moral, indicate that this may not be the case? I think it does.
If we are to accept that we are part of an evolutionary process, would it not to be fair to assume that our sense of morality has also evolved. If this is the case, then we could argue that what we see as moral/immoral behaviour stems from the conditions of life faced by our early hominid ancestors. These ancestors lived in small groups which were mostly related. To understand the behaviour of these ancestors, we must understand the evolutionary forces which governed them. This includes both kin selection and reciprocal altruism: The concepts for which have been studied by the likes of John Maynard Smith, Robert Trivers and Robert Axelrod, through evolutionary biology/psychology and game theory.
Kin selection is the process in which individuals favour their relatives as they share high proportions of their genes. Due to this, it is in the individual's interest to act altruistically towards their kin as they will also pass many of the same genes to their offspring. This leads individuals to form groups among relatives.
As these groups become larger and more successful their relatedness diminishes. This leads to the necessity of reciprocal altruism.
Reciprocal altruism is the process in which individuals require the assistance of others for tasks which they are not able to do alone, such as removing parasites in hard to reach areas of their bodies. If individuals were selfish by refusing to reciprocate, they would be punished by the group by the refusal of further assistance. Due to this punishment, selfishness will not prosper in favour of altruism. The successful altruistic genes will then be passed to the next generation, ultimately leading to an evolutionary stable strategy favouring reciprocal altruism.
These evolutionary forces governed the way we acted, as it was in our own self interest to care for those around us. Our sense of morality has not been shaped through altruism for the sake of altruism, but selfishness, masquerading as altruism. We protected our kin who shared many if our genes and built trust in those whose help we required. In such small groups, antisocial behaviour would not go unnoticed, which could lead to a member of a group being ostracised, leaving them without food and vulnerable to prey.
These ancestors weren't moral through a god-given code of conduct. They were moral because their lives depended on them being so.
Over millions of years, this moral code has been passed from generation to generation until it found itself in today's societies. As we no longer live in small groups, our immoral acts are less noticeable and therefore less likely to be punished. This leaves us questioning this inherited sense of morality; leading us to our own subjective conclusions.