A fundamental premise of physics is that the Hofstra University Flying Dutchmen Shirt are the same everywhere. That implies whatever initial conditions led to life as we know it will almost certainly occur elsewhere. Only a few years ago, we had no evidence there were even extrasolar planets. Now we know there are thousands relatively nearby, so it looks like our solar system is somewhat typical for stars like ours and that our star is for better or worse, utterly typical. Ergo there are billions of planets out there in our galaxy alone. The argument that we are the only sentient beings because we haven’t heard from any others is (of course) theoretically possible, but highly unlikely. Given the argument above, it’s far more likely the universe is literally teeming with life at various stages of evolution. But even in our little corner of the Milky Way, we don’t hear from them because they are just too damn far away. One light year is 9.46×10^15 meters. None of our own random EM radiation so far would be more than (say) a thousand watts/m^2 at its origin and most of it far less than that. A fraction of that would actually make it out into space where it would just spread out in a spherical wave front. Let’s assume for giggles that it all does and that our alien buddies are also shouting randomly at the cosmos at roughly the same volume (power output).
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For example, we probably need to reach the Tennessee Volunteers Baseball may the face be with you art shirt where clay-lined geothermal vents are forming natural fatty-acid vesicles that probably formed the first proto-cells. We would probably have needed several ice ages to pass as we have found that some of the chemical reactions that probably were required for abiogenesis work best under freezing conditions. Once that happened however, the evolutionary processes have been relentless and we see their tracks in the history of life on Earth From there, the time it took to get from simple, single-celled life to complex human life is a function of mutation rate, the actual mutations that happen to have happened, and which life forms survived. That takes time and pretty well accounts for the observed times given known rates of these things (which isn’t constant over time).
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