In the long history of humanity, no thought has been so common as personal immortality. At every time and in every place men and women have assumed they will live forever. Even Neanderthals, it seems, placed flowers in the graves of their dead, presumably to grace the afterlife.
It would be interesting to know who was the first person to accept that death is final. Certainly, empirical learning and awareness of mortality go hand in hand. None of the supposed evidence for an afterlife -- near death experiences, seances, channeling, hauntings, etc. -- holds up to experimental examination. No conclusion of science is more firmly established than that the soul is irretrievably embedded in the flesh and goes out like a light at the moment of death.
One-celled organisms are potentially immortal, in that death is not programmed into their existence. The inevitable death of multicellular organisms was -- from our point of view -- a marvelous breakthrough of natural selection, the mother of diversity and complexification. As the microbiologist Ursula Goodenough says: "It was the invention of death, the invention of germ/soma dichotomy, that made possible the existence of our brains."
We admire people who surrender their lives for a noble cause. What cause is more noble than the continuance of multicellular life in all of its diversity and grandeur? If we are to accept our personal mortality without despair, we need a worldview that emphasizes cosmic wholeness rather than the primacy of self. In Hymn of the Universe, the Jesuit scientist/mystic Teilhard de Chardin writes: "Man has every right to be anxious about his fate so long as he feels himself to be lost and lonely in the midst of the mass of created things. But let him once discover that his fate is bound up with the fate of nature itself, and immediately, joyously, he will begin again his forward march."