First we had the Indus Valley Civilization - a Bronze Age civilization - about 3300 BC. Then in 1500 BC approx. the Upanishads (Vedic texts or Vedanta, means the last chapters) were composed, containing the earliest of the central religious concepts of Hinduism, and Jainism - about the same time the prehistoric monument Stonehenge in England was completed. The major sacred works are the Rigveda, Yajurveda, Samaveda and Atharvaveda.
Of all Vedic literature, the Upanishads alone are widely known, and their central ideas are at the spiritual core of Hindus. The realization of "know that you are the Atman" is their message. The Bhagavad Gita and the Brahmasutra are some of the best known. The Upanishads is in our opinion one of the finest works of this age. Note that these works come from spiritual seekers who performed austerities for a long time and in full sincerity, to be receptive to the deepest Truths - which were distilled into the Vedas.. Thus we give a link to a free pdf translated by Swami Paramandanda (a disciple of Swami Vivekanada) in 1919 - by the Harvard Divinity School: https://www.vivekananda.net/PDFBooks/The_Upanishads.pdf