I've always found this equal parts fascinating and ominous, the thought that we haven't contacted any form of extraterrestrial life because every civilization that's existed in the universe has reached the point we are now then collapsed before being able to get off the planet.
https://en.wikipedia.org/wiki/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:
1) The right star system (including organics and potentially habitable planets)
2) Reproductive molecules (e.g. RNA)
3) Simple (prokaryotic) single-cell life
4) Complex (eukaryotic) single-cell life
5) Sexual reproduction
6) Multi-cell life
7) Tool-using animals with big brains
8) Where we are now
9) 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. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one, and the only things that appear likely to keep us from step 9 are some sort of catastrophe, an underestimation of the impact of procrastination as technology increasingly unburdens existence or resource exhaustion leading to the impossibility of making the step due to consumption of the available resources (like for example highly constrained energy resources).
https://en.wikipedia.org/wiki/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:
1) The right star system (including organics and potentially habitable planets)
2) Reproductive molecules (e.g. RNA)
3) Simple (prokaryotic) single-cell life
4) Complex (eukaryotic) single-cell life
5) Sexual reproduction
6) Multi-cell life
7) Tool-using animals with big brains
8) Where we are now
9) 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. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one, and the only things that appear likely to keep us from step 9 are some sort of catastrophe, an underestimation of the impact of procrastination as technology increasingly unburdens existence or resource exhaustion leading to the impossibility of making the step due to consumption of the available resources (like for example highly constrained energy resources).