  The build-up of stellar mass in galaxies is the consequence of their past
star formation and merging histories. Here we report measurements of rest-frame
optical light and calculations of stellar mass at high redshift based on an
infrared-selected sample of galaxies from the Hubble Deep Field North. The
bright envelope of rest-frame B-band galaxy luminosities is similar from 0<z<3,
and the co-moving luminosity density is constant to within a factor of 3 over
that redshift range. However, galaxies at higher redshifts are bluer, and
stellar population modeling indicates that they had significantly lower
mass-to-light ratios than those of present-day L* galaxies. This leads to a
global stellar mass density, Omega_*(z), which rises with time from z=3 to the
present. This measurement essentially traces the integral of the cosmic star
formation history that has been the subject of previous investigations. 50-75%
of the present-day stellar mass density had formed by z ~ 1, but at z ~= 2.7 we
find only 3-14% of today's stars were present. This increase in Omega_* with
time is broadly consistent with observations of the evolving global star
formation rate once dust extinction is taken into account, but is steeper at
1<z<3 than predicted by some recent semi-analytic models of galaxy formation.
The observations appear to be inconsistent with scenarios in which the bulk of
stars in present-day galactic spheroids formed at z>>2.
