  We present new results on the frequency distribution of projected HI column
densities f(N,X), total comoving covering fraction, and integrated mass
densities rho_HI of high redshift, HI `disks' from a survey of damped Lya
systems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the full
sample spanning z=2.2 to 5 [738 DLAs], f(N,X) is well fitted by a double
power-law with a break column density N_d = 10^(21.55 +/- 0.04) and
low/high-end exponents alpha = -2.00 +/- 0.05, -6.4^{+1.1}_{-1.6}. The shape of
f(N,X) is invariant during this redshift interval and also follows the
projected surface density distribution of present-day HI disks as inferred from
21cm observations. We conclude that HI gas has been distributed in a
self-similar fashion for the past 12Gyr. The normalization of f(N,X), in
contrast, decreases by a factor of two during the ~2Gyr interval from z=4 to
2.2 giving corresponding decreases in both the total covering fraction and
rho_HI. At z~2, these quantities match the present-day values suggesting no
evolution during the past ~10Gyr. We argue that the evolution at early times is
driven by `violent' processes that removes gas from nearly half the galaxies at
z~3 establishing the antecedants of current early-type galaxies. The perceived
constancy of rho_HI, meanwhile, implies that HI gas is a necessary but
insufficient pre-condition for star formation and that the global
star-formation rate is driven by the accretion and condensation of fresh gas
from the intergalactic medium.
