William Tenn: Of All Possible Worlds: Changing the world is simple; the trick is to do it before you have a chance to undo it!

"If his great-grandfather had not volunteered for the earliest time-travel experiments way back in the nineteen-seventies, back even before the Blight, it would never have been discovered that he and his seed possessed a great deal of immunity to extra-temporal blackout.

And if that had not been discovered, the ruling powers of Earth, more than a century later, would never have plucked Max Alben out of an obscure civil-service job as a relief guard at the North American Chicken Reservation to his present heroic and remunerative eminence. He would still be patrolling the barbed wire that surrounded the three white leghorn hens and two roosters—about one-sixth of the known livestock wealth of the Western Hemisphere—thoroughly content with the half-pail of dried apricots he received each and every payday."

And so the story tells of when the Blight had killed off almost all of earth's edible food stuffs and humans the world over struggled to get one meal a day. . But the immensely wealthy food monopolies now had banded together to use a time machine that would fix the problem. Or maybe it might not. Listen to this imaginative story told by a master.