The original emergence of life appears to meet these two criteria. As far as we know, it might have occurred only once, and it might have taken hundreds of millions of years for it to happen even after the planet had cooled down enough for a wide range of organic molecules to be stable. Later evolutionary history offers additional possible Great Filters. For example, it took some 1.8 billion years for prokaryotes (the most basic type of single-celled organism) to evolve into eukaryotes (a more complex kind of cell with a membrane-enclosed nucleus). That is a long time, making this transition an excellent candidate. Others include the emergence of multicellular organisms and of sexual reproduction.
If the Great Filter is indeed behind us, meaning that the rise of intelligent life on any one planet is extremely improbable, then it follows that we are most likely the only technologically advanced civilization in our galaxy, or even in the entire observable universe. (The observable universe contains approximately 1022 stars. The universe might well extend infinitely far beyond the part that is observable by us, and it may contain infinitely many stars. If so, then it is virtually certain that an infinite number of intelligent extraterrestrial species exist, no matter how improbable their evolution on any given planet. However, cosmological theory implies that because the universe is expanding, any living creatures outside the observable universe are and will forever remain causally disconnected from us: they can never visit us, communicate with us, or be seen by us or our descendants.)
The other possibility is that the Great Filter is still ahead of us. This would mean that some great improbability prevents almost all civilizations at our current stage of technological development from progressing to the point where they engage in large-scale space colonization. For example, it might be that any sufficiently advanced civilization discovers some technology–perhaps some very powerful weapons technology–that causes its extinction.
I will return to this scenario shortly, but first I shall say a few words about another theoretical possibility: that extraterrestrials are out there in abundance but hidden from our view. I think that this is unlikely, because if extraterrestrials do exist in any numbers, at least one species would have already expanded throughout the galaxy, or beyond. Yet we have met no one.
Various schemes have been proposed for how intelligent species might colonize space. They might send out “manned” spaceships, which would establish colonies and “terraform” new planets, beginning with worlds in their own solar systems before moving on to more distant destinations. But much more likely, in my view, would be colonization by means of so-called von Neumann probes, named after the Hungarian-born prodigy John von Neumann, among whose many mathematical and scientific achievements was the concept of a “universal constructor,” or a self-replicating machine. A von Neumann probe would be an unmanned self-replicating spacecraft, controlled by artificial intelligence and capable of interstellar travel. A probe would land on a planet (or a moon or asteroid), where it would mine raw materials to create multiple replicas of itself, perhaps using advanced forms of nanotechnology. In a scenario proposed by Frank Tipler in 1981, replicas would then be launched in various directions, setting in motion a multiplying colonization wave. Our galaxy is about 100,000 light-years across. If a probe were capable of traveling at one-tenth the speed of light, every planet in the galaxy could thus be colonized within a couple of million years (allowing some time for each probe that lands on a resource site to set up the necessary infrastructure and produce daughter probes). If travel speed were limited to 1 percent of light speed, colonization might take 20 million years instead. The exact numbers do not matter much, because the timescales are at any rate very short compared with the astronomical ones on which the evolution of intelligent life occurs.