There are two views about the existence of humans on this planet: one view says that human existence was inevitable, a natural culmination, just a matter of time; the other view says that human existence is an accident, an unpredictable anomaly, just a matter of luck (I am discounting theological ideas). I can think of myself as the kind of being whose existence was built into the mechanism of evolution, or I can think of myself as a bizarre aberration of evolution. The first view is often defended (or found natural) because evolution is thought to produce superiority, and we are superior—the pinnacle of the evolutionary process. Evolution is conceived as a process that tends towards superior intelligence, and we are the most intelligent creatures of all. The second view notes that our kind of intelligence is unique in the animal kingdom and therefore hardly a prerequisite for evolutionary success; indeed some of the most successful animals as judged by biological criteria are the least intelligent (bacteria do pretty well for themselves). Big brains are biologically costly and can be hazardous, hardly the sine qua non of survival and reproductive success. I hold to the second view of the evolutionary process (which is standard among evolutionary biologists) but I won’t try to defend it here; my aim is rather to adduce some considerations that support the view that human existence (and human success) are highly contingent in quite specific ways—we really are a complete anomaly, an extremely improbable biological phenomenon. It is a miracle that we are here at all (though a natural miracle). We might easily not have existed.
First, there are no other mammals like us on the planet: upright, bipedal, ground dwelling. Most land animals are quadrupeds (with the obvious exception of birds, whose forelimbs are wings, and who spend a lot of time in the air), and that body plan makes perfect sense given the demands of terrestrial locomotion. Our body plan, by contrast, makes little sense and no other species has followed us down this evolutionary path. Even our closest relatives don’t go around on their hind legs all the time existing in all manner of environments (are there any apes that live on the open plains or in the arctic?). There is no evolutionary convergence of traits here, as with eyes or a means of communication. Natural selection has not favored our bipedal wandering in other species (contrast the vastly many species of quadruped). This is by no means the natural and predictable mode of locomotion and posture that evolution homes in on. It is strange and unnatural (and fraught) not somehow logical or design-optimal. No sensible god would design his favorite species this way—unbalanced, top-heavy, swollen of head. (Note how slow even our fastest runners are compared to many other mammalian species.) Nor does evolution seem to have a penchant for large ingenious brains; it prefers compact efficient brains that stick to the point. Whatever the reason for these characteristics, it is not that our bodily design is a biological engineer’s dream: evolution has not all along been dying to get this design instantiated in its proudest achievement (as if expecting huge applause from the evolutionary judges of the universe—“And the first prize goes to…”). Cats, yes, who have been a long time in the making; but hardly humans, who arrived on the scene only yesterday and never looked the part to begin with.
Second, imagine what would happen if you drove gibbons down from the trees. Up there they are well adjusted, at home, finely tuned, grasping and swinging; but down on the ground they would be miserably out of place, athletically talentless, scarcely able to survive. Indeed, they would not survive—they would go rapidly extinct. They evolved to live in the trees not on the ground, and you can take a gibbon out of a tree but you can’t take a tree out of a gibbon. Yet we (or our ancestors) were driven down from the trees and forced to survive in alien territory, subject to terrifying predators, cut off from our natural food supply, poorly designed to deal with life on level ground. We should have gone extinct, but by some amazing accident we didn’t—something saved us from quick extinction (and it is possible to tell a plausible story about this). Descending from the trees is not something built into the evolutionary trajectory of tree-dwelling animals, as if it is a natural promotion or development, life on the ground being somehow preferable, like a fancy neighborhood and upward mobility. That’s why other species have not followed us—those gibbons are still happily up there, as they have been for millions of years. Our descent and eventual success was not a natural progression but a regression that happened to pan out against all odds. It could easily not have happened. There is certainly no general evolutionary trend that favors animals that make the descent—which is why birds haven’t abandoned their aerial life-style and taken up residence on the ground. There is no biological analogue of gravity causing animals to cling to the earth’s surface. That we made a go of it is more a reason for astonishment than confident confirmation.
Third, and perhaps most telling of all, the other evolutionary experiments in our line have not met with conspicuous success. We are the only one left standing (literally). We now know there were many hominid species in addition to the branch called Sapiens, which flourished (if that is the word) for a while, but they are all now extinct—things just didn’t work out for them. And it’s not like the dinosaurs where a massive catastrophe caused the extinction (of them as well as innumerable other species); no, these hominid species went extinct for more local and mundane reasons—they just couldn’t cut it in the evolutionary struggle. They just weren’t made of the right stuff, sadly. Slow, ungainly, unprotected, weak—they simply didn’t have what it takes. Yet we, amazingly, are still here: we made it through the wilderness despite the obstacles and our lack of equipment. How did we do it? That’s an interesting question, but the point I want to make here is that it is remarkable that we did—no other comparable species managed it. Evolution experimented with the hominid line and it didn’t work out too well in general (most mammal species living at the time of our early hominid relatives are still robustly around), but somehow we managed to beat the odds. We look like a bad idea made good—here by the skin of our teeth. The characteristics that set our extinct relatives apart from other animals did not prove advantageous in the long run, but by some miracle the Sapiens branch won out—we did what they could not. And we didn’t just survive; we dominated. Not only are we still here; we are here in huge numbers, everywhere, pushing other species around, the top of the pile. We have unprecedented power over other animals and indeed over the planet. But this is not because evolution came up with a product (the bipedal brainy animals descended for the trees) that had success written into its genes–most such animals fell by the wayside, with only us marching triumphantly forward. And notice how recently our dominance came about: we weren’t the alpha species for a very long time, a sudden success story once our innate talent shone forth; instead we scraped and struggled for many thousands of years before we started to bloom—the proverbial late developers. None of it was predictable: only in hindsight do we look like the evolutionary success we have turned out to be. Anyone paying a visit to the planet before and after our improbable rise would exclaim, “I never saw that one coming!” You could safely predict the continuing success of cats and elephants, sparrows and centipedes, given their track record; but the spectacular success of those weedy two-footed creatures seems like pure serendipity. You would have expected them to be extinct long ago! You would want to inquire into the reasons for their unlikely success, looking again at their distinguishing characteristics (language, imagination, a tendency to congregate, dangling hands). These characteristics turned out to be a lot more potent than anyone could have predicted. Certainly there is no general trend in evolution favoring animals designed this way. It is not as if being driven from one’s natural habitat and being made to start over is a recipe for biological success.
For these reasons, then, the existence and success of homo sapiens was not a foregone conclusion, a mere natural unfolding. It was vastly improbable and entirely accidental. It was like making a car from old bits of wood and newspapers that ends up winning the Grand Prix. 
 This essay recurs to themes explored in my Prehension: The Hand and the Emergence of Humanity (MIT Press, 2015). Of course, there is an enormous literature dealing with these themes. I think there is room for a type of writing about them that emphasizes the human significance of the scientific facts (one of the jobs of philosophy). It matters to us whether we are an accident or a preordained crescendo.