We have yet to discover a single trace of alien life, despite the extremely high probability that it exists somewhere. This contradiction is popularly known as the Fermi Paradox. A new theory attempts to solve this conundrum by suggesting that habitable planets are quite common in our galaxy, but nascent life gets snuffed out very quickly.
An oft-cited solution to the Fermi Paradox—that is, the lack of observational evidence that our galaxy has been colonized by an extraterrestrial civilization—is the Great Filter hypothesis. Devised by Robin Hanson of George Mason University, this theory suggests that some kind of cosmic-wide obstacle is preventing life from developing beyond a certain stage. Trouble is, we’re not entirely sure if this Great Filter actually exists, or what it looks like.
Some astrobiologists look to our planet’s ancient past and point to the presence of three possible filter points: the emergence of reproductive molecules, simple single-celled life, or complex single-celled life. If we could prove that any one of these critical evolutionary steps are true, that would be exceptionally good news—it would imply that the Great Filter is behind us. On the other hand, some pessimistic futurists fear that the Great Filter looms ahead of us, an event that will likely come in the form of a self-inflicted existential catastrophe.