"<a href=""https://www.youtube.com/watch?v=lVHRs3uTHNI&amp;t=1m30s"">1:30</a> I would argue that the development of the complex brain capable of toolmaking has happened twice", completely independently, on Earth. The peaks of those paths are primates represented by human and and cephalopods represented by octopus. (One could argue that some birds also exhibit toolmaking skills, but they would not qualify as independent as the common ancestor of the amniote clade had a well developed nervous system). <br><br>Relating to the  Fermi paradox, 2 is way more than 1.<br><br>The divergence between these paths was the protostome/deuterostome fork, which developmentally manifests at 8 cells, way before anything resembling a complex nervous system. Hence can be regarded as independent.<br><br>Although octopuses have not reached the level of humans (i.e. they don't, to our knowledge, have a space program), they are comparable to primates (aside from social context). So the right set of environmental pressures would do it (perhaps something like the current anthropogenic mass extinction). For the human path, this phase shift happened less than a million years before space travel.<br><br>Opposable thumb vs. tentacle? I'd put even money on the tentacle.