We have observed the UV upturn and its evolution in a large sample of cluster ETGs at 0 < z < 0.7. We show that the upturn is due to a population of hot/extreme horizontal branch stars and that the fraction and temperature of these stars increase as a function of galaxy luminosity (mass). We show that there is no significant evolution in the UV upturn out to z=0.55 but that it evolves rapidly to the red (i.e., the UV flux disappears) at z=0.7. This is only consistent with a stellar population (10% by mass) with enhanced He abundance (Y > 0.42): the timescales for evolution to the HB are such that the stellar population must form at z > 4. This suggests that ETGs are already very massive and have undergone considerable chemical evolution when the Universe was no more than 1 Gyr old.