Other civilizations would come, but they would be unable to keep up with these first five. All five co-evolved together, but most of the friction was around the proximity of Egypt and Sumer. Human history follows a path of competition and warfare. It seems to be dominated by the latest killing technique. First came bronze weapons, then came the chariot, iron and steel weapons, the phalanx, ships, cavalry, guns, the citizen soldiers, airplanes, and most recently, the nuclear bomb.
Humans can be a barbaric species with a great potential for violence.
While this violence was valuable for their survival on the Savanna, it has become detrimental. The final conquerors were the modern European nation-states, who used science and technology to conquer the world, bringing it together for the first time. Since the Europeans did not utterly destroy the cultures they were conquering, which was new, all of the various cultures, which were at their own stage of development, were brought together into a global empire. With the mixing of the various cultures and the findings of Modern Science, humanity has been left in the postmodern era with no identity, but globalization, scientism, constructivism, and Gaia recognition will lead to a conscious, world-wide reality reconstruction. Until then, they have suspended their reality and submerged into an orgy of greed and gluttony which could jeopardize their existence.