How can there be an objective right and wrong unless there exists a moral law giver?

This is a common view that is often posed as a challenge to those who point out the problems with religion being a basis for morality. The dilemma seems to be that without some law giver there cannot be an objective basis for morality. And, without an objective basis for morality, there is only moral relativism or subjectivism.

But, more and more research on morality and its origins seems to indicate that not only can there be a natural(as opposed to a supernatural) objective basis for morality, but that there is such a thing.

Morality seems to have arisen out of several basic behaviors that can be observed in other animal species such as reciprocity and cooperation. The outline of the argument for how these traits arose is somewhat involved and requires a fairly good understanding of evolution but from that basis it is possible to show how the moral principles that we have could have arisen naturally. I will recommend some good books below for more on this.
A good place to begin though might be with Sam Harris’ TED talk:

Science can answer moral questions

In this talk he lays out the basic ideas for an objective basis for morality based on the well being of conscious beings such as ourselves. Much of what we understand about how the brain works together with our understanding of evolution is lending evidence to the idea that there are objective moral principles and they are naturally evolved.

A good practical example of this is discussed in James Rachels book The Elements of Moral Philosophy. In his chapter on relativism he discusses some of the problems with that theory, one of which is that it denies any objective basis for morality.

But, consider several examples of moral principles that must be universal: truth telling, care of the young, rules against indiscriminate killing. How do we know these are universal? Because no culture could survive long without principles like this. Think about this in terms of your own community. Suppose you decided to gather a few hundred people together to disprove the objective basis of morality. All you would need to do is show that you could have a successful culture without these principles. So, you decide to operate with these principles instead: never tell the truth, do not care for young, kill anyone you want for any reason. How long do you think your culture will last?

So, the fact that any culture exists now from any time in the past must mean that they follow these principles. Yes, there are exceptions and not everyone will follow them but in general they have to be followed and most people most of the time have to respect them.
The reasons for the existence of these universal moral principles are quite easy to understand and they are quite natural. No divine origin need be postulated to explain them. The facts about what kind of beings we are together with the requirements of living on communities means these principles will evolve and successful communities will adopt them. Communities that reject them will die off.

But, how do we know that all of this isn’t being guided by a “divine moral lawgiver?” Of course, we can’t be absolutely 100% certain but the evidence we have suggests an entirely natural process. In explaining each step of the puzzle we do not need to postulate any supernatural intervention.

This seems impossible perhaps given what has to happen. How could it have all just occurred randomly? But, this misunderstands evolution in general and how moral principles evolved in particular. The best image to help explain this is provided by Richard Dawkins in his book Climbing Mount Improbable. In this book he relates the parable of Mount Improbable:

"Mount Improbable rears up from the plain, lofting its peaks dizzily to the rarefied sky. The towering, vertical cliffs of Mount Improbable can never, it seems, be climbed. Dwarfed like insects, thwarted mountaineers crawl and scrabble along the foot, gazing hopelessly at the sheer, unattainable heights. They shake their tiny, baffled heads and declare the brooding summit forever unscalable.

“Our mountaineers are too ambitious. So intent are they on the perpendicular drama of the cliffs, they do not think to look round the other side of the mountain. There they would find not vertical cliffs and echoing canyons but gently inclined grassy meadows, graded steadily and easily towards to distant uplands... The sheer height of the peak doesn't matter, so long as you don't try to scale it in a single bound. Locate the mildly sloping path and, if you have unlimited time, the ascent is only as formidable as the next step. The story of Mount Improbable is, of course, a parable."

But, the parable illustrates quite nicely the theory of evolution and how apparently designed creatures, can arise through a "non-random," natural process. And, it illustrates how moral principles evolved in the same slow, gradual “non-random” natural process.

Here are some good books to learn more about this fascinating subject:

The Moral Animal: Why We Are The Way We Are by Robert Wright
The Origins of Virtue: Human Instincts and the Evolution of Cooperation by Matt Ridley
The Selfish Gene by Richard Dawkins
Climbing Mount Improbable by Richard Dawkins
Moral Minds: How Nature Designed Our Universal Sense of Right and Wrong by Marc Hauser