  Though observationally rare, damped Lya absorption systems dominate the mass
density of neutral gas in the Universe. Eleven high redshift damped Lya systems
covering 2.8<z<4.4 were discovered in 26 QSOs from the APM z>4 QSO Survey,
extending these absorption system surveys to the highest redshifts currently
possible. Combining our new data set with previous surveys we find that the
cosmological mass density in neutral gas, omega_g, does not rise as steeply
prior to z~2 as indicated by previous studies. There is evidence in the
observed omega_g for a flattening at z~2 and a possible turnover at z~3. When
combined with the decline at z>3.5 in number density per unit redshift of
damped systems with column densities log N(HI)>21 atoms cm^-2, these results
point to an epoch at z>3 prior to which the highest column density damped
systems are still forming. We find that over the redshift range 2<z<4 the total
mass in neutral gas is marginally comparable with the total visible mass in
stars in present day galaxies. However, if one considers the total mass visible
in stellar disks alone, ie excluding galactic bulges, the two values are
comparable. We are observing a mass of neutral gas comparable to the mass of
visible disk stars. Lanzetta, Wolfe & Turnshek (1995) found that omega_g(z~3.5)
was twice omega_g(z~2), implying a much larger amount of star formation must
have taken place between z=3.5 and z=2 than is indicated by metallicity
studies. This created a `cosmic G-dwarf problem'. The more gradual evolution of
omega_g we find alleviates this. These results have profound implications for
theories of galaxy formation.
