Humankind as we know it will cease to exist by the year 3000 (in fact, I'd give a 50:50 chance of that happening by 2100). It will be replaced by a single incredibly powerful distributed computer wrapping Earth, containing all the knowledge of humanity and all memories of all humans who choose to join. The ones who refuse will be granted eternal youth and given a choice between taking a spaceship to a distant star, where they can try to start a new civilization, and taking a doorway to a parallel prehistoric Earth, where they would essentially be gods.
Long before this happens, advances in brain-computer interfacing, memory backup, memory cloning, will make us completely rethink our naive concepts of life, identity, and place in the universe.
And you know the fun part? Many of us will still be there to see this. In fact, some people who died will be there too. Ted Williams will be there, for example. Of course, "being there" won't amount to much more than having one's memories as part of the big computer. Human bodies will be discarded, boundaries between individual mind-streams will fall, and, aside from these three parts, there isn't really much left.
There are two alternatives. One is that we manage to kill ourselves, or at least knock ourselves 3000 years back. Was it Einstein who said that the fourth world war is going to be fought with sticks and stones?
The other is that there is a parallel universe whose properties are in some way preferable to this one and that's where all sentient civilizations retire. We'll all go there and join countless alien civilizations that walked this path before us.