Morality is simply an evolved survival strategy — cooperation helped our ancestors survive, so we feel it's 'right.' No God required.

Why people find it persuasive

Evolutionary accounts of altruism (kin selection, reciprocal altruism, group selection) are well-established science, and it seems parsimonious to conclude morality is fully explained by the same processes that explain everything else about us biologically.

Important distinctions

• • Explaining why we have moral feelings vs. explaining whether moral claims are objectively true.
• • Behavioral cooperation (a descriptive fact) vs. moral obligation (a normative claim about what one ought to do).
• • An evolutionary account of moral belief vs. an evolutionary account of moral facts themselves.
• • Evolutionary debunking of moral realism vs. evolutionary explanation compatible with moral realism.

Christian response

• • Grant the science entirely: evolutionary psychology plausibly explains why creatures like us have moral sentiments and cooperative instincts. The apologetic issue is not with the biology but with what conclusion follows from it — explaining the origin of a belief-forming mechanism does not by itself tell you whether the beliefs it produces are true or merely useful for survival, which is a separate philosophical question (a form of what's called the genetic fallacy if conflated).
• • Press the 'is-ought gap' explicitly: evolution can explain that humans came to feel that cooperation is obligatory, but 'this feeling helped our ancestors reproduce' is a descriptive fact about causal history, and it is a further, unlicensed step to 'therefore this feeling tracks a real moral truth.' Evolutionary forces optimize for survival and reproduction, not for truth-tracking about objective value, so on a purely naturalistic evolutionary story we would expect moral beliefs that are useful, not necessarily beliefs that are true, undermining our confidence that any of them latch onto real, objective moral facts at all.
• • Bring in the actual philosophical debate: this is precisely the argument made by moral philosophers like Sharon Street in her 'evolutionary debunking arguments' against moral realism, and by theistic philosophers who use a similar structure (echoing C.S. Lewis and later Alvin Plantinga's related evolutionary argument against naturalism) to suggest naturalism struggles to underwrite the objectivity of morality that most people, including most atheists, actually want to affirm.
• • Distinguish this from a claim that atheists cannot be moral (they obviously can and are) — the argument concerns whether atheistic naturalism can ground objective moral facts (as opposed to merely explaining subjective moral feelings), which is the distinction between moral epistemology/psychology and moral ontology that gets collapsed in casual conversation.
• • Point out that most people, on reflection, are unwilling to treat morality as 'just what helped ancestors survive' when pressed on hard cases — most people believe the Holocaust would be objectively wrong even if it had conferred a reproductive advantage on some population, which shows that our actual moral convictions outstrip what a purely descriptive evolutionary story is capable of grounding.

Christian counterresponse

Theists respond that positing free-floating non-natural moral facts that exist without any explanation for why they exist, why they are binding on persons, or why an unguided evolutionary process would reliably track them (rather than merely tracking survival-useful proxies for them) trades one mystery for a bigger one, and that theism offers a more unified and non-ad-hoc account by grounding both morality's existence and our reliable access to it in a single source — but this remains one of the genuinely live debates in contemporary metaethics.