The Japanese like many ancient cultures used word of mouth to pass their history and practices down from generation to generation. Unfortunately this led to a great deal of knowledge and wisdom being watered down and lost. Many people involved with Reiki believe that the techniques we use today for healing were first used in India by Buddha and later by Jesus.
Others look back even further to the civilisations of Mu and Atlantis for the birth and development of Reiki. Of course without written proof we can only speculate how humanity learnt to harness and develop the universal life force.