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　　托福阅读素材：生命的起源

　　内容回忆：

　　本文一共 6 段。第一段提出了问题，生命到底是怎么形成的?第二段说生命形成的环境一定是在深海的某处，因为大气层中的氧气会破坏生命最初形成的元素。第三段说高温细菌可能是生命形成的来源之一。第四段具体介绍了高温细菌形成生命过程。第五段介绍了高温细菌形成生命过程中的一些特点。例如，高温细菌会不断进行生化反应。另一些高温细菌则会将氧，硫和氢相结合。最后一段则说高温细菌很有可能是生命的来源。

　　参考阅读：

　　The origin of life is a scientific problem which is not yet solved. There are plenty of ideas, but few clear facts.[1] The Miller's 1953 experiment shows that abiotic synthesis of organic molecules is possible.

　　It is generally agreed that all life today evolved by common descent from a single primitive lifeform.[2] We do not know how this early form came about, but scientists think it was a natural process which took place perhaps 3,900 million years ago. This is in accord with a philosophy called naturalism: only natural causes are admitted.

　　We do not know whether metabolism or genetics came first. The main hypothesis which supports genetics first is RNA world hypothesis, and the one which supports metabolism first is Protein w orld hypothesis.

　　Another big problem is how cells develop. All existing forms of life are built out of cells.[3]

　　The Nobel Prize in Chemistry winner Melvin Calvin wrote a book on the subject,[4] and so did Alexander Oparin.[5] What links most of the early work on the origin of life was the idea that before biological evolution began there must have been a process of chemical evolution.[6] Another question which has been discussed by J.D. Bernal and others is the origin of the cell membrane. By concentrating the chemicals in one place, the cell membrane performs a vital function.[7]

　　Many religions teach that life did not evolve spontaneously, but was deliberately created by a God or several gods. Often they claim that this happened within the last few thousand years, which is much more recent than the fossil record suggests. The lack of evidence that this happened means that most scientists do not believe it. These theories are known as creationism.

　　The earliest claimed lifeforms are fossilized microorganisms (or microfossils). They were found in iron and silica-rich rocks which were once hydrothermal vents in the Nuvvuagittuq greenstone belt of Quebec, Canada.

　　These rocks are as old as 4.28 billion years. The tubular forms they contain are shown in a report.[8] If this is the oldest record of life on Earth, it suggests "an almost instantaneous emergence of life" after oceans formed 4.4 billion years ago.[9][10][11] According to Stephen Blair Hedges, "If life arose relatively quickly on Earth … then it could be common in the universe."

　　A scientific study from 2002 showed that geological formations of stromatolites 3.45 billion years old contain fossilized cyanobacteria.[13][14] At the time it was widely agreed that stromatolites were oldest known lifeform on Earth which had left a record of its existence. Therefore, if life originated on Earth, this happened sometime between 4.4 billion years ago, when water vapor first liquefied,[15] and 3.5 billion years ago. This is the background to the latest discovery discussed above.

　　Earliest evidence of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in the nearby Akilia Islands. This is because a high level of the lighter isotope of carbon is found there. Living things uptake lighter isotopes because this takes less energy. Carbon entering into rock formations has a concentration of elemental δ13C of about -5.5. of 12C, biomass has a δ13C of between -20 and -30. These isotopic fingerprints are preserved in the rocks. With this evidence, Mojzis suggested that life existed on the planet already by 3.85 billion years ago.

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