Astra Miliwotsit? You're in the Guard now son....
It's never been outright stated to the best of my knowledge, but it is heavily impled that the Eldar modified some Tau to create the Ethereals (who the Tau codex says basically came out of nowhere), which then led to the formation of the Tau Empire, and the sudden shift of the species from agrarian plains-dwellers to the super-advanced technology users we know them as today.
AUT TACE AUT LOQUERE MELIORA SILENTIO