Two years ago, I had the chance to debate an atheist professor at Weber State University in Utah on the best explanation for the existence of objective moral values. The writings of Bill Craig and Paul Copan have shaped a lot of my thinking in this area, as I'm sure you'll see below. In my opening argument, I made the case for God as the ontological foundation for objective morality. Then I raised five problems for an evolutionary view of ethics that make it an implausible alternative. Here are the problems I outlined in the debate:
(1) Evolution cannot account for moral values: Moral values do not fit in the ontology of naturalism. On a naturalistic view, our moral values are the result of biological evolution, purely for the purpose of survival and reproduction. But how would such a herd morality be binding and true?
Atheists recognize the unnatural fit. Philosopher of science Michael Ruse writes:
The position of the modern evolutionist...is that humans have an awareness of morality...because such an awareness is of biological worth. Morality is a biological adaptation no less than are hands and feet and teeth.... Considered as a rationally justifiable set of claims about an objective something, ethics is illusory. I appreciate that when somebody says “Love thy neighbor as thyself,” they think they are referring above and beyond themselves.... Nevertheless...such reference is truly without foundation. Morality is just an aid to survival and reproduction...and any deeper meaning is illusory…. 
J.L. Mackie, one of the most prominent atheist philosophers of the 20th century, said this: “Moral properties constitute so odd a cluster of qualities and relations that they are most unlikely to have arisen in the ordinary course of events, without an all-powerful god to create them.” 
(2) Evolution cannot account for moral obligations: If humans are simply more developed animals, why think there are moral duties to which they are obligated? Male great white sharks are under no obligation to refrain from forcibly copulating with female great whites. Male lions are under no obligation to refrain from killing all the young lion cubs in a pride they have just taken over. Notice, we do NOT use moral terms to describe such behavior. We do not call the shark’s behavior “rape” and we do not call the lion’s behavior “infanticide.”
Natural science is a descriptive enterprise, only telling us what is the case, not what ought to be the case. For example, nature can describe what it is to be healthy, but it cannot generate a moral obligation to be healthy.
Prominent American philosopher Richard Taylor recognizes this problem for naturalism:
The idea of political or legal obligation is clear enough…. Similarly, the idea of an obligation higher than this, referred to as moral obligation, is clear enough, provided reference to some lawgiver higher…than those of the state is understood. In other words, our moral obligations can…be understood as those that are imposed by God…. But what if this higher-than-human lawgiver is no longer taken into account? Does the concept of moral obligation…still make sense? … The concept of moral obligation [is] unintelligible apart from the idea of God. 
On a naturalistic view, there is nothing to issue moral commands and there is no one to serve as the appropriate authority standing behind our moral obligations. As Taylor states, in the absence of God, the concept of moral obligations is incoherent.
(3) Evolution cannot adequately explain human value: On a naturalistic evolutionary scenario, human beings are nothing special. The universe comes into existence through the Big Bang and, through a blind process of chance and necessity, evolves all the way through to us. The same process that coughed up humans also coughed up bacteria. Thus, there is nothing intrinsically valuable about being human. Indeed, on this view, to think humans beings are special is to be guilty of speciesism, the view that one’s own species is somehow superior to other species.
Atheist Richard Dawkins offers a bleak assessment of human worth on an atheistic worldview:
In a universe of blind physical forces and genetic replication, some people are going to get hurt, other people are going to get lucky, and you won’t find any rhyme or reason in it, or any justice. The universe that we observe has precisely the properties we should expect if there is, at bottom, no design, no purpose, no evil and no good, nothing but blind, pitiless indifference…. DNA neither knows nor cares. DNA just is. 
But what other result should we expect from valueless, cause-and-effect physical processes? There is no reason to think that an impersonal, valueless process could produce valuable, rights-bearing persons.
(4) Evolution cannot adequately explain moral accountability: If God does not exist, there is no basis for moral accountability. On naturalism, who or what imposes moral obligations upon us? And who or what would hold us to those obligations? In a purely material universe, there is no moral accountability. What difference would it make to disregard your moral obligations to whomever? In an atheistic universe, why would you ever set aside self-interest for self-sacrifice?
(5) Evolution cannot adequately explain human freedom: If we are the products of evolutionary forces, how did moral freedom and responsibility emerge? There is no reason to think, given our supposed materialistic and deterministic origins, that we have free will. Atheist philosopher Thomas Nagel states that there is “no room for agency in a world of neural impulses, chemical reactions, and bone and muscle movements”; naturalism strongly suggests that we are “helpless” and “not responsible” for our actions. 
Of course, if there is no free will, then no one is morally responsible for anything. Determinism puts an end to objective moral duties because on this worldview, we have no control over what we do. We are nothing more than puppets in a cause-and-effect universe.
 Michael Ruse, “Evolutionary Theory and Christian Ethics,” in The Darwinian Paradigm (London: Routledge, 1989), pp. 262, 268-9.
 J. L. Mackie, The Miracle of Theism (Oxford: The Clarendon Press, 1982), 115-116.
 Richard Taylor, Ethics, Faith, and Reason (Englewood Cliffs, N. J.: Prentice-Hall, 1985), p. 83-84.
 Richard Dawkins, River Out of Eden: A Darwinian View of Life (New York: Basic Books, 1995), 132-133.
 Thomas Nagel, The View from Nowhere (New York: Oxford University Press, 1986), 111, 113.