What does Eckels do in the past that has far-reaching consequences?
When Eckels is sent into the past he inadvertently disobeys one of the rules of Time Safari, Inc., the company running the time travel tours. The time travelers are told never to step off of the floating walkway that the company uses so that nothing in the past is affected. The fear is that even a very minor change in the distant past could have ripple effects in the present. Great care is taken to not change anything in the past, now that time travel has become a possibility. Eckels' group travels sixty million years into the past in order to hunt a dinosaur that Time Safari had determined was about to die anyway, thus not really changing anything. The anti-gravity walkway should keep their feet off any wildlife, but Eckels panics at the sight and sound of the Tyrannosaur. He steps off the path. When the group returns to the future, things have changed in various ways and only the returning group realizes it. On the sole of Eckels' shoe is a dead butterfly; the death of the one butterfly has changed human history, sixty million years later.