David Brooks rightly holds that the evolutionary picture of human nature is inadequate:
[The] strictly evolutionary view of human nature sells humanity short. It leaves the impression that we are just slightly higher animals — thousands of years of evolutionary processes capped by a thin layer of rationality. It lops off entire regions of human possibility.
According to Brooks, evolutionary biologists have reduced human nature to two distinct systems, one “to procreate or strut or think in certain ways” and another focused on reason and consciousness. Although biology depicts this dual nature, Brooks recognizes that morality mostly consists of reason ordering our more animalistic impulses:
Deep down we are mammals with unconscious instincts and drives. Up top there’s a relatively recent layer of rationality. Yet in conversation when we say someone is deep, that they have a deep mind or a deep heart, we don’t mean that they are animalistic or impulsive. We mean the opposite.
But without providing metaphysical cushioning to his objection, Brooks leaves room for naturalists to evade the real issue of evolutionary-based ethics.
The naturalist could argue that while animalistic impulses to procreate, protect our families, and avoid danger clearly maintain our survival, reason propagates our species as well. Animalistic impulses and rational ones essentially aim at the same end of survival.
Richard Dawkins has made this point in his book The Selfish Gene, arguing that even something as seemingly contrary to survival as altruism can be explained through survival-driven origins. The real problem is that even if evolution accounts for impulses to act in a certain way, it does not alone mean that we ought to act on or against a specific impulse. If a tendency for altruism is the product of biological processes, it by no means follows that altruism is therefore good.
On what grounds ought we follow the altruistic impulse?
There would have to be an “exterior” instinct to follow that rational impulse which tells us to resist the greedy temptation. But in a naturalistic worldview, ultimately, that instinct to follow reason would still be merely an instinct—giving it no real authority over any other instinct. Unless one impulse can justly be called “better” than another, moral judgment cannot be derived from evolutionary biology.
A system of morality must be established independently of studying our biological origins. To start, we would first need to establish the claim that “we ought to be rational”—itself a moral truth. If we admit that acting rationally is a foundational moral precept, then we already stray from a purely evolution-based theory of ethics.