All animals with eyes or the eyespots that have been studied so far share a gene in common. when the gene causes lack of eyes in fruit files, tiny eyes in mice, and missing irises (and other eye parts) in humans. the sequences in humans and mice is identical. there are so few sequences differences with fruit fly that the human/increases version can cause eye formation in eyeless fruit files, even though and last shared a common than 500 million years ago. pax-6 usually causes the production of a type of light-receptor pigment. in vertebrate eyes, though a different gene (the rh gene family) is responsible for the light-receptor pigments of the retina. the rh gene, like pax-6, is ancient. in marine ragworm, for example, the rh gene causes production of clock-opsin, which regulate the worm's biological clock. which of these most likely accounts for vertebrate vision? a) in animals more ancient than ragworms, the rh gene(s) coded for visual receptor pigments: in lineages more recent than ran ragworms rh has flip-flopped several times between producing biological clock opsins and visual receptor pigments. b) the pax-6 gene mutated to become the rh gene among early mammals. c) during vertebrate evolution, the rh for biological clock was co-opted as a gene for visual receptor pigments. d) pax-6 was lost from the mammalian genome and replaced by the rh gene much later.