LAUSR

Significance Since the time of Charles Darwin, explaining the stepwise evolution of the eye has been a challenge. Here, we describe the essential contribution of bacteria to the evolution of the vertebrate eye, via interdomain horizontal gene transfer (iHGT), of a bacterial gene that gave rise to the vertebrate-specific interphotoreceptor retinoid-binding protein (IRBP). We demonstrate that IRBP, a highly conserved and essential retinoid shuttling protein, arose from a bacterial gene that was acquired, duplicated, and neofunctionalized coincident with the development of the vertebrate-type eye >500 Mya. Importantly, our findings provide a path by which complex structures like the vertebrate eye can evolve: not just by tinkering with existing genetic material, but also by acquiring and functionally integrating foreign genes.