I will highlight what we have learned from the largest 850um survey to date, the SCUBA-2 Cosmology Legacy Survey, which for the first time allowed us to obtain a significant sample of dusty galaxies at z=4-6. Using the sample of ~2000 bright dusty star forming galaxies I will show that they form the high-mass extension of the 'main-sequence' (they are not 'starburst' outliers), and that they contribute substantially to the star-formation and mass budget of the Universe. I will also show that a significant fraction of all massive galaxies is in fact dust rich. Finally, I will discuss how huge amounts of dust in these sources could have been formed at high redshifts.