Shamans first appeared early in Human history while mankind still largely consisted of hunter-gatherers. These beings established a special relationship with the Warp, sensing its rhythm as it moved through all living things. Shamans were benevolent figures, using their powers to benefit their tribes. When a Shaman died, he was able to reincarnate into a new form and thus played a permanent role in their societies.
However as humanity began to develop away from a harmonious hunter-gatherer relationship with nature and towards real civilization, the Warp began to change. Power, ambition, greed, and lust took hold of the human race and this became reflected in the Warp, where sinister beings began to develop. To their horror, the Shamans discovered that not only were they losing their ability to tap into the Warp, but upon death their souls were often devoured by the new malignant warp entities instead of being reincarnated. Each subsequent reincarnation became more and more difficult, even for the most talented Shamans.
Terrified for the future of their race, all the Shamans of ancient Earth gathered in one place for the most important meeting in human history. The debate is said to have lasted centuries, after which they came to a consensus that they, along with humanity, were ultimately doomed and would soon fall prey to the growing psychic entities of the Warp. Thus they concocted a plan to create the "New Man", a being that would protect humanity from the malignancy of the Warp. The Shamans committed mass ritual suicide and as they died pooled their combined energies, reincarnating into a single body. Within a year this soul found its human host, giving birth to the being who would become the Emperor of Mankind. Upon his maturity, the Emperor began to remember the countless past lives of the Shamans who had created him.