No one knows. And as much as we try and predict it . . . we don't have a clue. I realize this is a much more macro example, but what percentage of the matter in the universe can we *not* account for, given our current cosomological theories? Something around 80% IIRC.
The earth is on cycles. Day, night. High tide, low tide. Winter, spring, summer, fall. And these are just micro examples. I am certain that there are cycles, on this earth, which have durations, that the human mind cannot concieve.
We, as a sentient species, are short sighted (social programming and genetics), arrogant (programming), and self destructive (comes from the short sightedness). Will we kill ourselves off before we can get some of us away from this planet? Who knows. We are already on the clock, it's just that we have somewhere between 7 and 14 billion years to get it done.
juju; what technology do we possess that can kill off everything? Ah sure, we can kill ourselves off no problem. maybe even a good portion of the rest of the species; but at this time, we possess nothing that can come close. It's not any weapons we possess today, and if there is anything that is a gradual process, we are a far more fragile species than the earth is a fragile planet, so we will kill ourselves off, and the earth will recover.
So yes, it's an assumption, but one I stand behind.