I'm trying to track down a story I read years ago, I think it was Bradbury but I'm not sure. Something about a time machine that these people took to visit prehistoric days, but had to stay on a path, if they stepped off the path they might change history. One person steps off, they go home and find the world totally different, run by dictators, and it has the guy looking at the bottom of his shoe to find a butterfly he stepped on, which started a long chain of events which led to all the dramatic world changes. IS THIS A BRADBURY STORY??