In the book Xenology a human, heretical scientist performs dissections on a number of aliens with accompanying speculation and background information, among them a Tau Ethereal. Many of the comments on the organs and physiology of the Ethereal implies heavily that it has been "uplifted" or genetically altered from some kind of bovine creature. He also describes a diamond shaped organ in the middle of the forehead packed with pheromones, and speculates that this organ might be capable of exerting some measure of control over lesser castes of Tau. Further comments by a visiting inquisitor states unambiguously that the Ethereals control the lesser castes by pheromones, but it's worth noting that the inquisitor is not entirely sane by that point in the book.
This diamond shaped organ is revisited later when it's discovered to also appear on an entirely different species called the Q'Orl, who originate in the opposite end of the galaxy from the Tau, and also works as a form of chemical / pheromone based form of communication. A story is retold where the Eldar help the Q'Orl fight off chaos-infected humans a long time ago, and subsequently made off with a Q'Orl queen.
As you said, GW hasn't officially come out and declared where the Ethereals are from, and I'm not certain how canon Xenology is any more. The book spells it out pretty clearly that the Eldar are behind it all, and they do have a good reason for wanting to seed a chaos-resistant empire, but it's also written in a way that makes the stories and information... less than reliable for various reasons. As it stands there isn't, as far as I know, anything 100% reliable to answer your question with.