Martin Rees:  Well, of course, the world could end with a bang all too soon, and if that happened, then this would foreclose the billion-year future of post-human evolution.  But of course, what I think is a more plausible scenario is that here on Earth, we will continue as humans, with some setbacks, and with some evolution, but that life will spread beyond the earth.  And that that life spreading beyond the earth will have a future of maybe billions of years, evolving in ways we can’t predict.  And evolving in ways that are not Darwinian selection but intelligent design by the creatures themselves, which will have human intelligence.  So that’s one scenario.  Of course, the other thing which will affect the long-term future is whether there’s life out there already.  Because it’s gratifying, in a way, that even if life was unique to this earth — making our pale blue dot in the cosmos important, not just to us but on a cosmic scale — then that doesn’t mean that life will forever be a sort of minor feature of the galaxy.  It could spread, in its far future, elsewhere.