600-555 million years ago
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).