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Oh I mean I could be way off, but the more I learn about how the Earth functions the more it seems like it's a whole bunch of spinning plates and upsetting any of them to the point that they stop spinning means the whole party (more or less) stops. There's certainly more to it than being roughly the right size, roughly the right composition, and in the "goldilocks zone". The only reason it works is that the plates (to continue the metaphor) self-stabilize when they get bumped, on account of the spinning. Take away even one of them and one of our cycles that keep things nice & cozy for complex life instead goes up-and-right or down-and-right... permanently. Things like plate tectonics to periodically send material down to the mantle, pull fresh material back up, and pump all kinds of stuff into the atmosphere, are really important for maintaining long-term stability. Imagine how much less life the oceans could support without rivers dumping disolved nutrients into them all the time—and those rivers have nutrients to dump, even billions of years into our planet's history, because of plate tectonics, because there's a water cycle at all, et c. There are other filters, too. Hard for an aquatic intelligent species to kick-start civilization—how will they get started using fire? One can come up with ways, maybe, kinda, but would they stumble on those methods the way we stumbled on fire, almost effortlessly (then still took many millennia to discover how to do much more with it than burn forests/grasslands, cook food, excavate shallow caves, and harden wooden points)? For some high-clouds ecosystems (as some speculate may exist) on planets that can't support (much) life at ground level... how would an intelligent species there build anything? The only materials available would be parts of other high-altitude life forms. That's assuming you could get enough biomass up there to supply the kind of readily-available energy and nutrients an active, complex, smart life-form needs to be able to gather from its environment in the first place. That sort of thing. Plus you're counting on the emergence of physical features necessary for tool use at all (like our hands) which is something that pretty much only climbing species develop, as a side-effect of needing to grip things all the time. I think there are lots of things working against the emergence of not just complex life, not just intelligent life, but coordinated civilizations with long-term retention and transmission of knowledge that can accomplish things we might associate with advanced alien life forms. Even if life turns out to be very common, I expect complex life is not, intelligent life even less so, and civilization vanishingly rare. But, again, I could be wrong—it's all guesswork, really. |