Evolution of the Eye
The evolution of the eye is a popular topic in the creation-evolution debate, perhaps because it is so easy for creationists to quote-mine Darwin on the subject - and credit to Answer in Genesis for acknowledging what Darwin actually said.
Here is the full quote, from The Origin of Species:
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of Spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certain the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, should not be considered as subversive of the theory.
The reality is that now, more than a century and a half later, we actually have a huge amount of data about eyes have evolved, and the great thing about science is that it provides a narrative that explains what we see so well, despite the many peculiarities of what we have found.
Some estimates say the eye has evolved independently over fifty times (eg here), but it kind of depends on where you start from. Recent genetic studies have shown that multicellular life developed opsins - light sensitive proteins - pretty earlier, and all (or most?) eyes evolved from that starting point.
Of course, they evolved in some very different ways. And some that look superficially similar are actually quite different.
Vertebrate eye verses cephalopod eye
In the evolutionary scenario, the first cephalopod happened to evolve an eye that was wired one way, and so every cephalopod since then has been stuck with the design. Meanwhile the first vertebrate happened to evolve an eye that was reverse-wired, and so every vertebrate since then has been stuck with that design, for good or bad.
How can creationism explain that? Why would the designer give inferior eyes to some of his creations? Why give all vertebrates reverse-wired eyes? Are they going to pretend that the reverse-wired eyes is superior for eagles AND for rabbits AND for fish AND etc.? And if they do, why did the creator give inferior eyes to cephalopods?
Either or the other is inferior, so the creator must have chosen an inferior in one instance.
It is also worth noting some other big differences in these eyes. The cephalopod eyes form as invaginations of the body surface, rather than outgrowths of the brain, and consequently the cornea lies over the top of the eye as opposed being a structural part of the eye.
Also, the cephalopod eye focuses by moving the lens (like a telescope), while the vertebrate eye focuses by changing the shape of the lens.
In the evolutionary scenario, these things were effectively written in stone once they were working. Anything else would lead to an intermediate species with reduced eye sight, and evolution cannot plan for the future.
However, if creationism was true, it would not be so neat. There would be no reason different "kinds" of vertebrates have all three of the features of a vertebrate eyes, while all cephalopods have all three of the features of cephalod eyes. We would expect all animals to have one type of eye because that is best, or we would expect each "kind" to have the best for its lifestyle. And that means that all animals with a certain lifestyle would have identical eyes. All bottom-dwelling scavengers would have the same eyes perfectly designed for bottom-dwelling scavengers. All night-time predators would have the same eyes perfectly designed for night-time predators.
And we do not see that.
There is a great article in Nature here.
Our Third Eye
Humans actually - kind of - have a third eye. We just cannot see with it.
A lot of vertebrates have an actual third eye - sharks, some bony fish, frogs, many reptiles, though not birds, mammals, turtles or crocodilians. It is on the top of the head, and as eyes go is very basic, but it serves an important role is regulating the day/night cycle of these animals.
In humans, the pineal gland does the same job, and is very closely related. However, it is buried deep in the brain, and takes its cues from the retina of our normal eyes.
Let us think about this from an evolutionary perspective. Our ancient ancestors were likely something like lampreys, feeling at the bottom of the sea or river. They evolved two normal eyes for regular vision, but a further two eyes on top of the head, one behind the other (assuming modern lampreys are the same) that might give warning of predators swimming above, the pineal and parapineal organs.
These top eyes are very different in "design" to the normal eyes, even down to not having rods and cones. Why would a creator do that?
The pineal organ in lampreys over time lost its visual acuity, and instead developed a secretory function as evolve went through fish, amphibian to mammal.
This is an area of active research - so utterly unlike creation science - and you can read more here and here.
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