Time Traveler steps on a butterfly, changes everything

Question

I heard of this in high school, but never learned its title or author- supposedly the aforementioned worm butterfly would have played a part in evolution that caused the extinction of the dinosaurs (instead of that meteor most people blame now). This story might even have been parodied on a certain Simpsons Halloween Special (the one where Homer accidentally makes a time machine trying to fix the toaster).

Answer 1

This is almost certainly Ray Bradbury's "A Sound of Thunder," a very famous short story originally published in 1952 and found (among other places) in his widely-read collection Golden Apples of the Sun.

Per the Wikipedia plot summary:

In the year 2055, time travel has become a practical reality, and the company Time Safari Inc. offers wealthy adventurers the chance to travel back in time to hunt extinct species such as dinosaurs. A hunter named Eckels pays $10,000 to join a hunting party that will travel back to the late Jurassic Era, on a guided safari to kill a Tyrannosaurus rex. As the party waits to depart, they discuss the recent presidential elections in which an apparently fascist candidate, Deutscher, has been defeated by the more moderate Keith, to the relief of many concerned. When the party arrives in the past, Travis (the hunting guide) and Lesperance (Travis’s assistant) warn Eckels and the two other hunters, Billings and Kramer, about the necessity of minimizing the events they change before they go back, since tiny alterations to the distant past could snowball into catastrophic changes in history. Travis explains that the hunters are obliged to stay on a levitating path to avoid disrupting the environment, that any deviation will be punished with hefty fines, and that prior to the hunt, Time Safari scouts had been sent back to select and tag their prey, which would have died within minutes anyway, and whose death has been calculated to have minimal impact on the future.

Although Eckels is initially excited about the hunt, when the monstrous Tyrannosaur approaches, he loses his nerve. Travis tells him he cannot leave, but Eckels panics, steps off the path and runs into the forest. Eckels hears shots, and on his return he sees that the two guides have killed the dinosaur, and shortly afterward the falling tree that would have killed the T-Rex has landed on top of it. Realizing that Eckels has fallen off the path, Travis threatens to leave him in the past unless he removes the bullets from the dinosaur’s body, as they cannot be left behind. Eckels obeys, but Travis remains furious, threatening on the return trip to shoot him.

Upon returning to 2055, Eckels notices subtle changes - English words are now spelled and spoken strangely, people behave differently, and Eckels discovers that Deutscher has won the election instead of Keith. Looking at the mud on his boots, Eckels finds a crushed butterfly, whose death has apparently set in motion a series of subtle changes that have affected the nature of the alternative present to which the safari has returned.

The Wikipedia article also notes:

The story was parodied in the Time and Punishment section of The Simpsons episode "Treehouse of Horror V".

This story has been previously asked about and answered here at Story involving a time machine in a museum and butterflies, but, that answer is unaccepted even though it's very likely to be correct.