God. Real. Right.

A note on terms in the argument: in this post, biology serves as the representative of material reductionism or just of naturalism, since it is the closest a reductionist can come to anything complex or progressive enough to explain the features which non-reductionists claim to exist. Similarly, evolution in this post is simply the most consistent model with which to explain biological advantages. In both cases, the position given to naturalism is intended to give it the most favor available.

Humans have many behavioral and functional characteristics which are biological by nature, none of which makes us human. To be clear, they may be necessary to being human, but they are not sufficient. For example, it is necessary to eat to be human. But many non-human things eat. So what separates humans from, say, pigs? Not much, biologically. But plenty, if Aristotle has anything to say about it; reason, to be specific. So Aristotle calls man the rational animal. From his perspective the definition is sufficient because it distinguishes humanity from every other thing.
There are characteristics other than reason which could be used to pull humanity out of the category of all other animals: consciousness, spirit, aesthetics, or ethics, for instance, depending on who is doing the pulling. But the one of most interest here is ethics.
Long since Aristotle, skeptics regarding human nature have argued that there is no cut-and-dry distinction between other animal species and humanity. There are differences of degree, but nothing absolute, they contend. Previously, value-laden characteristics like ethics seemed inexplicable in terms of biology alone. But once evolutionary theory takes on a social element, that explanation no longer seems so impossible. For instance, altruistic behavior is especially difficult to explain in terms of evolutionary biology at the level of individual organisms, since the whole purpose of such evolution is in the interest of the survival of the individual, and much altruistic behavior involves the sacrifice of the individual. In other words, it is hard to explain why a person would sacrifice his life to keep someone other than his own offspring alive when what should be motivating him above everything else is his own survival. However, social evolution puts survival at the level of the tribe, pack, or clan, rather than at the level of the individual. And tribes, packs, or clans which produce individuals willing to die in order to protect the pack would appear to have a survival advantage. All of a sudden, as the argument goes, even ethical behavior may not present an absolute line of distinction between humanity and other species. (That particular argument is not finished there, but it is not the point here to rebut it. On the other hand, it is worth mentioning that social-evolutionary explanation does nothing to address the rise of “value” at all in issues of ethics or of aesthetics, an accomplishment without which the great gap between real ethics and animal behavior remains unbridged.)
More importantly here, though, there is another, prominent feature of human behavior which makes perfect sense in terms of biology, but is condemned outright and regulated only by real ethics. That conflict (between biological and ethical products) is just as serious a threat to the effort to elide every real difference between animals and humanity as the maintenance of the non-evolutionary nature of altruism; that is, just as serious a threat to the claim that man is no more than the most complex of animals.
The behavior at issue, a very ugly one indeed, is this: people will cause others to suffer, then use that suffering as proof that the others deserved to suffer in the first place and that they deserve to suffer even more. It is a spiraling intensification of abusing the weak. The apostle Paul describes this behavior in the opponents of Christianity:

And in nothing terrified by your adversaries: which is to them an evident token of perdition, but to you of salvation, and that of God. Philippians 1:28.

Of course, Paul models his descriptions of how Christians live and what they face after Christ. So it is not remarkable that the same kind of behavior accompanies the crucifixion, for example as the Jewish leaders pronounce that Jesus is accursed and therefore incapable of being the Messiah because He is crucified according to their plan, an apparently deliberate effort to discredit Him based on a perverse interpretation of Deuteronomy 21:22-23:

And if a man have committed a sin worthy of death, and he be to be put to death, and thou hang him on a tree: His body shall not remain all night upon the tree, but thou shalt in any wise bury him that day; (for he that is hanged is accursed of God;) that thy land be not defiled, which the LORD thy God giveth thee for an inheritance.

More directly, as well, Jesus is assaulted for His weakness by the people who strike him and the people who observe His crucifixion. As oddly sadistic as these behaviors may seem to some sheltered rube, no one who has watched the herd mentality take hold of a crowd is surprised at all. Accounts of the mistreatment of prisoners during wartime, ethnic conflicts, or genocides are rife with supporting cases. The more compliant and suppliant a Jew, for instance, the more both abused and disdained he is by his Nazi guard. School-age bullying follows the same pattern. Cowering only produces more disdain, more bullying, and unfortunately more cowering, as the spiral intensifies.
The biological benefit is obvious. Weak members of a pack are a danger to the whole pack. The only reason to keep them around is to eat them later or offer them up for a predator so everyone else can escape. Either way, the weak are culled from the pack to the benefit of the pack. Whether such weakness would be passed down from one generation of the pack to the next is questionable, but makes no difference to this argument.
As obvious as the biological benefit of culling the weak is, so obvious also is the ethical contempt of it. It is the most despicable of human behaviors, the claim that it is a vice not even requiring justification.
So either Nietzschean, power-based abuse of the weak should receive ready acquiescence from those interested in the future of humanity (like the Nazis under Hitler or like Planned Parenthood in its origin under Margaret Sanger) or it must be condemned as the vice it is–as vicious, that is. But if it cannot be condemned on any sensible biological basis, then how?
The purely biological argument, where every form of weakness is to be discounted if not fully discarded, is perfectly rational and morally repugnant. Margaret Sanger’s purpose in founding Planned Parenthood was to cull the mentally infirm and genetically impure from the race. It is called eugenics, a word not properly despised until its atrocities were realized in Nazi practices during the 1930s and 1940s. (By the way, practices like contemporary sex-selection studies and a 90% abortion rate of babies with Down Syndrome have fully reintroduced eugenics to American society.) Every time someone with a weakness is preserved–every C-section birth, every tumor removed by surgery, every manic-depressive treated to promote “normal” functioning–every time a weakness is preserved the person in whom it was preserved becomes a weak link in the chain of human evolution. (Give me a moment to recuperate from the nausea which follows writing that paragraph. Ok. Thanks.)
Fortunately, once people recognize the practice of despising and punishing the weak outside of a herd, it receives almost universal condemnation–one cultural hallmark of an ethical reality. But if there really is something morally wrong with culling the weak, from where does it come? Answers may come from human nature, the Creator’s purpose, the nature of reason, or any other number of morally realistic sources. But one thing is clear: it will not come from the biological realm. A reasonable conclusion is that ethics is a non-biological feature of humanity.
We are certainly biological beings. And we can be human beings. But we cannot be the latter if we are only the former.