托福阅读背景材料(344)：地球是怎么产生生物的？

　　托福阅读背景知识/材料之地球是怎么产生生物的，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！

　　2014年6月15日托福真题再现：

　　版本一

　　1，地球早起大气成分及生物 早起甲烷与二氧化碳占主要地位，没有氧气。因为有了会光合作用的细菌，产生了大量氧气，消耗二氧化碳，提供臭氧层，为当今新生物钟提供必要环境。

　　版本二

　　有一个讲地球最早怎么产生生物的

　　大概有几点，首先是太阳当时不够热，地球当时气体组成像火山的，主要靠两种气体加温度，似乎一种是二氧化碳另一种是m开头的不认识

　　那种气体组成不适合生命也没什么氧，当时的organism有很大作用，进行光合作用产生氧气吸收二氧化碳，二氧化碳还有一部分被转移成非气态的，这样就形成了后来的大气组成

　　氧气可以形成臭氧层，保护生物不受紫外线辐射，这块提到了火星等其他星球就没有臭氧层或者臭氧不够多blabla不记得了，反正当时生物都去海里了因为水可以吸收紫外线辐射保护它们

　　解析：本文属于生命起源类型，是托福阅读资历很老的话题之一，写作角度涉及到巴斯德实验、生命起源的几种假说，以及过程的描述，同学在阅读中的难点是要克服对生僻词汇和背景知识的恐惧心理，这些都可以通过多读和精读来实现。

　　参考阅读：

　　There is no truly "standard" model of the origin of life. But most currently accepted models build in one way or another upon a number of discoveries about the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:

　　1. Plausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller-Urey experiment by Stanley L. Miller and Harold C. Urey in 1953, although it is now generally held that their laboratory conditions did not reflect the original Earth's atmosphere.

　　2. Phospholipids (of an appropriate length) can spontaneously form lipid bilayers, a basic component of the cell membrane.

　　3. The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis).

　　4. Selection pressures for catalytic efficiency and diversity result in ribozymes, which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent.

　　5. Protein out-compete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use.

　　There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models.

　　The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life was thought to have formed are commonly held to be methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43-). Molecular oxygen (O2) and ozone (O3) typically are considered to have been either rare or absent.

　　As of 2007, no one had yet synthesized a "protocell" using basic components that would have the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers working in this field have argued that a "top-down approach" is more feasible. One such approach involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist John Desmond Bernal coined the term biopoesis for this process, and suggested that there were a number of clearly defined "stages" that could be recognized in explaining the origin of life.

　　Stage 1: The origin of biological monomers

　　Stage 2: The origin of biological polymers

　　Stage 3: The evolution from molecules to cell

　　Bernal suggested that Darwinian evolution may have commenced early, some time between Stage 1 and 2.