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September 19, 2017 at 12:19 pm (This post was last modified: September 19, 2017 at 12:29 pm by The Grand Nudger.)
@OP.
I like to remind people that the odds of intelligent life occurring are unity, 1/1, it happened. I know that the impulse behind the question (and it's many answers) accepts that but contends "some of this stuff was super rare though, right?". Let's take a look. It starts at the macro level.
We're asked to imagine the vastness and emptiness of space. It is vast, it is empty, except for those things it;s full of. Galaxies. Galaxies are filled with stars. Stars, have solar systems. Solar systems are not so vast, and far from empty. Collisions inside the gravity well of a star appear to be routine. Our moon, which we're coming around to shortly, is evidence of that. Stepping back out, our galaxy appears to be on a collision course with another galaxy, itself. I suspect that when this course reaches it's terminus there will be a hell of a lot of collisions. In summary, it doesn't appear to matter what level of granularity you assess the universe. Collisions happen, and the more local the granularity the more frequent they appear to and must necessarily be.
Ah, the moon. Fun fact, it's escaping. It's apoapsis increases by roughly 3.8cm every year. The current theory is that a massive collision with a proto-planet either sheered it loose, or dove into our core expelling detritus opposite it's impact and just generally in a local wave of mass destruction. The question in parenthesis at the end is a misplacement of wonder. Planets of our size and composition fall into the solar orbits that they are in precisely due to their size and composition. If we were going to get hit by a mars sized object with comparable density and comp to earth, "earth's lane" on the solar freeway is -exactly- where you'd expect that to occur. We'll discuss that lane and the moon, again, shortly.
The convective rise and fall of heating and cooling iron, as well as whirlpools caused by coriolis generates the electrical current of the magnetosphere. Yes, the magnetosphere deflects some UV radiation..but what gets through is still a lethal dose. The earliest life put a more effective barrier between themselves and the sun. Water. The magnetosphere itself isn't a rarity, nor is it's interaction with uv radiation. This inevitability is borne out by the laws which govern where in the orbit of the sun a planet with an iron core will form, by what will happen when iron is subjected to heat, pressure, and rotation, and by the manner in which that active iron core produces energy. It's as common as any electromagnetic coil working.
Ah, the tide. The moon, initially, would have had a catastrophic effect. Magma tides more like tsunamis than highs and lows - there wouldn't exactly have been alot of water vapor on earth post collision. Tidal earthquakes on a clock. We sometimes forget the devastation that tides cause today, as well...then hurricane season hits and we remember. Far from being the peaceful benefactor of earthly life, the moon was once an even more effective agent of annihilation than it is today. It's a good thing that our genetic lineage wasn't around then, or for ages thereafter. One day, it's going to completely halt our rotation. One side of earth will be permanently fixed facing the moon...as the moon faces us. Life as we know it on earth will have been exterminated long before that work is done. Then, it will leave.
Now we're back in-lane. The Goldilocks Zone. It sounds small, like a little girl..and we all know the story. However, the character of the story and it;s narrative thrust have little, if any, resemblance to the usage of the term in astronomy. The HZ (habitable zone) at this point in time for a star like ours is at least .7AU wide. This doesn't properly account for extremophiles and certainly doesn't account for novel exo-geology. It's 70m miles wide. You could fit 9k earths shoulder to shoulder within it's inner and outer boundaries....and we're drifting at the outer edge of it. No point on that line is "just right" unless our tolerance for error is big enough to drive a star 9x the size of our own through.
Speaking of shooting the gaps, other orbital bodies do take some hits for us. The moon is famously pockmarked. They also pull those objects shooting the gaps into eventual collision courses with us. It seems, as we discussed before...that at least once those other planets -were- the things crashing into us, and none of them seemed to keen to take one for the dinosaurs.
Chemistry+Energy+Tidal forces+Time=Life sounds more like an invocation of inevitability. May not have needed the tidal forces, either...but at present it seems likely, at least. That's pretty much the only thing in that equation that could have gone another way. I say "could have" ignoring for purposes of discussion that...in this solar system...it couldn't have gone any other way. If there was a rock like ours formed in our band on a collision course with us, and that collision was going to carry x amount of force - then the moon and all of it's attendant effects were a predictable result waiting to happen. Chemistry, energy, and time aren't variables in this equation. The sun existed. Earth had a chemistry. Time was on the march.
Evolution is certain when errors in reproduction occur. It's not strange or odd or unlikely that, after billions of years, the accumulated differences of many billions of generations produce a vast array of forms. As to the ratio of living to dead....well, wouldn't that be expected as well? No day on earth is exactly like the last - and we've had many billions of those in the same time, with similar accumulated differences.
Now, about intelligence. This one is flat out wrong. We're far from the only example of intelligent life. -Plants- are intelligent. The entire kingdom of animalia is intelligent. I suppose there being little more to say on this one it;s a great time for summary. The chances of life occurring are unity. A known event that happened. It happened on a planet whose composition and density could support life, where other such planets could (and apparently did) form. A planet that is, like every other planet, the sole survivor of a cosmic game of road rage. A planet that, like the others, has an orbital body. A planet that sits within a range of potential habitation as wide as an astronomical unit as, potentially, 40b others outside of our solar system do. On a planet where the collisions are infrequent but still deleterious. A planet which only needed what was present in order to produce life in what was, referencing the age of our planet - a short timescale. A planet which now teems with intelligent life. The mind numbing commonality of it all is what strikes me, personally.
TLDR version: Sure. I guess we're special....at least in the same way that everyone else is special. We're all a bunch of intergalactic snowflakes.
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