The tweaked version of the rare earth hypothesis I present is this: We all know that the odds of life existing elsewhere in the universe is greater than ever before, and it is conceivable, perhaps even likely, that life can be found pretty much anywhere where there is a terrestrial body with liquid water present on it in some way (note: I`m restricting the notion of “life” to carbon and water based life like that found here on earth for obvious reasons), such as the moons of the outer solar system, perhaps even rogue planetary bodies or rogue moons as well, assuming they had oceans of liquid water present in their subsurface regions.
But I find it likely that while life may indeed be common in the universe, most of that life is going to be microbial, even on earth-like terrestrial planets orbiting stars not too different from our own. Look at it this way, for pretty much most of the history of life on earth (three and a half billion years, from the Hadean to the late Proterozoic), *all* of the biosphere was comprised of microbes, even today most of the biomass is again made up of microbes, multicellular eukaryotes make up a pithy fraction of the rest.
In the absence of necessary selective factors that are a prerequisite for multicellularity to arise in previously unicellular organisms, it can be assumed that the vast majority of life in the universe is microbial. There are about 10 or so billion earth like planets in our galaxy alone, and I’d wager that the number of those planets containing complex life on them is low, if we assume life is found everywhere where it can develop, then pretty much *all* of those planets would have life on them in some way, albeit being *microbial* life, so, using my generous assumption, there’d probably be a few hundred thousand to a few million planets with complex, multicellular life on them, that may seem like a big number but we’re dealing with 10 *billion* earth like planets out there, that’s a *pathetic* number indeed in comparison to ten *billion*.
Now how many years did it take for sapience to arise on earth (“sapience” in this context meaning “human-level intelligence,” obviously other animals are sapient as well but they probably *aren’t* human-level sapients, your pet cat or dog, while hardly idiots, don’t count, and for the sake of argument neither do dolphins or elephants)? It took roughly four billion years for the first members of the genus Homo to arise, for most of which our history we were nothing but hunter-gatherers, we’ve only been practicing agriculture for a mere ten millennia, we’ve only been recording history for about half of that, and we have only had an advanced technological civilization capable of radio astronomy for about a century now.
I think it obvious that the number of those planets with complex life on them *also* bearing sapient life at the same time we do to be *ridiculously* low, possibly even in the single digits, and most of those planets with sapient life on them odds are the sapients never left their stone age, because the development of civilization here on earth was dependent upon a number of *highly specific* circumstances that are probably, given how long we went without even developing agriculture, not that common among sapients, it was never a given *we* would develop civilization, and it is hardly a given *they* will either.
So assuming what I wrote above is true, the Fermi Paradox is hardly a paradox at all, everyone else as intelligent as we are are still stuck in the stone age and therefore have no hopes of becoming a spacefaring civilization, we’re the lucky ones, and I hope we live long enough to make us of our lucky status before climate change does us in.