The Tau are a native species of T'au, their homeworld on the eastern edge of the galaxy. The only interesting thing about their early development is that the Tau race as a whole evolved very rapidly into 4 distinct sub-species to better survive in the area of the planet they inhabited.
These subspecies eventually fought a massive war for dominance of T'au when they were technologically around the time of the Renaissance. The Ethereal Tau, a "Fifth" subspecies, then appeared and brokered a peace between the factions, and unified them under a single banner. The factions would become the four principle castes of the Tau: Fire (soldiers), Earth (manufacturers, laborers and scientists), Air (Messengers, Space captains and pilots), and Water (Diplomats). The Ethereal fifth caste were to be the leaders. The rest is history.
OT: For the Greater Good!