Three men and a woman onboard a timeship travel from 2070 AD to 12,000 BC - a journey that could never be repeated. For the passengers, all anthropologists, it was a once-in-a-million-lifetimes expedition... a chance to study primitive man as modern man never could. But none...
Ballantine, 1972. First edition of this title, wherein Farmer revisits the Tarzan theme, this time with time travel. Later revised and expanded - the new edition was published by Ballantine Del Rey in 1977.