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  • 生命出现的证据

    Where did life arise on Earth?
    ::地球上的生命在哪里出现?

    It is unclear whether the transition from chemistry to biology occurred on the land surface of Earth or in the oceans. Those intermediate species on our planets - entities that are more than chemistry, but less than life - no longer seem to exist . 
    ::从化学到生物学的转变是在地球的陆地表面还是在海洋中发生的,尚不清楚。 我们星球上的中间物种 — — 不仅仅是化学,但比生命还小的实体 — — 似乎已经不复存在。

    In the mid-1960s, Alexander Graham Cairns-Smith proposed that  complex prebiotic molecules  m ay have first organized around the regular crystal patterns found in minerals in clay. Cairns-Smith hypothesized that electrostatic forces in mineral crystals would help to concentrate and  align specific molecules to their surface. With the help of this clay, precursors of  RNA could have assembled,  triggering the start of an  RNA world.  Tide pools have also been proposed as the site  where chemistry developed the complexity needed for life . The cycles of  evaporation and subsequent  addition of water in tide pools might have concentrated  the organic materials necessary to form life. Higher concentrations increase the likelihood that these organic materials will react and form RNA or lipids.
    ::1960年代中期,Alexander Graham Cairns-Smith提出,复杂的生物前分子可能首先围绕在粘土矿物中发现的常规晶体形态进行组织。Cairns-Smith假设矿物晶体中的静电力量将有助于浓缩特定分子并使其与表面相匹配。在这种粘土的帮助下,RNA的先质可能已经组装起来,从而触发了RNA世界的诞生。还提议将潮化池作为化学发展生命所需复杂性的地点。水池中的蒸发循环和随后增加的水可能集中了形成生命所必需的有机材料。高浓度增加了这些有机材料反应和形成RNA或脂质的可能性。

    It is possible that, far from the rocky surface of Earth , life originated deep in the oceans  when our planet was a frozen  world. During the  phases where  Earth was  covered  by  a thick mantle of ice, t he ice may have  protected organic compounds be neath the ice from the damage of impacting asteroids or  ultraviolet  radiation from the sun.  
    ::在地球被冰层覆盖的阶段,冰层可能保护了冰层下的有机化合物免受小行星撞击或太阳紫外线辐射的破坏。

    D eep-sea   are hot spots that release gas from the Earth's interior. DNA sequencing   suggests that LUCA was a thermophilic microbe and deep sea vents  might have been ideal locations for nurturing that early life. At first glance, this seems an inhospitable place for life. These vents are  located in some of the darkest, highest pressure environments on  our world. However, the gas  that is  released from hydrothermal vents   creates  a natural chemical gradient that can be used by living cells  to generate the energy needed for metabolism . In deep hydrothermal vents, this chemical gradient flows around the rocks surrounding the vent and creates nooks where organic compounds  can collect and reach higher concentrations. M inerals that are  capable of acting as catalysts  have been found in deep sea vents. Therefore, h ydrothermal  vents seem to provide the needed ingredients:  high concentrations of organic compounds, natural catalysts, and a powerful energy source in the absence of sunlight.  T oday, extremely varied ecosystems are found around these vents.
    ::深海是释放地球内部气体的热点。 DNA测序表明, LUCA 是热性微生物, 深海喷口可能是培育早期生命的理想地点。 乍一看, 这似乎是不适宜生存的地方。 这些喷口位于世界上一些最黑暗、最高压力环境中。 然而, 热液喷口释放的气体产生天然化学梯度, 生命细胞可以使用它来产生新陈代谢所需的能量。 在深厚的热液喷口, 这种化学梯度在喷口周围的岩石上流动, 并创造 nook, 有机化合物可以收集和达到更高的浓度。 在深海喷口发现了能够作为催化剂的矿物。 因此, 热液喷口似乎提供了所需的成分: 高浓度的有机化合物, 天然催化剂, 以及缺乏阳光的强大能源源。 如今, 这些喷口周围的生态系统极为多样化。

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    A deep-sea hydrothermal vent. These vents release a cocktail of varied chemicals and ions that contain the starting materials for life and create a helpful chemical gradient that can be used as a source of natural energy.
    ::深海热液喷口:这些喷口释放出各种化学物和离子的鸡尾酒,这些化学物和离子含有生命的起始材料,并产生有用的化学梯度,可作为自然能源。

     

    When did life arise on Earth?
    ::生命何时在地球上出现?

    The evidence for timing  the initial rise of life on our planet is ambiguous.  The most ancient  evidence has been destroyed as the first forms of life  would have been single-celled organisms that did not leave behind fossil remains . However, suggestive evidence from  biology, chemistry, and geology  supports e stimates that life arose on Earth between  3 to 4 billion years ago. Since t he Earth formed about 4.56 billion years ago, the early appearance of life  hints  that the evolution from chemistry to life may be statistically probable. If so, this greatly increases the  odds that life has also evolved elsewhere.
    ::我们星球上生命最初崛起的时机证据是模糊的。最古老的证据已经被摧毁,因为第一种生命形式本会是没有留下化石的单细胞生物。然而,生物学、化学和地质学的暗示证据支持了30至40亿年前地球上生命诞生的估计。自地球在45.6亿年前形成以来,生命的早期出现暗示从化学到生命的演变在统计上可能是可能的。如果是这样,这大大增加了生命在其他地方也演变的可能性。

    1.  Carbon isotopes
    ::1. 碳同位素

    The  earliest line of evidence for timing the rise of life comes from carbon isotopes. Carbon naturally occurs in three isotopes, always with 6 protons, but with either   6 , 7,  or 8 neutrons, which are annotate d  12 C ,   13 C , and   14 C   respectively. The superscripts indicate the atomic mass number, or the total number of protons and neutrons in an element.  Of these isotopes, 14 C will undergo radioactive decay with a half life of 5730 years. However,  12 C   and   13 C   do not decay and t herefore , the ratio of   12 C   to   13 C   is   constant over time on the Earth . However, life preferentially  incorporates  the lighter   12 C, rejecting  13 C. Therefore, a low ratio of 13 C to 12 C  offers a tentative suggestion  that organic material existed and that isotopic imbalance can be  incorporated into the metamorphic structure of the rock. 
    ::生命升起的时间最早的证据线来自碳同位素。碳自然在三个同位素中产生,始终有6个质子,但有6、7或8个中子,分别有注解12C、13C和14C。上标表示原子质量数,或元素中的质子和中子总数。在这些同位素中,14C将经历放射性衰减,半衰期为5730年。然而,12C和13C不会衰减,因此,12C和13C在地球上的比例是恒定的。然而,生命优先包含较轻的12C,拒绝13C。因此,13C的低比例提供了一种初步的建议,即有机材料存在,而同位素不平衡可以纳入岩石的变形结构。

    Why does life prefer to use 12 C?  While the chemical reactivity of an atom is predominantly driven  by the electron configuration, more massive isotopes tend to have slightly slower reaction rates . That slight edge in the speed of reaction rates is enough of an advantage to favor the uptake of carbon-12 over carbon-13  in   organic biochemistry.
    ::生命为什么更喜欢使用 12C ? 虽然原子的化学反应主要由电子结构驱动,但更大规模同位素的反应速度往往略慢一些。 反应速度的微小边缘足以有利于有机生物化学中碳-12比碳-13的吸收。

    Zircon crystals are commonly used to assess ages in very ancient geological records because they are very durable minerals, resistant to both heat and corrosion. Trapped  minerals  can be preserved when zircons form. A group of scientists from the University of California, Los Angeles studied 10,000 zircons gathered from western Australia. Of these ten thousand zircons, one contained a graphite inclusion, a compound composed entirely of carbon atoms.
    ::在非常古老的地质记录中,Zircon晶体通常用来评估年代,因为它们是耐久的矿物,既耐热又耐腐蚀。当Zircons形成时,被覆盖的矿物可以得到保存。来自加利福尼亚大学的一组科学家研究从澳大利亚西部收集的10 000个Zircons。在这10 000个Zircons中,有一个包含一个石墨,一个完全由碳原子组成的化合物。

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    A hearty zircon crystal. Zircon crystals are not only capable of surviving through great stressors, but they are also good at keeping elements trapped inside them. Why are both of these properties important for accurate radiometric dating?
    ::Zircon晶体不仅能够通过巨大的压力来生存,而且能够将元素困在里面。 为什么这两个特性对精确的辐射计约会都很重要?

    Radiometric dating showed that this zircon crystal was 4.1 Gyr old and the carbon inclusion exhibited  a  larger ratio of   12 C to   13 C . , dating back to 4.1 Gya? On Earth today, this would be a good  indicator of organic material . But, interpreting this result from  so long ago is more controversial.   
    ::辐射测定的约会表明,这个zircon晶体是4.1 Gyr 的古代,碳的吸收比例为12C对13C,比大于12C对13C,可以追溯到4.1 Gya?在今天的地球上,这将是有机材料的好指标。 但是,解释如此久远之前的这一结果则更具争议性。

    2. Stromatolites
    ::2. 血压醇

    Ancient stromatolites offer a more secure  timeline for  the emergence of life. These fossilized structures  are found in shallow waters and look like modern structures formed by  cyanobacteria. The oldest stromatolites have been found in South Africa and Australia and date back to the  early  Archean Period between 3.2 - 3.5 Gya. By the end of the Archean and throughout the Proterozoic geological periods, stromatolites appear to have been abundant and formed the first reefs.
    ::古代的岩石石为生命的出现提供了更安全的时间表,这些化石结构在浅水中发现,看起来像是氰化物形成的现代结构,在南非和澳大利亚发现最古老的岩石石,可追溯到最初的阿切斯时期3.2-3.5格亚,到阿切斯群岛末期和整个Proterozoic地质时期,石化石似乎富饶,形成第一个珊瑚礁。

     

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    Fossilized stromatolites. This distinctive banding pattern resembles structures produced by living organisms today. This suggests that similar organisms existed at the time these fossils were formed.
    ::化石化的电磁石。 这种独特的带状模式类似于今天活生物体产生的结构。 这表明这些化石形成时也存在类似的生物。

    S tromatolites today  are composed of layers of microbial mats of  photosynthesizing cyanobacteria,  as shown in the YouTube video below .  Photosynthesis depletes carbon dioxide in the water, precipitating calcium carbonate deposits, which along with other sandy sediments,  is trapped  in the sticky bacterial film. The bacterial colonies grow upward  towards better exposure to the sun, and o ver time the layers of bacterial film and mineral precipitates build up to form distinctive layered stromatolites. Similar  stromatolite structures  are formed today.   
    ::今天的斯特罗玛托利物由相光合成氰化物的微生物垫层组成,如下面YouTube视频所示。光合成物在水中消耗二氧化碳,碳酸钙矿床和其他沙沉淀物一起被困在粘粘的细菌薄膜中。细菌聚集地向上增长,以更好地接触太阳,随着时间的推移,细菌薄膜层和矿物沉淀物层逐渐形成独特的层层状的磁铁。今天,Simimilar 磁铁结构已经形成。

    We guess that  ancient stromatolites were formed through biological processes, since we observe this behavior in  microbes today. T he processes required to make these structures seem to require  microbes that are already quite complex and capable of photosynthesis. Actual first life then, likely to be a  more  primitive organism,  would have  existed even earlier than these ~3.5 Gyr old structures.
    ::我们猜想古代的铁石石石是由生物过程形成的,因为我们在今天的微生物中观察了这种行为。 制造这些结构所需的过程似乎需要已经相当复杂和具有光合作用能力的微生物。 那时,实际的第一生命,很可能是一个更原始的有机体,比这些~3.5Gyr的古老结构早了。

    3. Microfossils
    ::3. 微量信息

    The  most direct evidence for life comes in the form of microfossils, fossils preserving micro-organisms that may have been among the first living creatures on Earth. The most convincing microfossils date back to approximately 3 billion years ago. However, these are difficult to identify both because rocks undergo erosion over time and because the structures that resemble microfossils  might  actually be formed by nonbiological processes. Many supposed microfossils turn out to be false positives after  careful chemical analysis. Though the discovery of a microfossil is an unmistakable mark of life, it is  easy to be  fooled . The   below gives you a good idea of the scant information content in microfossils.
    ::生命的最直接证据是微量信息,即保存微生物的化石,它们可能是地球上最早的生物。最有说服力的微量信息可以追溯到大约30亿年前。然而,由于岩石随着时间的流逝而不断侵蚀,而且类似微量信息的结构可能实际上是由非生物过程形成的,因此很难确定两者。许多假定的微量信息在经过仔细的化学分析后变成了虚假的阳性。虽然发现微量信息是生命的明显标志,但很容易被愚弄。下面可以很好地了解微量信息中很少的信息内容。

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    An example of possible microfossils. A micrometer is a millionth of a meter. Human hair is on average 80 micrometers thick.
    ::一个微米是一百万米,人的头发平均厚80米。

    4. Great Oxidation Event
    ::4. 大氧化氧化物事件

    The Great Oxidation Event is alternatively called the Oxygen Catastrophe depending on whether you ask aerobes or anaerobes . It marks a drastic change in the Earth's atmospheric composition that occurred about 2.5 Gya.
    ::大氧化事件则称为氧气灾难,这取决于是问厌食还是厌食。它标志着地球大气构成发生了巨变,大约2.5Gya。

    A n increase in atmospheric oxygen is seen in the geological record as a sudden onset of oxidation of iron occurred. Iron oxidation  coincides  with the  geological evidence for glaciation 2.5 billion years ago. Oxygen is not a greenhouse gas, but it is a highly reactive species  that would have interacted with the primitive methane atmosphere  produced by volcanic outgassing . Methane is a powerful greenhouse gas. A s oxygen levels rise, oxygen would  react with  methane to form carbon dioxide and water.  This mechanism for removing methane would have produced  the significant cooling  and  widespread glaciation.  An outcrop  of  ancient Canadian rocks shows evidence of a glaciation event between different rock layers. Below (and therefore older than) this glaciation event, the rocks are consistent with low levels of atmospheric oxygen. Younger rock above the glaciation layer show significantly higher oxidation.  Radiometric dating of the rock show that the  increase in  atmospheric oxygen, or the Great Oxidation Event,  occurred  2.45 billion years ago. 
    ::在地质记录中,大气氧的增加被视为铁的突然氧化,铁的氧化与25亿年前冰川变异的地质证据相吻合。氧不是温室气体,但是一种高度反应的物种,与火山喷气产生的原始甲烷大气相互作用。甲烷是一种强大的温室气体。随着氧气水平的上升,氧气会与甲烷发生反应,形成二氧化碳和水。这种去除甲烷的机制将产生显著的冷却和广泛的冰川化。加拿大古代岩石的外表显示不同岩石层之间的冰川变异事件的证据。以下(以及因此比以前),岩石与大气氧含量低的情况一致。冰川层以上的恒温岩显示出显著较高的氧化作用。岩石的辐射测定显示,大气氧或大氧化事件的增加发生在2,45亿年前。

    The addition of oxygen to the atmosphere is believed to be the result of photosynthesis from cyanobacteria. Oxygen did not accumulate immediately because of the enormous number of sinks, including reduced gases and minerals that would have overwhelmed its production. Those sinks for oxygen are now largely saturated; respiration of anaerobic organisms (including us) and decay of organic material take up most of the oxygen produced today.    
    ::据认为,大气中增加氧是氰化细菌光合作用的结果,而氧并没有立即积累,因为吸收汇数量巨大,包括气体和矿物减少,会超过其生产。 氧的吸收汇现在基本饱和;厌氧生物(包括我们)的呼吸和有机材料的衰减占用了今天产生的大部分氧气。

    As the atmosphere cooled, a  positive feedback loop ensued . Water vapor, which is another important greenhouse gas, would have condensed out of the atmosphere. The Earth eventually froze over and  became what is known as '' ." Volcanic activity  would have continued during this phase, releasing internal heat along with carbon dioxide into the air. With a sufficient amount of CO 2 , the  greenhouse warming   would have  melted the ice, allowing the Earth to recover from its frozen state. Assuming present rates of volcanism, the necessary build up of CO 2 would take 10 million years. Evidence in the rock record suggests that  one snowball Earth  event occurred around 2.2 billion years ago.
    ::随着大气的冷却,一个积极的回馈循环随之而来。水蒸气是另一个重要的温室气体,它会从大气中凝结出来。地球最终会冻僵,变成所谓的“'”。在这一阶段,火山活动会继续下去,释放内热和二氧化碳到空气中。如果有足够的二氧化碳,温室变暖就会使冰融化,使地球能够从冰冻状态中恢复。假设目前的火山化速度,那么必要的二氧化碳积聚需要1 000万年时间。 岩石记录中的证据表明,22亿年前就发生了一场雪球地球事件。

    An artist rendition of how the Snowball Earth may have appeared. The extent of glaciation would have been worldwide, causing a thick layer of ice from pole to pole.
    ::对雪球地球的出现进行艺术家的解说。 浮化的程度本来会波及全世界,导致极地之间的冰层稠密。