A group of time travelers journey from 2070 to around 12,000 BC. Among the three men and woman is the mysterious John Gribardsun, who seems ideally suited to the journey. He's strong, dark haired and grey-eyed and uses a lot of anachronistic sayings that puzzle a few of his companions. One of the anthropologists is intent on discovering the origins of various languages and the woman scientist, of course, falls for Gribardsun, which drives her husband into a jealous rage that eventually erupts. And Gribardsun gets more and more comfortable in this ancient land, to such an extent that he chooses not to return his own time after their four-year expedition ends. You see (and this is no huge surprise), his true identity is that of a certain Lord of the Jungle and he feels that he no longer fits in modern society. To make it even more interesting, we learn the various roles this immortal (he doesn't age, ever since drinking a potion by an African witch doctor) plays in history, including being a character from Farmer's HADON OF ANCIENT OPAR series. And once he catches up with his own time (avoiding the version of himself that hasn't time traveled yet), he is able to resurrect his wife, Jane, who had been gravely injured and was in suspended animation. Ultimately, we learn that the two travel to a frontier planet to spend their extremely long lives...
This is one of my favorite Farmer novels, particularly in that it fits so well with Edgar Rice Burrough's Tarzan character.