Star Gods: Book Four of Seeds of a Fallen Empire

In Star Gods, the action returns to an alien home world, where the alien Alessia is raised. Her mentor, Hinev, creates the immortality serum and tests it on his pupil, Alessia, who becomes one of her planet's intergalactic explorers. The explorers eventually visit the Earth of the ancient past on their long journey in space. On Earth, they influence the development of our ancient civilizations. Their journey leads them to believe that an ancient progenitor race 'seeded' the galaxy with life...