In the entire 4.5+ billion year history of the Solar System, how likely is it...?

Questions 3-6 are central to the Fermi paradox, which is perhaps the strongest argument against existence of ETI. If interstellar travel is possible (and the laws of physics do not prohibit it at sub-light speeds) and extraterrestrial intelligence emerges with any regularity within the Milky Way, then the entire galaxy could be colonized within only a few million years, and should have long since been complete. By this chain of reasoning,the apparent absence of ETI from the solar system can be taken as strong evidence of its nonexistence throughout the galaxy.