Evolution can never explain morality.
Here's 2 reasons why
1. The chemical/electrical/neural basis.
If, indeed, we have simply evolved from a pool of amino acids, then all our thought processes can be broken down to reactions between the chemicals and neurons in our brains. That includes all thoughts/feelings we may have about morality.
Therefore, all our thoughts about morality should be considered equal. They are all A. From the same source of evolution and B. Involve the same chemicals/neural reactions.
If the connections in your head tell you, "Hey, human sacrifice is probably bad," and the connections in a tribesman's head say, "Hey, human sacrifice is good," who are you to say that is wrong? What is inherently wrong with stopping the continued motion of some connected reactions/hormones/genes?
The same processes that say, for one person, "Racism is bad," can say for another person, "Racism is okay." If there is no higher authority to distinguish, then both views are of equal authority. They are both the product of the same amount of evolution, they both rely on reactions in the brain, etc. So, one may cause pain. What is inherently wrong with causing another evolved meat-sack pain? Their life-span is meaningless in the context of 4.5 billion years of evolution. As long as the survival of the species/gene pool is not at stake, there is not a *logical* argument to be made for an inherent issue with causing pain. The view of the majority (that pain is bad) does not make something true. It just means that more chemical reactions happen to result in one speech/thought pattern than a different one.
People who ascribe to the view may respond with, "how could you say that," or "I hope that the only reason you're not out there killing people is your ancient book".
This totally dodges the question. Because the only reason that can be cited for morality when God is discarded is the basic life processes and evolution that led to each individual's existence.
2. Natural selection and our moral beliefs are, in some cases, mutually exclusive.
For example, many people, from all beliefs/political ideologies, think that caring for those who cannot care for themselves is good. To some, it means loving your neighbor. To some, it means the government should provide healthcare. To some, it means philanthropy is admirable.
Yet, logically, this view could not come as a result of evolution. Those early hominids who expended resources to help the helpless would have been at a disadvantage, and therefore natural selection would have killed them off. The 'kindness to the helpless' gene would have been eradicated, because it would have been an unlivable lifestyle when resources were scarce, and survival was at stake.