The concept of the polis (city-state) began to evolve with the development of aristocratic clans to replace chiefdoms. Clan rivalries yielded single powerful figures who were termed tyrants because they achieved domination in outright power struggles within the aristocratic group and among clan centers. Because they often were marginal clan members, the success of the tyrants created a new criterion for power: ability rather than birth. This change was a crucial element in the development of the polis, which came to be a politically unified community covering an average of 200 square kilometers and based on a small urban center. When the tyrants were overthrown after one to three generations, the institutionalized structure they created remained and became an important legacy to the modern world.

In the eighth century and early seventh century B.C., Sparta began to develop as a militant polis with a rigid social structure and a government that included an assembly representing all citizens. Meanwhile, Athens became the largest polis, combining several regions of the peninsula of Attica. Under the leadership of the aristocrat Solon, Athens developed a social system in which power was based on wealth rather than aristocratic birth. Citizens of various wealth categories were allotted different positions of power. Thus, in different ways Sparta and Athens built states that included wider sectors of society in their political activity than had any previous society, and the basis of democracy was laid.