  To study the dust obscured phase of the galaxy evolution during the peak of
the Star Formation Rate (SFR) and the Black Hole Accretion Rate (BHAR) density
functions ($z = 1 - 4$), rest frame mid-to-far infrared (IR) spectroscopy is
needed. At these frequencies, dust extinction is at its minimum and a variety
of atomic and molecular transitions, tracing most astrophysical domains, occur.
The future IR space telescope mission, SPICA, fully redesigned with its $2.5\,
\rm{m}$ mirror cooled down to $T < 8\, \rm{K}$, will be able to perform such
observations. With SPICA, we will: 1) obtain a direct spectroscopic measurement
of the SFR and of the BHAR histories, 2) measure the evolution of metals and
dust to establish the matter cycle in galaxies, 3) uncover the feedback and
feeding mechanisms in large samples of distant galaxies, either AGN- or
starburst-dominated, reaching lookback times of nearly 12 Gyr. SPICA large-area
deep surveys will provide low-resolution, mid-IR spectra and continuum fluxes
for unbiased samples of tens of thousands of galaxies, and even the potential
to uncover the youngest, most luminous galaxies in the first few hundred
million years. In this talk a brief review of the scientific preparatory work
that has been done in extragalactic astronomy by the SPICA Collaboration will
be given.
