Two evolutionists have made a stunning admission: the human eye is not poorly engineered. This collapses a long-standing argument among critics of intelligent design. There are good reasons why vertebrate eyes have backward-pointing retinas, they say. In fact, human eyes may outperform the eyes of cephalopods, which have been held up as a smarter example of engineering design.
Our analysis of this major rethink will be divided into two parts. First, we will see why the design of the human eye, with its backward-pointing retina, makes sense. Second, we will see how the authors try to rescue Darwinism from this major rethink. Of special interest to this story is that one of the authors, Dan-Eric Nilsson, made a splash with Suzanne Pelger in 1994 with a graphic of eye evolution showing how a light-sensitive spot could evolve into a vertebrate eye in stages. Richard Dawkins made hay with that story. The episode required a lot of fact-checking to refute, as Jonathan Wells and David Berlinski remember.
Terms and Facts
A couple of terms and facts should be enunciated. Cephalopods (octopuses, squid, and cuttlefish) have everted retinas, with the photoreceptor cells pointing toward the light source. Vertebrates all have inverted retinas, with the photoreceptors pointed away from the light source. Some other invertebrates have either one arrangement or the other. Some animals, like zebrafish larvae, have no vitreous space between the lens and the retina. Humans exemplify most vertebrate arrangements with a fluid-filled vitreous humor between the lens and retina.
The new article by Tom Baden and Dan-Eric Nilsson appeared this month in Current Biology, under the title, “Is Our Retina Really Upside Down?” They do not argue that one form is better than the other, but rather, because of trade-offs owing to size, habitat, and behavior, what works for one animal may not be optimal for another.
But in general, it is not possible to say that either retinal orientation is superior to the other. It is the notions of right way or wrong way that fails. Our retina is not upside down, unless perhaps when we stand on our head.
Criticisms of the “Backward Retina” Disappear
Knowing that many of their evolutionary colleagues have criticized the inverted retina as a bad design, Baden and Nilsson elaborate on the criticism then state their thesis:
From an engineer’s perspective, these problems could be trivially averted if the retina were the other way round, with photoreceptors facing towards the centre of the eye. Accordingly, the human retina appears to be upside down. However, here we argue that things are perhaps not quite so black and white. Ranging from evolutionary history via neuronal economy to behaviour, there are in fact plenty of reasons why an inverted retinal design might be considered advantageous. [Emphasis added.]
For the remainder of the article, with 28 references, they consider advantages of the inverted retina, which humans share with all other vertebrates, and the everted retina shared by most invertebrates, with ample evolutionary storytelling thrown in. Here are specific criticisms of the inverted retina with their responses.
Blind spot. The inverted retina needs a place for bundling the nerves from the photoreceptors into a hole so they can join in the optic nerve to the brain. This so-called blind spot is “not all that bad,” they point out; it only occupies 1% of the visual field in humans and is filled in with data from the other eye. “Moreover, body movements can ensure suitable sampling of visual scenes despite this nuisance,” they say. “After all, when is the last time you have felt inconvenienced by your own blind spots?”
Optically compromised space. Surely the tangles of neural cells in front of the photoreceptors reduce optical quality, don’t they? Not really, say Baden and Nilsson, for several reasons:
Looking out through a layer of neural tissue may seem to be a serious drawback for vertebrate vision. Yet, vertebrates include birds of prey with the most acute vision of any animal, and even in general, vertebrate visual acuity is typically limited by the physics of light, and not by retinal imperfections. Likewise, photoreceptor cell bodies, which in vertebrate eyes are also in the way of the retinal image, do not seem to strongly limit visual acuity. Instead, in several lineages, which include species of fish, reptiles and birds, these cell bodies contain oil droplets that improve colour vision and/or clumps of mitochondria that not only provide energy but also help focus the light onto the photoreceptor outer segments.
They don’t specifically mention the Mueller cells that act as waveguides to the photoreceptors, but surely those are among the “surprising” ways that the “design challenges have been met” in the inverted retina.
Benefits of Inverted Retinas
The inverted retina also provides distinct advantages:
Preprocessing. One thing the everted retina cannot do as well is process the information before it gets to the brain. Baden and Nilsson spend some time discussing why this is so very beneficial.
To fully understand the merits of the inverted design, we need to consider how visual information is best processed. The highly correlated structure of natural light means that the vast majority of light patterns sampled by eyes are redundant. Using retinal processing, vertebrate eyes manage to discard much of this redundancy, which greatly reduces the amount of information that needs to be transmitted to the brain.This saves colossal amounts of energy and keeps the thickness of the optic nerve in check, which in turn aids eye movements.
For example, they say, a blue sky consists of mostly redundant information. The vertebrate eye “truly excels” at concentrating on new and unexpected information, like the shadow of a bird flying against the sky. Neurons that monitor portions of the visual field that have not changed can stay inactive, saving energy. What’s more, the vertebrate eye contains layers of specialized cells that preprocess the information sent to the brain, giving the eye “predictive coding” as reported elsewhere. They touch on that fact here:
The extensive local circuitry within the eye — enabled by two thick and densely interconnected synaptic layers, achieves an amazingly efficient, parallel representation of the visual scene. By the time the signal gets to the ganglion cells that form the optic nerve, spikes are mostly driven by the presence of the unexpected.
Useful space. The inverted retina uses to good advantage the vitreous space for placing the “extensive local circuitry” for the preprocessing cells. Squid eyes, with the photoreceptors smack against the optic lobe, lack that benefit. Here, Baden and Nilsson turn the tables on the “bad design” critics so eloquently one must read it in their own words:
Returning to our central narrative, the intraocular space of vertebrate eyes is an ideal location for such early processing, hinting that the vertebrate retina is in fact cleverly oriented the right way! … Taking larval zebrafish as the best studied example, the somata and axons of their ganglion cells are squished up right against the lens, while on the other end the outer segment of photoreceptors sits neatly inserted into the pigment epithelium that lines the eyeball. Clearly, in these smallest of perfectly functional vertebrate eyes, the inverted retina has allowed efficient use of every cubic micron of intraocular real estate. In contrast, now it is suddenly the cephalopod retina that appears to have an awkward orientation. … For tiny eyes, the everted design wastes extremely valuable space inside the eye whereas the inverted retinal design is a blessing. With this reasoning, cephalopods have an unfortunate retinal orientation and, contrary to the general notion, it is the vertebrate retina that is the right way up.
“Close to Perfect”
As a capstone to their argument, they conclude that “In terms of performance, vertebrate eyes come close to perfect.” This does not mean the poor octopus is the loser in this surprising upset. If scientists knew more about cephalopod eye performance within the animals’ own circumstances, a similar conclusion would probably be justified. “Both the inverted and the everted principles of retinal design have their advantages and their challenges, or shall we say ‘opportunities’.”
Surely with all this engineering and design talk, the authors are ready to jump the Darwin ship and join the ID community, right? One must never underestimate the stubbornness and storytelling ability of evolutionists. Next time we will look at how they explain all this engineering perfection in Darwinian terms.