Final answer:
Major ancient civilizations grew primarily due to the presence of rivers, which provided water for agriculture, transportation, and sustained economic and population growth, enabling the establishment of complex societies.
Step-by-step explanation:
The growth of the four major ancient civilizations was supported largely by rivers. Civilizations such as those in Mesopotamia, Egypt, the Indus Valley, and China all developed along major rivers which provided a stable water supply for agriculture, transportation routes, and facilitated trade. These river systems enabled sustained food production, which in turn supported increasing population numbers and the development of complex societies with division of labor, government structures, and cultural achievements such as monumental architecture and writing systems. While nomadic groups and animal domestication played roles in the identity and economy of these civilizations, it was the dependable water source and fertile land provided by rivers that were most crucial for their growth and stability.