i have a pessimistic view for humans, optimistic for the planet. we can quite easily wipe ourselves out but in my view it will take only a very short time (maybe an ice age or two) to fix things up again. wont be any help to us but at least in a sense we will leave the planet as we found it, even allowing for nuclear wars etc since even then radiation levels will die down in the blink of an eye compared to the age of the earth.
but the world will be totally different people might say. so what? the world is constantly changing anyway.
but geez, it would be nice to have a star trek-esque future...