在2014年6月29日的托福閱讀考試中有這樣一道題：眼睛的起源。出國留學網(www.liuxue86.com)小編提醒大家：該文屬于起源類別文章，通過考古證據提出理論，在TPO里類似的文章非常多，自然科學類和社會科學類均涉及，在理解時重點關注不同的證據支持的觀點，以及后續(xù)證據對前觀點的支持或反駁。
托福閱讀真題再現：
版本一：關于眼睛的起源
版本二：第一篇眼睛起源 不是來自多細胞動物 早起軟體動物化石提供證據
版本三：講生物眼睛的構造和進化什么的
解析：本文關注眼睛的起源，重復的是2012年8月19日閱讀。該文屬于起源類別文章，通過考古證據提出理論，在TPO里類似的文章非常多，自然科學類和社會科學類均涉及，在理解時重點關注不同的證據支持的觀點，以及后續(xù)證據對前觀點的支持或反駁。
托福閱讀背景知識：眼睛的起源
The common origin (monophyly) of all animal eyes is now widely accepted as fact. This is based upon the shared genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago, and the PAX6 gene is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race" among all species that did not flee the photopic environment. Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel (except those of groups, such as the vertebrates, that were only forced into the photopic environment at a late stage).
Eyes in various animals show adaptation to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can detect ultraviolet radiation. The different forms of eye in, for example, vertebrates and molluscs are examples of parallel evolution, despite their distant common ancestry. Phenotypic convergence of the geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging cephalopod eye, but this is not the case, as the reversed roles of their respective ciliary and rhabdomeric opsin classes and different lens crystallins show.
The very earliest "eyes", called eyespots, were simple patches of photoreceptor protein in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source.
Through gradual change, the eyespots of species living in well-lit environments depressed into a shallow "cup" shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes. However, the ancestors of modern hagfish, thought to be the protovertebrate were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. This would have led to a somewhat different evolutionary trajectory for the vertebrate eye than for other animal eyes.
The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into a transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.
The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the cornea and iris. Separation of the forward layer again formed a humour, the aqueous humour. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.
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