  The photometric redshift distributions, spectral types, Sersic indices, and
sizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field
(UDF) are studied in order to understand the environment and nature of star
formation in the early Universe. Clumpy disk galaxies that are bright at short
wavelengths (restframe <5000 Angstroms) dominate the UDF out to z~5.5. Their
uniformity in V/V_max and co-moving volume density suggest they go even
further, spanning a total time more than an order of magnitude larger than
their instantaneous star formation times. They precede as well as accompany the
formation epoch of distant red galaxies and extreme red objects. Those
preceding could be the pre-merger objects that combined to make red spheroidal
types at z~2 to 3. Clumpy disks that do not undergo mergers are likely to
evolve into spirals. The morphology of clumpy disks, the size and separation of
the clumps, and the prevalence of this type of structure in the early Universe
suggests that most star formation occurs by self-gravitational collapse of disk
gas.
