When we were hunting animals with no fear, Eckel went to the past to hunt dinosaurs getting yelled at for everything he did. When he walks off the path a person nearly kills him. When he tries to run and then they kill the dinosaur they force him to get the bullets and stick his hands in blood when he also had to pay to get into the time machine.
The thing is that I see why they treat him so bad but if you get paid to do something I think you have to treat the payer nicer. So when the present changed he notices a clump of dino dirt on his shoe and when he picked it up the dirt showed a butterfly and somehow that butterfly changed the future a lot and that the words changed and the world changed.