The two regimes of the cosmic sSFR evolution are due to spheroids and
  discs
  This paper aims at explaining the two phases in the observed specific star
formation rate (sSFR), namely the high (>3/Gyr) values at z>2 and the smooth
decrease since z=2. In order to do this, we compare to observations the
specific star formation rate evolution predicted by well calibrated models of
chemical evolution for elliptical and spiral galaxies, using the additional
constraints on the mean stellar ages of these galaxies (at a given mass). We
can conclude that the two phases of the sSFR evolution across cosmic time are
due to different populations of galaxies. At z>2 the contribution comes from
spheroids: the progenitors of present-day massive ellipticals (which feature
the highest sSFR) as well as halos and bulges in spirals (which contribute with
average and lower-than-average sSFR). In each single galaxy the sSFR decreases
rapidly and the star formation stops in <1 Gyr. However the combination of
different generations of ellipticals in formation might result in an apparent
lack of strong evolution of the sSFR (averaged over a population) at high
redshift. The z<2 decrease is due to the slow evolution of the gas fraction in
discs, modulated by the gas accretion history and regulated by the Schmidt law.
The Milky Way makes no exception to this behaviour.
