Time's Last Gift (Wold Newton Prehistory) (original 1972; edition 2012)

It was a timeship, with three men and a woman on board. Without moving a micron in space, it had traveled from 2070 A.D. to 12,000 B.C. — a journey that could never be repeated.
For the three men and the woman, all anthropologists, it was once-in-a-million-lifetimes expedition... a chance to study primitive man as modern man never could.
But spending four years in another millennium was a complex proposition, especially when two of the principals were having marital problems. But, then, no one was prepared for the startling truths about themselves and their subjects ... or for the reverberations in a time that had yet to come!