re: #89 silverdolphin

The “Great Filter” hypothesis appeals to me greatly.

en.wikipedia.org

With no evidence of intelligent life in places other than the Earth, 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 intelligence
8. A civilization advancing toward the potential for a colonization explosion (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 is not an early step (i.e., in the past), then the implication is that the improbable step lies in the future and humanity’s 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 (for example highly constrained energy resources).[6] So by this argument, finding multicellular life on Mars (provided it evolved independently) would be bad news, since it would imply steps 2-6 are easy, and hence only 1, 7, 8 or 9 (or some unknown step) could be the big problem.[4]