600-555 million years ago

worm with eye

Ancestral coelomates evolved a number of genes which vertebrates use in their eyes and ears. The eyes of primitive animals are homologous to those of vertebrates in that the common ancestor of coelomates possessed primitive eyes whose developmental mechanisms are shared across its modern descendants.

Invertebrate coelomates lack hair cells, although they possess homologs of the ion channels which hair cells utilize and even possess ciliate epithelial cells surrounded by a potassium rich environment (as are hair cells). Drosophila chordotonal organs share a number of characteristics with the vertebrate inner ear including importance of TGF, FGF, Hox, zinc finger, and Pax (and possibly Iroquois Hox) genes in its development (Fritzch, 2001).

Are the eyes of jellyfish, worms, and insects homologous to human eyes? The vertebrate eye did not evolve from the compound eye of insects, and so in that sense the answer must be no. However, there are a number of shared genetic mechanisms in both vertebrate and invertebrate eyes, indicating that the common ancestor of modern lineages possessed some sort of primitive eye in its head whose genetic mechanisms have been conserved in its descendants. One of the first genes involved in the development of the eye is the Optx2 homeobox gene (a member of the sine-oculis-Six family) which is expressed in the eye field which forms during gastrulation. It is expressed in both invertebrates and vertebrates in the early tissues which develop into eyes (Toy, 1998). Sine oculis is a gene which functions downstream of Pax-6 and requires eyes absent in Drosophila . Sine oculis is a homeobox gene in the family with Six genes which function in the development of eyes in vertebrates. Planarians also involve sine oculis in eye development and regeneration (Pineda, 2000; Furukawa, 1997). In addition to Pax6, other members of the Pax gene family function in the formation of eyes such as the PaxB gene of jellyfish and the Eye gone gene of Drosophila (Kozmik, 2005). A PaxB homolog was identified in sponges, although sponges lack both eyes and nervous tissue (Kozmik, 2005). DRx/Rx is a paired-like homeobox gene which is conserved in both protostomes such as Drosophila and vertebrates. Inappropriate expression of Rx in frogs results in ectopic retinal tissue. It is expressed in both fly and vertebrate brains at an early stage, before the expression of Pax-6/eyeless (Eggert, 1998).
A number of developmental genes are shared in eye formation between protostomes and deuterostomes such as Pax-6 (eyeless), sine oculis, and eyes absent (the latter two being downstream in the cascade controlled by eyelss) (Eggert, 1998). Tunicates express Pax genes in their eyes, as do vertebrates and fruit flies. Ectopic expression of Pax genes can cause the formation of additional eyes (Glardon, 1997).