  Earth-mass ``microhalos'' may be the first objects to virialize in the early
universe. Their ability to survive the hierarchical clustering process as
substructure in the larger halos that form subsequently has implications for
dark matter detection experiments. We present a large N-body simulation of
early substructure in a supersymmetric cold dark matter (SUSY-CDM) scenario
characterized by an exponential cutoff in the power spectrum at M_c=10^-6 Msun.
The simulation resolves a 0.014 Msun parent ``SUSY'' halo at z=75 with 14
million particles. On these scales the effective index of the power spectrum
approaches -3, and a range of mass scales collapse almost simultaneously.
Compared to a z=0 galaxy cluster substructure within our SUSY host is less
evident both in phase-space and in physical space, and it is less resistant
against tidal disruption. As the universe expands by a factor of 1.3, we find
that between 20 and 40 percent of well-resolved SUSY substructure is destroyed,
compared to only about 1 percent in the low-redshift cluster. Nevertheless SUSY
substructure is just as abundant as in z=0 galaxy clusters, i.e. the normalized
mass and circular velocity functions are very similar. The dark matter
self-annihilation gamma-ray luminosity from bound subhalos and other deviations
from a smooth spherical configuration is at least comparable to the
spherically-averaged signal in the SUSY host, and at least three times larger
than the spherically-averaged signal in the cluster host. Such components must
be taken into account when estimating the total cosmological extragalactic
gamma-ray annihilation background.(ABRIDGED)
