  Present-day clusters are massive halos containing mostly quiescent galaxies,
while distant protoclusters are extended structures containing numerous
star-forming galaxies. We investigate the implications of this fundamental
change in a cosmological context using a set of N-body simulations and
semi-analytic models. We find that the fraction of the cosmic volume occupied
by all (proto)clusters increases by nearly three orders of magnitude from z=0
to z=7. We show that (proto)cluster galaxies are an important, and even
dominant population at high redshift, as their expected contribution to the
cosmic star-formation rate density rises (from 1% at z=0) to 20% at z=2 and 50%
at z=10. Protoclusters thus provide a significant fraction of the cosmic
ionizing photons, and may have been crucial in driving the timing and topology
of cosmic reionization. Internally, the average history of cluster formation
can be described by three distinct phases: at z~10-5, galaxy growth in
protoclusters proceeded in an inside-out manner, with centrally dominant halos
that are among the most active regions in the Universe; at z~5-1.5, rapid star
formation occurred within the entire 10-20 Mpc structures, forming most of
their present-day stellar mass; at z<~1.5, violent gravitational collapse drove
these stellar contents into single cluster halos, largely erasing the details
of cluster galaxy formation due to relaxation and virialization. Our results
motivate observations of distant protoclusters in order to understand the
rapid, extended stellar growth during Cosmic Noon, and their connection to
reionization during Cosmic Dawn.
