Pohl, Frederick. "Let the Ants Try"
Divergence: c 40,000,000 BCE
Summary: Following a nuclear war, a scientist carries some mutated ants 40 Myr into the past and returns to find his present irrevocably altered.
Comments: Originally published under the pseudonym James MacCreigh.
Published: In Planet Stories Winter 1949; Beyond the End of Time, Doubleday 1952, PermaBooks 1952; Alternating Currents, Ballantine 1956, 1969; Mutants (ed. Robert Silverberg), T. Nelson 1974; In the Problem Pit, Bantam 1976 (0553088572); In the Problem Pit and Other Stories, Corgi 1976 (0552103349); and The SFWA Grand Masters Volume 3 (ed. Pohl), Tor 2001 (0312868774w88 android).