What elements of earlier belief systems do you see integrated into Hinduism? What were the external conditions...

One of the traditions that was incorporated into Hinduism was the Vedic religion of the Iron Age, which was practiced among the Indo-Aryans of northern India from about 1750 BCE to 500 BCE. Their rituals were based on the four Vedas (including the Vedic Samhitas and some of the older Upanishads). These texts became the founding texts of Hinduism. The Vedas, written in Sanskrit, include the Rigveda, composed of about 1,000 hymns for the priestly families...

One of the traditions that was incorporated into Hinduism was the Vedic religion of the Iron Age, which was practiced among the Indo-Aryans of northern India from about 1750 BCE to 500 BCE. Their rituals were based on the four Vedas (including the Vedic Samhitas and some of the older Upanishads). These texts became the founding texts of Hinduism. The Vedas, written in Sanskrit, include the Rigveda, composed of about 1,000 hymns for the priestly families who conserved the literature; the Yajuveda, which contains prose for certain rituals; the Samaveda, which contains verses from the Rigveda with music; and the Atharvaveda, which includes incantations and magic spells. The Vedic religion was a polytheistic religion that involved sacrifices to gods connected with nature.

During the later Vedic period, only Brahmins could carry out the rituals in the Vedas. The four varnas, or castes, of Hinduism--the Brahmans (priests), Kshatriyas (warriors), Vaishyas (merchants), and Shudras (servants)—developed during the Vedic Age. The Vedic religion ended around 500 BCE, when the formation period of Hinduism developed. 

Hinduism developed from a combination of the Vedic religion and the religious cultures of the indigenous inhabitants of India. The religion likely evolved following the Aryan invasion of the Indian subcontinent around 1500 BCE and the eventual merging of the Aryan culture (the Vedic religion) with the indigenous beliefs already existing in India. The Aryan invasion, which some experts still dispute, is thought by other experts to have occurred at the time when the Indus Valley Civilization declined after their water sources dried up. After the Aryans, originally nomads, settled in the Indus Valley, their culture began to spread. This period, referred to as the Hindu synthesis, brought Hindu beliefs to southern India. In addition, the spread of Hinduism was facilitated by the granting of land to Brahmans by local rulers and by the incorporation of non-Vedic gods into the religion. As the culture of Hinduism developed, along with cities, the religion became increasingly complex and philosophical.