Visual systems are present in six phyla.  These visual systems are very different and are believed to have evolved independently.  From phylogenies based on morphological characteristics, taxonomists think that eyes have evolved more than 40 times.
 

Evolution has come up with two main alternatives to the problem of seeing.  The solution favoured by all insects is a compound eye with multiple lenses, which has evolved at least twice in the arthropods and three times elsewhere.  Camera eyes like our own are even more interesting.  Not only have they evolved independently at least seven times, but typically they are found in active, predatory and intelligent animals.  All vertebrates have them, but so too do octopuses and squids - mulluscs whose ancestry is very far removed from ours.

Simon Conway Morris, New Scientist Nov 16, 2002, pg 26.

 See also:    How could an eye evolve?

 

But the Pax6 regulatory gene (called Ey in flies) that controls development of visual systems is very similar in five of the six phyla that have visual systems.
 
 
 

Explanation 1.  Convergence - the five different phyla evolved nearly the same gene independently by chance.  This is impossibly improbable, as attested to by the statements below:
 

"Because Pax-6 is involved in the genetic control of eye morphogenesis in both mammals and insects, the traditional view that the vertebrate eye and the compound eye of insects evolved independently has to be reconsidered."

R. Quiring, U. Walldorf, U. Kloter, W. J. Gehring   Science 265, 1994, 785.

"the hypothesis that the eye of cephalopods has evolved by convergence with the vertebrate eye is challenged by our recent finding of Pax-6-related sequences in the squid Loligo vulgaris.

R. Quiring, U. Walldorf, U. Kloter, W. J. Gehring   Science 265, 1994, 788.

 

Explanation 2.  Common ancestor - the Pax6 gene that regulates eye development existed in a common ancestor before eyes had evolved, presumably serving another function.  Somehow five phyla independently adopted this same gene as a regulator and developed very different visual systems.  But what was a gene that regulates eye developement doing prior to the existence of visual systems, and if it had a different function what selective advantage was there for adopting it as a regulator for visual systems?