  Evolution in the measured rest frame ultraviolet spectral slope and
ultraviolet to optical flux ratios indicate a rapid evolution in the dust
obscuration of galaxies during the first 3 billion years of cosmic time (z>4).
This evolution implies a change in the average interstellar medium properties,
but the measurements are systematically uncertain due to untested assumptions,
and the inability to measure heavily obscured regions of the galaxies. Previous
attempts to directly measure the interstellar medium in normal galaxies at
these redshifts have failed for a number of reasons with one notable exception.
Here we report measurements of the [CII] gas and dust emission in 9 typical
(~1-4L*) star-forming galaxies ~1 billon years after the big bang (z~5-6). We
find these galaxies have >12x less thermal emission compared with similar
systems ~2 billion years later, and enhanced [CII] emission relative to the
far-infrared continuum, confirming a strong evolution in the interstellar
medium properties in the early universe. The gas is distributed over scales of
1-8 kpc, and shows diverse dynamics within the sample. These results are
consistent with early galaxies having significantly less dust than typical
galaxies seen at z<3 and being comparable to local low-metallicity systems.
