Have you ever heard of the butterfly effect? This is a phenomenon that refers to chaos theory but also has a significance in psychology. It states the idea that a minute localized change in a complex system can have large effects elsewhere. The classic example speaks of the fluttering of a butterfly's wings and the great impacts that small action could have elsewhere in the world - i.e. the evolution of a tornado or an airplane experiencing turbulence.
The butterfly effect was popularized by Edward Lorenz when he used weather simulation data to demonstrate that even the smallest change of wind speed or air temperature has significant impacts on weather for up to two months after said change. From a psychological perspective, the theory makes almost even more sense: every small act in turn results in another act that would have been distinct had the previous act also been different. Apply this to the history of the world, and be prepared to be blown away.