This post builds on previous ones, such as here.
In the year 507 B.C., the Athenian leader Cleisthenes introduced a system of political reforms that he called demokratia, or “rule by the people.” This system was comprised of three separate institutions: the ekklesia, a sovereign governing body that wrote laws and dictated foreign policy; the boule, a council of representatives from the ten Athenian tribes; and the dikasteria, the popular courts in which citizens argued cases before a group of lottery-selected jurors. (reference)
The ancient Roman Senate was composed of patricians, members of the ruling families, who wielded varying amounts of influence and power in the Roman monarchy, republic, and empire. This aristocratic body is the forerunner of upper chambers of legislatures in the modern age.
Modern parliaments are descendants of the ancient ekklesia in single-chamber democracies. A legislative body of democratically-elected representatives is sufficient for this kind of democracy. A broader democracy includes two legislative chambers, with the lower chamber representing the people and an upper chamber representing tribes, ruling families, or key subdivisions of the country, that is, the land.
Representation of the traditional tribal, familial, or territorial alliances is important since they are the gluten than holds society together. While political principles and traditions are important, they alone cannot keep a society from separating, since they have no inherent attachment to a people or a place. There must be something so that a group of people are invested in the good of the country.
Hence a legislative body is needed that is tied to something tribal, familial, or territorial. In order to go beyond mere tribal or familial alliances, the territories of the people must be represented. A legislative body whose representation is not based on population will also mute the influence of gerrymandering. The democratic approach to representation is through election so legislative divisions by territory are represented by the people who live in each territory.
One could go further and require that the electorate consist of those who live on land they own in the territory — or those who own their residence in the territory. These people are invested in the place. A republic includes both territorial-based representation and population-based representation. Hence a republic needs two legislative bodies with two different kinds of representation.