I was going to say a few things about basic ethics, and SJ's tweet seems like a good jumping off point. She is asking why we have a sense of ethics, and suggests biology doesn't explain it.
There's a confusion here we need to clear up. Biology can explain a good deal about WHY we have certain in-built judgments, but it cannot GROUND them as MORAL JUDGMENTS.
For example, we have a disgust reaction to feces. That's a natural reaction, but not a moral judgment.
For example, we have a disgust reaction to feces. That's a natural reaction, but not a moral judgment.
However, we also have a natural disgust reaction to such things as incest, pedophilia, and—being honest, and also scientific, because this can be empirically shown—towards homosexual acts. But it depends on how this reaction is INTERPRETED before it means anything.
Even if you don't think there is a nature disgust reaction towards incest or same-sex acts, assume for a moment there is. So what? If this disgust reaction is MERELY BIOLOGICAL, in the sense of naturalism, then it HAS NO MORAL IMPLICATIONS AT ALL.
One could argue that, morally, it would be necessary to "get over" one's natural disgust reaction towards same-sex acts or incest. That our biology doesn't rule us. And since there is "no reason" to morally condemn same sex acts or incestuous acts, we need to IGNORE NATURE.
This is the point where all naturalistic ethics break down: you cannot appeal to nature as any kind of moral norm that is (1) reliable or (2) binding-obligating — in the naturalist's sense of "nature."
Only on the condition that human nature was CREATED BY GOD (whether by evolutionary means or not) can human nature function as a NORMATIVE standard.
The short version is: Any moral system that lacks a ground in the transcendent-sacred, i.e. God—by whatever name—lacks any ground for MORAL OBLIGATION.
You have a think left that is called "morality," but it is now OPTIONAL.
You have a think left that is called "morality," but it is now OPTIONAL.
Husserl once said "Naturalism decapitates philosophy." I add: "Naturalism decapitates ethics" because what a naturalistic ethics lacks is a ground of normativity. It can attempt to arbitrarily assert one, but the arbitrariness can be dismissed by counter-arbitrariness.
One naturalist might say that the good of the group is the sole criterion of goodness. Another might say it is the sovereign will of the individual. Another might say it is some sort of balance between the good of the group and the individual. None can say "Thou shalt not kill."
This is the place where the naturalistic fallacy kicks in. IF mere nature is itself morally meaningless, THEN it cannot supply the passage from an IS to an OUGHT. And if you have no OUGHT, you have no CATEGORICAL IMPERATIVES. Which means your "morality" is merely optional advice.
But most people rightly recognize that morality without the categorical necessity of "thou shalt not" and "thou shalt" isn't really a morality.
I use the example of rape, because there is, as I understand it, a fairly strong case that rape is somewhat successful evolutionary strategy for low-status males to pass on their genes—successful enough to have been selected for enough to persist anyway. What does that imply?
I can't see how it doesn't imply "rape is okay" if one also holds that "evolutionary biology is the ground of morality." I'm not sure what is NOT OKAY in that context, since any activity that exists, presumable survived evolution.
Phenomenologically, I think the evidence is very strong that the BEST EXPLANATION of human moral behavior is the Christian teaching of the FALL. That is, we are creatures made good, who became broken or damaged in our nature, such that our once-good nature is tainted.
This explains why, although almost all human beings who were raised well KNOW what is right, so few of us DO IT. There is a clear disconnect between how we ARE and how we OUGHT TO BE.
This is one of the most obvious things to anyone with sufficient experience of human beings. Thus, the TWO ERRORS: to think human nature totally corrupt, and to think human nature GOOD. The latter error is more dangerous. We aren't capable of being good, but we can recognize it.
If the completion or perfection of human nature is part of the divinely established order, then it IS normative. If we are fallen beings, then completion or perfection of human nature requires God's grace.
Only if our nature is divinely ordained by an all-good, all-knowing Holy One, can it be normative. It cannot be normative in a merely biological sense. In THAT sense, there just is nothing normative or moral.
So, it is my view that a purely secular ethic is impossible. On could not consistently exist, as it would lack normativity. It could appeal to a pseudo-normativity such as the principle of utility or reason (as Kant did), but neither can answer "Why should I obey that?"
Morality is connected to God in that it share a certain absoluteness. This is no accident. The absolute character of "Thou shalt not murder" comes from God. Because the world was created with a moral order, BY God, who is the absolute standard, there is an absolute standard.
God is, so to speak, the True North of our moral compass. If there is no God, there is no non-arbitrary moral compass—although one can pick an arbitrary one, and somewhat navigate, one's standard will be wrong, so one will always make moral errors.
The Nazis taught that "whatever is good for the German Volk is good". They actually thought that every people-state (united as one in fascism) should strive for its own selfish evolutionary benefit with other nation-state-people-organisms. A morality that deifies the state.
Communism deifies the idea of "the people", which is why it is Rousseauianism. Another might deify "diversity." No shortage of those today. A more libertarian sort deifies the self and personal choice. All errors, all arbitrary "fake norths" on broken moral compasses.
C. S. Lewis made the point that modern "moralities" are fragments of the older unified Western-Christian morality, parts disconnected from the whole and grown monstrous. Chesterton made the point before Lewis: 
