I read the book from my local library in the early- to mid- seventies. It was hardcover (library bound) and the dust jacket had a printed burlap or canvas background. The back flap mentioned that the author's name was a pseudonym. The book had lots of conversation, anecdotes and opinions with little to no 'action'. I came away with impression that the author wasn't a professional writer, but rather more like a history professor with an academic reputation to protect.
It begins shortly after the formation of a world government, and the President of Earth is presiding over the trial of Earth's only immortal man, for the crime of withholding the secret of immortality. Much of the book is taken up by his narration of stories of his life, from his birth in prehistoric times to the present. This could almost be the backstory of 'The Man From Earth', but IIRC he was never a famous person.
One anecdote I remember is what he called 'Pavlov's Other Experiment', with the supposed discoveries by the scientist after the surprising results of the 1924 Leningrad flood. The novelist added a vitamin deficiency factor that became an important plot point. There was also a story I recall as being very amusing about a British military steamship in Egypt before or during the occupation in the eighteen hundreds.
And finally, as a major plotline (such as it is), everybody is taught the simple trick to become immortal. When dead people are cloned and become 99.99% similar, their soul and memories are reunited with their bodies and they are taught immortality; time-traveling robots collect DNA from everyone who has ever lived and they become immortal as well.
This is not Bixby's "A Man From Earth", Anderson's "Boat of a Million Years", Heinlein's Lazarus Long, Borges' "The Immortal", Zelanzny's "This Immortal". The main character was not a time traveler or the Wandering Jew. It is possible I'm getting the similarity thing confused with van Vogt's Null-A, but there was something like that.
The immortal man was likely 6000-8000 years old, and the time of the novel in a future not far from now. His prehistoric tribal roots being near the invention of agriculture? (maybe)