The CONSTANS (CO) gene plays a central role in the regulation of flowering time in Arabidopsis, and is a member of a family of 17 CO-like genes. CO and CO-like genes have been found in all flowering plants, but not in yeast and animals. To address the question of the origin of CO, we analysed this gene family in the moss Physcomitrella patens, a phylogenetically distant organism. Database searches in EST libraries that almost completely covered the Physcomitrella transcriptome, and Southern blotting, identified only three genes that had all of the hallmarks of CO. Further analysis demonstrated that these are most similar to CO-like genes AtCOL3/AtCOL4/AtCOL5, a group of Arabidopsis genes closely related to, but distinct from CO, suggesting that the CO branch of the AtCOL phylogeny does not exist in the Physcomitrella genome. Since 17 COL genes occur in Arabidopsis and only three closely related and two distantly related genes were found in Physcomitrella, the family of CO-like proteins appears to be smaller in Physcomitrella than in Arabidopsis, in agreement with observations made with other gene families. The data also indicate that CO-like genes must have existed in the common ancestor of bryophytes and flowering plants, and that CO originated in the group of CO-like genes represented by AtCOL3/AtCOL4/AtCOL5. Furthermore, expression of the three closely related Physcomitrella homologues is regulated by light, suggesting that the role of CO in flowering time control was probably derived from an ancestral function in light signal transduction.