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We present new results on the frequency distribution of projected HI columndensities f(N,X), total comoving covering fraction, and integrated massdensities rho_HI of high redshift, HI `disks' from a survey of damped Lyasystems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the fullsample spanning z=2.2 to 5 [738 DLAs], f(N,X) is well fitted by a doublepower-law with a break column density N_d = 10^(21.55 +/- 0.04) andlow/high-end exponents alpha = -2.00 +/- 0.05, -6.4^{+1.1}_{-1.6}. The shape off(N,X) is invariant during this redshift interval and also follows theprojected surface density distribution of present-day HI disks as inferred from21cm observations. We conclude that HI gas has been distributed in aself-similar fashion for the past 12Gyr. The normalization of f(N,X), incontrast, decreases by a factor of two during the ~2Gyr interval from z=4 to2.2 giving corresponding decreases in both the total covering fraction andrho_HI. At z~2, these quantities match the present-day values suggesting noevolution during the past ~10Gyr. We argue that the evolution at early times isdriven by `violent' processes that removes gas from nearly half the galaxies atz~3 establishing the antecedants of current early-type galaxies. The perceivedconstancy of rho_HI, meanwhile, implies that HI gas is a necessary butinsufficient pre-condition for star formation and that the globalstar-formation rate is driven by the accretion and condensation of fresh gasfrom the intergalactic medium.

On the (Non)Evolution of HI Disks over Cosmic Time