The Time Masters (Lancer SF, No. 75-290)

Second paperback edition, 1971. Slightly revised from the old Signet edition. ~ ~~ ~ Ten thousand years ago, a spaceship from the stars crashed on Earth. Several survivors found that one critical element that kept them from dying prematurely was missing here, and eventually the number of the humanoid alien survivors was reduced to one. ~ ~~ ~ Jump forward to 1947. In Oak Ridge, Tennessee lives a man named Gilbert Nash, the last survivor. He knows that soon the lack of that critical element will take his life, and he has accepted his fate. Until a client walks into his detective agency and describes a problem he's having with his wife. From what the man says about the woman, Nash KNOWS that she is another survivor. Perhaps she knows a way for them to survive, or even to get home again? Or, perhaps, she's a cold-blooded killer who has no use for sharing survival.