What power do humans in A Sound of Thunder try to control? What happened In result?

The "Sound of Thunder" is based upon time travel, specifically, traveling back in time to hunt animals that are now extinct. In the story, Eckels is incredibly excited about the idea of conquering the beast Tyrannosaurus. In the end, Eckels becomes frightened and cannot shoot, putting himself and everone else in great danger. The men kill the T-Rex, but they also kill something else..... a butterfly. As a result, the future they return to...... their present, is dramatically altered.