Senate of the Roman Kingdom

The Senate of the Roman Kingdom was a political institution in the ancient Roman Kingdom. The word senate derives from the Latin word senex, which means "old man". Therefore, senate literally means "board of old men" and translates as "Council of Elders". The prehistoric Indo-Europeans who settled Rome in the centuries before the legendary founding of Rome in 753 BC were structured into tribal communities. These tribal communities often included an aristocratic board of tribal elders, who were vested with supreme authority over their tribe. The early tribes that had settled along the banks of the Tiber eventually aggregated into a loose confederation, and later formed an alliance for protection against invaders.

The early Romans were deeply patriarchal. The early Roman family was called a gens or "clan". Each clan was an aggregation of families under a common living male patriarch, called a pater (the Latin word for "father"). The pater was the undisputed master of his clan. He had the absolute power to resolve any disputes, and to make any decisions for the collective gens. When the early Roman gens were aggregating to form a common community, the patres from the leading clans were selected for the confederated board of elders (what would become the Roman Senate). Legend states that the senate grew to a membership of 300 after three blocks of 100 senators were added at fixed points in time. What likely happened, however, was a gradual aggregation of patres over time, as more clans achieved high status. The early senate derived its ultimate sovereignty from the fact that it was composed of the patriarchal heads of the leading families. As the individual patres led their families, the board of patres led the confederation of those families. In time, the patres came to recognize the need for a single leader. Therefore, they elected a king (rex), and vested in him their sovereign power. The king presided over the senate, appointed individuals to the senate (for life), and expelled individuals from the senate. When the king died, his sovereign power naturally reverted to the patres.

...
Wikipedia