Harari posits that the first major human revolution was the Cognitive Revolution, wherein humans developed an ability to communicate that was far more powerful and nuanced compared to animals. This ability allowed them to talk about things that do not exist, such as mythologies and deities that could help bind a tribe together under one culture. “Such myths gave Sapiens the unprecedented ability to cooperate flexibly in large numbers.” (Harari, p.25)
Expounding on that Cognitive Revolution came the Agricultural Revolution. It was at this time when Sapiens transitioned, for better or for worse, away from a hunter-gatherer society to an agricultural one. This led to a great increase in the amount of food available, allowing for larger and larger populations to settle into the land now adapted for their use. “Cultivating wheat provided much more food per unit of territory, and thereby enabled Homo sapiens to multiply exponentially.” (Harari, p.83) So in a short and reductionist summary, these two revolutions enabled Sapiens to work and live together in ever-increasing populations.
Harari credits “imagined orders” as the next key step that allowed for even larger populations of Sapiens to coexist and work together. Harari posits that shared myths such as the power of the government or a religion shared between people are what allows for civilizations such as the Roman Empire to exist (Harari, p.105). I personally find Harari’s arguments to be mostly persuasive, but I think some more attention should have been paid on the psychology of people organized into different groups, i.e. in how just by being in different groups people tend to prefer those in their group and deal more harshly with those outside of the group, as shown in the Robbers Cave Experiment.