It can be unsettling to contemplate the unlikely nature of your own existence, to work backward causally and discover the chain of blind luck that landed you in front of your computer screen, or your mobile, or wherever it is that you are reading these words. For you to exist at all, your parents had to meet, and that alone involved quite a lot of chance and coincidence. If your mother hadn’t decided to take that calculus class, or if her parents had decided to live in another town, then perhaps your parents never would have encountered one another. But that is only the tiniest tip of the iceberg. Even if your parents made a deliberate decision to have a child, the odds of your particular sperm finding your particular egg are one in several billion. The same goes for both your parents, who had to exist in order for you to exist, and so already, after just two generations, we are up to one chance in 1027. Carrying on in this way, your chance of existing, given the general state of the universe even a few centuries ago, was almost infinitesimally small. You and I and every other human being are the products of chance, and came into existence against very long odds.

And just as your own existence seems, from a physical point of view, to have been wildly unlikely, the existence of the entire human species appears to have been a matter of blind luck. Stephen Jay Gould argued in 1994 that the detailed course of evolution is as chancey as the path of a single sperm cell to an egg. Evolutionary processes do not innately tend toward Homo sapiens, or even mammals. Rerun the course of history with only a slight variation and the biological outcome might have been radically different. For instance, if the asteroid hadn’t struck the Yucatán 66 million years ago, dinosaurs might still have run of this planet, and humans might have never evolved.