Upon seeing the gigantic, terrifying Tyrannosaurus Rex, Eckles is overwhelmed with fear and immediately sprints towards the Time Machine. Unfortunately, Eckels leaves the floating Path and accidentally steps on a butterfly. When Travis, Lesperance, and the other hunters arrive back at the Time Machine, Travis notices Eckels's boots and realizes that he has run off the Path. After a brief argument, Travis makes Eckels retrieve the bullets from the deceased dinosaur before they travel back to the present. When they return to the present, Travis and Eckels realize that the trajectory of human history has dramatically changed after they read the Time Safari sign, which is spelled wrong. Eckels then holds up a piece of dirt from his boot and discovers that he stepped on a butterfly. Bradbury writes,
Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and very dead (10).
The death of the prehistoric butterfly has significantly altered the trajectory of human history in what is referred to as the "butterfly effect," which is a phenomenon where every living being or organism is connected in a complex relationship. According to the theory, a single occurrence can have a chain reaction and impact every living being in the world. This change can dramatically alter the course of human history, which is what happens when Eckels kills the butterfly.