托福阅读之生命起源

　　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 molecular¹ and cellular² components⁴ for life, which are listed in a rough order of postulated⁵ emergence⁶:

　　1. Plausible⁷ pre-biotic conditions result in the creation of certain basic small molecules⁸ (monomers) of life, such as amino acids. This was demonstrated in the Miller⁹-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 component³ of the cell membrane¹⁰.

　　3. The polymerization of nucleotides into random¹¹ 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 catalysts¹². Thus the first ribosome is born, and protein synthesis becomes more prevalent.

　　5. Protein out-compete ribozymes in catalytic ability, and therefore become the dominant¹³ 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 metabolism¹⁴. Many models fall into the "genes¹⁵-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid¹⁶ 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 methane¹⁷ (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43-). Molecular oxygen (O2) and ozone¹⁸ (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 minimal¹⁹ 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.

　　本文中中难点词汇解析：

　　molecular ：adj. [化学] 分子的;由分子组成的

　　例句：You do not get this line from molecular hydrogen.

　　从氢分子中我们得不到这种光线。

　　demonstrated ：演示、展现

　　例句：Have they, for example, demonstrated a commitment to democracy?

　　例如，他们展现了对民主的奉献吗?

　　Phospholipids：n. 磷脂质

　　例句：What drove the transition from such membranes²⁰ to modern ones, which are based on the more complex phospholipids?

　　这种的简单膜结构转变成现代的基于磷脂的版本是由什么驱使的?

　　oligopeptides：n. [生化] 寡肽(oligopeptide的复数);寡生肽

　　例句：This paper reports preparation process of high Fischer ratio oligopeptides from corn gluten meal ( CGM ) by subtilisin and carboxypeptidase from Aspergillus oryzae in brief.

　　本文简述了酶法水解玉米醇溶蛋白制备高F值寡肽的主要工艺步骤。

　　predominantly：adv. 主要地; 显著地

　　例句：So, those are the reasons predominantly why people cannot get out themselves.

　　这就是人们为什么无法从利比亚撤出的主要原因。

　　progressively ：adv. 渐进地; 日益增多地

　　例句：Remember to take it easy, and your skill will progressively grow.

　　请记住，这些需要慢慢来，你的技能将逐步增长。

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