A few fossil bones in clay and limestone have opened a greater vista back into Time than the Indian imagination ventured upon for its Gods: and every day turns up something new.

—Edward FitzGerald to E.B. Cowell, January 28, 18451

In Mark Twain’s Letters from the Earth, God gathers the archangels and announces that He has made animals. Satan—who else?—asks, “What are they for?” Perhaps you can hear the strangeness, the dissonance in this question, which is the sort that marks the boundary between theology and science. Scientists have no trouble asking what the various parts of an organism are for or what function it has in a food web or an ecosystem. But they tend not to ask Satan’s question because it offers no hypotheses to be tested. What are animals for? Here is God’s chilly answer: “They are an experiment in Morals and Conduct. Observe them, and be instructed.” So Satan goes to Earth and soon concludes that “the people are all insane, the other animals are all insane, the earth is insane, Nature itself is insane.”

You might say of Twain, as Walter Benjamin said of Charles Baudelaire, that his “satanism must not be taken too seriously”—that speaking in the voice of a disillusioned archangel merely allowed Twain “to sustain a nonconformist position.” Yet Letters from the Earth was withheld from publication by Twain’s daughter until 1962, and it tends to come festooned with editorial disclaimers blaming its antireligious cynicism on the circumstances of his old age, as if the book were merely a late, funebral fugue, unrelated to the rest of his work. In fact, Satan is the Connecticut Yankee in extremis, a rational being in an irrational world.

Why do I mention all this? As I was reading some recent books on dinosaurs, I kept wondering, “What were dinosaurs for?” It’s a ridiculous question, and I wondered why I was wondering it. After all, dinosaurs were “for” exactly what we are “for,” what every organism has been “for” since life began. Every species that has ever lived is a successful experiment in the enterprise of living, and every species is closely kinned at the genetic level with all other species. This is harder to grasp than it seems, partly because the logic of that Satanic preposition—“for”—is so insidious, so woven through the problem of time. Teleology is the moralizing of chronology, and nowadays science tries…