The human eye evolved.

Thank you for accepting, and welcome to your first debate. Additional thanks you for conceding that the eye appears not to be "intelligently designed", (saved me the time) human and animal life is full of imperfections, evolution needs not to strive for perfection merely success. Our vast amount of junk DNA is testament to that. As to the often appropriated and misused Darwin quote; creationists rarely finish it. "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." Arguments: A. Argument of Incredulity and B. Argument of Beauty A.Evolution of the eye: Firstly some key points about evolutionary/genetic theory that may enable easier understanding. 1.Not everything is an adaption: A part may mutate and be generationally passed down, simply because there is no selective pressure to remove said part. A part or expression thereof may be a ‘side effect' of another trait. A gene variant may spread/exist due to its relation to a successful gene. 2. Natural selection is not the only way for evolution to progress: DNA when copied creates errors. Human embryos will contain around 100 new mutations; natural selection will eliminate the most harmful. Most mutations will not matter, occurring within our junk DNA that makes up the majority of our genome. The majority of neutral mutations will die out but a few will survive and spread through generations by chance in a process called genetic drift. 3. A mutation may be used differently to its previous precursors (e.g. fins to fingers, the infamous flagellum precursor). "What is the use of half an eye?" Half an eye is just 1 per cent better than 49 per cent of an eye, which is already better than 48 per cent, and the difference is significant. Serviceable, image-forming eyes have evolved between 40 and 60 times, independently from scratch, in many different invertebrate groups. Among these 40-plus independent evolutions, at least nine distinct design principles have been discovered, including pinhole eyes, two kinds of camera-lens eyes, curved-reflector ("satellite dish") eyes, and several kinds of compound eyes. The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body. Geneticists have discovered that eye development in animal embryos is almost always initiated by a gene called pax6, which like many developmental genes is found in many different lineages. The prototype eye was very simple, one photoreceptor cell and one pigment cell. The C. elegans nematode has lost its eyes because it lives underground, but it has retained its Pax6 gene. The reason for retaining the Pax6 gene lies in the pleiotropic function of Pax6, which specifies not only the eye but also the nose and parts of the brain. Therefore, there is selective pressure to maintain Pax6 despite of the reduction of the eyes (point 1). Various descendants of this organism, including trilobites, went on to realise unique ways of improving on this prototype to create fully fledged eyes with lenses. It has been discovered that two types of light-sensitive cells existed in our early animal ancestors: rhabdomeric and ciliary. In most animals, rhabdomeric cells became part of the eyes, and ciliary cells remained embedded in the brain. But the evolution of the human eye is peculiar – it is the ciliary cells that were recruited for vision which eventually gave rise to the rods and cones of the retina. Studying a "living fossil," Platynereis dumerilii, a marine worm that still resembles early ancestors that lived up to 600 million years ago. By mapping "molecular fingerprints" ( a fingerprint is a unique combination of molecules that is found in a specific cell), it can be shown that if cells between species have matching molecular fingerprints, then the cells are very likely to share a common ancestor cell. Scientist Kristin Tessmar- determined the molecular fingerprint of the cells in the worm's brain. She found an opsin, a light-sensitive molecule, in the worm that strikingly resembled the opsin in the vertebrate rods and cones (European Molecular Biology Laboratory, 2004, November, 1; Gehring, 2005. Journal of Heredity, 96(3), 171–184). Time line: Stage 1: 580 Million years ago (Mya). Animals with bilateral symmetry exist, numerous families of genes exist, the opsin has evolved into 3 major classes: rhabdomeric, photoisomerase and ciliary opsins. Protostomes seprate from our line (deuterstomes). Stage 2: (580-550 Mya). Ciliary photoreceptor and ciliary opsin continue to evolve similar to those in extant and ascidian larvae. Cephalochordates and tunicates separate from our line (chordates). Stage 3: (550-530 Mya). Ciliary photoreceptors make synaptic contact onto projection neurons that might have been descendants of rhabdomeric photoreceptors. The eye-field region of the diencephalon bulges to form ‘eye vesicles'. These lateral vesicles invaginate bringing the proto-retinal into opposition with the retinal pigment epithelium. A primordial; lens develops, preventing pigmentation of the overlying skin. Myxiniformes (hagfish etc) separate from our line (vertebrates). Stage 4: lamprey-like ancestors (530-500 Mya). Photo receptors develop cone-like features: Ordered sac/disc membranes, Mitochondria becomes concentrated within the ellipsoid region, coloured filter material is incorporated into the inner segment for spectral tuning, ribbon synapses evolve into the synaptic terminal, cell classes diverge to give 5 separate cone like photoreceptors each with own cliary opsin and with isoforms of transduction proteins. Retinal computer power increases: Cone bipolar cells evolve, either from proto neurons or photoreceptors, bi-plexiform ganglion cells develop, highly organised 3 layered neuronal structure with 2 intervening plexiform layers develops. Ganglion-cell axoms project to the thalamus, the optics evolve (lens accommodation and eye movement), the lens placode invaginates and develops to form a lens, the iris develops and papillary constriction becomes possible, innervated extra ocular muscles evolve. Petromyzoniformes separate from our line Stage 5 (500-530 Mya). Myelin evolves and is incorporated throughout the nervous system. Rod photoreceptors evolve: Rhopsidin evolves from cone opsin, free floating discs pinch off within the plasma membrane, rod bipolar cells evolve probably from rod photoreceptors, the sceptic rod pathways evolves with amacrine cells (input via pre existing cone pathway), highly contractile iris evolves that can adjust eye levels, intrinsic eye muscles develop that permit accommodation of the lens. The eye currently possesses a duplex retina, rods & cones, retinal wiring, colour coded photopic pathways. The last jawless fish separate from our line Stage 6: Tetrapods (430>Mya). The lens develops an elliptical shape to compensate for the added refractive power that is provided by the cornea in air, the dermal component of the split cornea is lost and the eyelids evolve, certain opsin classes are lost due to nocturnal conditions. (Nature Reviews Neuroscience 8, 960-976 December 2007) B.Argument from Beauty The eye is far from the only unique component in our body. "Window to the soul" sounds nothing more than tribal mysticism. What you may think as reading emotion in the eye is nothing more than basic body language as a whole.