Of course, long before transhumanism and even before space travel, science fiction writers were speculating on the implications of just such a notion of posthuman destiny. In his 1956 short story "The Last Question," Isaac Asimov considers the inevitability of limits, ends, endings, and beginnings. The story presages the metaphysical spiritualism of Arthur C. Clarke's 2001 and related science fiction, as well as that of many of the later transhumanists. Read it for the provocation of thought (and the hilarious anachronism of planet-sized computers).
(Hat tip: Mark Reitblatt.)