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Using the DR12 public release of APOGEE data, we show that thin and thickdisk separate very well in the space defined by [$\alpha$/Fe], [Fe/H] and[C/N]. Thick disk giants have both higher [C/N] and higher [$\alpha$/Fe] thando thin disk stars with similar [Fe/H]. We deduce that the thick disk iscomposed of lower mass stars than the thin disk. Considering the fact that at agiven metallicity there is a one-to-one relation between stellar mass and age,we are then able to infer the chronology of disk formation. Both the thick andthe thin disks - defined by [$\alpha$/Fe] -- converge in their dependance on[C/N] and [C+N/Fe] at [Fe/H]$\approx$-0.7. We conclude that 1) the majority ofthick disk stars formed earlier than did the thin disk stars 2) the formationhistories of the thin and thick disks diverged early on, even when the [Fe/H]abundances are similar 3) that the star formation rate in the thin disk hasbeen lower than in the thick disk, at all metallicities. Although these generalconclusions remain robust, we also show that current stellar evolution modelscannot reproduce the observed C/N ratios for thick disk stars. Unexpectedly,reduced or inhibited canonical extra-mixing is very common in field stars.While subject to abundance calibration zeropoint uncertainties, this implies astrong dependence of non canonical extra-mixing along the red giant branch onthe initial composition of the star and in particular on the $\alpha$ elementalabundance.

Carbon, nitrogen and $\alpha$-element abundances determine the formation sequence of the Galactic thick and thin disks