When the safari participants return, many things have changed in the year 2055, due to an act of carelessness on Eckels's part.
In Ray Bradbury's short story "A Sound of Thunder," Eckels, the main character, pays for a time travel safari to the Cretaceous period to hunt a tyrannosaurus rex. The safari guide, Travis, warns the participants not to leave the metal, anti-gravity path that has been laid by Time Safari, Inc., to ensure that nothing is touched in the past. When Eckels asks why Travis explains the chain reaction of events in great detail:
"Eventually, it all boils down to this: Fifty nine million years later, a cave man, one of a dozen in the entire world, goes hunting wild boar or saber toothed tiger for food. But you, friend, have stepped on all the saber toothed tigers in that region. By stepping on one single mouse. So the cave man starves. And the cave man, please note, is not just any expendable man, no! He is an entire future nation. From his loins would have sprung ten sons. From their loins one hundred sons. And thus onward toward civilization."
Despite the detailed warnings, Eckels becomes so awestruck and afraid when he sees the tyrannosaurus rex, that he steps off the path and into the jungle moss. In doing this, he killed a butterfly, which changed the course of human history through a chain reaction of events.
Some things that changed as a result of Eckels killing the butterfly were: the chemical taint to the air when he returned, the man sitting at the desk was "not quite the same," the spelling on the sign at Time Safari, Inc., had changed, and of course, the biggest change was that Deutscher, the anti-everything man, had been elected president instead of Keith.