  We use accurate estimates of aluminium abundance provided as part of the
APOGEE Data Release 17 and Gaia Early Data Release 3 astrometry to select a
highly pure sample of stars with metallicity $-1.5\lesssim {\rm [Fe/H]}\lesssim
0.5$ born in-situ in the Milky Way proper. We show that the low-metallicity
([Fe/H]$\lesssim -1.3$) in-situ component that we dub Aurora is kinematically
hot with an approximately isotropic velocity ellipsoid and a modest net
rotation. Aurora stars exhibit large scatter in metallicity and in a number of
element abundance ratios. The median tangential velocity of the in-situ stars
increases sharply with increasing metallicity between [Fe/H]$=-1.3$ and $-0.9$,
the transition that we call the spin-up. The observed and theoretically
expected age-metallicity correlations imply that this increase reflects a rapid
formation of the Milky Way disk over $\approx 1-2$ Gyrs. The transformation of
the stellar kinematics as a function of [Fe/H] is accompanied by a qualitative
change in chemical abundances: the scatter drops sharply once the Galaxy builds
up a disk during later epochs corresponding to [Fe/H]$>-0.9$. Results of galaxy
formation models presented in this and other recent studies strongly indicate
that the trends observed in the Milky Way reflect generic processes during the
early evolution of progenitors of MW-sized galaxies: a period of chaotic
pre-disk evolution, when gas is accreted along cold narrow filaments and when
stars are born in irregular configurations, and subsequent rapid disk
formation. The latter signals formation of a stable hot gaseous halo around the
MW progenitor, which changes the mode of gas accretion and allows development
of coherently rotating disk.
