> It seems quite reasonable to believe that life inevitably evolves into intelligent life if given enough time. Intelligence is an arbitrary yardstick. Compare

> It seems quite reasonable to believe that life inevitably evolves into intelligent life if given enough time. Intelligence is an arbitrary yardstick. Compared to a yeast cell that just sits on a surface, a tree that grows toward light, strengthens its stem in response to wind forces etc, could be considered intelligent. More useful is tool use: animals that use sticks, stones etc. to get at food unreachable without those tools. But even that won't do: a planet full of life like that, but it never gets off that planet. Bits of life getting spread by meteor strikes etc? Possible. But chances of that drop off sharply with distance (within solar system -> interstellar -> between galaxies & up). The missing bit? Technology itself evolving from simple -> complex much like life evolved before that. In our (1) case: simple tools -> use of mechanical devices to replace muscle power -> industrial revolution -> automation -> entering the AGI age (where biological life may not be a strict requirement for a civilization to advance further). Quite possible that tool-using intelligent life is relatively common, but some of those extra steps to 'get off the home planet' are rare. And even if all of that happens: well... the universe is big. And so are timescales. Blips on the radar are easily missed.