but that is why the giant squid's case is intriguing... Squids are a mollusk and a invertebrate that have only connections with mammals form the most primitive of species. The giant squid lives in extremely deep water, and have developed eyes very similar to a mammals, with balls and eye-sockets even though they come from completely different lines in completely different circumstances, no other invertebrate species have such complex eyes. This may suggest that some evolutionary forms are more prevalent to exist then others, due to scientifically inconclusive reasons... but obviously just because it is not yet able to be explained right now, doesn't mean it won't... and there are a lot of strange tendency's that could happen strictly because of the gravity our planet is at and that we are carbon based.
This is not limited to Squids, other Cephalopods have similar eyes. You can see a primitive form of cephalopod eye development in the Nautilus, its eye is basically a pin hole camera, and is similar to other cephalopod eyes, just not as developed. Regardless, there are differences between cephalopod eyes and invertebrate eyes, such as Cephalopods do not have a cornea, and they do not have rods and cones. but rather structures unique to them that serve a similar purpose. This is an example of convergent evolution, just as a bats wing and a birds wing are very similar because they serve the same function.
Just because the function of the eye develops in one lineage, this does not mean that a similar structure could not arise elsewhere. This is especially true when the gene9s) that controls eye formation and structures is shared by creatures as diverse as humans, fruit flies, and cephalopods (notably Pax-6 - discovered quite early on in the advent of genetic research in fruit flies).A separate yet convergent development is also supported when you look at the embryonic development of a cephalopod eye versus a vertebrate eye. The genes that control the development of the eyes for both cephalopods and invertebrates seems to have come from a basal ancestor to both invertebrates and mollusks (a cephalopod, and consequently a squid are both mollusks).
To trace the evolutionary changes that are potentially responsible for camera eye formation, we also compared octopus-eye ESTs with the completed genome sequences of other organisms. We found that 1019 out of the 1052 genes had already existed at the common ancestor of bilateria, and 875 genes were conserved between humans and octopuses. It suggests that a larger number of conserved genes and their similar gene expression may be responsible for the convergent evolution of the camera eye.
Same genes, with the same basic blueprint inherited from an ancestral lineage that are then expressed in a very similar fashion, although there are enough differences to point that they developed completely separate from each other.
From the same paper:
Although the morphology of the ancestral eye cannot be inferred from this study, we were able to provide strong support for the hypothesis that these genes having had an important role in the function of camera eyes in both humans and octopuses were present in the last common ancestor of these two lineages. Taking this observation into account, we can reasonably contend that the convergent evolution of camera eyes is caused by the already-abundant presence of the commonly shared genes as the ancestral gene set and the remarkable similarity of expression profiles of their derived genes
Comparative Analysis of Gene Expression for Convergent Evolution of Camera Eye Between Octopus and Human
So in summation.. we do not know the specifics, but there are shared genes that controls the development of the eye in the separate lineages, and they stem from a common ancestor - but here I am quoting from the paper, I will let it provide the summation:
Our results indicate that most of the genes, including several gene pathways necessary for the evolution of the camera eye, might be shared between human and octopus lineages. Therefore, there is strong evidence that the evolutionary mechanisms for the camera eyes of humans and octopuses are subjected to similar gene expression profiles of the commonly conserved gene set, although the developmental processes of the human and octopus eyes are a bit different.
Thanks for leading me to some fascinating information Celticwar. I am likely to spend some time over the next couple of days delving deeper into this, it touches on an area I have more than a passing interest in (not specifically cephalopods, I have had limited exposure to them and have not studied them all that much, but rather many of their marine invertebrate cousins). - This is not a damnation of evolution in the slightest however.
edit.. somehow this got chopped - a picture to show how dissimilar embryonic eye development is between cephalopods and vertebrates: