Words like Hindu or Hinduism are anachronisms--convenient terms coined to suit various needs at different points in history. These terms do not exist in the natural Indian cultural lexicon, and nowhere in the scriptures is there any reference to 'Hindu' or 'Hinduism.' Hinduism does not have anyone founder and it does not have a Bible or a Koran to which controversies can be referred for resolution. Consequently, it does not require its adherents to accept any one idea. It is thus cultural, not creedal, with a history contemporaneous with the people with which it is associated.
also read Hinduism: Origin and Evolution as the early history
Writings we now categorize as Hindu scriptures include not just books relating to spirituality, but also secular pursuits such as science, medicine, and engineering. This is another reason why Hinduism defies classification as a religion per se. Further, it cannot be claimed to be essentially a school of metaphysics. Nor can it be described as 'otherworldly.' In fact, one could almost equate Hinduism with broad human civilization itself as it now exists. The Aryan Invasion Theory, once popular, has now been largely discredited. It cannot be assumed that Hinduism was the pagan faith of invaders belonging to a race called Aryans who imposed it on the Indian subcontinent. Rather, it was the common metafaith of people of various races, including Harappans.
Evidence that Hinduism must have existed even circa 10000 BCE. is available--the importance attached to the river Saraswati and the numerous references to it in the Vedas indicates that the Rig Veda was being composed well before 6500 BCE. The first vernal equinox recorded in the Rig Veda is that of the star Ashwini, which is now known to have occurred around 10000 BCE. Subhash Kak, a computer engineer and a reputed Indologist, 'decoded' the Rig Veda and found many advanced astronomical concepts within it. The technological sophistication required to even anticipate such concepts is unlikely to have been acquired by a nomadic people, as the Invasionists would like us to believe. In his book Gods, Sages and Kings, David Frawley provide compelling evidence to substantiate this claim.
also read Buddhism: Dharmachakra the oldest symbol's significance and information