A new study, however, has concluded that chimp murders are a part of their natural evolutionary development. Here's the relevant excerpt from the NYT:
The study’s authors argue that a review of all known cases of when chimpanzees or bonobos in Africa killed members of their own species shows that violence is a natural part of chimpanzee behavior and not a result of actions by humans that push chimpanzee aggression to lethal attacks. The researchers say their analysis supports the idea that warlike violence in chimpanzees is a natural behavior that evolved because it could provide more resources or territory to the killers, at little risk.
We balk at that conclusion, of course, but if we're naturalists (i.e.atheists) and if we embrace the Darwinian view of humanity, what do we think makes humans different such that murder should have a moral dimension for us that it doesn't have for those animals most closely related to us? Researchers in the field may be repulsed by chimp violence and saddened by the deaths, but do they believe that murderous chimps have transgressed some objective moral law? If not, why are human murderers thought to transgress such a law? If there is no such law for chimps and other animals why do we think there is such a law for humans?
It seems to me that the naturalist has no good answer to this. If we're just animals then there is no objective moral law. If, however, one insists on believing that there is an objective moral law, a genuine right and wrong, then that person must also accept the idea that there must be a lawgiver, a personal transcendent moral authority (PTMA), who promulgates the moral law and holds us accountable to it. Apart from this belief in what amounts to a God our conviction that it's wrong to murder, to be cruel, to torture, to abuse children, is completely without objective foundation. When we voice that conviction we're simply doing no more than stating our personal predilections, and our personal, subjective tastes can hardly be the standard for morality.
Once again, it seems to me, the naturalist is between a rock and a hard place, metaphysically speaking. Either he can give up his belief that there is an objective moral duty not to murder and embrace a nihilistic view of morality, or he can give up his naturalism and embrace the idea that there must be a PTMA.
If he does neither, if he clings to his naturalism while at the same time clinging to his conviction that murder is objectively wrong, then he's behaving irrationally. He may hold on to his atheism, but he can't claim that it's the reasonable position, and he certainly can't claim that either the nihilist or the theist are behaving less rationally than he is.