If Post Filter, We Are Alone
Me four years ago:
Imagine that over the entire past and future history of our galaxy, human-level life would be expected to arise spontaneously on about one hundred planets. At least it would if those planets were not disturbed by outsiders. Imagine also that, once life on a planet reaches a human level, it is likely to quickly (e.g., within a million years) expand to permanently colonize the galaxy. And imagine life rarely crosses between galaxies. In this case we should expect Earth to be one of the first few habitable planets created, since otherwise Earth would likely have already been colonized by outsiders. In fact, we should expect Earth to sit near the one percentile rank in the galactic time distribution of habitable planets – only ~1% of such planets would form earlier. …
If we can calculate the actual time distribution of habitable planets in our galaxy, we can then use Earth’s percentile rank in that time distribution to estimate the number of would-produce-human-level-life planets in our galaxy! Or at least the number of such planets times the chance that such a planet quickly expands to colonize the galaxy. (more)
New results:
The Solar System formed after 80% of existing Earth-like planets (in both the Universe and the Milky Way), after 50% of existing giant planets in the Milky Way, and after 70% of existing giant planets in the Universe. Assuming that gas cooling and star formation continues, the Earth formed before 92% of similar planets that the Universe will form. This implies a < 8% chance that we are the only civilisation the Universe will ever have. (more; HT Brian Wang)
Bottom line: these new results offer little support for the scenario where we have a good chance of growing out into the universe and meeting other aliens before a billion of years have passed. Either we are very likely to die and not grow, or we are the only ones who could grow. While it is possible that adding more filters like gamma ray bursts could greatly change this analysis, that seems to require a remarkable coincidence of contrary effects to bring Earth back to being near the middle of the filtered distribution of planets. The simplest story seems right: if we have a chance to fill the universe, we are the only ones for a billion light years with that chance.
"Properly understood, evolutionary pressure refers to the increase in a population of genetic traits that mitigate environmental threats"
That is a secondary effect, evolution only responds to "environmental threats" by how much it affects the ability of that organism to reproduce. Evolution is just the almost tautological observation that those that are better at reproducing become more numerous over time. There is a common myth that "removing environmental threats stops evolution", which is pure nonsense, it only shifts selection from survival traits to raw fertility. Modern living seems to neutralize some reproductive instincts in the present breed of humans but this will not be permanent over time humans will evolve genetic adaptations to modern living just as they did to farming etc, and like all life forms reproduce exponentially until they have filled the available habitat if we get something like fusion rockets then that could be the resources of the entire solar system, turn all the minerals in asteroids into cartwheel habitats. Before the solar system is totally mined out it would make sense for habitats to migrate to other solar systems as they will all be descended from those who had the strongest drive to reproduce of each generation.
If robots take over and wipe out organic life its just the same thing the robots that make the most copies of themselves become more numerous over time.
As we have no idea how hard the filters are to cross we dont know how many filters are needed. Could be a single trillion to one filter. Maybe the creation of the first cell and genetic replicator, it involves several components coming together. There could be a lot of lifeless, sterile yet potentially fertile and Earth like worlds out there just waiting for us to seed them.