I just had a thought. What if the Eldar took primordial Necrontyr DNA and seeded it to create the Tau as an anti-Chaos weapon (and also to troll the Necrons)? I think I need to file that away with my other "Crazy yet plausible 40K Conspiracy Theories" in a box. Well there is only 'Custodes are Golden Men' one in there so far. The box makes it so much easier to bury!