I've often wondered why it is that we discover a particular technology when we're just barely sensible enough to use it, but before we've got the sense not to use it at all.

For instance, imagine how the 19th century would have gone if somebody had the A-bomb. The civil war would've left North America a smoking ruin. But if electricy or the internal combustion engine were new science today, we would be to scared of them to use either. (This isn't an original idea. Bob Newhart did a skit on what would happen if the airplane or smoking were invented in the 1960's.)

So my theory is that we'll figure out how to move to another planet right around time we've just about destroyed this one, but also figured out how not to destroy the next one. If we moved right now, we'd just destroy the new one, so what good would that do us?