It is true that natural selection is descriptive, not prescriptive.

I’m more interested in the scientific worth of a human being. It is a question of justification in the broader fields (history, philosophy, logic, etc.). I guess the only thing I wonder about in this conversation is this:

If you can only justify the worth of a human based on their capacity to reason and experience emotions and the universe, then what about the infant, the insane/senile, or the mentally ill. Is their value not justified because their experience of the universe is retarded? For example, there are humans who are unable to reason, and even diseases that inhibit the some people to feel any emotion. What about people who are so depressed that they have no will to live or survive? Do we allow them to end their life because they have lost the will to live? There are people living who have an extremely weak capacity to do these things that you say make us “human.”

No one ever says, “But if you fall off a cliff, gravity kills you; how can we reconcile the value of life with knowledge of universal gravitation?” This is an obvious non sequitur, because gravity is just a natural law of the universe; it has nothing to do with ethics. Why this is not equally obvious in the case of evolution, I’m not sure.

As for the idea of “a scientific explanation for the value of a human being,” I do not think there is such a thing, but then, I think the question is asked so narrowly that we should not expect an affirmative answer.

Science, at least the theory/experiment science that we usually mean when we use the term “science,” really isn’t all that concerned with questions of value. Certain values—truth, honesty, curiosity—are taken as given, axioms without which inquiry could not occur—and the rest is left up to philosophy. Indeed, an “explanation for value” strikes me as an odd notion: isn’t what you want a *justification,* a reason, rather than an *explanation,* a cause?

Now, if you mean the broader field of rational knowledge, which includes history, philosophy, logic, and mathematics—then I think it is quite possible to give a justification for the value of human beings, and indeed support and extend this justification far further than most religions would take it.

We can say, for instance, that human beings are valuable because they are rational, and valuable because they are emotional; human beings are valuable because they *experience* the universe, and can act based upon these experiences. Where rocks and stars—made of much the same stuff—merely act as natural law requires of them, human beings actually consider alternatives and seek out goals. We have needs and desires, and we make choices based on these. Our right to exist, then, comes from the simple fact that we *want* to exist—most things don’t want anything, but we do, and one of the things we want is to live.

From this, we can also ask what else wants to live, or appears to do so, and attribute value to these in the same way; the answer is obvious: animals. From the brightest chimp through the lowliest ant, animals express a desire to live, a will to survive, that rivals our own. When a plant is cut down, it doesn’t care; but when an ant’s leg is removed, she writhes in pain, struggles to fight, struggles to survive. Mammals even mourn their dead: When an ant dies, it is not clear that any other ants care, but when a wolf dies, his parents and his siblings will become depressed for weeks. When a sow loses her piglet, she sulks for months. Heifers have been known to cry out to their calves long after they are gone.

Both religious and rational morality agree that humans are valuable; but where religion would grant only us this pedestal, rationality shares it with many of our siblings and cousins.