Crossover quintessence and cosmological history of fundamental "constants"

Crossover quintessence predicts that the time variation of fundamental couplings is substantially faster at redshift $z\approx 2$ than over the past few billion years. This could reconcile the reported time variation of the fine structure constant from quasar absorption lines with severe bounds from archeo-nuclear physics and high precision tests of the equivalence principle. We present a model that is consistent with all present data on time varying couplings, tests of the equivalence principle and cosmology.