Great filter

The Great Filter, in the context of the Fermi paradox, is whatever prevents "dead matter" from giving rise, in time, to "expanding lasting life".The concept originates in Robin Hanson's argument that the failure to find any extraterrestrial civilizations in the observable universe implies the possibility something is wrong with one or more of the arguments from various scientific disciplines that the appearance of advanced intelligent life is probable; this observation is conceptualized in terms of a "Great Filter" which acts to reduce the great number of sites where intelligent life might arise to the tiny number of intelligent species actually observed (currently just one: human). This probability threshold, which could lie behind us or in front of us, might work as a barrier to the evolution of intelligent life, or as a high probability of self-destruction.The main counter-intuitive conclusion of this observation is that the easier it was for life to evolve to our stage, the bleaker our future chances probably are.

The idea was first proposed in an online essay titled, "The Great Filter — Are We Almost Past It?" written by economist Robin Hanson. The first version was written in August 1996 and the article was last updated on September 15, 1998. Since that time, Hanson's formulation has received recognition in several published sources discussing the Fermi paradox and its implications.

Fermi paradox

There is no evidence aliens have visited earth and we have observed no intelligent extraterrestrial life with current technology nor has SETI found any transmissions from other civilizations. The universe, apart from the Earth, seems "dead"; Hanson states:
Our planet and solar system, however, don't look substantially colonized by advanced competitive life from the stars, and neither does anything else we see. To the contrary, we have had great success at explaining the behavior of our planet and solar system, nearby stars, our galaxy, and even other galaxies, via simple "dead" physical processes, rather than the complex purposeful processes of advanced life.
All life expands to fill all available niches. With technology, such as self-replicating spacecraft, these niches would include neighbouring star systems and even, on longer time scales which are still small compared to the age of the universe, other galaxies. Hanson notes, "If such advanced life had substantially colonized our planet, we would know it by now."

The Great Filter

With no evidence of intelligent life other than ourselves, it appears that the process of starting with a star and ending with "advanced explosive lasting life" must be unlikely. This implies that at least one step in this process must be improbable. Hanson's list, while incomplete, describes the following nine steps in an "evolutionary path" that results in the colonization of the observable universe:
The right star system (including organics & potentially habitable planets)
Reproductive molecules (e.g. RNA)
Simple (prokaryotic) single-cell life
Complex (archaeatic & eukaryotic) single-cell life
Sexual reproduction
Multi-cell life
Tool-using animals with big brains
Where we are now
Colonization explosion
According to the Great Filter hypothesis at least one of these steps — if the list were complete — must be improbable. If it's not an early step (i.e. in our past), then the implication is that the improbable step lies in our future and our prospects of reaching step 9 (interstellar colonization) are still bleak.