The photoreceptor genes arose froma single ancestral
gene (protogene). The rhodopsin–transducin
pair is found in invertebrates and is at
least 700 million years old. The blue receptor is
almost as old as rhodopsin, about 500 million
years. The separation into a receptor for green
and one for red must have occurred only about
30 million years ago, after the Old World and
New World apes separated, since man and the
Old World apes have three cone pigments
whereas NewWorld apes have two.
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