Because of our culture and the technology it's developed, we can contemplate immortality--or as close to immortality as matters.

From our own selfish perspective, our civilization outlasts individual humans, and outlasts even peoples. America could disappear from the face of the Earth today, but the totality of Western Civilization to which it contributed would go on. We expect in even the worst case scenarios, such as a nuclear war, that civilization could be reconstructed much faster than it was constructed, based on the records and memories that would survive (ideas as the seeds of civilization; just add water.) The knowledge of civilization is more robust than the human body, and by relying on civilization our species becomes more robust. It should give us a life expectency much greater than 800K years. Of course, it also gives us the power to cut it short in an instant. But we've managed to beat the odds so far, so maybe our formula is working.

And then there's an even longer view. Because our civilization exists outside of us, as an expression of our phenotype, it can outlast even our species. A bird's nest is an example of an extended phenotype, something an organism builds outside its body; and just as bird's nests can be passed on and used by more than one bird, human civilization can, and probably will, be passed on to our inheritors. Our descendents may be a species descended from our DNA, or a species we created using some of our DNA; it may be entirely artificial heirs in the form of AI; it might be an intelligence that arose around another star, joined us, and carries on when we're gone. Even if the human species were to perish and leave Earth empty of intelligence, perhaps some day the ruins would be discovered by others, and through our records our civilization could be brought to life again and incorporated into another living culture. Our civilization is a creation greater than our species.