  We report the first robust measurement of the Milky Way star formation
history using the imprint left on chemical abundances of long-lived stars. The
formation of the Galactic thick disc occurs during an intense star formation
phase between 9.0 (z~1.5) and 12.5 Gyr (z~4.5) ago and is followed by a dip (at
z~1.1) lasting about 1 Gyr. Our results imply that the thick disc is as massive
as the Milky Way's thin disc, suggesting a fundamental role of this component
in the genesis of our Galaxy, something that had been largely unrecognized.
This new picture implies that huge quantities of gas necessary to feed the
building of the thick disc must have been present at these epochs, in
contradiction with the long-term infall assumed by chemical evolution models in
the last two decades. These results allow us to fit the Milky Way within the
emerging features of the evolution of disc galaxies in the early Universe.
