One word that defines Hinduism history is time immemorial.
It must be noted that the term ‘Hindu’ and ‘Hinduism’ is not found in any of the important ancient religious texts associated with Hinduism. The word ‘Hindu’ came into being with the arrival of Muslims invaders. They called the people who lived along the Sindhu River as ‘Hindu’ and the place ‘Hindustan.’ The term Hindu and Hinduism was first widely used by the British.
Most historians shy away from fixing a particular period mainly because their claims will be easily challenged by other scholars. Every theory regarding the origin of Hinduism has been challenged and it is an ongoing controversy.
The earliest evidence of Hinduism is found in the Indus Valley civilization (3300–1700 BC). Several figurines unearthed from here are 3-faced indicating them to be the forerunner of Hindu trinity and some quite similar to Lingas – the worshipping form of Lord Shiva.
But it must be understood here that the cultural level and style of living of the people of Indus Valley Civilization was of the highest kind and it cannot be attained overnight. They might have attained such high level through steady progress over the years. So it would be right to say the earliest seed of Hinduism was laid hundreds of years before Indus valley civilization flourished.