针对托福考试中出现过不少关于生命起源的话题，前程百利小编为大家带来关于生命起源的托福阅读材料，这样的背景知识有助于考生在面对这类题目时作答。也提醒大家：大家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.
以上就是前程百利小编为大家带来的托福阅读材料，平时的阅读练习是托福考试阅读备考的重要一环。希望大家利用好每份材料，尽可能多地从中获取知识。预祝大家托福考试取得好成绩。
