  We present the first results obtained from the identification of ~30,000
sources in the 24mic observations of the COSMOS field at S24>~80muJy. Using
accurate photo-z's and extrapolations of the counts at faint fluxes we resolve
with unprecedented detail the build-up of the mid-IR background across cosmic
ages. We find that ~50% and ~80% of the 24mic background intensity originate
from galaxies at z<~1 and z<~2 respectively, supporting the scenario where
highly obscured sources at very high redshifts (z>~2) contribute only
marginally to the CIB. Assuming flux-limited selections at optical wavelengths,
we also find that the fraction of i-band sources with 24mic detection strongly
increases up to z~2 as a consequence of the rapid evolution that star-forming
galaxies have undergone with lookback time. Nonetheless this rising trend shows
a clear break at z~1.3, probably due to k-correction effects implied by the
complexity of SEDs in the mid-IR. Finally, we compare our results with the
predictions from different models of galaxy formation. We note that
semi-analytical formalisms fail to reproduce the redshift distributions
observed at 24mic. Furthermore the simulated galaxies exhibit R-K colors much
bluer than observed and the predicted K-band fluxes are systematically
underestimated at z>0.5. Unless these discrepancies result from an incorrect
treatment of extinction they may reflect an underestimate of the predicted
density of high-z massive sources with strong star formation, which would point
to more fundamental processes that are still not fully controlled in the
simulations. The most recent backward evolution scenarios reproduce reasonably
well the flux/redshift distribution of 24mic sources up to z~3, although none
of them is able to exactly match our results at all redshifts. [Abridged]
