The Boat of a Million Years
by Poul Anderson
The novel follows a group of eleven immortals from the ancient past to the distant future. Most of the novel follows the various immortals throughout their lives as they try to find others like themselves, avoid being killed, and remain quiet about their gift.
Gradually, the immortals begin to meet across the world and form a family of sorts. After they share their secret of immortality with the rest of humanity, the ensuing years result in a human culture they no longer relate to or fit in with. Consequently, they create and crew a starship to explore new civilizations within the galaxy.