There is an interesting theory that the reason we haven’t seen aliens is that there is some sort of evolutionary bottleneck or hurdle.
The pacifist view is that nuclear weapons or some of other tendency to war wipes these civilizations out before they become interstellar.
Another theory I read was that it might be the development of mitrochondria and thus complex life. There are single celled organisms that feed off sulfur compounds via volcanic vents, methane loving bacteria that feed off methane hydrates in the ocean, anaerobic bacteria and aerobic bacteria. Lots of diversity in the single celled biosphere. Even bacteria as large as small multi-celled.
But everything with multiple cells shares a large part of its genome. Mitrochondria are biological power houses, essentially getting a free ride for their DNA in return for fueling the larger cell. It appears less like a radical evolutionary step than a cellular accident, an incomplete division OR partial absorption of another cell that became part of a new, improved whole.
Out of all our biological history, this has only happened once to be passed on to all complex organisms.
And it may be such a fluke that other words may be teaming with single celled life, even oxygen bearing algae analogs. But we might be the only thing more advanced than that, because few worlds see the accident to make complex life possible.
This theory also explains “the boring billion”. Complex life was a fluke that could only happen after the accidental combination that led to mitrochondria.
That never gets discussed. There always seems to be that human need for something more advanced.
/johnny