A Sound of Thunder was a short story by Ray Bradbury. It tells the story of a man who steps on a butterfly in the past, and the future is dramatically changed. On April 15, 1986, Marcus Irving travels to January 8, 19654 B.C. in his Yugo time machine, temporarily stranding Marty McFly and Doc Brown. Doc used Bradbury's short story as inspiration to form a plan to get back home. He told Irving that their presence might wreck the ecosystem shortly before he left. They managed to escape sabre-tooth tigers shortly before Irving returned to bring them forward to 1997, as Doc had made him afraid of the butterfly effect their presence could cause that far in the past.