Complex life may only exist because of millions of years of groundwork by fungi
This is tangentially related:I've had a pet theory for a while (which may not be original, idk) that a possible explanation for the Fermi Paradox Great Filter is literally the existence of huge amounts of Fossil Fuels. It took millions of years of dead biological life on our planet to build up the supplies we have, and the big ol'  meteor that hit us some 60 million years ago helped not only kill off a bunch of that life to create oil but set the stage for actual intelligent life. Without lots of oil, we wouldn't have hit the industrial age, we couldn't manufacture renewable energy sources, we couldn't make plastic, we couldn't make rocket fuel, electricity wouldn't be as widespread, basically everything would be different.If oil were substantially less common, its feasible that we'd still live in a 1800s-esque society with some form of civilization development, but would be have radio? Certainly not space travel.Point being: M-class planets might be pretty common in the universe, maybe cellular and even multicellular life is less common, maybe intelligent/sentient live is even less common, maybe civilization is even less common. And we've only really "existed" for a few tens of thousand years in the grand scheme of billions of years, those years littered with possible extinction-like events both man-made and natural. Combine all of those probabilities together, with the reality that we may have just "missed" another species on the timeline of the universe, and it seems more likely than not that we're alone in this galaxy.Which is actually pretty awesome if you think about it. The whole playground is our's; let's go take it.