  We use chemistry ([alpha/Fe] and [Fe/H]), main sequence turnoff ages, and
kinematics determined from H3 Survey spectroscopy and Gaia astrometry to
identify the birth of the Galactic disk. We separate in-situ and accreted stars
on the basis of angular momenta and eccentricities. The sequence of high-alpha
in-situ stars persists down to at least [Fe/H]=-2.5 and shows unexpected
non-monotonic behavior: with increasing metallicity the population first
declines in [alpha/Fe], then increases over the range -1.3<[Fe/H]<-0.7, and
then declines again at higher metallicities. The number of stars in the in-situ
population rapidly increases above [Fe/H]=-1. The average kinematics of these
stars are hot and independent of metallicity at [Fe/H]<-1 and then become
increasingly cold and disk-like at higher metallicities. The ages of the
in-situ, high-alpha stars are uniformly very old (13 Gyr) at [Fe/H]<-1.3, and
span a wider range (8-12 Gyr) at higher metallicities. Interpreting the
chemistry with a simple chemical evolution model suggests that the
non-monotonic behavior is due to a significant increase in star formation
efficiency, which began 13 Gyr ago. These results support a picture in which
the first 1 Gyr of the Galaxy was characterized by a "simmering phase" in which
the star formation efficiency was low and the kinematics had substantial
disorder with some net rotation. The disk then underwent a dramatic
transformation to a "boiling phase", in which the star formation efficiency
increased substantially, the kinematics became disk-like, and the number of
stars formed increased tenfold. We interpret this transformation as the birth
of the Galactic disk at z~4. The physical origin of this transformation is
unclear and does not seem to be reproduced in current galaxy formation models.
