From back cover Del Ray paperback March 1984:

... TWIXT THE CUP AND THE LIP

On Earth, six million B.C., two species of alien ruled, the graceful humanoid Tannu and their twisted brethren, the Firvulag. Then men from twenty-second century Earth arrived through a one-way time tunnel --and soon the aliens were locked in a battle to the death, for the humans had upset the precarious balance of power that existed between them.

But then the tides of combat had receded, no one group held firm control, though Aiken Drum, man of no woman born, had declared himself the Nonborn King...