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The rise of powerful new technology means that humanity must confront the risk of its own demise. The invention of nuclear weaponry, for example, has already shown just how quickly humanity’s destructive power could grow. The atomic bomb was a thousand times more powerful than conventional explosives; many hydrogen bombs were a thousand times more powerful again. Within decades, the USA and USSR between them had created over ten thousand nuclear bombs. The next generation of weapons of mass destruction, such as bioweapons by engineered viruses, could dramatically increase humanity’s destructive power again—to the point that an all-out war could threaten all human life.

If Homo sapiens were to go extinct, what would that mean from a cosmic perspective? Would some other species evolve to become technologically capable, and discover science, create art, and build civilization in our place? Ultimately, I don’t think that’s at all guaranteed. The end of Homo sapiens would therefore not merely be an unimaginable loss from our perspective; it would fundamentally change the story of the universe.

It took 200 million years for humans to evolve from the first mammals. The last common ancestor of humans and chimps was alive only 8 million years ago, and there are still hundreds of millions of years remaining (at least) until the sun’s increasing brightness renders the earth uninhabitable to human-sized animals. Given this, you might think that, if Homo sapiens went extinct and chimps survived, a technologically capable species should be able to evolve from chimps, like Planet of the Apes, in 8 million years or less. Similarly, as long as some mammals survived, even if all primates went extinct, shouldn’t we expect a technologically capable species to evolve within around 200 million years? This is a long time, but it’s still easily short enough for such evolution to occur before the earth is no longer habitable.

This argument is too quick. We don’t know how unlikely the major evolutionary transitions were, and some of them—including, potentially, the evolution of a technologically capable species—were very unlikely indeed.

This reasoning is based on the Fermi paradox: the paradox that, even though there are at least hundreds of millions of rocky habitable-zone planets in the galaxy, and even though our galaxy is 13.5 billion years old—ample time for an interstellar civilization to spread widely across it—we see no evidence of alien life. If the galaxy is so vast and so old, why is it not teeming with aliens?

One answer is that something about our evolutionary history was exceptionally unlikely to occur. Perhaps planets that are conducive for life are in fact extremely rare (perhaps needing to be in a safe zone in the galaxy, with plate tectonics, a large moon, and the right chemical composition), or certain steps on the path from the formation of the earth 4.5 billion years ago to the evolution of Homo sapiens were extraordinarily uncommon. Potentially improbable steps include the creation of the first replicators from inorganic matter, the evolution of simple cells into complex cells with a nucleus and mitochondria (called “eukaryotes”), the evolution of sexual reproduction, and possibly even the evolution of a species, like Homo sapiens, that is distinct from other primates by virtue of being unusually intelligent, hypercooperative, culturally evolving, and capable of speech and language. Recent research by my colleagues at the Future of Humanity Institute suggests that once we properly account for our uncertainty about just how unlikely these evolutionary transitions might be, it actually becomes not all that surprising that the universe is empty, even though it is so vast.