The Vedic Age (1500 - 500 B.C.) rose after the decline of the Indus Valley Civilization, a decline that started in 1900 B.C. and escalated by 1800 B.C. During the Vedic Age, Indo-European nomads, who called themselves Aryans, began migrating into India and, in the early period, settled first in Punjab; then, in the later period, the Aryans spread into eastern Afghanistan, the outskirts of Uttar Pradesh, and the Middle and Eastern Gangetic Valley. The Aryans were attracted to the fertile land fed by the river Kubha, and all five branches of the river Indus. Historians call the Aryan period of history the Vendic Age because the Aryans were responsible for authoring the Vedas sometime between 1800 B.C. and 600 B.C., a collection of religious writings believed by the Hindus to be the word of God. The early Vedic society was organized first by clans and later by tribes.
A clan consists of a group of people who believe they all stem from one "common, founding ancestor" and all "share a common social and political identity" (University of Oslo, "The Vedic Age, 1500 - 500 B.C."). Clans are patriarchal, and their lineages are traced through the male ancestry. To keep the genetics of a clan strong, members of one clan can only marry outside of the clan. The Aryan clan, called a vish, was ruled by a chief, called a raja.
Soon, multiple clans grouped together to form tribes, a tribe being called a jana. As the Aryan clans settled in more territories, it became necessary for the clans to fight the locals and for the rajas to protect the clans. As the clans conquered the locals, dark-skinned peoples the Aryans called dasus and dasyus, they socially and ethnically integrated with these peoples, and it became socially necessary for the clans to group people by social differences to form a system of hierarchy. This hierarchical system became their caste system and consisted of priests, called brahmins; rulers and warriors, called kshatriyas; wealthy farmers and merchants, called vaisyas; and peasants, called thesudras. Hence, soon, the Aryan tribe was structured by a caste system.
Many scholars believe that the early Rig Vedic society was not made up of a caste, or socio-economic class system, but instead determined political hierarchy by rank, the highest social status being given to the eldest son with the most "leadership qualities" (University of Oslo). However, as the Aryans became more agricultural, the need developed to enforce more control over territories, as well as over the tribes and clans residing in those territories. The need for more control led rajas to develop into the kingships we are now familiar with, which also led to the creation of the lower class divisions of the caste system.