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I'm working on a simple computational model of Primate eye: as we know, it presents a central part, the fovea, that is very rich of receptors in comparison with eye periphery. I'm wondering if other species eye, like octopus, present a similar structure: a central fovea. Thanks
Yes, they do have an area of densely packed receptors, although not point-like as the human fovea.
Talbot and Marshall (2011) performed a retinal topography analysis in three species of cephalopods: a cuttlefish, a squid and an octopus.
According to the authors:
It was found that all species possessed an increase in photoreceptor density in a horizontal streak approximately placed at the position of a potential horizon in the habitat.
Here is an image from the same paper:
The retinal topography of each of the three species in this study: (a) S. plangon and (b) O. cyanea show a prominent, horizontally orientated band of increased photoreceptor density across the horizontal equator of the retina; (c) S. lessoniana shows a more centrally positioned area of increased photoreceptor density.
Source: Talbot, C. and Marshall, J. (2011). The retinal topography of three species of coleoid cephalopod: significance for perception of polarized light. Philosophical Transactions of the Royal Society B: Biological Sciences, 366(1565), pp.724-733.
One major argument for the existence of a creator is the “argument from design,” a conclusion that means the design existing in creation proves the existence of an intelligent designer. Darwinists try to disprove the argument from design by providing examples of what they claim are poor design to argue that the living world is not designed but is the result of blind, natural, and impersonal forces. This view is called the blind watchmaker thesis by Oxford professor Richard Dawkins (1986). The blind watchmaker is in reference to William Paley who used the illustration of that, just as a watch proves a watchmaker, so too the natural world proves an intelligent creator.
Top 5 Mammals with Magnificent Eyesight
Like birds, mammals in the animal kingdom, too, have a splendid vision. In this section, we will explore some of the best mammals with superb eyesight.
1. Cheetah – best vision on the run
Cats’ eyes are some of the most evolved in the animal kingdom.
Cheetahs, the fastest mammals on Earth, are also known for having a vision of the best quality. There are several aspects to cheetah vision:
- Due to the location of their eyes, they have binocular vision.
- Cheetahs can spot prey that is located up to 5 km away.
- To support the prey during the chase, cheetahs have additional structures in their vestibular regions that help them hold their head straight and keep the focus.
- Cheetah’s tear lines absorb sunrays and protect them from being blinded by bright light.
- One reason that cheetah has the best vision in the feline family is that these big cats have the highest density of photoreceptor cells in their retinas than their other relatives.
- Unlike their other cat relatives, cheetahs have poor night vision, as they are daytime hunters.
2. Domestic goat – best panoramic vision
Virtually everybody has had a goat encounter at least once in their life. And many were probably scared by the goats` eyes at first.
Indeed, the goat’s visual system is unique:
- Unlike the eyes of most mammals, their pupils are horizontal and rectangular.
- The retinas of goats are shaped as rectangles, too.
- Due to this unique shape, goats can have a panoramic vision – their field of vision can reach up to 320-340 degrees.
- Goats also can see at night due to a particular layer in the eyes called tapetum lucidum. This structure reflects the light that comes into the eyes back, “lighting up” the field.
- To see better while grazing, the goat’s eyes can rotate and control their surroundings when they are vulnerable.
- This panoramic vision adaptation comes at a price. The goats have limited color vision. They have only two types of cones in their retinas and decipher only a limited number of colors, including violet, blue, yellow, orange, and green. They cannot really interpret red color.
- The goats have also lost the depth of vision.
3. Bornean Tarsier – most prominent eyes among mammals
The tarsier is a small primate. It looks a bit similar to lemurs and is active at night. A tarsier is easily recognizable by its large, round eyes.
This feature does not exist just for cuteness – this is an essential adaptation for its nocturnal lifestyle:
- Unlike most nocturnal animals, tarsiers do not have the specialized tapetum lucidum layer that helps other animals like owls, cats, and goats see at night.
- Instead, in the course of evolution, the tarsiers have developed the most enormous eyes among mammals.
- If humans had eyes of tarsier’s proportions, they would be the size of large grapefruits.
- Tarsiers also have an exceptionally high density of photoreceptors in the retina – about 2.5 times more than humans.
- These adaptations allow tarsiers to effectively hunt insects and small reptiles at night.
4. Arctic Reindeer – best visual adaptations for life in polar regions
As arctic reindeer tend to live in arctic and subarctic regions, they face some unique vision problems. They live in areas covered with snow for prolonged periods, and snow is highly reflective.
Also, regions close to the North Pole have prolonged periods of darkness – polar nights lasting for several months. This means that deer need to orient themselves in semi-darkness always. Because of this, deer have two adaptations:
- Their tapetum lucidum, the reflective structure typical for mammals active at night, changes color in winter. This way, the deer have a golden color in summer and blue eye color in winter.
- The changes in the tapetum lucidum in winter make the eyes more sensitive to light in this period.
- Reindeer eyes can react to UV light as well as standard colors. It is a useful adaptation, as many predators may be undetectable in regular light because of their white fur.
- Fortunately, fur also has a UV signature, and this can be detected by deer eyes.
- As UV light can potentially damage the eyes, reindeer also have protection mechanisms in place that allow them to view ultraviolet light waves relatively safely.
5. Asian short-clawed Otter – amphibious vision
An otter is a small, sleek mammal that spends a lot of time in the water, fishing for food.
Their vision reflects that:
- Otters have quality vision both on land and in the water, though it is still better.
- The otters’ amphibious vision can be possible due to the specialized focusing mechanism.
- The otters have muscles that change the cornea’s shape, adjusting for the different behavior of light in the water and in the air.
- The otters also have color vision, though they perceive a limited number of colors.
The number of contigs and unique genes is larger (almost triple) in Nautilus than in squid. One possibility for the higher number of contigs in Nautilus is that the coverage of RNA-seq reads is too small to construct full-length genes for Nautilus and resulted in gapped fragments from the same gene. As the total reads of Nautilus and squid RNA-seq are almost equal, difference of numbers of contigs could come from the insufficient coverage of RNA-seq. To assess this problem, we utilized the two following approaches. First, if a small number of genes are highly expressed and occupy sequence reads, low-expression genes tend to be missed in RNA-seq data. We, therefore, checked the distribution of gene expression frequencies for each species. We counted "Fragments Per Kilobase of transcript per Million mapped reads (FPKM)" 15 , for each gene and drew a distribution graph (fig. S2). The average FPKM for Nautilus is higher than that for squid and the mean FPKM for Nautilus is smaller than that of squid. This result indicates that the proportion of highly expressed genes is larger in Nautilus and RNA-seq coverage is slightly better in squid. Second, to assess the influence of RNA-seq coverage to find lowly expressed but important genes, such as transcription factors, we performed the following tests. We searched for EFTFs that are essential for eye development in vertebrates. We searched for otx2, tbx3, pax6 and lhx2 in our RNA-seq data, as these genes are already known to be involved in eye development in molluscs. As a result, all genes were found to be expressed with a FPKM of 1.7
2.5. These FPKMs do not differ between Nautilus and squid. Then, we assembled contigs using 1/4, 1/2, 3/4 random data sets of sequence reads and counted the number of unique homologous genes in the human eye EST database. As shown in fig. S3, the number of unique homologs obtained at 3/4 data is equivalent to that at the complete data set, indicating that the variation in genes is almost saturated at the 3/4 dataset level. In conclusion, despite differences in RNA-seq coverage, our data can be used to detect lowly expressed genes in both species.
From our results, downstream genes and networks of the Pax6/Six3 complex appear to have been lost in Nautilus due to the loss of six3, resulting in the inactivation of the lens formation process during Nautilus evolution. Thus our data support the first scenario presented in the introduction that most likely the Nautilus lineage lost its lens and cornea and that its pinhole eye might have evolved from a camera-type eye by deregulation of a particular regulatory network, in this case the six3/6 one, which is well conserved from the common ancestor of cephalopods and vertebrates. Furthermore our approach and results strongly argue that whole transcriptome studies are quite useful to delineate the mechanisms or evolution of the eye. Also, our results indicate that despite a common master gene for eye evolution, perturbance of downstream networks and factors might account for the diversity of eye types during evolution.
5 RESULTS: SIMULATIONS OF THE COMPUTATIONAL MODELS
Simulations of the models of navigation described in Section 4 are described in this Results section.
5.1 Navigation using spatial view cells
In this strategy, navigation is implemented by proceeding via a series of landmarks, to which spatial view cells respond. The navigational task simulated using spatial view cells is illustrated in Figure 4a and was implemented with program NavSVC.m. The simulation worked to perform navigation using spatial view cell information as illustrated in Figure 4b, and this can be viewed as a video by running NavSVC.mp4. The results of the simulation can also be seen by running program NavSVC.m, which allows the start point to be altered, and the details of the implementation to be seen. In the program NavSVC.m, for each leg or node of the route, the navigator looks for the relevant landmark for that leg with spatial view cells, and moves towards the direction specified by the spatial view using error correction of the Navigational Direction (“NavDir”) by the spatial view direction of the landmark. When the individual is very close to the landmark, the next leg starts.
5.2 Navigation using “allocentric-bearing-to-a-landmark” cells
An example of a navigational task performed with “allocentric-bearing-to-a-landmark” cells is illustrated in Figure 5a, and the results of the simulation are shown in Figure 5b, which can be viewed as a video by running NavABL.mp4. The results of the simulation can also be seen by running program NavABL.m, which allows the start point to be altered, and the details of the implementation to be seen.
5.3 Navigation using combinations of allocentric-bearing-to-a-landmark cells: Triangulation
The navigational task used to illustrate this navigational strategy by triangulation uses the route illustrated in Figure 5 and described in Section 4.3. Places in this Euclidean space are defined by their X,Y coordinates, and this type of geometry is not needed in the strategies described previously. The task is to navigate from an X,Y start place “Start” to Waypoint 1 (W1), and then via the places specified by W2, and W3 to reach the goal at W4. The results for this type of navigation are illustrated in Figure 6, the corresponding video is NavTRI.mp4, and program is NavTRI.m.
The navigation can be completed successfully as shown in Figure 6, but at the cost of requiring a topological map in Euclidean space, and the ability to perform trigonometry.
Instead of using geometrical computation in a Euclidean space of the type implemented using triangulation as implemented in NavTRI.m, it is suggested that in primates including humans, simultaneously active spatial view cells for different landmarks in a scene can be associated together to form a spatial representation of a scene, seen from a particular place. As a primate traverses through different places and the scene defined by the landmarks gradually changes, storage of a few such scenes (using for example, the hippocampus to store such episodic memories) could enable later recall of the place, given the set of spatial view cells that are active by comparison with the stored representations. It is proposed that such a neural mechanism might enable spatial view cells to contribute to the lookup in an association memory of a place where the individual is located. This is proposed as another biologically plausible way for spatial view cells to be involved in navigation, by using the viewed scene to recall a place. Such a mechanism might operate to provide useful accuracy even without the need to store too many scenes. Although allocentric-bearing-to-a-landmark cells (which might also encode distance) might be used in addition to or as an alternative to spatial view cells, there is the considerable disadvantage that very many allocentric-bearing-to-a-landmark cells could be required, as a number of bearings need to be specifiable for each landmark.
Research maze puts images on floor, where rodents look
A rodent in a maze is a staple -- even a stereotype -- of experimental psychology research. But the maze in the lab of Rebecca Burwell, professor of cognitive, linguistic, and psychological sciences at Brown University, is not your grandfather's apparatus. In a new video article published in the Journal of Visualized Experiments, Burwell's research group demonstrates in full detail how the maze can be used to perform automated visual cognitive research tasks with great efficiency.
The maze is part real and part virtual. There are actual walls -- often in the shape of a giant piece of farfalle -- but researchers can project any imagery they want onto the floor from below. The use of digital projections makes the maze versatile, but using the floor for projections makes it particularly well-designed for rodent subjects.
"We've known since Lashley's classic studies on the mechanisms of vision that rats pay more attention to stimuli presented near the ground, but the field has persisted in presenting vertical 2-D images or 3-D objects," Burwell said. "What's new is the idea that presentation of images to the ground is the best way to present stimuli to rats and mice. Rodents do not have a fovea [a small depression in the retina where visual acuity is highest] like primates do, but they do have more retinal ganglion cells and photoreceptors in the upper retina, indicating that they can see items in the lower visual field better."
The system makes a real difference in the speed with which rats learn tasks in the maze, Burwell said. In different experiments, the article notes, rats have learned to respond properly to visual stimuli in a fourth to a sixth the number of trials when stimuli were projected onto the floor rather than onto walls.
While rodents behave in the maze, they are tracked with an overhead camera and software that monitors their behavior. Implanted neural sensors in the rodents' brains allow for precise recording of brain activity during sessions so that it can be correlated with behavior.
Implants also allow for delivery of a rewarding stimulus. Rewards sent directly to the brain can be more effective than food rewards, Burwell said, because rats can be rewarded in the moment of desired performance, and they don't become satiated.
Burwell first published a paper about the floor projection maze in 2009 and has been using it ever since. She owns the intellectual property and said a company is interested in licensing it.
"For anyone interested in using rats or mice as a model for visual information processing, presenting the information on the floor makes good sense behaviorally and biologically," she said.
In morphology, analogous traits arise when different species live in similar ways and/or a similar environment, and so face the same environmental factors. When occupying similar ecological niches (that is, a distinctive way of life) similar problems can lead to similar solutions.    The British anatomist Richard Owen was the first to identify the fundamental difference between analogies and homologies. 
In biochemistry, physical and chemical constraints on mechanisms have caused some active site arrangements such as the catalytic triad to evolve independently in separate enzyme superfamilies. 
In his 1989 book Wonderful Life, Stephen Jay Gould argued that if one could "rewind the tape of life [and] the same conditions were encountered again, evolution could take a very different course."  Simon Conway Morris disputes this conclusion, arguing that convergence is a dominant force in evolution, and given that the same environmental and physical constraints are at work, life will inevitably evolve toward an "optimum" body plan, and at some point, evolution is bound to stumble upon intelligence, a trait presently identified with at least primates, corvids, and cetaceans. 
In cladistics, a homoplasy is a trait shared by two or more taxa for any reason other than that they share a common ancestry. Taxa which do share ancestry are part of the same clade cladistics seeks to arrange them according to their degree of relatedness to describe their phylogeny. Homoplastic traits caused by convergence are therefore, from the point of view of cladistics, confounding factors which could lead to an incorrect analysis.    
In some cases, it is difficult to tell whether a trait has been lost and then re-evolved convergently, or whether a gene has simply been switched off and then re-enabled later. Such a re-emerged trait is called an atavism. From a mathematical standpoint, an unused gene (selectively neutral) has a steadily decreasing probability of retaining potential functionality over time. The time scale of this process varies greatly in different phylogenies in mammals and birds, there is a reasonable probability of remaining in the genome in a potentially functional state for around 6 million years. 
Parallel vs. convergent evolution Edit
When two species are similar in a particular character, evolution is defined as parallel if the ancestors were also similar, and convergent if they were not. [b] Some scientists have argued that there is a continuum between parallel and convergent evolution, while others maintain that despite some overlap, there are still important distinctions between the two.   
When the ancestral forms are unspecified or unknown, or the range of traits considered is not clearly specified, the distinction between parallel and convergent evolution becomes more subjective. For instance, the striking example of similar placental and marsupial forms is described by Richard Dawkins in The Blind Watchmaker as a case of convergent evolution, because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences. 
Protease active sites Edit
The enzymology of proteases provides some of the clearest examples of convergent evolution. These examples reflect the intrinsic chemical constraints on enzymes, leading evolution to converge on equivalent solutions independently and repeatedly.  
Serine and cysteine proteases use different amino acid functional groups (alcohol or thiol) as a nucleophile. In order to activate that nucleophile, they orient an acidic and a basic residue in a catalytic triad. The chemical and physical constraints on enzyme catalysis have caused identical triad arrangements to evolve independently more than 20 times in different enzyme superfamilies. 
Threonine proteases use the amino acid threonine as their catalytic nucleophile. Unlike cysteine and serine, threonine is a secondary alcohol (i.e. has a methyl group). The methyl group of threonine greatly restricts the possible orientations of triad and substrate, as the methyl clashes with either the enzyme backbone or the histidine base. Consequently, most threonine proteases use an N-terminal threonine in order to avoid such steric clashes. Several evolutionarily independent enzyme superfamilies with different protein folds use the N-terminal residue as a nucleophile. This commonality of active site but difference of protein fold indicates that the active site evolved convergently in those families.  
Cone snail and fish insulin Edit
Conus geographus produces a distinct form of insulin that is more similar to fish insulin protein sequences than to insulin from more closely related molluscs, suggesting convergent evolution. 
Na,K-ATPase and Insect resistance to cardenolides Edit
Many examples of convergent evolution exist in insects in terms of developing resistance at a molecular level to toxins. One well-characterized example is the evolution of amino acid substitutions at well-defined positions in the structure of the Na,K-ATPase α-subunit spanning 15 genera and 4 orders. The synergistic relationship between the Q111 and N122 substitutions are highlighted. Convergent evolution in this case does not depend on the type of selection or time frame in which it can occur, but has more to do with the co-evolutionary relationship causing a sort of soft selection between cardenolide-producing plants and the insects that prey on them. 
Nucleic acids Edit
Convergence occurs at the level of DNA and the amino acid sequences produced by translating structural genes into proteins. Studies have found convergence in amino acid sequences in echolocating bats and the dolphin  among marine mammals  between giant and red pandas  and between the thylacine and canids.  Convergence has also been detected in a type of non-coding DNA, cis-regulatory elements, such as in their rates of evolution this could indicate either positive selection or relaxed purifying selection.  
Swimming animals including fish such as herrings, marine mammals such as dolphins, and ichthyosaurs (of the Mesozoic) all converged on the same streamlined shape.   A similar shape and swimming adaptations are even present in molluscs, such as Phylliroe.  The fusiform bodyshape (a tube tapered at both ends) adopted by many aquatic animals is an adaptation to enable them to travel at high speed in a high drag environment.  Similar body shapes are found in the earless seals and the eared seals: they still have four legs, but these are strongly modified for swimming. 
The marsupial fauna of Australia and the placental mammals of the Old World have several strikingly similar forms, developed in two clades, isolated from each other.  The body and especially the skull shape of the thylacine (Tasmanian tiger or Tasmanian wolf) converged with those of Canidae such as the red fox, Vulpes vulpes. 
As a sensory adaptation, echolocation has evolved separately in cetaceans (dolphins and whales) and bats, but from the same genetic mutations.  
One of the best-known examples of convergent evolution is the camera eye of cephalopods (such as squid and octopus), vertebrates (including mammals) and cnidaria (such as jellyfish).  Their last common ancestor had at most a simple photoreceptive spot, but a range of processes led to the progressive refinement of camera eyes — with one sharp difference: the cephalopod eye is "wired" in the opposite direction, with blood and nerve vessels entering from the back of the retina, rather than the front as in vertebrates. As a result, cephalopods lack a blind spot. 
Birds and bats have homologous limbs because they are both ultimately derived from terrestrial tetrapods, but their flight mechanisms are only analogous, so their wings are examples of functional convergence. The two groups have powered flight, evolved independently. Their wings differ substantially in construction. The bat wing is a membrane stretched across four extremely elongated fingers and the legs. The airfoil of the bird wing is made of feathers, strongly attached to the forearm (the ulna) and the highly fused bones of the wrist and hand (the carpometacarpus), with only tiny remnants of two fingers remaining, each anchoring a single feather. So, while the wings of bats and birds are functionally convergent, they are not anatomically convergent.   Birds and bats also share a high concentration of cerebrosides in the skin of their wings. This improves skin flexibility, a trait useful for flying animals other mammals have a far lower concentration.  The extinct pterosaurs independently evolved wings from their fore- and hindlimbs, while insects have wings that evolved separately from different organs. 
Flying squirrels and sugar gliders are much alike in their body plans, with gliding wings stretched between their limbs, but flying squirrels are placental mammals while sugar gliders are marsupials, widely separated within the mammal lineage. 
Hummingbird hawk-moths and hummingbirds have evolved similar flight and feeding patterns. 
Insect mouthparts Edit
Insect mouthparts show many examples of convergent evolution. The mouthparts of different insect groups consist of a set of homologous organs, specialised for the dietary intake of that insect group. Convergent evolution of many groups of insects led from original biting-chewing mouthparts to different, more specialised, derived function types. These include, for example, the proboscis of flower-visiting insects such as bees and flower beetles,    or the biting-sucking mouthparts of blood-sucking insects such as fleas and mosquitos.
Opposable thumbs Edit
Opposable thumbs allowing the grasping of objects are most often associated with primates, like humans, monkeys, apes, and lemurs. Opposable thumbs also evolved in giant pandas, but these are completely different in structure, having six fingers including the thumb, which develops from a wrist bone entirely separately from other fingers. 
Convergent evolution in humans includes blue eye colour and light skin colour.  When humans migrated out of Africa, they moved to more northern latitudes with less intense sunlight.  It was beneficial to them to reduce their skin pigmentation.  It appears certain that there was some lightening of skin colour before European and East Asian lineages diverged, as there are some skin-lightening genetic differences that are common to both groups.  However, after the lineages diverged and became genetically isolated, the skin of both groups lightened more, and that additional lightening was due to different genetic changes. 
Lemurs and humans are both primates. Ancestral primates had brown eyes, as most primates do today. The genetic basis of blue eyes in humans has been studied in detail and much is known about it. It is not the case that one gene locus is responsible, say with brown dominant to blue eye colour. However, a single locus is responsible for about 80% of the variation. In lemurs, the differences between blue and brown eyes are not completely known, but the same gene locus is not involved. 
Carbon fixation Edit
While convergent evolution is often illustrated with animal examples, it has often occurred in plant evolution. For instance, C4 photosynthesis, one of the three major carbon-fixing biochemical processes, has arisen independently up to 40 times.   About 7,600 plant species of angiosperms use C4 carbon fixation, with many monocots including 46% of grasses such as maize and sugar cane,   and dicots including several species in the Chenopodiaceae and the Amaranthaceae.  
A good example of convergence in plants is the evolution of edible fruits such as apples. These pomes incorporate (five) carpels and their accessory tissues forming the apple's core, surrounded by structures from outside the botanical fruit, the receptacle or hypanthium. Other edible fruits include other plant tissues  for example, the fleshy part of a tomato is the walls of the pericarp.  This implies convergent evolution under selective pressure, in this case the competition for seed dispersal by animals through consumption of fleshy fruits. 
Seed dispersal by ants (myrmecochory) has evolved independently more than 100 times, and is present in more than 11,000 plant species. It is one of the most dramatic examples of convergent evolution in biology. 
Carnivory has evolved multiple times independently in plants in widely separated groups. In three species studied, Cephalotus follicularis, Nepenthes alata and Sarracenia purpurea, there has been convergence at the molecular level. Carnivorous plants secrete enzymes into the digestive fluid they produce. By studying phosphatase, glycoside hydrolase, glucanase, RNAse and chitinase enzymes as well as a pathogenesis-related protein and a thaumatin-related protein, the authors found many convergent amino acid substitutions. These changes were not at the enzymes' catalytic sites, but rather on the exposed surfaces of the proteins, where they might interact with other components of the cell or the digestive fluid. The authors also found that homologous genes in the non-carnivorous plant Arabidopsis thaliana tend to have their expression increased when the plant is stressed, leading the authors to suggest that stress-responsive proteins have often been co-opted [c] in the repeated evolution of carnivory. 
Phylogenetic reconstruction and ancestral state reconstruction proceed by assuming that evolution has occurred without convergence. Convergent patterns may, however, appear at higher levels in a phylogenetic reconstruction, and are sometimes explicitly sought by investigators. The methods applied to infer convergent evolution depend on whether pattern-based or process-based convergence is expected. Pattern-based convergence is the broader term, for when two or more lineages independently evolve patterns of similar traits. Process-based convergence is when the convergence is due to similar forces of natural selection. 
Pattern-based measures Edit
Earlier methods for measuring convergence incorporate ratios of phenotypic and phylogenetic distance by simulating evolution with a Brownian motion model of trait evolution along a phylogeny.   More recent methods also quantify the strength of convergence.  One drawback to keep in mind is that these methods can confuse long-term stasis with convergence due to phenotypic similarities. Stasis occurs when there is little evolutionary change among taxa. 
Distance-based measures assess the degree of similarity between lineages over time. Frequency-based measures assess the number of lineages that have evolved in a particular trait space. 
Process-based measures Edit
Methods to infer process-based convergence fit models of selection to a phylogeny and continuous trait data to determine whether the same selective forces have acted upon lineages. This uses the Ornstein-Uhlenbeck (OU) process to test different scenarios of selection. Other methods rely on an a priori specification of where shifts in selection have occurred. 
Does octopus eye presents the equivalent of fovea in primates? - Biology
The Evolution of the Human Eye
No discussion of evolution seems complete without bringing up the topic of the human eye. Despite its deceptively simple anatomical appearance, the human eye is an incredibly complicated structure. Even in this age of great scientific learning and understanding, the full complexity of the human eye has yet to be fully understood. It seems that with increased learning comes increased amazement in that the complexity that once seemed approachable continues to be just as incomprehensible as ever, if not more so. It is well documented that Darwin stood in wonder at the complexity of the eye, even from what little he knew of it in comparison to modern science. And yet, though he could not explain exactly how, he believed that such amazing complexity could be developed through a naturalistic process of evolution. Very small changes, selected as advantageous, could be passed on and multiplied over many generations to produce major miracles of complexity such as the human eye.
Obviously, Darwin was not crazy. His proposed theory of evolution and his basic explanations concerning the gradual development of complex structures, such as the eye, have convinced the vast majority of modern scientists. So, what exactly did he propose to explain the complexity of such structures as the human eye? Consider the following quote from Darwin.
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 certainly 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, though insuperable by our imagination, should not be considered as subversive of the theory. 1
Darwin was at a loss to explain exactly what was happening, but he proposed a stepwise evolution of the human eye by showing examples of differences in the eyes of other creatures that seemed to be less complex. These differences were ordered in a stepwise fashion of progression from the most simple of eyes to the most complex. There did in fact appear to be a good number of intermediaries that linked one type of eye to another type in an evolutionary pattern. Some of the most "simple" eyes are nothing more than spots of a small number of light sensitive cells clustered together. This type of eye is only good for sensing light from dark. It cannot detect an image. From this simple eye, Darwin proceeded to demonstrate creatures with successively more and more complex eyes till the level of the complexity of the human eye was achieved.
This scenario certainly seems reasonable. However, many theories that initially seem reasonable on paper are later disproved. Such theories need direct experimental evidence to support them before they are accepted outright as "scientific". Do complex structures such as eyes actually evolve in real life? As far as I could find, there is no documented evidence of anyone evolving an eye or even an eye spot through any sort of selection mechanism in any creature that did not have an eye before. Also, I have not seen documented evidence for the evolution of one type of eye into a different type of eye in any creature. As far as I can tell, no such evolution has ever been directly observed. Of course the argument is that such evolution takes thousands or even millions of years to occur. Maybe so, but without the ability for direct observation and testing, such assumptions, however reasonable, must maintain a higher degree of faith.
The necessary faith in such a scenario increases even more when one considers the fact that even a simple light sensitive spot is extremely complicated, involving a huge number of specialized proteins and protein systems. These proteins and systems are integrated in such a way that if one were removed, vision would cease. In other words, for the miracle of vision to occur, even for a light sensitive spot, a great many different proteins and systems would have to evolve simultaneously, because without them all there at once, vision would not occur. For example, the first step in vision is the detection of photons. In order to detect a photon, specialized cells use a molecule called 11-cis-retinal. When a photon of light interacts with this molecule, it changes its shape almost instantly. It is now called trans-retinal. This change in shape causes a change in shape of another molecule called rhodopsin. The new shape of rhodopsin is called metarhodopsin II. Metarhodopsin II now sticks to another protein called transducin forcing it to drop an attached molecule called GDP and pick up another molecule called GTP. The GTP-transducin-metarhodopsin II molecule now attaches to another protein called phosphodiesterase. When this happens, phosphodiesterase cleaves molecules called cGMPs. This cleavage of cGMPs reduces their relative numbers in the cell. This reduction in cGMP is sensed by an ion channel. This ion channel shuts off the ability of the sodium ion to enter the cell. This blockage of sodium entrance into the cell causes an imbalance of charge across the cell's membrane. This imbalance of charge sends an electrical current to the brain. The brain then interprets this signal and the result is called vision. Many other proteins are now needed to convert the proteins and other molecules just mentioned back to their original forms so that they can detect another photon of light and signal the brain. If any one of these proteins or molecules is missing, even in the simplest eye system, vision will not occur. 2
The question now of course is, how could such a system evolve gradually? All the pieces must be in place simultaneously. For example, what good would it be for an earthworm that has no eyes to suddenly evolve the protein 11-cis-retinal in a small group or "spot" of cells on its head? These cells now have the ability to detect photons, but so what? What benefit is that to the earthworm? Now, lets say that somehow these cells develop all the needed proteins to activate an electrical charge across their membranes in response to a photon of light striking them. So what?! What good is it for them to be able to establish an electrical gradient across their membranes if there is no nervous pathway to the worm's minute brain? Now, what if this pathway did happen to suddenly evolve and such a signal could be sent to the worm's brain. So what?! How is the worm going to know what to do with this signal? It will have to learn what this signal means. Learning and interpretation are very complicated processes involving a great many other proteins in other unique systems. Now the earthworm, in one lifetime, must evolve the ability to pass on this ability to interpret vision to its offspring. If it does not pass on this ability, the offspring must learn as well or vision offers no advantage to them. All of these wonderful processes need regulation. No function is beneficial unless it can be regulated (turned off and on). If the light sensitive cells cannot be turned off once they are turned on, vision does not occur. This regulatory ability is also very complicated involving a great many proteins and other molecules - all of which must be in place initially for vision to be beneficial.
Now, what if we do not have to explain the origin of the first light sensitive "spot." The evolution of more complex eyes is simple from that point onward. . . right? Not exactly. (See discussion of the Nilsson and Pelger paper below):
The Nilsson and Pelger Theory of Eye Evolution
In 1994 Nilsson and Pelger published what was to become an oft-referenced classic paper on the evolution of the complex camera-type eye starting from a simple light sensitive eyespot. 22 In their paper they argued that a series of insensible gradations, 1829 steps in all separated by 1% changes in visual acuity, could be crossed by an evolving population in about 350,000 generations - - or around 500,000 years. The following figures illustrate their theory:
The illustration above is layered with a dark backing and has a translucent epithelial covering in front of the light sensitive cells. Examples of creatures with simple flat eyespots include cnidarian medusa, turbellaria (flatworms that have eyespots that function as both photo- and chemoreceptors), annelids (i.e segmented worms), caterpillars, and starfish. Earthworms and sea urchins have eyespots consisting of single-celled photoreceptors scattered all over their surface epithelium ( Link ). The entire bodies of some creatures with eyespots are largely translucent. Some of these creatures have no associated pigmented cells and therefore cannot tell any sort of directionality for determining the source of light. All they can tell is if the environment around them is light or dark. However, other largely translucent creatures do have pigmented cells. This feature allows for the direction of the source of light to be determined so that the creature can deliberately head toward or away from the source of light.
Next, the eyespot dimples inward. This increases visual acuity by allowing the eye to sense the direction the light is coming from better than a flat eyespot. Planarians (flatworms) have such dimpled eyes.
Next, the rim of the pit begins to constrict to form a narrower opening or "aperture".
Around this point the pit begins to fill with a clear jelly-like material. It is thought that producing this jelly would be rather simple for most creatures - probably no more than one or two mutations. It is suggested that this jelly or slime helps to hold the shape of the pit, and helps to protect the light sensitive cells from chemical damage. And, the jelly might also keep mud and other debris out of the eye.
The aperture continues to decrease. Visual acuity increases until the aperture gets so small that it begins to shut out too much light. There will come a point when the aperture is the perfect size. A bigger aperture gives worse eyesight, and a smaller one gives worse eyesight. (The exact size that is "perfect" depends on the brightness of the lighting in a particular environment.) An example of a narrow aperture lensless eye is found in the chambered nautilus.
Next, a lens is needed. To get a lens, a ball-shaped mass of clear cells with a slight increase in the refractive index is needed. Once this mass is formed, it can be refined with very slight increases in the refractive index to produce greater and greater visual acuity.
An example of such an eye with a "primitive" lens is found in the Roman garden snail (Helix aspersa ) or slug.
Now that the eye has a lens, the aperture is in the wrong place. The eye will be more acute if the lens moves towards the center of curvature of the light-sensitive surface. So, over time, the lens not only moves, but increases in refractive index with a great index in the center of the lens vs. the edges of the lens. This is possible because the lens is made from a mixture of proteins. The ratio of the proteins can be different in different places, so the lens material is not optically uniform. It is common for a biological lens to have a higher refractive index at the center than at the edges. This "graded index" significantly improves image quality in that it is able to correct for distortion.
And viola! - the evolution of a camera-type eye is complete after a series of Darwin's "insensible gradations".
The the following video where Nilsson explains his eye-evolution theory:
Problems with the Paper Theory
There are just a few problems with this "theory" of eye evolution however. The argument is that the morphologic gaps are so narrow that it would be a very simple process to step from one gradation in visual acuity to the next with no more than one or two genetic mutations. In fact, it is often argued that these gradations already exist in a population that expresses one of the above listed steps. For example, a population that has flat eyespots is said to have at least some individuals within the population that have slightly dimpled eyespots. If a change in selective pressures favored a dimpled eyespot with a slight increase in visual acuity, pretty soon the majority of the population would have dimpled eyespots. The problem with this notion is that no population of creatures with flat eyespots shows any sort of intra-population range like this were even a small portion of the population has dimpled eyespots to any selectable degree. This is a common assertion, but it just isn't true.
Now, if these 1,829 gradations really evolutionary steps that are in fact small enough to cross in fairly short order (a few generations each under selective conditions), it seems quite likely that such ranges in morphologic expression would be seen within a single gene pool of a single species. But, they aren't. Species that have simple flat light-sensitive eyespots only have flat light-sensitive eyespots. No individual within that species shows any sort of dimpled eye that would have any selective advantage with regard to increased visual acuity. This fact alone suggests that these seemingly small steps probably aren't that simple when it comes to the coordinated underlying genetic changes that would be needed to get from one step to the next.
A big problem with these morphologic steps is that they do not take into consideration the fact that vision is more involved than what goes on just within the eye. In order to take any advantage of improved visual acuity within the eye, the brain must also change in such a way that it is able to interpret the information the eye is sending it. Otherwise, if the brain is still step up to appreciate only differences in light from dark sent from the eye, without being able to interpret specific patterns of light and dark on the retina, there would be no selective advantage from a dimpled vs. a flat eyespot. Because of this requirement, whatever evolution happens to take place in the eye, must be backed up by equivalent evolution in brain development and interpretive powers.
Another interesting problem with the argument for a selective advantage for a dimpled eye over a flat eyespot is the fact that determining the general direction of a light source can be achieved with a flat eyespot. Dimpling is not needed to determine the relative direction from which a beam of light is coming. All that is needed is an ability to rotate the eyespot relative to the source of light combined with the brain's ability to associate differences in the intensity of light with the change in orientation of the eyespot relative to the source of light. This sort of associative ability could produce essentially the same effect of being able to localize and even follow or move toward a source of light without the need for producing a dimpled or cup-shaped eye. In fact, the species Euglena, with just a flat patch of light-sensitive cells, can swim toward a source of light - - no dimpling needed ( Link ). In fact, some creatures, like starfish and sea urchin have no eyespots at all yet are still sensitive to light to the degree that they can move toward sources of greater light intensity ( Link ).
Another potential problem is getting thousands of light-sensitive cells to work together in coordination at the same time to produce a dimpled effect. What sort of simple mutation would produce such an effect among thousands of cells where each must be specifically oriented relative to all the others to form a "dimple" instead of a "protrusion" or some sort of other irregular surface? - at exactly the right spot to affect the light-sensitive spot in an orderly manner? Some argue that one or two mutations can and often do produce large morphologic changes. The problem with this argument is that all examples of large morphologic changes that result from small mutations are based on losses in pre-established morphologic features. There simply are no examples where a small mutation produces a large morphologic difference where an entirely new unique system of function is produced or a new structural modification, not just a loss of pre-existing structures, actually results in an improvement of function. When it comes to producing actual gains in novel beneficial structural alterations involving large numbers of cells (or even subcellular building blocks) the underlying coded information involved simply isn't that simple. The same thing is true for producing a lens or lens-like structure - even a "primitive" one. Getting a bunch of translucent epithelial cells to form a spherical structure and then to develop an increased refractive index isn't so easy - to any selectable level of improved visual acuity.
These are just a few of the reasons why the work of Nilsson and Pelger is still nothing more than a "paper theory" all these years later. What seems to work very well on paper may not work so well when it comes to putting the paper theory to a real life test. No such tests have actually been successful even though testing this theory isn't so hard to do. All that would have to be done is to take a creature with a flat eyespot and have it produce a bunch of offspring, artificially select the offspring with the most dimpled eyespots, have them produce the next generation, again select those offspring with the most dimpled eyespots, and so on. Very quickly, within a few generations, it should be very easy to demonstrate the evolution of dimpled eyespots and to show that these eyespots are actually functionally advantageous with respect to localizing sources of light vs. the use of a simple flat eyespot in the evolved creatures.
Such experimental demonstration has yet to be done. If it were ever done, successfully, it would certainly create a sensation within the scientific community. Creationism and intelligent design theorists would take a huge hit if such an experiment were actually successful. Until this actually happens, however, the eye-evolution theory of Nilsson and Pelger isn't really a true scientific theory since it hasn't actually been subject to any potentially falsifying real life test. It remains, therefore, a working hypothesis - a paper theory at best.
Oh, but what about the "design flaws" of the human eye? It is a common argument in favor of evolution that no intelligent designer would design anything with flaws. Evolution on the other hand, being a naturalistic process of trial and error, easily explains the existence of flaws in the natural world. Although many are convinced by this argument, this argument in and of itself assumes the motives and capabilities of the designer. To say that everything designed should match our individual conceptions of perfection before we can detect design, is clearly misguided.
Some might question the design of a Picasso painting, but no one questions the fact that it was designed, even having never met Picasso. A child might build a box car for racing the neighborhood kids in a box car derby. His car might not meet anyoneâ€™s idea of perfection, but most would not question the idea that it was designed. Or, someone might deliberately alter the design of a previous designer for personal reasons. This alteration itself is designed by a new designer and can be detected as such. Although not "beneficial" to overall function or the intentions of the original designer, the alteration might still be understood to be designed. For example, if someone slices the tires on a car with a razor blade, would it be accurate for someone walking by afterward to automatically assume that an evolutionary process was at work because of the presence of this current supposed design flaw? While a sliced up tire might not seem logical for a designer of tires to create, the flaw itself does not automatically rule out a designer. A very intelligent designer of flaws might be at work and the calling card might be the abundant evidence of high intelligence and purpose. Or, design flaws might be the result of natural decay and not representative of the original purpose or creation of the designer. A car tire that has 50,000 miles on it might have a few more "flaws" than it had when it was first made. Everything wears out. People grow old, have low back pain, arthritis, senile dementia, and dental decay. Are these design flaws or the wearing out of a great design that just did not last forever? Simply put, just because someone can think of a better design or an improvement upon an old design, does not mean that the old design was not designed.
Another problem with finding design flaws in nature is that we do not know all the information there is to know. What seems to us to be a design flaw initially, might turn out to be an advantage once we learn more about the needs of a particular system or creature or designer. In any case, lets take a closer look at the supposed design flaws in the human eye.
In his 1986 book, "The Blind Watchmaker," the famous evolutionary biologist Richard Dawkins posses this design flaw argument for the human eye:
"Any engineer would naturally assume that the photocells would point towards the light, with their wires leading backwards towards the brain. He would laugh at any suggestion that the photocells might point away, from the light, with their wires departing on the side nearest the light. Yet this is exactly what happens in all vertebrate retinas. Each photocell is, in effect, wired in backwards, with its wire sticking out on the side nearest the light. The wire has to travel over the surface of the retina to a point where it dives through a hole in the retina (the so-called 'blind spot') to join the optic nerve. This means that the light, instead of being granted an unrestricted passage to the photocells, has to pass through a forest of connecting wires, presumably suffering at least some attenuation and distortion (actually, probably not much but, still, it is the principle of the thing that would offend any tidy-minded engineer). I don't know the exact explanation for this strange state of affairs. The relevant period of evolution is so long ago." 3
Dawkins's argument certainly does seem intuitive. However, the problem with relying strictly on intuition is that intuition alone is not scientific. Many a well thought out hypothesis has seemed flawless on paper, but in when put to the test, it turns out not to work as well as was hoped. Unforeseen problems and difficulties arise. New and innovative solutions, not previously considered, became all important to obtaining the desired function. Dawkins's problem is not one of reasonable intuition, but one of a lack of testability of his hypothesis. However reasonable it may appear, unless Dawkins is able to test his assumptions to see if in fact "verted" is better than "inverted" retinal construction for the needs of the human, this hypothesis of his remains untested and therefore unsupported by the scientific method. Beyond this problem, even if he were to prove scientifically that a verted retina is in fact more reasonable for human vision, this still would not scientifically disprove design. As previously described, proving flaws in design according to a personal understanding or need does not disprove the hypothesis that this flawed design was none-the-less designed.
Since a designer has not been excluded by this argument of Dawkins, the naturalistic theory of evolution is not an automatic default. However true the theory of evolution might be, it is not supported scientifically without testability. This is what evolutionists need to provide and this is exactly what is lacking. The strength of design theory rests, not in its ability to show perfection in design, but in its ability to point toward the statistical improbability of a naturalistic method to explain the complexity of life that is evident in such structures as the human eye. Supposed flaws do not eliminate this statistical challenge to evolutionary theories. Dawkins's error is to assume that the thinking, knowledge and motivation of all designers are similar to his thinking, knowledge and motivation.
Dawkins's problems are further exacerbated by his own admission that the inverted retina works very well. His argument is not primarily one that discusses the technical failures of the inverted retina, but of aesthetics. The inverted retina just does not seem right to him regardless of the fact that the inverte d retina is the retina used by the animals with the most acute (image forming) vision systems in the world.
The most advanced verted retinas in the world belong to the octopus and squid (cephalopods). An average retina of an octopus contains 20 million photoreceptor cells. The average human retina contains around 126 million photoreceptor cells. This is nothing compared with birds who have as much as 10 times as many photoreceptors and two to five times as many cones (cones detect color) as humans have. 4,5 Humans have a place on the retina called a "fovea centralis." The fovea is a central area in the central part of the human retina called the macula. In this area humans have a much higher concentration photoreceptors, especially cones. Also, in this particular area, the blood vessels, nerves and ganglion cells are displaced so that they do not interpose themselves between the light source and the photoreceptor cells, thus eliminating even this minimal interference to the direct path of light. This creates an area of high visual acuity with decreasing visual acuity towards the periphery of the human retina. The cones in the macula (and elsewhere) also have a 1:1 ratio to the ganglion cells. Ganglion cells help to preprocess the information received by the retinal photoreceptors. For the rods of the retina, a single ganglion cell handles information from many, even hundreds of rod cells, but this is not true of cones whose highest concentration is in the macula. The macula provides information needed to maximize image detail, and the information obtained by the peripheral areas of the retina helps to provide both spatial and contextual information. Compared with the periphery, the macula is 100 times more sensitive to small features than in the rest of the retina. This enables the human eye to focus in on a specific area in the field of vision without being distracted by peripheral vision too much. 6
Bird retinas, on the other hand, do not have a macula or fovea centralis. Visual acuity is equal in all areas. Octopus retinas also lack a fovea centralis, but do have what is called a linea centralis. The linea centralis forms a band of higher acuity horizontally across the retina of the octopus. A unique feature of octopod eyes is that regardless of the position of their bodies, their eyes always maintain the same relative position to the gravitational field of the earth using an organ called a statocyst. The reason for this appears to be related to the fact that octopods retinas are set up to detect horizontal and vertical projections in their visual fields. 7 This necessitates a predictable way to judge horizontal and verticalness. Octopods use this ability, not so much to form images as vertebrates do, but to detect patterns of movement. It is interesting to note that regardless of the shape of an object, octopods will respond to certain movements as they would to prey that make similar movements. However, if their normal prey is not moving, an octopus will not generally respond. 8,9 In this respect, the vision of octopods is similar to an insect-type compound eye. The octopod eye has in fact been referred to as a compound eye with a single lens. 10 In some other respects, it is also more simple in its information processing than is the vertebrate eye. The photoreceptors consist only of rods, and the information transmitted by these rods does not pass through any sort of peripheral processing ganglion cell(s). 11 Octopod eyes are not set up for the perception of small detail, but for the perception of patterns and motion thus eliminating the need for the very high processing power seen in human and other vertebrate eyes.
The high processing power of human and other vertebrate eyes is not cheep. It is very expensive and the body pays a high price for the maintenance of such a high level of detection and processing power. The retina has the highest energy demands/metabolic rate of any tissue in the entire body. The oxygen consumption of the human retina (per gram of tissue) is 50% greater than the kidney, 300% greater than the cerebral cortex (of the brain), and 600% greater than cardiac muscle. These are numbers for the retina as a whole. The photoreceptor cell layer, taken alone, has a significantly higher metabolic demand. 12,13 All this energy must be supplied quickly and efficiently. Directly beneath each photoreceptor lies the choroid layer. This layer contains a dense capillary bed called the choriocapillaris. The only thing separating the capillaries from direct contact with the photoreceptors is the very thin (one cell thick) retinal-pigmented epithelial (RPE) layer. These capillaries are much larger than average being 18-50 microns in diameter. They provide a huge relative blood supply per gram of tissue and as much as 80% of the total blood supply for the entire eye. On the other hand, the retinal artery that passes through the "blind spot" and distributes across the anterior retina supplying the needs of the neural layer, contributes only 5% of the total blood supply to the retina. 15 The close proximity of the choroidal blood supply to the photoreceptor cells without any extra intervening tissue or space such as nerves and ganglion cells (ie: from a "verted" system) allows the most rapid and efficient delivery of vital nutrients and the removal of the tremendous quantities of waste generated. The cells that remove this waste and re-supply several needed elements to the photoreceptors are the RPE cells.
Everyday rods and cones shed around 10% of their segmented disks. Rods average 700 to 1,000 disks while cones average 1,000 to 1,200 disks. 16 This in itself creates a very large metabolic demand on the RPE cells who must recycle this huge number of shed disks. Conveniently, these disks do not have to travel too far to reach the RPE cells since they are sloughed from the end of the photoreceptor that directly contacts the RPE cell layer. If these disks were sloughed off in the opposite direction (toward the lens and cornea), their high level of sloughing would soon create a cloudy haze in front of the photoreceptors, which could not be cleared as rapidly as would be needed to maintain the highest degree of visual clarity. This high rate of recycling maintains the very high sensitivity of the photoreceptors. RPE cells also contain retinol isomerase. Trans-retinal must be converted back to 11-cis-retinal in the visual molecular cascade. With the help of vitamin-A and retinol isomerase, the RPE cells are able to do this and then transfer these rejuvenated molecules back to the photoreceptors. 17 The funny thing is, the RPE cells in the retinas of cephalopods do not have retinol isomerase. 18 However, the retinas of all sighted vertebrates do have this important enzyme. All of these functions require large amounts of energy and so the RPE cells, like the photoreceptor cells, must be in close proximity to a very good blood supply, which of course they are. Also, as the name implies, RPE cells are pigmented with a very dark/black pigment called melanin. This melanin absorbs scattered light, thus preventing stray reflections of photons and the indirect activation of photoreceptors. This aids significantly in the creation of a clear/sharp image on the retina. There is a different system for some other vertebrates such as the cat who have a reflective layer called the tapetum lucidus, which allows for better night vision (six times better than humans) but poor day vision. 19
So we see that inverted retinas seem to have some at least marginal if not significant advantages based on the needs of their owners. We also have the evidence that the best eyes in the world for image detection and interpretation are all inverted as far as their retinal organization. As far as the disadvantages are concerned, they are generally not of practical significance in comparison to overall relative function. Even Dawkins seems to admit that his uneasiness is mostly one of aesthetics. Consider the following admission from Dawkins:
With one exception, all the eyes I have so far illustrated have had their photocells in front of the nerves connecting them to the brain. This is the obvious way to do it, but it is not universal. The flatworm keeps its photocells apparently on the wrong side of their connecting nerves. So does our own vertebrate eye. The photocells point backwards, away from the light. This is not as silly as it sounds. Since they are very tiny and transparent, it doesn't much matter which way they point: most photons will go straight through and then run the gauntlet of pigment-laden baffles waiting to catch them. 20
As it turns out, the supposed problems Dawkins finds with the inverted retina become actual advantages in light of recent research published by Kristian Franze et. al., in the May 2007 issue of PNAS (see illustration above ). As it turns out, " cells are living optical fibers in the vertebrate retina." 21 Consider the observations and conclusions of the authors in the following abstract of their paper:
Although biological cells are mostly transparent, they are phase objects that differ in shape and refractive index. Any image that is projected through layers of randomly oriented cells will normally be distorted by refraction, reflection, and scattering. Counterintuitively, the retina of the vertebrate eye is inverted with respect to its optical function and light must pass through several tissue layers before reaching the light-detecting photoreceptor cells. Here we report on the specific optical properties of glial cells present in the retina, which might contribute to optimize this apparently unfavorable situation. We investigated intact retinal tissue and individual Muller cells, which are radial glial cells spanning the entire retinal thickness. Muller cells have an extended funnel shape, a higher refractive index than their surrounding tissue, and are oriented along the direction of light propagation. Transmission and reflection confocal microscopy of retinal tissue in vitro and in vivo showed that these cells provide a low-scattering passage for light from the retinal surface to the photoreceptor cells. Using a modified dual-beam laser trap we could also demonstrate that individual Muller cells act as optical fibers. Furthermore, their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Thus, Muller cells seem to mediate the image transfer through the vertebrate retina with minimal distortion and low loss. This finding elucidates a fundamental feature of the inverted retina as an optical system and ascribes a new function to glial cells. 21
And Dawkins would have us believe that no "intelligent" designer would have done it that way? Really?
To say then that the human eye is definite proof of a lack thoughtful design, is a bit presumptuous I would think. This seems to be especially true when one considers the fact that the best of modern human science and engineering has not produced even a fraction of the computing and imaging capability of the human eye. How can we then, ignorant as we must be concerning such miracles of complex function, hope to accurately judge the relative fitness or logic of something so far beyond our own capabilities? Should someone who cannot even come close to understanding or creating the object that they are observing think to critique not to mention disparage the work that that lies before them? This would be like a six-year-old child trying to tell an engineer how to design a skyscraper or that one of his buildings is "better" than the others. Until Dawkins or someone else can actually make something as good or better than the human eye, I would invite them to consider the silliness of their efforts in trying to make value judgments on such things such things that are obviously among most beautiful and beyond the most astounding works of human genius and art in existence.
If and when humans do achieve and surpass this level of creativity and genius and are able to experimentally prove the existence of actual defects in the function of human eyes and other such marvels, would this evidence rule out a designer? No. Intuitively, such complexity as we see in living things seems to speak for design in that it has the obvious appearance of design. Richard Dawkins as much as admits this in the title of his book, "The Blind Watchmaker." For those who wish to propose a naturalistic mechanism to explain complexity, the burden of proof cannot be relieved by appealing to supposed design "flaws." The best that evolutionist can do to disprove the theory of design is to demonstrate some real examples of evolution in action where a purely naturalistic mechanism actually works to form a comparably complex function of interacting parts. I have yet to see this done. As it currently stands, the theory of evolution is based only on correlation and inference, but not on actual demonstration. The best examples of evolution in action deal with the evolution of very simple enzymatic functions, such as the evolution of the enzyme galactosidase in E. coli. and even this evolution has its clear limitations. I have yet to see an "irreducibly complex" system of function evolve were the function in question requires more than a few hundred fairly specified amino acid "parts" working together at the same time. For example, the flagellar bacterial motility system requires several thousand fairly specified amino acid "parts" in the form of a couple dozen individual proteins, working together in unified harmony at the same time. Of course, there are many different kinds of bacterial motility systems possible, but all of them require several thousand fairly specified amino acids working together at the same time before the function of motility can be realized. Such a level of functional complexity has never been observed to evolve through any sort of naturalistic process.
If one looks carefully at the average time required for the evolution of such a multipart system of function, Dawkins and other evolutionists will most likely be waiting for a very long time for any experimental confirmation. No wonder hypothetical claims of design flaws are so common. There does not seem to be too much else to go on as far as a significant example of real evolution in action. The statistics are against such a process actually working in real life (kind of like a perpetual motion machine). So, evolutionists are left with the design flaw argument - an argument that relies upon the assumed understanding of the identity, motives, and abilities of any possible designer or collection of designers. Such arguments prove nothing except for the arrogance of those who use such arguments - especially when the very ones proposing such arguments cannot make anything even remotely comparable to much less better than that which they are disparaging.
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Controversy exists regarding the safety of scleral collagen cross-linking (CXL) by riboflavin/ultraviolet A (UVA), such as the ultraviolet-induced retinal damage. Compared with accelerated CXLs using shorter treatment times with higher UVA irradiations [24–25], the conventional CXL approach (3.0mW/cm 2 , 30 min) was applied in the present research for it has been generally studied and applied both in vivo experiments of scleral CXL [11, 13, 20] and in clinical practice of corneal CXL [26–27]. According to previous studies [11–13], the equatorial sclera was chosen as the treatment area of the scleral CXL procedures, and the scleral biomechanical strength was proved to be increased for more than 8 months postoperatively. While the safety results and the potential risk to the retina were still in doubt . In this study, biological parameters of retina and choroid were investigated by SD-OCT and OCTA examinations, aiming to verify the safety of this scleral CXL technique in vivo primates.
In the analysis of retinal thickness, SD-OCT was performed pre- and postoperatively. There was no statistical difference between CXL eyes and control eyes at different pre-/post-operative periods in retinal thickness of the nine ETDRS subfields (each P>0.05). These outcomes indicated that the retinal thickness in measured area was not affected by scleral CXL in rhesus monkeys.
Flow density is the index reflecting quantitative measurement of vascular density . In the present study, no statistical difference was noted between two groups at different pre-/post-operative periods in flow density of retinal superficial vascular networks (each P>0.05). It was demonstrated that the superficial vascular density in macular area was not affected by scleral CXL in rhesus monkeys.
The choroidal thickness in 1500μm temporal to the fovea center of CXL eyes revealed a significant reduction in 1 week postoperatively (P<0.05), but it subsequently increased from 1 month postoperatively, and no statistical difference was found between two groups in the following periods (each P>0.05). In other zones, there was no statistical difference between two groups postoperatively (each P>0.05). In previous studies [30–32], many factors could influence choroidal thickness, including age, axial length, sex and eye pressure, and it is still unknown how or whether these factors affect the local choroidal geometry. Hypothesis has been demonstrated that the choroidal circulation might play a role in the distribution of choroidal thickness [33–34] thus, the decreased volume in the choroidal vascular bed would led to a reduction of choroidal thickness.
In this study, no statistical difference was noted in IOP, spherical equivalent and axial length between two groups in different pre-/post-operative periods. Thus, considering the sclera is a kind of tissue with sparse vasculature and penetrated by blood vessels supplying the choroid, we speculate that local capillaries within scleral tissue might be affected temporarily by CXL surgery, leading to reduced blood flow in irradiation zone. Therefore, the local choroidal circulation could be transiently impaired. It is possible that the decrease choroidal circulation caused thinner choroidal thickness in acute postoperative phase, and it may return to the normal level concomitant with the autonomic regulation [33, 35] subsequently according to outcomes in the present study. While the vascular flow density and thickness of retina were not affected by scleral CXL postoperatively.
Potential limitations of our study should be mentioned. First, retinal and choroidal parameters measured in vivo in this study is limited to posterior pole of eyeball, where is not underlying the treatment. However, there has not been any OCT equipment providing the software for measurement in equatorial region so far. Potential adverse effects at the direct vicinity of the application site should be investigated in further histological examination. Second, whereas manual choroidal thickness measurement is one of drawbacks of this study, until now, no automated measurement software is supplied by existing equipment. Finally, these results might be valid for rhesus monkeys, and it has not been clarified whether the results are applicable in other species.
In conclusion, the present study found that the choroidal thickness near crosslinked region may change temporarily following scleral CXL. This may be the result of reversible transient microcirculatory dysfunction of the choroid, and it might recover gradually after 1 month postoperatively. The vascular flow density and thickness of retina were not affected by scleral CXL. The potential role of scleral CXL in the effect of this change remain to be investigated. Further study should be performed to evaluate the pathological result and long-term effect of scleral CXL in clinical application.