Evolution has shaped our evaluative judgments to the point that we are not justified in believing that there really are mind-independent moral facts. The atheist might respond and say that it was evolutionarily beneficial for humans to detect moral truths--our ability to grasp truth has aided us in our survival.
"According to the tracking account, making certain evaluative judgments rather than others promoted reproductive success because these judgments were true. But let's now look at this. How exactly is this supposed to work? Exactly why would it promote an organism's reproductive success to grasp the independent evaluative truths posited by the realist? The realist owes us an explanation here. It is not enough to say, "Because they are true."...Consider truths about a creature's manifest surroundings--for example, that there is a fire ranging in front of it, or a predator rushing toward it. It is perfectly clear why it tends to promote reproductive success for a creature to grasp such truths: the fire might burn it to a crisp; the predator might eat it up. But there are many other kinds of truths such that it will confer either no advantage or even a disadvantage for a given creature to be able to grasp them."