We have carried out two extremely deep surveys with SPIRE, one of the two cameras on Herschel, at 250 mu m, close to the peak of the far-infrared background. We have used the results to investigate the evolution of the rest-frame 250-mu m luminosity function out to z = 2. We find evidence for strong evolution out to z similar or equal to 1 but evidence for at most weak evolution beyond this redshift. Our results suggest that a significant part of the stars and metals in the universe today were formed at z <= 1.4 in spiral galaxies.
