The earliest manifestation of what we now call Hinduism seems to be the product of the melding of two religious and cultural influences: the Indus Valley Civilization that was located in what is today northwest India and eastern Pakistan and that dates to between 2500 and 1500 B.C.E.; and the Aryan culture and religion that arose between 1500 and 500 B.C.E. Although there has been considerable scholarly (and political) debate about the relative influence of these two cultures, it seems clear enough that Hinduism emerged out of a complex combination of elements of each of these religious cultures.
Relatively little is in fact known about the details of the religious world of the Indus Valley civilization. Based on archaeological remains, however, it seems that this was a religious world that was particularly focused on ritual bathing and animal sacrifice, elements that may be the source of later Hinduism's attention to the purifying qualities of water and the centrality of sacrifice. Furthermore, a great many female figurines have been discovered in the ruins of the cities that date to this period. These seem to have been goddesses, and may have been particularly associated with fertility rituals. Scholars have speculated that these figures are origins of the many goddesses who populate the vast Hindu pantheon. Male figures have also been found on stone seals. Some of these seals depict a seated figure surrounded by a variety of animals, including bulls. These images lead some scholars to label these "proto-Shiva" figures, since the great god Shiva is generally associated with animals (he is sometimes called "Pashupati," the Lord of the animals) and more particularly linked with the bull, which later becomes his special "vehicle."
In recent years, there has been a tremendous amount of debate about the influence of the Aryans. Part of the debate has been about who these people really were. Over one hundred years ago European scholars speculated that the Aryans were invaders who came from northeastern Europe, and were a warlike, highly mobile people engaged in herding and breeding animals who brought with them a new, "foreign" religion that supplanted the indigenous Indus Valley Civilization. Many scholars, both in India and in the West, have seen in this explanation—the "Aryan Invasion Thesis"—a western, colonialist agenda at work, one that wants to see all that is good in India as having its ultimate source in Europe. These scholars have argued that the Aryans were not in fact outsiders, and that they did not invade and supplant the Indus Valley Civilization, but instead blended with it. The generally held scholarly position is somewhere in the middle: that there was a migration of Aryan people and a diffusion of Aryan culture into South Asia, but not a single invasion (if there was an "invasion" at all).
Regardless of who the Aryans were or where they came from, it is clear that their religious world focused on ritual sacrifice, particularly sacrifice involving fire. They also worshipped a variety of gods, many of them linked to natural forces. These religious rituals and myths were eventually formulated as the Vedas, a genre of orally-transmitted texts that would come to form the foundation of Hinduism.