In one such conversation I am currently having over on Facebook, a young man said:
The gist of his question/statement, if it is to be nut-shelled, is: Where does the moral nature of man come from? Evolution does not provides a satisfactory answer for all mankind sharing a common ethical set of principles. Man's semi-common sense of "right" and "wrong" must have come from a higher source."
What about the "laws of nature"? Nowadays, when we talk of the 'laws of nature' we usually mean things like gravitation, or heredity, or the laws of chemistry. but the older "thinkers" called the law of right and wrong 'the Law of Nature,' they really meant the Law of Human Nature. The idea is that, just as all bodies are governed by the law of gravitation, and organisms by biological laws, so the creature called man also had his law--with this great difference, that a body could not choose whether it obeyed the law of gravitation or not, but a man could choose either to obey the Law of Human Nature or to disobey it. [...] There have been differences between their moralities, but these have never amounted to anything like a total difference. If anyone will take the trouble to compare the moral teaching of, say, the ancient Egyptians, Babylonians, Hindus, Chinese, Greeks and Romans, what will really strike him will be how very like they are to each other and to our own. [...] Selfishness has never been admired. Men have differed as to whether you should have one wife or four. But they have always agreed that you must not simply have any woman you liked.
The problem in this statement is that a much stronger claim is being made than is actually warranted by any of the evidence. There isn't a consistent code of ethics being followed by billions of people, although there are consistent ethical principles that people follow. There are principles against killing the innocent in most societies, but those codes inconsistently define who is "innocent" and who is not. There are many principles against theft in most societies, but those codes inconsistently define the nature of private property and what is to be done with it.
Both the consistency of the general principles as well as the inconsistency of the codes which express those principles are quite understandable in the context of evolution.
The reason why there are laws against murder in all cultures is because nobody wants to be murdered (believe it or not...). In every culture, people got together and agreed that having a law against murder is a good idea (although that society will still kill people based on other "moral" laws held by that same group). With a law against murder they can all worry a little bit less about being murdered themselves within their particular social group as long as they adhere to the other moral precepts and codes. Same thing with theft: People do not like having things stolen from them. They got together and enacted laws against theft (although people will still steal if they are short-sighted enough to think of their own survival rather than the societies' survival). It is evolution that is responsible for the desire to survive (not be murdered) and thrive (one part of which is keeping the the things we own and not having them stolen from us). The evolutionary process on each individual has made us all similar enough as a society (due to those particular individual traits surviving in individuals in those groups, expressing those traits would have been more desirable for mating as they were better for the group and/or society where potential mates were located) in wanting similar things. So when we get together and decide on a moral or legal code, there are things that will end up being illegal or "immoral" in all cultures IN PRINCIPLE. On an individual level, the societies mentioned still have the same basic laws (again, in principle), and if you ask the individuals in those cultures, they will tell you they have a conscience and know right from wrong within the context of their definitions of private property, innocence, public property, et cetera.
Religious morality claims to be based on absolute moral law given down by a god. However, it is easy to look at the varying moralities of members of the same religions over history and in present day, and see that in fact RELIGIOUS morality constantly changes as secular societies change and interpretations of religious texts change as well (and thus, the moral stances of that religion changes). One of the implications of this sort of moral code is that when you harm someone, the wrong you have done is disobeying God, not harming that person. The harm to that person becomes fairly inconsequential. If an action is to be objectively wrong, then it is to be wrong for anyone--including God.
One example is slavery. Using the bible, there is not one passage that flat out condemns slavery. Slavery existed for thousands of years under societies that were strictly controlled by religious forces. The bible was used in the defense of slavery before and during (and after!) the civil war. Nowadays, however, most Christians will say that the bible opposes slavery. They have simply chosen to take different passages and interpret them in a different manner than those who have preceded us for thousands of years. Both groups can quote from the bible to support their arguments, yet both are completely convinced that theirs is (or was, since pro-slavery forces have mostly died away and/or become unpopular) the God-given moral law.
There are hundreds more examples besides slavery, from basic rights that were once considered morally necessary but are now laughed at, to major issues like equality amongst the genders.
The fact is, religious morality stems from the same place atheistic morality stems from. People. There is no "natural" or "moral" law. If morality WERE part of the order of nature, it would be inviolable, like gravity, or the laws of thermodynamics. What is called "natural law" is actually social consensus, arising out of the fact that people need to cooperate with one another in order to survive.
Morality is credited to evolution, and it is worthwhile to remember that large society is a very new thing (mostly due to medical and technological sciences based on those very evolutionary truths). Before 10,000 years ago, we lived in small hunter-gatherer type groups with no formal government per se. And this is much too short a time span for any evolutionary change to occur. Natural selection (one part of the equation that makes evolution what it is) works on the individual and their genes, not on societies or populations as a whole. The catch is, just because a group is a small, hunter-gather society does NOT mean there is no government-entity to enforce socially-agreed upon moral precepts. It may be informal, yes, but a government it is. If you lived 100,000 years ago and you stole food from an individual in your group, then it doesn't take lawyers to decide you're harming the group--one pack elder could frown and have you killed as a liability to the group! These behaviors are not unique: they have been witnessed in other animals, such as ravens and great apes and others. This type of proto-legal government would quite rapidly tend to favor those with group-friendly behaviors. Thus, a common across-species "morality" is carried through the generations across the species who survive due to socially-accepted mores and laws which are then taught, through both words and actions to the next generation, not necessarily genetically (although genetic predisposition cannot necessarily be ruled out either!).
Saying morality and goodness must flow from a deity not only puts the cart before the horse, it disregards the horse (evolution) completely as the driving force of the cart (carries our common ethical principles).