Photosynthesis converts solar energy into forms that support life on earth. In the November 22 Science Raymond et al. present a comparative genomic approach to exploring the origins of photsynthesis (Science 2002, 298:1616-1620). Analysis of five different bacterial phyla capable of photosynthesis (cynaobacteria, proteobacteria, green sulfur bacteria, green filamentous bacteria, and Gram-positive heliobacteria), led them to conclude that horizontal gene flow played a major role in the evolution of bacterial phototrophs. Extensive, whole-genome BLAST comparisons were used to select nearly 200 sets of putative orthologs for phylogenetic analysis. The genomes look like mosaics, with even highly conserved genes showing very different evolutionary histories. Raymond et al. developed a method to identify 'photosynthesis-specific' genes and provide evidence for the importance of horizontal gene transfer in the evolution of phototrophic prokaryotes.