Still, one cosmic Truth holds in Hindu thought, and that is that all things are simply a part of a greater, whole One. In early Hindu belief, which still holds true, for nothing in Hinduism is ever discarded, this Universal whole was called Brahmam. All beings and things, from the gods and demons, through humans, on to the lowliest pebble on the beach, were and are part of this One. In later times, the neuter Brahmam became equated with the masculine Brahma, but the original idea is still very much a part of Hindu thought.
The history of Hindu mythology can be broken up into several different ages, all of which have contributed to the faith as a whole. The first is the pre-Vedic age, which goes back to the time of the early Indus valley civilizations of Harappa and Mohenjo-daro, which were established around 2400 BCE. These cities were destroyed by 1700.
Some scholers have suggested that the Indo-European invaders known as the Indo-Aryans came and conquered both much of India and Persia by about 1500 BCE. They brought with them new gods and hymns dedicated to them. These hymns came to be called collectively the Vedas. The Vedic age is when Hinduism proper begins. The Indo-Aryans became the lords of India, and their gods became the most important in the pantheon, but earlier gods were still revered; they were just given different roles. The Aryans also brought with them a distinct class structure, which included a priestly class, a warrior or ruling class, and the trade or merchant class. The native peoples who were subject to Aryan rule were incorporated into a fourth class. This is the basis for the caste system which still is very much a part of Indian life. By the end of the Vedic period, these castes were called, respectively: Brahmans, Kshatriyas, Vaisyas, and Sudras. The Vedic gods were led by Indra, the archetypical thunder god, and they got their strength from the drink Soma, a form of ambrosia.
From around 900 BCE to 500 BCE, as Aryan culture spread further into the sub-continent, Hinduism underwent some major changes. This period has been referred to as the Brahmanic Age, for it was during this time that the Brahmans and the Kshatriyas fought for supremacy over Indian life. New thought had been adopted, with the idea of the soul or atman becoming a major part of Hinduism and the transmigration of that soul becoming a foundation of the religion. It was during this time that the Brahman caste asserted that the gods need human priests to keep their power, and some of the rishis, or sages, became more powerful than the gods. Sacrifice became the chief form of worship. The major Vedic deities began to fall from their high positions and were slowly usurped by the cults of the three gods who came to dominate Hinduism: Brahma, Vishnu, and Shiva.
From 500 BCE to about 100 CE, the age of Buddhism and Jainism put Hinduism in decline. The Buddha's doctrine took India by storm, and the older religion almost was suppressed entirely. Hinduism still included its child into itself, however, and was able to survive the storm with new ideas. Sacrifice went out of favor, and influence by the ascetic worshipers of Jainism and Buddhism led to the composition of the Upanishads. It was also during this time that Vishnu and Shiva completed their eclipse of Indra and the other Vedic gods.
The next age was the Epic or Classical period, the time of the great Hindu epics the Ramayana and the Mahabharata. These great works were compiled into their present form during thins time, but their origins go back at least to Vedic times. The Puranas were also composed at this time. Finally, around 1000 CE we come to modern Hinduism, when the religion once again became the dominant faith on the sub-continent.