  We present deep spectroscopy with the Keck telescope of eight galaxies in the
luminous X-ray cluster MS1054-03 at z=0.83. The data are combined with imaging
observations from the Hubble Space Telescope (HST). The spectroscopic data are
used to measure the internal kinematics of the galaxies, and the HST data to
measure their structural parameters. Six galaxies have early-type spectra, and
two have "E+A" spectra. The galaxies with early-type spectra define a tight
Fundamental Plane (FP) relation. The evolution of the mass-to-light ratio is
derived from the FP. The M/L ratio evolves as \Delta log M/L_B \propto -0.40 z
(Omega_m=0.3, Omega_Lambda=0). The observed evolution of the M/L ratio provides
a combined constraint on the formation redshift of the stars, the IMF, and
cosmological parameters. For a Salpeter IMF (x=2.35) we find that z_form>2.8
and Omega_m<0.86 with 95% confidence. The constraint on the formation redshift
is weaker if Omega_Lambda>0: z_form>1.7 if Omega_m=0.3 and Omega_Lambda=0.7. At
present the limiting factor in constraining z_form and Omega from the observed
luminosity evolution of early-type galaxies is the poor understanding of the
IMF. We find that if Omega_m=1 the IMF must be significantly steeper than the
Salpeter IMF (x>2.6).
