Scientists have been trying to decide for a long time when vision evolved in animals (you might say they have never lost "sight" of their goal). Therefore, this is a huge step forward in understanding when vision first developed in animals, and how today we are able to see.
The article in the Proceedings of the National Academy of Science (PNAS) is entitled "Metazoan opsin evolution reveals a simple route to animal vision".
It is the published work of researchers Roberto Feudaa, Sinead C. Hamiltona, James O. McInerneya, and Davide Pisania (from the School of Biological Sciences and School of Earth Sciences, University of Bristol, United Kingdom).
These Irish researchers are all from the Department of Biology, National University of Ireland Maynooth, Kildare, Ireland.
And, "We found that the Placozoa have opsins, and that the opsins share a common ancestor with the melatonin receptors."
Placozoa (scientific classification: Phylum: Placozoa; Kingdom: Animalia) is a basal form of invertebrate; the simplest in structure of all non-parasitic, multicellular animals.
The press release from the University of Bristol, "New study sheds light on how and when vision evolved" stated, "Using this information, the researchers developed a timeline with an opsin ancestor common to all groups appearing some 700 million years ago. This opsin was considered 'blind' yet underwent key genetic changes over the span of 11 million years that conveyed the ability to detect light."
Opsins are a group of light-sensitive proteins that are critical to the development of vision. They trap light inside of the eyes. Actually, 5 groups of opsins are involved in vision, helping to convert a photon of light into an electrochemical signal to the brain to create vision.
Dr. Pisania, one of the authors of the study, stated in the University of Bristol press release, "The great relevance of our study is that we traced the earliest origin of vision and we found that it originated only once in animals."
Pisania added, "This is an astonishing discovery because it implies that our study uncovered, in consequence, how and when vision evolved in humans."
The PNAS paper was published online, before the print issue, on October 29, 2012 (doi: 10.1073/pnas.1204609109).