From the Eternal Golden Braid, some quotes about Space Colonisation. An example from Carl Sagan :
Since, in the long run, every planetary civilization will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring--not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive... If our long-term survival is at stake, we have a basic responsibility to our species to venture to other worlds.I'd amend that - it's not just our species that's at stake, it's the whole biosphere as we know it.
When Mother Nature presses the "Reset" button, as she does now and then, sometimes it only kills all land-dwelling life larger than a Guinea Pig. But sometimes she leaves it pressed down, and nothing larger than bacteria survive. We owe it not just to ourselves, but to our fellow travellers on Spaceship Earth to make a backup.
Most people are too busy with day-to-day life, arguing the whys are wherefores of events-of-the-day, how many angels can dance on a pinhead, let alone the intricacies of Postmodernism, to worry about this type of stuff. Fair enough - but it's not a matter of if a (literally) world-shattering (or at least biosphere-destroying) event happens, it's a matter of when, and how bad. Tomorrow? A million years hence? The point is, it will happen, as surely as it will rain sometime where you're reading this. Faith is not required, nor belief. It's as immutable as an avalanche that's roaring down on us. But we probably have some breathing space. Probably. And probably enough.
In the next few thousand years, we're going to see a lot of changes. Cyborgs, genetic engineering directing our own evolution, probably genuine AI, what Vernor Vinge terms the Singularity. Everything I've read and learned in the way of hard-nosed, practical engineering (as well as Science), tells me that this is not just a possibility, it's inevitable, barring some global catastrophe.
A worldwide Thermonuclear war would delay it a few millenia, but no more than that. A Dinosaur-killer asteroid strike that killed most of humanity would delay things a bit longer, maybe a hundred millenia in the worst case. But no longer. Unfortunately, there are far worse things than that that could happen (like a Gamma Ray Burster going off nearby, as in "somewhere in the same Galaxy"). Some of these possibilities would be non-survivable without Godlike technology. But most of them would be quite survivable just by not being in the immediate vicinity. If Sol burps and licks the Solar System clean, as long as we have backups in other systems, Earth life will survive. Vastly more probably, if Earth takes a hit from a glorified snowball the size of Ireland, only a few lithophillic bacteria might ride it out here, but anyone on Mars or even the Moon would be perfectly safe.
The only question is what is going to happen first? An Extinction Event? Us getting off this rock (and taking our companions, from dogs to dandelions, with us)? Or a singularity that makes things like travel to the stars, solar Dyson spheres etc a matter of will rather than know-how?
Beats me. But just in case, let's have a decent space exploration and colonisation programme going, shall we? Just in case something as stupid as a couple of cubic kilometres of rock hits the joint before we "become as Gods".