1 and type 2 opsins respectively. Both types are called opsins, because at one time it was thought that they were related: Both are seven-transmembrane receptors and bind covalently retinal as chromophore, which turns them into photoreceptors sensing light. However, both types are not related on the sequence level. In fact, the sequence identity between animal and mirobial opsins is no greater than could be accounted for by random chance. However, in recent years new methods have been developed specific to deep phylogeny. As a result, several studies have found evidence of a possible phylogenetic relationship between the two. However, this does not necessarily mean that the last common ancestor of microbial and animal opsins was itself light sensitive: All animal opsins arose (by gene duplication and divergence) late in the history of the large G-protein coupled receptor (GPCR) gene family, which itself arose after the divergence of plants, fungi, choanflagellates and sponges from the earliest animals. The retinal chromophore is found solely in the opsin branch of this large gene family, meaning its occurrence elsewhere represents convergent evolution, not homology. Microbial rhodopsins are, by sequence, very different from any of the GPCR families. According to one hypothesis, both microbial and animal opsins belong to the transporter-opsin-G protein-coupled receptor (TOG) superfamily, a proposed clade that includes G protein-coupled receptor (GPCR), Ion-translocating microbial rhodopsin (MR), and seven others. Most microbial opsins are ion channels or pumps instead of proper receptors and do not bind to a G protein. Microbal opsins are found in all three domains of life: Archaea, Bacteria, and Eukaryota. In Eukaryota, microbial opsins are found mainly in unicellular organisms such as green algae, and in fungi. In most complex multicellular eukaryotes, microbial opsins have been replaced with other light-sensitive molecules such as cryptochrome and phytochrome in plants, and animal opsins in animals. Microbial opsins are often known by the rhodopsin form of the molecule, i.e., rhodopsin (in the broad sense) = opsin + chromophore. Among the many kinds of microbial opsins are the proton pumps bacteriorhodopsin (BR) and xanthorhodopsin (xR), the chloride pump halorhodopsin (HR), the photosensors sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII), as well as proteorhodopsin (PR), Neurospora opsin I (NOPI), Chlamydomonas sensory rhodopsins A (CSRA), Chlamydomonas sensory rhodopsins B (CSRB), channelrhodopsin (ChR), and archaerhodopsin (Arch). Several microbal opsins, such as proteo- and bacteriorhodopsin, are used by various bacterial groups to harvest energy from