This is the 1985 Tor paperback edition. First published in 1972, this novel was revised and expanded in 1977. "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 of them was prepared for what they would discover - or for the impact of their travels in a time that had yet to come...