托福阅读背景知识(291)：眼睛的起源

　　托福阅读背景知识/材料之眼睛的起源，更多托福阅读背景材料/知识尽在新东方在线托福考试频道！

　　在2014年6月29日的托福阅读考试中有这样一道题：眼睛的起源。针对这道托福考题，新东方谢真真老师来为大家普及一下关于眼睛的起源的背景知识，这样有助于考生在面对这类题目时方便作答。新东方谢真真老师指出：该文属于起源类别文章，通过考古证据提出理论，在TPO里类似的文章非常多，自然科学类和社会科学类均涉及，在理解时重点关注不同的证据支持的观点，以及后续证据对前观点的支持或反驳。

　　托福阅读真题再现：

　　版本一：关于眼睛的起源

　　版本二：第一篇眼睛起源 不是来自多细胞动物 早起软体动物化石提供证据

　　版本三：讲生物眼睛的构造和进化什么的

　　新东方谢真真解析：

　　解析：本文关注眼睛的起源，重复的是2012年8月19日阅读。该文属于起源类别文章，通过考古证据提出理论，在TPO里类似的文章非常多，自然科学类和社会科学类均涉及，在理解时重点关注不同的证据支持的观点，以及后续证据对前观点的支持或反驳。

　　相关背景：

　　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.