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“生命的起源”在这里重定向。关于生命起源的非科学观点，请参阅创世神话。 不要与Biogenesis混淆。 对于最古老的生命形式，请参见最早的已知生命形式。 生物学，或非正式地生命起源，[3] [4] [5] [注释1]是生命源于非生命物质的自然过程，如简单的有机化合物。[6] [4] [7] [8]从非生物体到生物体的过渡不是一个单一的事件，而是一个逐渐增加复杂性的过程，涉及分子自我复制，自组装，自催化和细胞膜。[9] [10] [11]尽管在科学家中发生血管生成是没有争议的，但是没有单一的，普遍接受的生命起源模型，本文提出了如何发生血管生成的几个原则和假设。 研究人员通过分子生物学，古生物学，天体生物学，海洋学，生物物理学，地球化学和生物化学相结合的方式研究血管生成，旨在确定生命前化学反应如何产生生命。[12]自然发生的研究可以是地球物理，化学，或生物，[13]与更近的方法尝试所有三个的合成，[14]因为生活是在与今天地球上的生活条件截然不同的条件下产生的。生命通过碳和水的专门化学作用发挥作用，并在很大程度上建立在四个关键的化学家族：脂质（脂肪细胞壁），碳水化合物（糖，纤维素），氨基酸（蛋白质代谢）和核酸（自我复制的DNA和RNA）。任何成功的血管生成理论都必须解释这些分子类别的起源和相互作用。[15]许多关于血管生成的方法研究了自我复制 分子的方式或者它们的组成部分成立。研究者普遍认为，地球上现在的生活从下降RNA世界，[16]虽然RNA为基础的生命可能不是第一个生命存在过。[17] [18] 经典的1952年Miller-Urey实验和类似的研究表明，大多数氨基酸，即所有生物体中使用的蛋白质的化学成分，都可以在无机化合物的条件下，在复制早期地球的那些条件下合成。科学家提出了各种可能引发这些反应的外部能量来源，包括闪电和辐射。其他方法（“代谢优先”假设）着重于理解早期地球上化学系统中的催化如何提供自我复制所必需的前体分子。[19]复杂的有机分子存在于太阳系和星际空间中，这些分子可能为地球上的生命发展提供了起始材料。[20] [21] [22] [23] 该生物化学生活可能不久后也开始大爆炸一中，13.8十亿年前，可居住时期当宇宙的年龄只有10至17万年前。[24] [25]所述的胚种论假说认为，微观寿命被分配到早期地球空间粉尘，[26] 流星，[27] 小行星和其他小太阳系机构和寿命可在整个存在宇宙。[28]panspermia假说认为生命起源于地球之外，但并未明确解释其起源。 然而，地球仍然是唯一的地方，宇宙中已知的藏匿生活，[29] [30] ，并从地球的化石证据通知偶发的大多数研究。地球的年龄约为45.4亿年; [31] [32] [33]地球上最早的无可争议的生命证据至少可以追溯到35亿年前[34] [35] [36]，也可能早在Eoarchean时代（年龄在36到40亿年之间）之后，地质地壳开始凝固后，熔化的Hadean Eon。2017年5月，科学家们在西澳大利亚皮尔巴拉克拉通（Pilbara Craton）发现了34.8亿年前的岩石和其他相关矿藏（通常在温泉和间歇泉附近发现）的土地早期生命的可能证据。[37] [38] [39] [40]然而，许多发现表明生命可能更早出现在地球上。2017年[更新]，微体化石，化石或微生物，内水泄沉淀物从3.77日期为4.28十亿岁发现魁北克加拿大可能是地球上最古老的生命记录，暗示生命是在44亿年前海洋形成后不久开始的。[1] [2] [41] [42] [43]根据生物学家斯蒂芬·布莱尔·赫奇斯的说法，“如果生命在地球上相对较快地出现......那么它在宇宙中可能是常见的。” [44] [45] [46] 据认为，Hadean地球具有二次大气层，是通过从星际撞击物中积累的岩石脱气而形成的。起初，人们认为地球的大气层由氢化合物 - 甲烷，氨和水蒸气组成 - 并且在这种还原条件下开始生命，这有利于有机分子的形成。根据后来的模型，通过对古代矿物的研究表明，哈迪恩时期晚期的大气主要由水汽，氮和二氧化碳组成。 ，含少量的一氧化碳，氢和硫化合物。[47]在其形成过程中，地球失去了其初始质量的重要部分，其中原行星盘的较重岩石元素的核仍然存在。[48]因此，地球缺乏重力将大气中的任何分子氢保持在其中，并且在大规模的原始惰性气体中，在哈代期间迅速失去了氢。认为二氧化碳在水中的溶液使海水呈微酸性，使其pH值约为5.5。[ 引证需要 ]当时的气氛被称为“巨大的，富有成效的户外化学实验室”。[49]它可能类似于今天由火山释放的气体混合物，火山仍然支持一些非生物化学。[49] 在地球形成后的两亿年（200 Ma），在温度高达100°C（212°F）的环境中，海洋可能首先出现在Hadean Eon中，并且pH值约为5.8。中性。[50]这已经由4.404约会支持 嘎 -old 锆石晶体变质石英岩的山Narryer在澳大利亚西部杰克山的的皮尔巴拉地区，这是证据表明，海洋和陆壳 150中存在 马地球形成的。[51]尽管火山活动可能增加并且存在许多较小的构造 “血小板”，但有人认为，在4.4到4.3 Ga（十亿年）之间，地球是一个水世界，几乎没有任何大陆地壳，极度湍流的大气层和水太阳光受到强烈的紫外线（UV），来自T Tauri阶段的太阳，宇宙辐射和持续的火花撞击。[52] 哈迪恩环境对现代生活将是非常危险的。经常与直径达500公里（310英里）的大型物体发生碰撞，足以对地球进行消毒，并在撞击后的几个月内使海洋蒸发，热蒸汽与岩石蒸气混合成为高空云层覆盖这个星球。几个月后，这些云的高度将开始减少，但云底仍将在未来一千年左右升高。在那之后，它将开始在低空下雨。再过两千年，降雨将慢慢降低云层的高度，在撞击事件发生后仅3000年就将海洋恢复到原来的深度。[53] 生命最早的生物学证据编辑主要文章：最早的已知生命形式 对于细菌门的分枝，请参见细菌门。 生命之树根最常被接受的位置是单系域细菌和由古细菌和真核生物形成的进化枝之间的基于从C.Woese开始的几个分子研究的所谓“传统生命之树”。 。[54]极少数研究得出的结论不同，即根在域细菌中，或者在Firmicutes门中[55]，或者Chloroflexi门是与Archaea Eukaryotes和其他细菌分支的基础。由Thomas Cavalier-Smith提出。[56]最近Peter Ward已经建立了一种替代观点，其根植于非生物RNA合成中，其被封闭在胶囊内，然后产生RNA 核酶复制物。有人提出，然后在Dominion Ribosa（RNA生命）之间，以及在作为Domain Viorea和Dominion Terroa [ 需要澄清 ]的核酶RNA病毒丢失后分叉，在脂质壁内产生大细胞后，产生DNA 20基于氨基酸和三联体编码，被确立为早期系统发育树的最后普遍共同祖先或LUCA。[57] 地球上最早的生命存在于35亿年前，[34] [35] [36]在Eoarchean时代，当熔化的Hadean Eon之后有足够的地壳固化。最早的实物证据迄今发现的由微化石在Nuvvuagittuq绿岩带北魁北克的，在“条带状含铁建造”石头至少3.77十亿，可能4.28十亿岁。[1] [58]这一发现表明，在海洋形成后，生命几乎立即发展。注意到微生物的结构类似于在热液喷口附近发现的细菌在现代时代，并支持在热液喷口附近开始生成血管的假设。[42] [1] 同样值得注意的是来自格陵兰西南部的37亿年变质沉积岩[59]中的生物成因石墨，以及来自西澳大利亚的34.8亿年来的砂岩中发现的微生物垫石化石。[60] [61]格陵兰西南部Isua 地壳带附近的Akilia岛岩石早期生命的证据，可追溯到37亿年前，已显示出生物碳同位素。[62] [63]在Isua上壳带的其他部分，石墨夹杂物被困在石榴石中晶体与生命的其他元素相连：氧气，氮气，以及磷酸盐形式的磷，为37亿年前的生命提供了进一步的证据。[64] 在澳大利亚西部皮尔巴拉地区的Strelley Pool，在化石海滩中的含黄铁矿砂岩中发现了早期生命的令人信服的证据，其显示了圆形管状细胞，其在没有氧气的情况下通过光合作用氧化硫。[65] [66] [67] 2015年对西澳大利亚锆石的进一步研究表明，至少41亿年前地球上的生命可能存在。[44] [68] [69] 传统上认为在4.28 [1] [2]和3.8 Ga 之间的时期，巨行星轨道的变化可能引起了小行星和彗星的猛烈轰击[70]，这使得月球和其他内行星被麻痹了。（水星，火星，大概是地球和金星）。这可能会反复消毒地球，在此之前就已经出现了生命。[49]从地质学角度来看，哈迪恩地球的历史将远远超过历史上任何时候。对陨石的研究表明放射性同位素例如半衰期为7.17×10 5（717千）年的铝-26，半衰期为1.250×10 9（12.5亿）年的钾-40，主要在超新星中产生的同位素，更多共同。[71]核心与地幔之间的重力分选导致的内部加热会引起大量的地幔对流，可能导致比现在更多更小，更活跃的构造板块。 这些破坏性环境事件之间的时间段为早期环境中可能的生命起源提供了时间窗口。如果深海海水热带环境是生命起源的地方，那么早在4.0到4.2 Ga就可能发生血管生成。如果这个地点位于地球表面，那么血管生成只能发生在3.7到4.0 Ga之间。[72] 2016年，确定了可能存在于地球上所有生物的最后世界共同祖先（LUCA）中的355个基因。[73]对来自不同系统发育树的总共610万个原核蛋白编码基因进行了测序，从286,514个蛋白质簇中鉴定了可能与LUCA相同的355个蛋白质簇。结果''描述了LUCA作为厌氧，CO 2固定，H 2依赖于Wood-Ljungdahl途径，N 2 -固定和嗜热。LUCA的生物化学充满了FeS簇和激进反应机制。它的辅助因素 揭示了对过渡金属，黄素，S-腺苷甲硫氨酸，辅酶A，铁氧还蛋白，钼喋呤，corrins和硒的依赖性。它的遗传密码需要核苷修饰和S-腺苷甲硫氨酸依赖的甲基化。结果表明产气 梭菌作为355个系统发育中的基础进化枝，表明LUCA 在富含H 2，CO 2和铁的地球化学活性环境中居住于厌氧水热通风设置。[74] MD Brazier已经证明，早期的微型化石来自一个炎热的气体世界，如甲烷，氨，二氧化碳和硫化氢，它们对当前的生命有毒。[75]对传统三重生命树的另一种分析表明，嗜热和超嗜热细菌和古菌最接近根，这表明生命可能在炎热的环境中进化。[76] 概念历史编辑自发生成编辑主要文章：自发的一代 从非生命物质中自发产生某种生命形式的信念可以追溯到亚里士多德和古希腊哲学，并一直支持西方学术直到19世纪。[77]这种信念与异质性的信念相结合，即一种生命形式来自不同的形式（例如，来自花的蜜蜂）。[78]自发生成的经典概念认为某些复杂的生物有机体是通过腐烂有机物质产生的。根据亚里士多德的说法，这是一个容易观察到的事实，即蚜虫是由落在植物上的露水产生的，苍蝇来自腐烂的物质，来自脏干草的老鼠，来自水体底部的腐烂原木的鳄鱼，等等。[79]在17世纪，人们开始质疑这种假设。1646年，托马斯·布朗爵士发表了他的Pseudodoxia Epidemica（副标题是对许多收到的原则的调查，通常是推定的真相），这是对错误信念和“粗俗错误”的攻击。他的当代人亚历山大·罗斯错误地驳斥了他，他说：“质疑这种[自发的一代]，就是质疑理性，理智和经验：如果他对此表示怀疑，就让他去Ægypt，在那里他会找到田野与老鼠蜂拥而生的Nylus泥，对于居民的巨大灾难。[80] [81] 1665年，罗伯特胡克发表了第一批微生物图纸。Hooke于1676年由Antonie van Leeuwenhoek跟踪，他绘制并描述了现在被认为是原生动物和细菌的微生物。[82]许多人认为微生物的存在是支持自发生成的证据，因为微生物对于有性繁殖似乎过于简单，并且尚未观察到通过细胞分裂的无性繁殖。Van Leeuwenhoek对跳蚤和虱子可能由腐败自发产生的常见想法提出质疑而且那些青蛙也可能来自粘液。利用密封和开放式肉类孵化以及昆虫繁殖的密切研究等广泛的实验，到1680年代，他确信自发生成是不正确的。[83] 反对自发生成的第一个实验证据出现在1668年，当时弗朗切斯科雷迪表明，当苍蝇无法产卵时，肉中没有蛆虫出现。逐渐表明，至少在所有较高且容易看见的生物体的情况下，先前关于自发生成的情绪是错误的。另一种选择似乎是生物发生：每一个生物都来自一个已存在的生物（omne vivum ex ovo，拉丁语为''来自鸡蛋的每一个生物''）。 1768年，Lazzaro Spallanzani证明微生物存在于空气中，可以通过煮沸杀死。1861年，路易斯巴斯德进行了一系列实验，证明细菌和真菌等生物不会自发地出现在无菌，营养丰富的培养基中，但只能通过入侵而出现。 相信通过自发生成进行自我排序是不可能的。到19世纪中叶，由于巴斯德和其他人的工作，生物发生理论积累了如此多的证据支持，即自发生成的替代理论已经被有效地证明了。X射线晶体学的先驱约翰·德斯蒙德·伯纳尔（John Desmond Bernal）认为，诸如自发生成之类的早期理论是基于一种解释，即生命是偶然因机会事件而产生的。[84] 词源编辑主要文章：生物发生 生物发生这个术语通常归功于Henry Charlton Bastian或Thomas Henry Huxley。[85]巴斯蒂安在与约翰廷德尔的未发表的交换中使用1869年左右的术语来表示“生命起源或开始”。1870年，作为英国科学促进会新任主席的赫胥黎发表了题为“ 生物发生与生殖发育”的演讲。[86]在其中他引入了生物发生这个术语（与巴斯蒂安的含义相反）以及生物发生：
随后，在巴斯蒂安1871年出版的“最低生物的起源模式”一书的序言中，[87]巴斯蒂安提到了可能与赫胥黎的用法混淆并明确放弃了自己的意思：
路易斯巴斯德和查尔斯达尔文编辑查尔斯达尔文于1879年 路易斯·巴斯德（Louis Pasteur）评论说，他在1864年发现了一个他认为是权威的发现，“自发生成的教义永远不会从这个简单实验所致的致命打击中恢复过来。” [89] [90]另一种选择是生命在地球上的起源来自宇宙中的其他地方。定期复活（参见上面的Panspermia），伯纳尔说，这种方法“等同于断言形而上学的精神实体的运作......它开启了创造者或创造者设计创造的论点。” [91]伯纳尔说，这种理论是不科学的。一个同时流行的理论是，生命是内在生命力的结果''，在19世纪后期由亨利柏格森支持。 的思想演变由自然选择所提出查尔斯达尔文杜绝这些形而上学的神学。在1871年2月1日致约瑟夫道尔顿胡克的一封信中，[92]达尔文讨论了这样的建议，即原始的生命火花可能是在一个温暖的小池塘里开始的，它有各种氨和磷酸盐，光，热，电， ＆c。，礼物，蛋白质化合物是化学形成的，准备进行更复杂的变化。他接着解释说，“现在这种物质会立即被吞噬或吸收，而在生物形成之前就不会这样了。” 他曾于1863年写信给胡克，指出“这只是垃圾，目前正在思考生命的起源; 人们可能会想到物质的起源。在“物种的起源”中，他提到生命被“创造”，他“真的意味着”出现了“一些完全未知的过程”，但很快就后悔使用了旧约的术语“创造”。[93] ''原始汤''假设编辑主条目：原始汤 更多信息：Miller-Urey实验 亚历山大·奥帕林（右）在他的实验室，1938年 直到1924年，亚历山大·奥帕林（Alexander Oparin）认为大气中的氧气阻止了某些有机化合物的合成，而这些有机化合物是生命进化的必要组成部分，因此没有出现关于该主题的新的值得注意的研究或假设。在他的着作“生命的起源”中，[94] [95] Oparin提出路易斯·巴斯德尔攻击的“自发生命”确实发生过一次，但现在已经不可能了，因为早期地球上发现的条件已经发生了改变了，先前存在的生物会立即消耗任何自发产生的生物。Oparin认为，有机分子的“原始汤”可以通过阳光的作用在无氧气氛中产生。这些将以更复杂的方式结合，直到它们形成凝聚液滴。这些液滴会通过与其他液滴融合而“ 生长 ”，并通过裂变“ 再生 ”成子液滴，因此具有原始代谢，其中促进“细胞完整性”的因子存活，而那些不会灭绝的因子。许多关于生命起源的现代理论仍然以奥帕林的思想为出发点。 Robert Shapiro总结了Oparin和JBS Haldane的“原始汤”理论，其成熟形式如下：[96]
大约在这个时候，霍尔丹表示，地球的益生元海洋（与现代海洋完全不同）会形成一种“热稀汤”，其中可能形成有机化合物。伯纳尔将这种观点称为生物生物学或生物生物学，生物物质从自我复制但非生命的分子发展而来[84] [97]，并提出生物生成经历了许多中间阶段。 1952年，“汤”理论最重要的实验性支持之一出现。斯坦利·米勒和哈罗德·C·尤里进行了一项实验，证明了有机分子是如何在无条件前提下由无机前体自发形成的。 Oparin-Haldane假说。现在着名的Miller-Urey实验使用了高度还原的气体混合物 - 甲烷，氨和氢以及水蒸气 - 形成简单的有机单体，如氨基酸。[98]气体混合物循环通过一个向混合物输送电火花的装置。一周后，发现系统中约10％至15％的碳是有机化合物的外消旋混合物形式，包括氨基酸，它们是蛋白质的组成部分。这为“汤”理论的第二点提供了直接的实验支持，并且围绕理论的剩余两点，现在大部分争论都集中在这里。 伯纳尔表明，基于这一和随后的工作，原则上形成大多数我们认为是无机前体生命必需分子的分子并不困难。例如，Oparin，Haldane，Bernal，Miller和Urey所持的基本假设是原始地球上的多种条件有利于从这种简单前体合成同一组复杂有机化合物的化学反应。2011年对使用当前和更先进的分析设备和技术进行的Miller和Urey实验产生的原始提取物的保存小瓶进行了重新分析，发现了比20世纪50年代最初发现的更多的生物化学品。其中一个更重要的发现是23种氨基酸，远远超过最初发现的5种氨基酸。[99]然而，伯纳尔说，“仅仅解释这些分子的形成是不够的，必要的是对这些分子起源的物理化学解释，这表明存在合适的自由能源和汇。” [100] 2017年10月的最新研究支持了这样一种观念，即地球形成后的生命可能是从“温暖的小池塘”中出现的RNA分子开始的。[46] 蛋白质类微球编辑主条目：Proteinoid In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal, Sidney W. Fox in the 1950s and 1960s studied the spontaneous formation of peptide structures (small chains of amino acids) under conditions that might plausibly have existed early in Earth''s history. In one of his experiments, he allowed amino acids to dry out as if puddled in a warm, dry spot in prebiotic conditions. He found that, as they dried, the amino acids formed long, often cross-linked, thread-like, submicroscopic polypeptide molecules now named ''proteinoid microspheres''.[101] In another experiment to set suitable conditions for life to form, Fox collected volcanic material from a cinder cone in Hawaii. He discovered that the temperature was over 100 °C (212 °F) just 4 inches (100 mm) beneath the surface of the cinder cone, and suggested that this might have been the environment in which life was created—molecules could have formed and then been washed through the loose volcanic ash into the sea. He placed lumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials, and baked the lava over the amino acids for a few hours in a glass oven. A brown, sticky substance formed over the surface, and when the lava was drenched in sterilized water, a thick, brown liquid leached out. The amino acids had combined to form proteinoids, and the proteinoids had combined to form small globules that Fox called ''microspheres''. His proteinoids were not cells, although they formed clumps and chains reminiscent of cyanobacteria, but they contained no functional nucleic acids or any encoded information. Based upon such experiments, Colin S. Pittendrigh stated in December 1967 that ''laboratories will be creating a living cell within ten years,'' a remark that reflected the typical contemporary naivety about the complexity of cell structures.[102] Current modelsEditThere is no single, generally accepted model for the origin of life. Scientists have proposed several plausible hypotheses, which share some common elements. While differing in the details, these hypotheses are based on the framework laid out by Alexander Oparin (in 1924) and by J. B. S. Haldane (in 1925), who postulated the molecular or chemical evolution theory of life.[103] According to them, the first molecules constituting the earliest cells ''were synthesized under natural conditions by a slow process of molecular evolution, and these molecules then organized into the first molecular system with properties with biological order''.[103] Oparin and Haldane suggested that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43−), with molecular oxygen (O2) and ozone (O3) either rare or absent. According to later models, the atmosphere in the late Hadean period consisted largely of nitrogen (N2) and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen (H2), and sulfur compounds;[104] while it did lack molecular oxygen and ozone,[105] it was not as chemically reducing as Oparin and Haldane supposed. In the atmosphere proposed by Oparin and Haldane, electrical activity can produce certain small molecules (monomers) of life, such as amino acids. The Miller–Urey experiment reported in 1953 demonstrated this. Bernal coined the term biopoiesis in 1949 to refer to the origin of life.[106] In 1967, he suggested that it occurred in three ''stages'':
Bernal suggested that evolution commenced between stages 1 and 2. Bernal regarded the third stage – discovering methods by which biological reactions were incorporated behind a cell''s boundary – as the most difficult. Modern work on the way that cell membranes self-assemble, and the work on micropores in various substrates may be a halfway house towards the development of independent free-living cells.[107][108][109] The chemical processes that took place on the early Earth are called chemical evolution. Since the end of the nineteenth century, ''evolutive abiogenesis'' means increasing complexity and evolution of matter from inert to living state.[110] Both Manfred Eigen and Sol Spiegelman demonstrated that evolution, including replication, variation, and natural selection, can occur in populations of molecules as well as in organisms.[49] Spiegelman took advantage of natural selection to synthesize the Spiegelman Monster, which had a genome with just 218 nucleotide bases, having deconstructively evolved from a 4500-base bacterial RNA. Eigen built on Spiegelman''s work and produced a similar system further degraded to just 48 or 54 nucleotides – the minimum required for the binding of the replication enzyme.[111] Following on from chemical evolution came the initiation of biological evolution, which led to the first cells.[49] No one has yet synthesized a ''protocell'' using simple components with the necessary properties of life (the so-called ''bottom-up-approach''). Without such a proof-of-principle, explanations have tended to focus on chemosynthesis.[112] However, some researchers work in this field, notably Steen Rasmussen and Jack W. Szostak. Others have argued that a ''top-down approach'' is more feasible. One such approach, successfully attempted by Craig Venter and others at J. Craig Venter Institute, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life are reached.[113][114][115] The NASA strategy on abiogenesis states that it is necessary to identify interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems.[116] Emphasis must continue to map the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection likely was a critical step in the emergence of prebiotic chemical evolution.[116] In October 2018, researchers at McMaster University announced the development of a new technology, called a Planet Simulator, to help study the origin of life on planet Earth and beyond.[117][118][119][120] It consists of a sophisticated climate chamber to study how the building blocks of life were assembled and how these prebiotic molecules transitioned into self-replicating RNA molecules.[117] Chemical origin of organic moleculesEditThe elements, except for hydrogen and helium, ultimately derive from stellar nucleosynthesis. On 12 October 2016, astronomers reported that the very basic chemical ingredients of life — the carbon-hydrogen molecule (CH, or methylidyne radical), the carbon-hydrogen positive ion (CH ) and the carbon ion (C ) — are largely the result of ultraviolet light from stars, rather than other forms of radiation from supernovae and young stars, as thought earlier.[121] Complex molecules, including organic molecules, form naturally both in space and on planets.[20] There are two possible sources of organic molecules on the early Earth: 根据最近的计算机模型的研究中，复杂的有机分子所必需的生命可能形成在原行星盘的尘埃颗粒围绕太阳地球的形成之前。[124] [125]根据计算机研究，同样的过程也可能发生在获得行星的其他恒星周围。（另见外星有机分子）。 从这些来源估算有机物的产量表明，在早期大气层中3.5 Ga之前的晚期重轰击使得有机物的数量与地面来源产生的相当。[126] [127] 据估计，晚期重型轰击还可以有效地对地球表面进行数十米的深度消毒。如果生命进化得比这更深，它也可以避免来自太阳演化的T Tauri阶段的早期高水平的紫外线辐射。地热加热的海洋地壳的模拟产生的有机物远远多于Miller-Urey实验中发现的有机物（见下文）。在深层热液喷口中，Everett Shock发现“有一种巨大的热力学驱动形成有机化合物，因为远离平衡的海水和热液流体混合并向更稳定的状态移动。” [128]休克已经发现，可利用的能量最大化在大约100-150摄氏度，恰好是发现极端嗜热细菌和嗜热 古菌的温度，位于最接近最后的普遍共同祖先（LUCA）的生命系统树的基础上。）。[129] 有机分子在行星表面上的积累和浓度也被认为是生命起源的必要早期步骤。[116]识别和理解导致在各种环境中产生益生元分子的机制对于建立生命起源于地球的成分清单至关重要，假设分子的非生物生产最终影响生命出现的分子选择。[116] 化学合成编辑虽然自组织和自我复制的特征通常被认为是生命系统的标志，但是存在许多非生物分子在适当条件下表现出这种特征的情况。Stan Palasek基于理论模型提出，由于热液喷口中的物理因素，核糖核酸（RNA）分子的自组装可以自发发生。[130] 宿主细胞内的病毒自组装对生命起源的研究具有意义[131]，因为它进一步证实了生命本来可以作为自组装有机分子开始的假设。[132] [133] Multiple sources of energy were available for chemical reactions on the early Earth. For example, heat (such as from geothermal processes) is a standard energy source for chemistry. Other examples include sunlight and electrical discharges (lightning), among others.[49] Computer simulations also suggest that cavitation in primordial water reservoirs such as breaking sea waves, streams and oceans can potentially lead to the synthesis of biogenic compounds.[134] Unfavourable reactions can also be driven by highly favourable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for carbon fixation (the conversion of carbon from its inorganic form to an organic one).[note 2] Carbon fixation via iron-sulfur chemistry is highly favourable, and occurs at neutral pH and 100 °C (212 °F). Iron-sulfur surfaces, which are abundant near hydrothermal vents, are also capable of producing small amounts of amino acids and other biological metabolites.[49] As early as the 1860s, experiments have demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. In particular, experiments by Butlerov (the formose reaction) showed that tetroses, pentoses, and hexoses are produced when formaldehyde is heated under basic conditions with divalent metal ions like calcium. The reaction was scrutinized and subsequently proposed to be autocatalytic by Breslow in 1959. Similar experiments (see below) demonstrate that nucleobases like guanine and adenine could be synthesized from simple carbon and nitrogen sources like hydrogen cyanide and ammonia. Formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Formamide is ubiquitous in the Universe, produced by the reaction of water and hydrogen cyanide (HCN). It has several advantages as a biotic precursor, including the ability to easily become concentrated through the evaporation of water.[135][136] Although HCN is poisonous, it only affects aerobic organisms (eukaryotes and aerobic bacteria), which did not yet exist. It can play roles in other chemical processes as well, such as the synthesis of the amino acid glycine.[49] In 1961, it was shown that the nucleic acid purine base adenine can be formed by heating aqueous ammonium cyanide solutions.[137] Other pathways for synthesizing bases from inorganic materials were also reported.[138] Leslie E. Orgel and colleagues have shown that freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide.[139] Research by Stanley L. Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures.[140] Research by the Miller group notes the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide were left in a freezer from 1972 to 1997.[141][142] Other work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source).[143] The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often. Mechanistic exploration using quantum chemical methods provide a more detailed understanding of some of the chemical processes involved in chemical evolution, and a partial answer to the fundamental question of molecular biogenesis.[144] At the time of the Miller–Urey experiment, scientific consensus was that the early Earth had a reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen (e.g., CH4 and NH3 as opposed to CO2 and nitrogen dioxide (NO2)). However, current scientific consensus describes the primitive atmosphere as either weakly reducing or neutral[145][146] (see also Oxygen Catastrophe). Such an atmosphere would diminish both the amount and variety of amino acids that could be produced, although studies that include iron and carbonate minerals (thought present in early oceans) in the experimental conditions have again produced a diverse array of amino acids.[145] Other scientific research has focused on two other potential reducing environments: outer space and deep-sea thermal vents.[147][148][149] The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the ''soup'' theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments.[150] The Miller–Urey experiment, for example, produces many substances that would react with the amino acids or terminate their coupling into peptide chains.[151] A research project completed in March 2015 by John D. Sutherland and others found that a network of reactions beginning with hydrogen cyanide and hydrogen sulfide, in streams of water irradiated by UV light, could produce the chemical components of proteins and lipids, as well as those of RNA,[152][153] while not producing a wide range of other compounds.[154] The researchers used the term ''cyanosulfidic'' to describe this network of reactions.[153] AutocatalysisEditMain article: Autocatalysis Autocatalysts are substances that catalyze the production of themselves and therefore are ''molecular replicators.'' The simplest self-replicating chemical systems are autocatalytic, and typically contain three components: a product molecule and two precursor molecules. The product molecule joins together the precursor molecules, which in turn produce more product molecules from more precursor molecules. The product molecule catalyzes the reaction by providing a complementary template that binds to the precursors, thus bringing them together. Such systems have been demonstrated both in biological macromolecules and in small organic molecules.[155][156] Systems that do not proceed by template mechanisms, such as the self-reproduction of micelles and vesicles, have also been observed.[156] It has been proposed that life initially arose as autocatalytic chemical networks.[157] British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor''s Tale.[158] In his book, Dawkins cites experiments performed by Julius Rebek Jr. and his colleagues in which they combined amino adenosine and pentafluorophenyl esters with the autocatalyst amino adenosine triacid ester (AATE). One product was a variant of AATE, which catalyzed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of natural selection.[159][160] In the early 1970s, Manfred Eigen and Peter Schuster examined the transient stages between the molecular chaos and a self-replicating hypercycle in a prebiotic soup.[161] In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery of ribozymes capable of catalyzing their own chemical reactions. The hypercycle theory requires the existence of complex biochemicals, such as nucleotides, which do not form under the conditions proposed by the Miller–Urey experiment. Geoffrey W. Hoffmann has shown that an early error-prone translation machinery can be stable against an error catastrophe of the type that had been envisaged as problematical for the origin of life, and was known as ''Orgel''s paradox''.[162][163][164] Hoffmann has furthermore argued that a complex nucleation event as the origin of life involving both polypeptides and nucleic acid is compatible with the time and space available in the primitive oceans of Earth[165] Hoffmann suggests that volcanic ash may provide the many random shapes needed in the postulated complex nucleation event. This aspect of the theory can be tested experimentally. HomochiralityEditMain article: Homochirality Homochirality refers to a geometric uniformity of some materials composed of chiral units. Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands. Living organisms use molecules that have the same chirality (''handedness''): with almost no exceptions,[166] amino acids are left-handed while nucleotides and sugars are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enantiomers (called a racemic mixture). Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth''s rotation, statistical fluctuations during racemic synthesis,[167] and spontaneous symmetry breaking.[168][169][170] Once established, chirality would be selected for.[171] A small bias (enantiomeric excess) in the population can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction.[172] In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.[173] Clark has suggested that homochirality may have started in outer space, as the studies of the amino acids on the Murchison meteorite showed that L-alanine is more than twice as frequent as its D form, and L-glutamic acid was more than three times prevalent than its D counterpart. Various chiral crystal surfaces can also act as sites for possible concentration and assembly of chiral monomer units into macromolecules.[174][175] Compounds found on meteorites suggest that the chirality of life derives from abiogenic synthesis, since amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias, the same as found in living organisms.[176] Self-enclosement, reproduction, duplication and the RNA worldEditProtocellsEditMain article: Protocell The three main structures phospholipids form spontaneously in solution: the liposome (a closed bilayer), the micelle and the bilayer. A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life.[177] A central question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation driving the evolution of life. Although a functional protocell has not yet been achieved in a laboratory setting, there are scientists who think the goal is well within reach.[178][179][180] Self-assembled vesicles are essential components of primitive cells.[177] The second law of thermodynamics requires that the universe move in a direction in which entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate life processes from non-living matter.[181] Researchers Irene A. Chen and Jack W. Szostak amongst others, suggest that simple physicochemical properties of elementary protocells can give rise to essential cellular behaviours, including primitive forms of differential reproduction competition and energy storage. Such cooperative interactions between the membrane and its encapsulated contents could greatly simplify the transition from simple replicating molecules to true cells.[179] Furthermore, competition for membrane molecules would favour stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today.[179] Such micro-encapsulation would allow for metabolism within the membrane, the exchange of small molecules but the prevention of passage of large substances across it.[182] The main advantages of encapsulation include the increased solubility of the contained cargo within the capsule and the storage of energy in the form of a electrochemical gradient. A 2012 study led by Armen Y. Mulkidjanian of Germany''s University of Osnabrück, suggests that inland pools of condensed and cooled geothermal vapour have the ideal characteristics for the origin of life.[183] Scientists confirmed in 2002 that by adding a montmorillonite clay to a solution of fatty acid micelles (lipid spheres), the clay sped up the rate of vesicles formation 100-fold.[180] Another protocell model is the Jeewanu. First synthesized in 1963 from simple minerals and basic organics while exposed to sunlight, it is still reported to have some metabolic capabilities, the presence of semipermeable membrane, amino acids, phospholipids, carbohydrates and RNA-like molecules.[184][185] However, the nature and properties of the Jeewanu remains to be clarified. Electrostatic interactions induced by short, positively charged, hydrophobic peptides containing 7 amino acids in length or fewer, can attach RNA to a vesicle membrane, the basic cell membrane.[186] RNA worldEditMain article: RNA world Molecular structure of the ribosome 30S subunit from Thermus thermophilus.[187] Proteins are shown in blue and the single RNA chain in orange. The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins.[188] It is widely accepted that current life on Earth descends from an RNA world,[16][189] although RNA-based life may not have been the first life to exist.[17][18] This conclusion is drawn from many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical groups and information transfers required for life.[18][190] The structure of the ribosome has been called the ''smoking gun,'' as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed.[17] The concept of the RNA world was first proposed in 1962 by Alexander Rich,[191] and the term was coined by Walter Gilbert in 1986.[18][192] Possible precursors for the evolution of protein synthesis include a mechanism to synthesize short peptide cofactors or form a mechanism for the duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.[193] Eugene Koonin said, ''Despite considerable experimental and theoretical effort, no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system. The MWO [''many worlds in one''] version of the cosmological model of eternal inflation could suggest a way out of this conundrum because, in an infinite multiverse with a finite number of distinct macroscopic histories (each repeated an infinite number of times), emergence of even highly complex systems by chance is not just possible but inevitable.''[194] Viral originsEditRecent evidence for a ''virus first'' hypothesis, which may support theories of the RNA world, has been suggested.[195] [196] One of the difficulties for the study of the origins of viruses is their high rate of mutation; this is particularly the case in RNA retroviruses like HIV.[197] A 2015 study compared protein fold structures across different branches of the tree of life, where researchers can reconstruct the evolutionary histories of the folds and of the organisms whose genomes code for those folds. They argue that protein folds are better markers of ancient events as their three-dimensional structures can be maintained even as the sequences that code for those begin to change.[195] Thus, the viral protein repertoire retain traces of ancient evolutionary history that can be recovered using advanced bioinformatics approaches. Those researchers think that ''the prolonged pressure of genome and particle size reduction eventually reduced virocells into modern viruses (identified by the complete loss of cellular makeup), meanwhile other coexisting cellular lineages diversified into modern cells.[198] The data suggest that viruses originated from ancient cells that co-existed with the ancestors of modern cells. These ancient cells likely contained segmented RNA genomes.[195][199] Although the virus-first hypothesis is highly controversial today, some astrobiologists have suggested looking for viruses on other celestial bodies such as Mars if they do emerge before cells.[200] RNA synthesis and replicationEditA number of hypotheses of formation of RNA have been put forward. As of 1994[update], there were difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil.[201] Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals.[135][136] Early cell membranes could have formed spontaneously from proteinoids, which are protein-like molecules produced when amino acid solutions are heated while in the correct concentration of aqueous solution. These are seen to form micro-spheres which are observed to behave similarly to membrane-enclosed compartments. Other possible means of producing more complicated organic molecules include chemical reactions that take place on clay substrates or on the surface of the mineral pyrite. Factors supporting an important role for RNA in early life include its ability to act both to store information and to catalyze chemical reactions (as a ribozyme); its many important roles as an intermediate in the expression of and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the RNA molecule under the conditions that approximated the early Earth. Relatively short RNA molecules have been synthesized, capable of replication.[202] Such replicase RNA, which functions as both code and catalyst provides its own template upon which copying can occur. Jack W. Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. If these conditions were present, Darwinian natural selection would favour the proliferation of such autocatalytic sets, to which further functionalities could be added.[203] Such autocatalytic systems of RNA capable of self-sustained replication have been identified.[204] The RNA replication systems, which include two ribozymes that catalyze each other''s synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the conditions that existed in the experiment.[205] In evolutionary competition experiments, this led to the emergence of new systems which replicated more efficiently.[17] This was the first demonstration of evolutionary adaptation occurring in a molecular genetic system.[205] Depending on the definition, life started when RNA chains began to self-replicate, initiating the three mechanisms of Darwinian selection: heritability, variation of type, and differential reproductive output. The fitness of an RNA replicator (its per capita rate of increase) would likely be a function of its intrinsic adaptive capacities, determined by its nucleotide sequence, and the availability of resources.[206][207] The three primary adaptive capacities may have been: (1) replication with moderate fidelity, giving rise to both heritability while allowing variation of type, (2) resistance to decay, and (3) acquisition of process resources.[206][207] These capacities would have functioned by means of the folded configurations of the RNA replicators resulting from their nucleotide sequences. Carl Zimmer has speculated that the chemical conditions, including the presence of boron, molybdenum and oxygen needed for the initial production of RNA, may have been better on early Mars than on early Earth.[208][209][210] If so, life-suitable molecules originating on Mars may have later migrated to Earth via meteor ejections. Pre-RNA worldEditIt is possible that a different type of nucleic acid, such as PNA, TNA or GNA, was the first to emerge as a self-reproducing molecule, only later replaced by RNA.[211][212] Larralde et al., say that ''the generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity.''[213] and they conclude that their ''results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability.'' The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.[214] Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesized by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by using nitrogenous or oxygenous chemistries. Sutherland has demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater.[215] This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallized out from a mixture of the other pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization with UV light allows for inversion about the 1'' anomeric centre to give the correct beta stereochemistry.[216] In 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2'',3''-cyclic pyrimidine nucleotides directly, which are known to be able to polymerize into RNA. This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2'',3''-cyclic phosphates.[217] Origin of biological metabolismEditMetabolism-like reactions could have occurred naturally in early oceans, before the first organisms evolved.[19][218] Metabolism may predate the origin of life, which may have evolved from the chemical conditions in the earliest oceans. Reconstructions in laboratories show that some of these reactions can produce RNA, and some others resemble two essential reaction cascades of metabolism: glycolysis and the pentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids and lipids.[218] A study at the University of Düsseldorf created phylogenic trees based upon 6 million genes from bacteria and archaea, and identified 355 protein families that were probably present in the LUCA. They were based upon an anaerobic metabolism fixing carbon dioxide and nitrogen. It suggests that the LUCA evolved in an environment rich in hydrogen, carbon dioxide and iron.[219] Following are some observed discoveries and related hypotheses. Iron–sulfur worldEditMain article: Iron–sulfur world theory In the 1980s, Günter Wächtershäuser, encouraged and supported by Karl R. Popper,[220][221][222] postulated his iron–sulfur world, a theory of the evolution of pre-biotic chemical pathways as the starting point in the evolution of life. It systematically traces today''s biochemistry to primordial reactions which provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds. In contrast to the classical Miller experiments, which depend on external sources of energy (simulated lightning, ultraviolet irradiation), ''Wächtershäuser systems'' come with a built-in source of energy: sulfides of iron (iron pyrite) and other minerals. The energy released from redox reactions of these metal sulfides is available for the synthesis of organic molecules, and such systems may have evolved into autocatalytic sets constituting self-replicating, metabolically active entities predating the life forms known today.[19][218] Experiments with such sulfides in an aqueous environment at 100 °C produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) although under the same conditions, dipeptides were quickly broken down.[223] Several models reject the self-replication of a ''naked-gene'', postulating instead the emergence of a primitive metabolism providing a safe environment for the later emergence of RNA replication. The centrality of the Krebs cycle (citric acid cycle) to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, suggests that it was one of the first parts of the metabolism to evolve.[224] Concordantly, geochemist Michael Russell has proposed that ''the purpose of life is to hydrogenate carbon dioxide'' (as part of a ''metabolism-first,'' rather than a ''genetics-first,'' scenario).[225][226] Physicist Jeremy England of MIT has proposed that life was inevitable from general thermodynamic considerations: ''... when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. This could mean that under certain conditions, matter inexorably acquires the key physical attribute associated with life.''[227][228] One of the earliest incarnations of this idea was put forward in 1924 with Oparin''s notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. Variants in the 1980s and 1990s include Wächtershäuser''s iron–sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman''s notion of collectively autocatalytic sets, discussed later that decade. Orgel summarized his analysis by stating, ''There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral.''[229] It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the ''open'' acetyl-CoA pathway (another one of the five recognized ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organization on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase, harbours mixed nickel-iron-sulfur clusters in its reaction centres and catalyzes the formation of acetyl-CoA (similar to acetyl-thiol) in a single step. There are increasing concerns, however, that prebiotic thiolated and thioester compounds are thermodynamically and kinetically unfavourable to accumulate in presumed prebiotic conditions (i.e. hydrothermal vents).[230] It has also been proposed that cysteine and homocysteine may have reacted with nitriles resulting from the Stecker reaction, readily forming catalytic thiol-reach poplypeptides.[231] Zn-world hypothesisEditThe Zn-world (zinc world) theory of Armen Y. Mulkidjanian[232] is an extension of Wächtershäuser''s pyrite hypothesis. Wächtershäuser based his theory of the initial chemical processes leading to informational molecules (RNA, peptides) on a regular mesh of electric charges at the surface of pyrite that may have facilitated the primeval polymerization by attracting reactants and arranging them appropriately relative to each other.[233] The Zn-world theory specifies and differentiates further.[232][234] Hydrothermal fluids rich in H2S interacting with cold primordial ocean (or Darwin''s ''warm little pond'') water leads to the precipitation of metal sulfide particles. Oceanic vent systems and other hydrothermal systems have a zonal structure reflected in ancient volcanogenic massive sulfide deposits (VMS) of hydrothermal origin. They reach many kilometres in diameter and date back to the Archean Eon. Most abundant are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). ZnS and MnS have a unique ability to store radiation energy, e.g. from UV light. During the relevant time window of the origins of replicating molecules, the primordial atmospheric pressure was high enough (>100 bar, about 100 atmospheres) to precipitate near the Earth''s surface, and UV irradiation was 10 to 100 times more intense than now; hence the unique photosynthetic properties mediated by ZnS provided just the right energy conditions to energize the synthesis of informational and metabolic molecules and the selection of photostable nucleobases. The Zn-world theory has been further filled out with experimental and theoretical evidence for the ionic constitution of the interior of the first proto-cells before archaea, bacteria and proto-eukaryotes evolved. Archibald Macallum noted the resemblance of body fluids such as blood and lymph to seawater;[235] however, the inorganic composition of all cells differ from that of modern seawater, which led Mulkidjanian and colleagues to reconstruct the ''hatcheries'' of the first cells combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. The authors conclude that ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K , Zn2 , Mn2 , and phosphate. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in what we today call marine settings but is compatible with emissions of vapour-dominated zones of what we today call inland geothermal systems. Under the oxygen depleted, CO2-dominated primordial atmosphere, the chemistry of water condensates and exhalations near geothermal fields would resemble the internal milieu of modern cells. Therefore, the precellular stages of evolution may have taken place in shallow ''Darwin ponds'' lined with porous silicate minerals mixed with metal sulfides and enriched in K , Zn2 , and phosphorus compounds.[236][237] Deep sea vent hypothesisEditDeep-sea hydrothermal vent or black smoker The deep sea vent, or alkaline hydrothermal vent, theory posits that life may have begun at submarine hydrothermal vents,[238][239] William Martin and Michael Russell have suggested ''that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments,...''[240] These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see Iron–sulfur world theory. These are highly exothermic reactions.[238][note 3] Michael Russell demonstrated that alkaline vents created an abiogenic proton motive force (PMF) chemiosmotic gradient,[240] in which conditions are ideal for an abiogenic hatchery for life. Their microscopic compartments ''provide a natural means of concentrating organic molecules,'' composed of iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Wächtershäuser.[224] This movement of ions across the membrane depends on a combination of two factors:
These two gradients taken together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. The proton motive force can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential). Jack W. Szostak suggested that geothermal activity provides greater opportunities for the origination of life in open lakes where there is a buildup of minerals. In 2010, based on spectral analysis of sea and hot mineral water, Ignat Ignatov and Oleg Mosin demonstrated that life may have predominantly originated in hot mineral water. The hot mineral water that contains bicarbonate and calcium ions has the most optimal range.[241] This case is similar to the origin of life in hydrothermal vents, but with bicarbonate and calcium ions in hot water. This water has a pH of 9–11 and is possible to have the reactions in seawater. According to Melvin Calvin, certain reactions of condensation-dehydration of amino acids and nucleotides in individual blocks of peptides and nucleic acids can take place in the primary hydrosphere with pH 9-11 at a later evolutionary stage.[242] Some of these compounds like hydrocyanic acid (HCN) have been proven in the experiments of Miller. This is the environment in which the stromatolites have been created. David Ward of Montana State University described the formation of stromatolites in hot mineral water at the Yellowstone National Park. Stromatolites survive in hot mineral water and in proximity to areas with volcanic activity.[243] Processes have evolved in the sea near geysers of hot mineral water. In 2011, Tadashi Sugawara from the University of Tokyo created a protocell in hot water.[244] Experimental research and computer modelling suggest that the surfaces of mineral particles inside hydrothermal vents have catalytic properties similar to those of enzymes and are able to create simple organic molecules, such as methanol (CH3OH) and formic, acetic and pyruvic acid out of the dissolved CO2 in the water.[245][246] The research reported above by William F. Martin in July 2016 supports the thesis that life arose at hydrothermal vents,[247][248] that spontaneous chemistry in the Earth’s crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life’s origin[249][250] and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism.[251] Martin suggests, based upon this evidence that LUCA ''may have depended heavily on the geothermal energy of the vent to survive''.[252] ThermosynthesisEditToday''s bioenergetic process of fermentation is carried out by either the aforementioned citric acid cycle or the Acetyl-CoA pathway, both of which have been connected to the primordial Iron–sulfur world. In a different approach, the thermosynthesis hypothesis considers the bioenergetic process of chemiosmosis, which plays an essential role in cellular respiration and photosynthesis, more basal than fermentation: the ATP synthase enzyme, which sustains chemiosmosis, is proposed as the currently extant enzyme most closely related to the first metabolic process.[253][254] First, life needed an energy source to bring about the condensation reaction that yielded the peptide bonds of proteins and the phosphodiester bonds of RNA. In a generalization and thermal variation of the binding change mechanism of today''s ATP synthase, the ''first protein'' would have bound substrates (peptides, phosphate, nucleosides, RNA ''monomers'') and condensed them to a reaction product that remained bound until after a temperature change it was released by thermal unfolding. The energy source under the thermosynthesis hypothesis was thermal cycling, the result of suspension of protocells in a convection current, as is plausible in a volcanic hot spring; the convection accounts for the self-organization and dissipative structure required in any origin of life model. The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis. By phosphorylating cell membrane lipids, this ''first protein'' gave a selective advantage to the lipid protocell that contained the protein. This protein also synthesized a library of many proteins, of which only a minute fraction had thermosynthesis capabilities. As proposed by Dyson,[13] it propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Functioning daughters consisted of different amino acid sequences. Whereas the Iron–sulfur world identifies a circular pathway as the most simple, the thermosynthesis hypothesis does not even invoke a pathway: ATP synthase''s binding change mechanism resembles a physical adsorption process that yields free energy,[255] rather than a regular enzyme''s mechanism, which decreases the free energy. It has been claimed that the emergence of cyclic systems of protein catalysts is implausible.[256] Other modelsEdit-13 — – -12 — – -11 — – -10 — – -9 — – -8 — – -7 — – -6 — – -5 — – -4 — – -3 — – -2 — – -1 — – 0 — Clay hypothesisEditMontmorillonite, an abundant clay, is a catalyst for the polymerization of RNA and for the formation of membranes from lipids.[257] A model for the origin of life using clay was forwarded by Alexander Graham Cairns-Smith in 1985 and explored as a plausible mechanism by several scientists.[258] The clay hypothesis postulates that complex organic molecules arose gradually on pre-existing, non-organic replication surfaces of silicate crystals in solution. At the Rensselaer Polytechnic Institute, James P. Ferris'' studies have also confirmed that clay minerals of montmorillonite catalyze the formation of RNA in aqueous solution, by joining nucleotides to form longer chains.[259] In 2007, Bart Kahr from the University of Washington and colleagues reported their experiments that tested the idea that crystals can act as a source of transferable information, using crystals of potassium hydrogen phthalate. ''Mother'' crystals with imperfections were cleaved and used as seeds to grow ''daughter'' crystals from solution. They then examined the distribution of imperfections in the new crystals and found that the imperfections in the mother crystals were reproduced in the daughters, but the daughter crystals also had many additional imperfections. For gene-like behaviour to be observed, the quantity of inheritance of these imperfections should have exceeded that of the mutations in the successive generations, but it did not. Thus Kahr concluded that the crystals ''were not faithful enough to store and transfer information from one generation to the next.''[260] Gold''s ''deep-hot biosphere'' modelEditIn the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometres below the surface. It is claimed that discovery of microbial life below the surface of another body in our Solar System would lend significant credence to this theory. Thomas Gold also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold''s theory is that the flow of such food is due to out-gassing of primordial methane from the Earth''s mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks. PanspermiaEditMain article: Panspermia Panspermia is the hypothesis that life exists throughout the universe, distributed by meteoroids, asteroids, comets,[261] planetoids,[262] and, also, by spacecraft in the form of unintended contamination by microorganisms.[263][264] The panspermia hypothesis does not attempt to explain how life first originated, but merely shifts it to another planet or a comet. The advantage of an extraterrestrial origin of primitive life is that life is not required to have formed on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact.[265] Evidence to support the hypothesis is scant, but it finds support in studies of Martian meteorites found in Antarctica and in studies of extremophile microbes'' survival in outer space tests.[266][267][268][269] (See also: List of microorganisms tested in outer space.) Extraterrestrial organic moleculesEditMethane is one of the simplest organic compounds An organic compound is any member of a large class of gaseous, liquid, or solid chemicals whose molecules contain carbon. Carbon is the fourth most abundant element in the Universe by mass after hydrogen, helium, and oxygen.[270] Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets.[271] Organic compounds are relatively common in space, formed by ''factories of complex molecular synthesis'' which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation.[20][272][273][274] Based on computer model studies, the complex organic molecules necessary for life may have formed on dust grains in the protoplanetary disk surrounding the Sun before the formation of the Earth.[124] According to the computer studies, this same process may also occur around other stars that acquire planets.[124] Observations suggest that the majority of organic compounds introduced on Earth by interstellar dust particles are considered principal agents in the formation of complex molecules, thanks to their peculiar surface-catalytic activities.[275][276] Studies reported in 2008, based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite, suggested that the RNA component uracil and related molecules, including xanthine, were formed extraterrestrially.[277][278] On 8 August 2011, a report based on NASA studies of meteorites found on Earth was published suggesting DNA components (adenine, guanine and related organic molecules) were made in outer space.[275][279][280] Scientists also found that the cosmic dust permeating the universe contains complex organics (''amorphous organic solids with a mixed aromatic–aliphatic structure'') that could be created naturally, and rapidly, by stars.[281][282][283] Sun Kwok of The University of Hong Kong suggested that these compounds may have been related to the development of life on Earth said that ''If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life.''[281] Glycolaldehyde, the first example of an interstellar sugar molecule, was detected in the star-forming region near the centre of our galaxy. It was discovered in 2000 by Jes Jørgensen and Jan M. Hollis.[284] In 2012, Jørgensen''s team reported the detection of glycolaldehyde in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422 400 light years from Earth.[285][286][287] Glycolaldehyde is needed to form RNA, which is similar in function to DNA. These findings suggest that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.[288] Because sugars are associated with both metabolism and the genetic code, two of the most basic aspects of life, it is thought the discovery of extraterrestrial sugar increases the likelihood that life may exist elsewhere in our galaxy.[284] NASA announced in 2009 that scientists had identified another fundamental chemical building block of life in a comet for the first time, glycine, an amino acid, which was detected in material ejected from comet Wild 2 in 2004 and grabbed by NASA''s Stardust probe. Glycine has been detected in meteorites before. Carl Pilcher, who leads the NASA Astrobiology Institute commented that ''The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare.''[289] Comets are encrusted with outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by ionizing radiation. It is possible that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth.[290][291][292] Amino acids which were formed extraterrestrially may also have arrived on Earth via comets.[49] It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year.[49] Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the observable universe, and are considered a likely constituent of the primordial sea.[293][294][295] In 2010, PAHs, along with fullerenes (or ''buckyballs''), have been detected in nebulae.[296][297]In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like PAHs, the most carbon-rich chemical found in the Universe, may have been formed in red giant stars or in interstellar dust and gas clouds.[298] A group of Czech scientists reported that all four RNA-bases may be synthesized from formamide in the course of high-energy density events like extraterrestrial impacts.[299] Lipid worldEditMain article: Gard model The lipid world theory postulates that the first self-replicating object was lipid-like.[300][301] It is known that phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favourably. Studies on vesicles from potentially prebiotic amphiphiles have so far been limited to systems containing one or two types of amphiphiles. This in contrast to the output of simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures of compounds.[177]Within the hypothesis of a lipid bilayer membrane composed of a mixture of various distinct amphiphilic compounds there is the opportunity of a huge number of theoretically possible combinations in the arrangements of these amphiphiles in the membrane. Among all these potential combinations, a specific local arrangement of the membrane would have favoured the constitution of a hypercycle,[302][303] actually a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles which would have allowed Darwinian natural selection.[304] PolyphosphatesEditA problem in most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphates.[305][306] Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4−3. Several mechanisms of organic molecule synthesis have been investigated. Polyphosphates cause polymerization of amino acids into peptides. They are also logical precursors in the synthesis of such key biochemical compounds as adenosine triphosphate (ATP). A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate. There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early Earth.[307] PAH world hypothesisEditMain article: PAH world hypothesis Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called ''polycyclic aromatic hydrocarbons'' (PAHs), causing them to fluoresce. (Spitzer space telescope, 2018) Polycyclic aromatic hydrocarbons (PAH) are known to be abundant in the universe,[293][294][295] including in the interstellar medium, in comets, and in meteorites, and are some of the most complex molecules so far found in space.[271] Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of PAHs in a nebula.[308] In 2010, another team also detected PAHs, along with fullerenes, in nebulae.[296] The use of PAHs has also been proposed as a precursor to the RNA world in the PAH world hypothesis.[citation needed] The Spitzer Space Telescope has detected a star, HH 46-IR, which is forming by a process similar to that by which the Sun formed. In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. In September 2012, NASA scientists reported that PAHs, subjected to interstellar medium conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics—''a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively.''[309][310] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons ''for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.''[309][310] NASA maintains a database for tracking PAHs in the universe.[271][311] More than 20% of the carbon in the universe may be associated with PAHs,[271][271] possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe,[293][294][295] and are associated with new stars and exoplanets.[271] Radioactive beach hypothesisEditZachary Adam claims that tidal processes that occurred during a time when the Moon was much closer may have concentrated grains of uranium and other radioactive elements at the high-water mark on primordial beaches, where they may have been responsible for generating life''s building blocks.[312] According to computer models,[313] a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the Oklo uranium ore seam in Gabon. Such radioactive beach sand might have provided sufficient energy to generate organic molecules, such as amino acids and sugars from acetonitrile in water. Radioactive monazite material also has released soluble phosphate into the regions between sand-grains, making it biologically ''accessible.'' Thus amino acids, sugars, and soluble phosphates might have been produced simultaneously, according to Adam. Radioactive actinides, left behind in some concentration by the reaction, might have formed part of organometallic complexes. These complexes could have been important early catalysts to living processes. John Parnell has suggested that such a process could provide part of the ''crucible of life'' in the early stages of any early wet rocky planet, so long as the planet is large enough to have generated a system of plate tectonics which brings radioactive minerals to the surface. As the early Earth is thought to have had many smaller plates, it might have provided a suitable environment for such processes.[314] Thermodynamic dissipationEditThe 19th-century Austrian physicist Ludwig Boltzmann first recognized that the struggle for existence of living organisms was neither over raw material nor energy, but instead had to do with entropy production derived from the conversion of the solar spectrum into heat by these systems.[315] Boltzmann thus realized that living systems, like all irreversible processes, were dependent on the dissipation of a generalized chemical potential for their existence. In his book ''What is Life'', the 20th-century Austrian physicist Erwin Schrödinger[316] emphasized the importance of Boltzmann’s deep insight into the irreversible thermodynamic nature of living systems, suggesting that this was the physics and chemistry behind the origin and evolution of life. However, irreversible processes, and much less living systems, could not be conveniently analyzed under this perspective until Lars Onsager,[317] and later Ilya Prigogine,[318] developed an elegant mathematical formalism for treating the ''self-organization'' of material under a generalized chemical potential. This formalism became known as Classical Irreversible Thermodynamics and Prigogine was awarded the Nobel Prize in Chemistry in 1977 ''for his contributions to non-equilibrium thermodynamics, particularly the theory of dissipative structures''. The analysis of Prigogine showed that if a system were left to evolve under an imposed external potential, material could spontaneously organize (lower its entropy) forming what he called ''dissipative structures'' which would increase the dissipation of the externally imposed potential (augment the global entropy production). Non-equilibrium thermodynamics has since been successfully applied to the analysis of living systems, from the biochemical production of ATP [319] to optimizing bacterial metabolic pathways [320] to complete ecosystems.[321][322][323] In his ''Thermodynamic Dissipation Theory of the Origin and Evolution of Life'',[324][325][326][327] Karo Michaelian has taken the insight of Boltzmann and the work of Prigogine to its ultimate consequences regarding the origin of life. This theory postulates that the hallmark of the origin and evolution of life is the microscopic dissipative structuring of organic pigments and their proliferation over the entire Earth surface.[327] Present day life augments the entropy production of Earth in its solar environment by dissipating ultraviolet and visible photons into heat through organic pigments in water. This heat then catalyzes a host of secondary dissipative processes such as the water cycle, ocean and wind currents, hurricanes, etc.[325][328] Michaelian argues that if the thermodynamic function of life today is to produce entropy through photon dissipation in organic pigments, then this probably was its function at its very beginnings. It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 230–290 nm wavelength (UV-C) region, which is a part of the Sun''s spectrum that could have penetrated the prebiotic atmosphere.[329] In fact, not only RNA and DNA, but many fundamental molecules of life (those common to all three domains of life) are also pigments that absorb in the UV-C, and many of these also have a chemical affinity to RNA and DNA.[330][331] Nucleic acids may thus have acted as acceptor molecules to the UV-C photon excited antenna pigment donor molecules by providing an ultrafast channel for dissipation. Michaelian has shown using the formalism of non-linear irreversible thermodynamics that there would have existed during the Archean a thermodynamic imperative to the abiogenic UV-C photochemical synthesis and proliferation of these pigments over the entire Earth surface if they acted as catalysts to augment the dissipation of the solar photons.[332] By the end of the Archean, with life-induced ozone dissipating UV-C light in the Earth’s upper atmosphere, it would have become ever more improbable for a completely new life to emerge that didn’t rely on the complex metabolic pathways already existing since now the free energy in the photons arriving at Earth’s surface would have been insufficient for direct breaking and remaking of covalent bonds. It has been suggested, however, that such changes in the surface flux of ultraviolet radiation due to geophysical events affecting the atmosphere could have been what promoted the development of complexity in life based on existing metabolic pathways, for example during the Cambrian explosion [333] Many salient characteristics of the fundamental molecules of life (those found in all three domains) all point directly to the involvement of UV-C light in the dissipative structuring of incipient life.[326] Some of the most difficult problems concerning the origin of life, such as enzyme-less replication of RNA and DNA, homochirality of the fundamental molecules, and the origin of information encoding in RNA and DNA, also find an explanation within the same dissipative thermodynamic framework by considering the probable existence of a relation between primordial replication and UV-C photon dissipation. Michaelian suggests that it is erroneous to expect to describe the emergence, proliferation, or even evolution, of life without overwhelming reference to entropy production through the dissipation of a generalized chemical potential, in particular, the prevailing solar photon flux. Multiple genesisEditDifferent forms of life with variable origin processes may have appeared quasi-simultaneously in the early history of Earth.[334] The other forms may be extinct (having left distinctive fossils through their different biochemistry—e.g., hypothetical types of biochemistry). It has been proposed that:
Fluctuating hydrothermal pools on volcanic islands or proto-continentsEditArmid Mulkidjanian and co-authors think that the marine environments did not provide the ionic balance and composition universally found in cells, as well as of ions required by essential proteins and ribozymes found in virtually all living organisms, especially with respect to K /Na ratio, Mn2 , Zn2 and phosphate concentrations. The only known environments that mimic the needed conditions on Earth are found in terrestrial hydrothermal pools fed by steam vents.[238] Additionally, mineral deposits in these environments under an anoxic atmosphere would have suitable pH (as opposed to current pools in an oxygenated atmosphere), contain precipitates of sulfide minerals that block harmful UV radiation, have wetting/drying cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of polymers of nucleic acids, polyesters[336] and depsipeptides,[337] both by chemical reactions in the hydrothermal environment, as well as by exposure to UV light during transport from vents to adjacent pools. Their hypothesized pre-biotic environments are similar to the deep-oceanic vent environments most commonly hypothesized, but add additional components that help explain peculiarities found in reconstructions of the Last Universal Common Ancestor (LUCA) of all living organisms.[338] Bruce Damer and David Deamer have come to the conclusion that cell membranes cannot be formed in salty seawater, and must therefore have originated in freshwater. Before the continents formed, the only dry land on Earth would be volcanic islands, where rainwater would form ponds where lipids could form the first stages towards cell membranes. These predecessors of true cells are assumed to have behaved more like a superorganism rather than individual structures, where the porous membranes would house molecules which would leak out and enter other protocells. Only when true cells had evolved would they gradually adapt to saltier environments and enter the ocean.[339] Colín-García et al. (2016) discuss the advantages and disadvantages of hydrothermal vents as primitive environments.[238] They mention the exergonic reactions in such systems could have been a source of free energy that promoted chemical reactions, additional to their high mineralogical diversity which implies the induction of important chemical gradients, thus favoring the interaction between electron donors and acceptors. Colín-García et al. (2016) also summarize a set of experiments proposed to test the role of hydrothermal vents in prebiotic synthesis.[238] Information theoryEditA theory that speaks to the origin of life on Earth and other rocky planets posits life as an information system in which information content grows because of selection. Life must start with minimum possible information, or minimum possible departure from thermodynamic equilibrium, and it requires thermodynamically free energy accessible by means of its information content. The most benign circumstances, minimum entropy variations with abundant free energy, suggest the pore space in the first few kilometres of the surface. Free energy is derived from the condensed products of the chemical reactions taking place in the cooling nebula.[340] See alsoEdit
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