All of these civilizations were connected by coastal shipping, thus creating the first major metropolitan network. China also had civilizations, but remained isolated from the Nile-Indus corridor. In the Americas, civilizations appeared mainly in Mesoamerica, although much later than in Eurasia, such as the Olmecs and the Maya. None of these civilizations merged, but their interactions created the first subway network in the Americas. Decipherable written documents seen in Mesopotamia (cuneiform) and Egypt (hieroglyphics) are missing in both South America and the Indus Valley, preventing too much from being known about their civilizations. The Sumerian cities of Mesopotamia were perhaps the most advanced of their time, and their religion played an important role in their daily lives. The temples were the most distinguished parts of the city and pleasing the gods was always a priority until the military rulers reduced their importance. Sumerian cities were also centers of new technologies such as tools, textiles, and weapons. City dwellers had the advantage of these superior weapons and herding groups had the advantage of mobility, leaving agricultural villages to submit to powerful, organized warfare. To maintain their new power, rulers established tax collectors and military specialists. This started a social system with social classes based on rank and wealth (luxury goods). Developments in Egypt and the Indus Valley contributed to the construction of the metropolitan network around Sumer. The Nile was the basis of all life in Egypt, as it allowed transportation by boat and was a source of water for successful irrigation and agriculture. The whole of Egypt was politically united under a single pharaoh, who was considered a divine ruler. Egypt's geography kept them safe from external threats, and they found little interest in business outside their borders. THE