We have identified a large sample of spectroscopically-confirmed z=5 UV-luminous star forming galaxies. These are generally young (<100Myr) starbursts undergoing their first significant burst of star formation. Although the galaxies contain about 3*10^9 M_sun of stars each and only a few percent of the baryons in today's galaxies, they will eventually form the most massive z=0 galaxies. Galaxies at z>5 are often clustered in redshift, but not not so tightly spatially that they can interact on a timescale comparable to their star formation lifetimes. They are therefore the tip of an iceberg, small UV luminous regions marking out much larger masses of structure in the young universe. By searching for CO (2-1) emission we will complement our CO(1-0) observations, probing the much larger mass of dark baryons in the large-scale structure hosting these galaxies.