Time’s Last Gift by Philip Jose Farmer
This classic is back in print, so if you haven’t read it yet, go do so! In the not so far future from now, time travel is discovered. You can only travel backwards in time, and there is a limit of how far back you can go. So a team of scientist is sent back as far as possible, to the early days of Homo Sapiens. So far back, North Africa was lush plains and forests. Here is the plot twist, one of the team members is not quite what he presents himself as. He is in fact, much older than his other team members, since he been given the gift of immortality (or an extremely long life span) by a Witch Doctor he saved from a leopard in the African jungles he was born in. If you haven’t guessed who the tall gray eyed ringer is by now, you haven’t read enough Farmer.