[yamtaddle]

Basically we have a bunch of things going on that cause some graphs to cycle rather than run away, and if you break any of them—which can be as simple as tweaking a value here or there—most of our biosphere's gone.

Carbonate-silicate cycle breaking is what's considered the most likely thing to end complex life on Earth, for instance. All it takes to cause that is the Sun getting slightly brighter. In a few hundred million years it'll become increasingly difficult (inefficient) for plants to photosynthesize, reducing energy available to the biosphere, until finally photosynthesis stops completely.

[wizeman]

That's very interesting, thanks.

So you're saying that in such a scenario, it would be impossible for complex life to exist, even if it looks different from the complex life we have now?

I am just wondering whether we are not confusing "complex life as we've seen to exist" with "complex life that can possibly exist".

For example, I am wondering whether the Great Oxidation Event is not a counterexample where we had a run-away biological process that caused a mass extinction (which, if it were possible for us to be alive at the time, might have looked like it would be impossible to recover from) but still lead to complex life existing despite oxygen being so toxic (at least from the point of view of the kind of life that existed at the time).

I know this is not the best example, as at the time complex life didn't exist yet. But I'm wondering if it wouldn't have been possible for complex life to exist in an oxygen-poor environment. Well, in fact some multicellular species that exist today are anaerobic and therefore do thrive without oxygen, right? Perhaps such anaerobic life would have evolved a lot more if our atmosphere had never been oxygenated.

Unfortunately I know nothing about the examples you mentioned, so I can't tell whether they are comparable.

Edit: I just read a bit about the carbonate-silicate cycle and I think I know what you mean now. Yes, if surface temperatures were more extreme it would indeed be quite difficult for complex life to exist (at the surface, at least). After all, some limits to the existence of interesting life must surely exist and temperature does seem like an inherent one.

[yamtaddle]

Carbonate-silicate cycle breaking means photosynthesis can't, chemically, happen, not just that its being hotter makes plants less happy.

My point isn't that life can't exist otherwise (ocean vents exist—though, you do need an active core for that—and anaerobic life is a thing, as you point out) but that only certain routes, even assuming there are some wildly different paths that might work, will yield enough energy to support complex life. You need a system that's reasonably stable (i.e. tends to prevent a variety of runaway events, turning them into cycles instead) even when life itself is changing the environment. Sometimes life will step in to do that if you get lucky (as in the oxygenation crisis, which you mentioned) but the planet itself needs a bunch of stuff to be just right, too—not just conditions, but processes.

I suspect there are lots of planets with some bio-goo on them, maybe even quite a few with things as complex as some insects or crustaceans. I further suspect there are very few (perhaps about as close as one can get to "none" without reaching it) that could support something like (even for generous values of "like") our giant tree of vertebrates. Not enough energy available, or, there is, but the world lacks the processes to stop life's own effects from wrecking the environment and stunting its own potential.

Then there's the fact that even complex life doesn't seem to necessarily lead to advanced intelligence and civilization. I mean, look at how long it took humans to develop writing! All that other stuff had gone just right, and intelligence emerged, and we happened to be a tribal species (probably), but we still puttered around for, what, a few hundred thousand years? The dinosaurs had no space program, nor the birds, nor the whales, nor fish, despite existing for tens or hundreds of millions of years—intelligence and the right kind of cooperative pack/tribe behavior to let it flourish seems to be a really rare combo, even when everything else has worked out very well for complex life.

[wizeman]

It's very rare and extremely interesting for me to read a well-argued opinion that is the polar opposite of mine.

Furthermore, I don't even have any good counter-arguments (at least not any that wouldn't just be pure speculation).

You definitely gave me something new to think about.

Thank you for your thoughts!

[yamtaddle]

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.