Given what we know about the composition of elements in space and the self-organizing principles that give rise to organic life, it’s vanishingly unlikely that we’re the only planet on which life has arisen. But it’s also vanishingly unlikely that we’ll ever encounter another technological species. We humans have had technology for the blink of an eye in terms of evolution, and we’re the only species in the three-billion-plus year history of life on Earth that’s ever developed sophisticated tools. We’re likely to be gone in another blink of an eye. So the chances of two technological civilizations overlapping in both proximity and time period are virtually zero. Hence we should stop fixating on Star Trek sci-fi space aliens and focus more on looking for bacteria-type organisms, so that we can begin to learn more about how life evolves under different conditions to those pertaining on our home planet. The biggest question of all is: how rare is it for eukaryotic analogs to appear? Prokaryotic life seems relatively simple to get started, but the jump to eukaryotic life is far more challenging (Lane’s book The Vital Question explores this problem in layman’s terms and is a great primer on bioenergetics). It seems to have happened just once on our planet, so we can’t estimate the probability of analogous jumps elsewhere. Finding even a single eukaryotic analog world would be hugely indicative of its probability elsewhere.