No Enemy But Time

John Monegal, a.k.a. Joshua Kampa, is torn between two worlds , the Early Pleistocene Africa of his dreams and the twentieth-century reality of his waking life. These worlds are transposed when a government experiment sends him over a million years back in time. Here, John builds a new life as part of a tribe of protohumans. But the reality of early Africa is much more challenging than his fantasies. With the landscape, the species, and John himself evolving, he reaches a temporal crossroads where he must decide whether the past or the future will be his present.
No Enemy But Time won the 1982 Nebula Award for best science fiction novel of the year.