4. Life and its Requirements
4. 生命及其所需条件

4.1. Does ‘Life’ Have a Definition?
4.1. “生命”有定义吗？

The question “What is life?” is foundational to biology, but it is especially important to astrobiologists searching for signs of life in the cosmos. Previous attempts to define life have faced serious counterexamples, and may generate as many problems as they solve. To answer the question “What is life?”, we require a general theory of the nature of living systems. In absence of such a theory, we like alchemists during the European Enlightenment trying to define water before the advent of atomic theory. The best description of water at the time would be to list water’s properties:
“生命是什么？” 这个问题是生物学的基础，但对于在宇宙中寻找生命迹象的天体生物学家来说尤为重要。 之前 attempts to define life have faced serious counterexamples, and may generate as many problems as they solve. 要回答“生命是什么？”这个问题，我们需要一个关于生命系统本质的通用理论。 在缺乏这种理论的情况下，我们就像欧洲启蒙运动时期试图在原子理论出现之前给水下定义的炼金术士一样。 当时对水最好的描述是列出水的性质：

• wet, 潮湿的、

• transparent, 透明的、

• odorless, 无味的、

• tasteless, 无味的、

• thirst quenching, and 解渴的，以及

• a good solvent. 良好的溶剂。

Although, finding water that was odorless and tasteless might have been a struggle for the average 16th century investigator. No amount of (macroscopic) observational or conceptual analysis of these features will reveal that water contains two hydrogens and one oxygen (${\mathrm{H}}_2\mathrm{O}$). The most scientifically informative answer to the question “What is water?” is most definitely ${\mathrm{H}}_2\mathrm{O}$. In the absence of a good theory of the nature of living systems, the best we can do is to describe its features until we have a more precise intuition.
然而，对于 16 世纪的研究者来说，想要找到无色无味的水可能并非易事。 仅仅依靠肉眼观察或概念分析这些特征，并不能揭示水是由两个氢原子和一个氧原子组成的 ( ${\mathrm{H}}_2\mathrm{O}$ )。对于“水是什么？”这个问题，最具科学价值的答案绝对是 ${\mathrm{H}}_2\mathrm{O}$ 。在缺乏对生命系统本质的完整理论的情况下，我们所能做的就是描述它的特征，直到我们对其有更精确的直觉。

4.1.1. Attempts to Define ‘Life’
4.1.1. 尝试定义“生命”

There have been many attempts to define life dating back at least to Aristotle, who defined life in terms of the capacity to reproduce. To this day, there is not a broadly accepted definition of life, where the scientific literature is filled with suggestions. Carl Sagan catalogued physiological, metabolic, biochemical, Darwinian (or genetic) and thermodynamic definitions, along with their counterexamples. All attempts typically face problems, in that they include phenomena that most are reluctant to consider alive, or exclude entities that clearly are alive. Figure 4.1 shows a definition, but it actually relies on many definitions to form a complete picture.
人们一直试图给“生命”下定义，最早可以追溯到亚里士多德时期，他认为拥有繁殖能力就意味着生命。但直到今天，我们依然没有一个普遍接受的生命定义，各种科学文献中充斥着各式各样的说法。卡尔·萨根就曾对生理学、代谢学、生物化学、达尔文进化论（或遗传学）以及热力学等不同角度的生命定义进行过分类，并一一列举了反例。所有这些尝试都不可避免地会遇到问题，要么把一些大多数人不愿视为生命的现象囊括进来，要么把一些明显有生命的实体排除在外。图 4.1 展示了一种定义，但实际上它也需要借助许多其他定义才能构成一幅完整的图景。

• A metabolic definition could be based on the ability to consume and convert energy in order to move, grow, or reproduce. But fire might be said to satisfy some or all of these criteria.
  新陈代谢的定义可以基于生物体消耗和转化能量以移动、生长或繁殖的能力。但火似乎也能满足其中的一些或全部标准。

• A thermodynamic definition might describe a living system as one that takes in energy in order to create order locally, but this would include crystals that are not generally considered alive.
  热力学的定义可能会将生命系统描述为吸收能量以在局部创造秩序的系统，但这将包括通常不被视为生命的晶体。

• A biochemical definition would be based on the presence of certain types of biomolecules, yet one must worry that any such choice could in the future face exceptions when the category of favored biomolecules changes. In that case, once living things would become non-living.
  生物化学的定义将基于某些类型生物分子的存在，但人们必须担心，任何此类选择在未来都可能面临例外，因为偏好的生物分子类别会发生变化。在这种情况下，曾经的生命将变成非生命。

• Genetic or Darwinian definitions are systems that are capable of evolution by natural selection, but these also face drawbacks. The taming (i.e. altered behavior) of wolves into dogs appears to fit this definition, but a behavior is not considered alive by itself.
  遗传学或达尔文主义的定义认为，生命系统能够通过自然选择进行进化，但这些定义也存在缺陷。例如，狼被驯化成狗似乎符合这个定义，但行为本身并不能被视为生命。

Fig. 4.1 Definition of cellular life according to Budisa, Kubyshkin, & Schmidt (2020).
图 4.1 根据Budisa、Kubyshkin 和 Schmidt（2020）对细胞生命的定义。

The philosophical question of the definition of life has increasing practical importance, as laboratory experiments approach the synthesis of life, and as greater attention is focused on the search for life within the Solar System and beyond. Definitions of life are often explicit or implicit in planning remote in situ searches for extraterrestrial life. The design of life-detection experiment that could be performed on Europa or Mars by spacecraft landers depends on decisions about what life is, and what observations will count as evidence of a detection.
随着实验室实验接近合成生命，以及人们越来越关注在太阳系内外寻找生命，生命定义这一哲学问题变得越来越具有现实意义。在规划远程原位寻找地外生命的过程中，对生命的定义往往是明确或隐含的。能够由航天器着陆器在木卫二或火星上进行的生命探测实验的设计，取决于对生命是什么以及哪些观测结果可以作为探测到生命的证据的判断。

4.1.1.1. Viking’s Search for Life on Mars
4.1.1.1. 海盗号在火星上寻找生命

The Viking mission’s search for life on Mars in the mid-1970s was the first dedicated in situ search for extraterrestrial life. The basic approach was to conduct experiments with the martian soil to test for the presence of metabolizing organisms. The results of the labeled release experiment (see the LR in Fig. 4.2) in particular were not unlike what had been expected for the presence of life. In the end, the Viking biology team’s consensus was for a nonbiological interpretation, which was strongly influenced by the failure of the Viking gas chromatograph mass spectrometer (GCMS) to find any organic molecules to its limits of detection in the soil with sample heating up to $500{}^{\circ }\mathrm{C}$.
海盗号任务于 20 世纪 70 年代中期开展了对火星生命的探索，这是人类首次原位寻找地外生命的尝试。其基本方法是通过对火星土壤进行实验，以测试是否存在代谢生物。 尤其是标记释放实验（见图 4.2 中的 LR）的结果，与预期中存在生命迹象的结果十分相似。最终，海盗号生物小组一致认为这是一种非生物的解释，这是因为海盗号气相色谱-质谱仪 (GCMS) 未能在土壤中检测到任何有机分子（即使将样品加热到 $500{}^{\circ }\mathrm{C}$

Fig. 4.2 NASA Viking Lander Biological Experiment Package. Image Credit: Wikipedia:Viking lander biological experiments/NASA.
图 4.2 美国宇航局海盗号着陆器生物实验包。图片来源：维基百科：海盗号着陆器生物实验/NASA。

The GCMS was not intended to conduct a “life-detection” experiment, but actually did so by implicitly employing a biochemical definition. The GCMS would not have detected as many as $\sim {10}^6$ bacterial cells per gram of soil. It now appears that oxidation of the meteoritic organics on the martian surface may have produced nonvolatile organic compounds that would not have been easily detectable. The result was psychologically powerful: no (detected) organics, no life. A lesson learned is that any in situ search for extraterrestrial life should employ more than one definition of life so that results can be intercompared. If there were really one correct known definition of life, then the extra effort and design would be unnecessary.
气相色谱-质谱联用仪 (GCMS) 并非用于进行“生命探测”实验，但实际上它通过隐含地采用生化定义做到了这一点。该仪器无法检测到每克土壤中多达 $\sim {10}^6$ 个细菌细胞。现在看来，火星表面陨石有机物的氧化可能产生了不易检测到的非挥发性有机化合物。结果在心理上是强烈的：没有（检测到）有机物，就没有生命。由此吸取的教训是，任何原位地外生命搜索都应该采用多种生命定义，以便对结果进行相互比较。如果真的存在一个关于生命的正确已知定义，那么额外的努力和设计将是不必要的。

4.1.1.2. The Darwinian Definition
4.1.1.2. 达尔文定义

Darwinian (sometimes called genetic) definitions of life hold that life is a system capable of evolution by natural selection. One popular version is the chemical Darwinian definition, where life is a self-sustained chemical system capable of Darwinian evolution. The notion of Darwinian evolution includes processes of self-reproduction, material continuation over a historical lineage (i.e., gene transfer), genetic variation, and natural selection. The requirement the system is self-sustained refers to living systems that contain all the genetic information necessary for their own constant production (i.e., metabolism). The chemical Darwinian definition excludes computer or artificial life through its demand that chemical exchanges are responsible for the self-sustaining property. It also excludes biological viruses by virtue of the self-sustained requirement.
达尔文主义对“生命”的定义（有时也被称为“遗传”定义）认为，生命是一个**能够通过自然选择进行进化的系统**。其中一个流行的版本是**化学达尔文主义**定义，它将生命定义为一个能够进行达尔文式进化的、自我维持的化学系统。达尔文进化论的概念包括自我复制、物质在历史谱系中的延续（即基因传递）、遗传变异和自然选择等过程。系统自我维持的要求是指生命系统包含了自身持续产生（即新陈代谢）所需的所有遗传信息。化学达尔文主义的定义由于强调化学交换是自我维持特性的基础，因此排除了计算机或人工“生命”。同时，由于自我维持的要求，它也不包括生物病毒。

Some researchers (e.g., Dawkins (1983); Dennett (1995)) do not restrict Darwinian evolution to chemical systems, explicitly leaving open the possibility of computer life. This characterizes Darwinian evolution by how it functions, or how it is more a general process that can be abstracted from any particular physical realization. In this functionalist view, it is not the computer that is alive but rather the processes themselves. This is similar to how energy propagates through a medium using waves, where the material at the wave source does not interact directly with the receiver. The artificial vehicle of the computer has a status no different from that of the artificial glassware that might be used in a laboratory synthesis of organic life. A computer simulation of cellular biochemistry is a simulation of biochemistry, and not biochemistry itself. No computer simulation of photosynthesis is actually photosynthesis since it does not yield authentic carbohydrates. In the functionalist view, the simulation is life because life is an abstract process independent of any particular physical realization.
一些研究者，比如道金斯（1983）和丹尼特（1995），并不认为达尔文进化论只适用于化学系统，他们明确地认为计算机生命也是有可能存在的。在他们看来，达尔文进化论的本质在于其运作方式，它更像是一个可以从任何特定物理实现中抽象出来的通用过程。这种功能主义的观点认为，活着的不是计算机本身，而是运行在计算机上的那些过程。这就好比能量通过波在介质中传播，波源处的物质并不直接与接收器相互作用。计算机作为一种人造载体，其地位与实验室合成有机生命时使用的人造玻璃器皿没有什么不同。计算机模拟细胞生物化学只是对生物化学的模拟，而不是生物化学本身。 虽然计算机可以模拟光合作用，但这终究只是模拟，因为模拟无法真正产生碳水化合物。不过，从功能主义者的角度来看，这种模拟本身就代表着生命，因为他们认为生命是一个抽象的过程，独立于任何特定的物理实现形式。

There are other problems with Darwinian definitions, where it is possible (though not generally favored) that early cellular life on Earth or some other work passed through a period reproduction without DNA-type replication and Darwinian evolution could not yet operate (e.g., Dyson (1985)). In this hypothesis, protein-based creatures capable of metabolism predated the development of exact replication based on nucleic acids. If such entities, where discovered on another world, then Darwinian definitions would preclude them from being considered alive. For more recent details (as of 2016), see How to build life in a pre-Darwinian world.
达尔文定义还存在其他问题。例如，地球上的早期细胞生命或其他生命形式可能经历过一段没有 DNA 复制的繁殖时期，因此达尔文进化论无法适用（例如，参见 Dyson 1985 年的论述）。在这个假设中，能够进行新陈代谢的蛋白质生物早于基于核酸的精确复制机制的出现。如果在其他星球上发现了这样的实体，那么根据达尔文的定义，它们将不被视为生命。有关更多最新细节（截至 2016 年），请参阅“如何在达尔文之前的世界中构建生命”。

A simple objection to the Darwinian definition considers that individual sexually reproducing organisms in our DNA-protein do not themselves evolve and would categorize many living entities in our world are not examples of life. The Darwinian definition refers to a system that at least in some cases must contain more than one entity, which would preclude Victor Frankenstein’s unique creation from being considered alive, even though the tale clearly describes a living entity. A practical drawback to Darwinian definitions is that they depend on how long a system should wait before demonstrating that it is capable of Darwinian evolution. How do we decide what are the proper conditions for a system to be capable of Darwinian evolution?
一种对达尔文定义的简单反对意见认为，我们 DNA-蛋白质世界中进行有性繁殖的个体生物本身并不会进化，并且会将我们世界中的许多生物实体归类为非“生命”的例子。达尔文定义指的是一个至少在某些情况下必须包含多个实体的“系统”，这将排除将维克多·弗兰肯斯坦的独特创造物视为生命的可能性，即使这个故事清楚地描述了一个活生生的实体。达尔文定义的一个实际缺陷是，它们取决于系统需要等待多长时间才能证明它能够进行达尔文进化。我们如何确定一个系统能够进行达尔文进化的适当条件是什么？

4.1.2. Definitions
4.1.2. 定义

Definitions are concerned with language and concepts, at least philosophically. For example, the definition of a “bachelor” is an unmarried human male, which explains the meaning of the word by dissecting the concept that we associate with it. Every definition has two parts: (1) the definiendum is the expression being defined and (2) the definiens is the expression doing the defining.
至少从哲学上讲，定义与语言和概念有关。例如，“单身汉”的定义是未婚男性，它通过剖析我们与之相关的概念来解释这个词的含义。每个定义都有两部分：（1）被定义项是被定义的表达式，（2）定义项是进行定义的表达式。

4.1.2.1. Varieties of Definition
4.1.2.1. 定义的种类

Lexical definitions report on the standard meaning of terms in a natural language (e.g., dictionary definitions). Lexical definitions are in contrast to stipulative definitions, which explicitly introduce new meanings for terms. The stipulative definition from physics for the term ‘work’ refers to the magnitude of an acting force and the displacement due to its action. Stipulative definitions are also used to introduce invented terms (e.g., electron or gene). Unlike lexical definitions, stipulative definitions are arbitrary in the sense that rather than reporting on the existing meanings of terms, they explicitly introduce new meanings.
词汇定义解释自然语言中术语的标准含义（例如字典定义）。与规定性定义不同，词汇定义不为术语引入新的含义。物理学中对“功”的规定性定义是指作用力的大小及其作用产生的位移。规定性定义也用于引入新创术语（例如电子或基因）。与词汇定义不同，规定性定义是任意的，因为它们没有解释术语的现有含义，而是明确地引入了新的含义。

Another type of definition is the ostensive definition that specify the meaning of a term merely by indicating a few prototypical examples within its extension, or the class of all things to which it applies. An adult who explains the meaning of the word ‘dog’ to a child by pointing to a dog and saying “that is a dog” is providing an ostensive definition.
另一种定义方式是指物定义，它仅通过在其外延内，或者说在其所适用的所有事物的类别中，指出几个原型例子来指定一个术语的含义。一个成年人指着一条狗对一个孩子说“那是一条狗”来解释“狗”这个词的意思，就是在进行指物定义。

Operational definitions are like ostensive definitions, where they explain meanings through representative examples. Operational definitions do not directly indicate examples, but instead specify procedures that can be performed on something to determine whether it falls into an extension of the definiendum. For example, an acid can be operationally defined as something that turns litmus paper red. The definiens specifies a procedure that can be used to determine whether an unknown substance is an acid. Some astrobiologists (e.g., Chyba & McDonald (1996); Conrad & Nealson (2001)) have called for the use of operational definitions in searches for extraterrestrial life. The problem with operational definitions is that they do not tell one very much about what the items falling under the definiendum have in common. The fact that litmus paper turns red when placed in a liquid doesn’t tell us much about the nature of acidity, only that a particular liquid is something called acid.
“操作定义”类似于“列举定义”，都是通过代表性示例来解释含义。 不过，操作定义并不*直接*列举示例，而是规定可以对某事物执行的操作步骤，从而确定该事物是否属于被定义项的外延。 比如说，我们可以把酸操作定义为：能使石蕊试纸变红的东西。 这个定义描述了一个操作步骤，可以用来判断未知物质是否是酸。 一些天体生物学家（例如 Chyba 和 McDonald [1996]；Conrad 和 Nealson [2001]）呼吁在天外生命的研究中使用操作定义。 操作定义的问题在于，它没有说明被定义项下的事物具有哪些共同点。 石蕊试纸放入液体后变红，这只能说明这种液体是酸性物质，而不能说明酸的本质。

The most informative definitions specify the meanings of terms by analyzing concepts and supplying a noncircular synonym for the term begin defined. In philosophy, such definitions are know as full or complete definitions. Philosophers sometimes uses these expressions to designate more fine-grained distinctions, in which the term ideal definition is used instead.
信息量最大的定义是通过分析概念并为被定义的术语提供非循环同义词来指定术语的含义。在哲学中，这种定义被称为完全或完整定义。哲学家有时会用这些表达来指代更细微的区别，在这种情况下，会使用理想定义一词。

4.1.2.2. Ideal Definitions
4.1.2.2. 理想定义

Ideal definitions explain the meanings of terms by relating them to expression that we already understand. It is important that the definiens make use of neither the term being defined nor one of its close cognates; otherwise the definition will be circular (e.g., defining a line as a linear path or a cause as something that produces an effect). Someone that does not understand the meaning of the definiens ‘cause’ will also not understand the meaning of ‘effect’ since the definition of ‘effect’ may be something that is caused.
理想的定义是通过将一个词与我们已知的表达联系起来解释其含义。重要的是，定义的描述部分不能使用被定义的词语本身或与其相近的词语，否则就会陷入循环定义的陷阱。例如，将“线”定义为“线性路径”，或者将“原因”定义为“产生结果的事物”。如果一个人不理解“原因”的含义，那么他也不会理解“结果”的含义，因为“结果”的定义可能是“由原因导致的事物”。

The definition of ‘bachelor’ is not circular since the concept of being unmarried, human, and male does not presuppose an understanding of the concept of bachelor. The definiens provides an informative analysis of the meaning of the definiendum ‘bachelor’. An ideal definition specifies the meaning of a term by reference to a logical conjunction of properties as opposed to representative examples (i.e., ostensive definition), or a procedure for recognizing examples (i.e., operational definition). The conjunction of descriptions determines the extension of the definiendum by specifying necessary and sufficient conditions for its application.
“单身汉”的定义并非循环论证，因为“未婚、男性、人类”的概念并不以理解“单身汉”的概念为先决条件。被定义项“单身汉”的意义可通过定义项进行信息性分析得到。“理想定义”指的是以逻辑属性组合（而非代表性例子（即“列举定义”）或识别例子的程序（即“操作定义”））来阐明一个术语的意义。描述性条件的组合通过详细说明其应用的必要条件和充分条件来确定被定义项的外延。

• A necessary condition for falling into the extension of a term is a condition in whose absence the term does not apply. For example, “Without a microscope, a person cannot see viruses.”
  构成某个术语外延的必要条件，指的是缺少该条件，则该术语不适用的条件。例如，“没有显微镜，人就看不到病毒”。

• A sufficient condition is a condition in whose presence the term cannot fail to apply. For example, “For a shape to be a square, each of its sides is equal in length to each of the other sides.”
  充分条件是指只要条件具备，术语就一定适用的条件。例如，“如果一个形状是正方形，那么它的每条边都和其他边等长”。

Most proposed ideal definitions face borderline cases in which it is uncertain as to whether something satisfies the conjunction of predicates supplied by the definiens. A good example is the question of whether a ten-year-old boy is a bachelor. Even if this case is resolved by adding an additional condition (e.g., adult) to the definiens, there wil be other borderline cases because language is vague.
大多数提出的理想定义都会面临边界情况，在这些情况下，我们不确定某事物是否满足定义项提供的谓词的组合。一个很好的例子是，一个十岁的男孩是否可以被称为单身汉。即使通过在定义项中添加一个附加条件（例如，成年）来解决这个问题，也会出现其他的边界情况，因为语言本身就具有模糊性。

Try to distinguish a bald man from a man who is not bald in terms of the number of hairs on his head. the fact that we cannot specify a crisp boundary does not show that there is no difference between being bald and not being bald. Ideal definitions that specify necessary and sufficient conditions are rare. We can often construct fairly satisfactory approximations of an ideal definition despite the problem of borderline cases. If definitions of life faced nothing more serious than counterexamples (e.g., viruses), there might not be insurmountable problems. Good definitions of life must deal with quartz crystals and candle flames, which are presumably not alive.
试着从头发数量的角度区分秃顶的男人和不秃顶的男人。我们无法确定一个清晰的界限，但这并不意味着秃顶和不秃顶之间没有区别。能够确定必要充分条件的理想定义是很少见的。尽管存在边界案例的问题，我们还是可以经常构建出对理想定义相当满意的近似值。如果对生命的定义只需要面对反例（例如病毒），那么可能就不会存在无法克服的问题。对生命的良好定义必须能够处理石英晶体和烛焰，而这些东西想必是没有生命的。

4.1.3. Natural Kinds
4.1.3. 自然类

Ideal definitions specify meaning by providing a complete analysis of the concepts associated with terms. However, they do not supply good answers to question about the identity of natural kinds, or categories carved out by nature. It is important to astrobiology because it seems likely that life is a natural kind term and represents an objective fact about the natural world.
理想定义通过全面分析与术语相关的概念来界定含义。然而，它们并不能很好地回答关于**自然类**（即自然划分出的类别）的本质问题。这对天体生物学来说非常重要，因为**生命**很可能就是一个自然类术语，代表着关于自然世界的客观事实。

Consider trying to answer the question “What is water?” by defining the natural kind term ‘water.’ The term ‘water’ could be defined by reference to its sensible properties (see Sect. 4.1). This definition of ‘water’ is not simply a matter of linguistic convention. A reference to a list of sensible properties cannot exclude things that superficially resemble water but are not in fact water. For example, the alchemists identified nitric acid (known as aqua fortis, or strong water) and mixtures of hydrochloric acid (known as aqua regia, or royal water) as water. A mixture of alcohols was called aqua vitae, or water of life. Even today, we classify various liquids in terms of their sensible properties (e.g., salt water, muddy water, and distilled water). Which of the sensible properties (e.g., transparency or tastelessness) are more important to the definition of water?
试想一下，如果要回答“水是什么？”这个问题，我们可能会尝试给“水”这个词下个定义。一种方法是参考水的感官特性来定义“水”（参见第 4.1 节）。但这种定义方式并不仅仅是语言上的约定俗成。仅仅列出一堆感官特性，并不能真正区分水和其他看起来像水但实际上并非水的物质。 历史上就有不少例子。比如，炼金术士就曾经将硝酸（被称为“aqua fortis”，意为“强水”）以及盐酸的混合物（被称为“aqua regia”，意为“王水”）也归类为水。酒精的混合物也被称为“aqua vitae”，意为“生命之水”。即使在今天，我们仍然会根据液体的感官特性来进行分类，例如盐水、泥水和蒸馏水等等。 那么问题来了：在水的定义中，哪些感官特性更为重要呢？是透明度？还是无味？

And so it [water] is sometimes sharp and sometimes strong, sometimes acid and sometimes bitter, sometimes sweet and sometimes thick or thin, sometimes it is seen bringing hurt or pestilence, sometimes health-giving, sometimes poisonous. So one would say that it suffers change into as many natures as are the different places through which it passes. And as the mirror changes with the colour of its object so it changes with the nature of the place through which it passes: health-giving, noisome, laxative, astringent, sulphurous, salt, incarnadined, mournful, raging, angry, red, yellow, green, black, blue, greasy, fat, thin.
水有时尖锐，有时强劲，有时酸涩，有时苦涩，有时甘甜，有时浓稠，有时稀薄。有时它带来伤害和瘟疫，有时它带来健康，有时它带来毒害。可以说，水会根据它所经过的地方的不同，而呈现出相应的变化。如同镜子会随着物体颜色而改变自身颜色一样，水也会随着它所经过的地方的性质而改变：有时带来健康，有时令人厌恶，有时让人通便，有时却让人便秘；有时它像硫磺，有时像盐，有时呈现肉色，有时忧郁，有时狂暴，有时愤怒，有时是红色，有时是黄色，有时是绿色，有时是黑色，有时是蓝色，有时油腻，有时浓稠，有时稀薄。

—Leonardo da Vinci (1513)
——列奥纳多·达·芬奇 (1513)

Without an understanding of the intrinsic nature of water, there is not a definitive answer to the question “What is water?” But an understanding of the molecular structure of matter causes such quandaries to disappear. The identification of water with ${\mathrm{H}}_2\mathrm{O}$ explains why liquids can resemble water and not be water because their molecular composition is more than ${\mathrm{H}}_2\mathrm{O}$ alone. In addition, the identification holds regardless of whether the water is in any of its familiar forms (solid, liquid, or gas), and it wil hold equally well in the less familiar high-pressure phases. The Aristotelian view of water as one of the basic elements began to fall into disfavor with a paper from Antoine Lavoisier (1783) that showed water is not an element, but can be decomposed and recombined.
如果不了解水的本质，就无法确切地回答“水是什么”这个问题。但是，只要理解了物质的分子结构，这种困惑就会迎刃而解。认识到水是由 ${\mathrm{H}}_2\mathrm{O}$ 构成，就能解释为什么有些液体看起来像水却不是水，因为它们的分子组成不仅仅是 ${\mathrm{H}}_2\mathrm{O}$ 。此外，无论水是以哪种常见形态存在（固态、液态或气态），甚至是在不太常见的高压状态下，这种认识都成立。亚里士多德认为水是基本元素之一，但随着安托万·拉瓦锡在 1783 年发表的一篇论文证明水并非元素，而是可以分解和重组的，这种观点开始逐渐被推翻。

The identification of water with ${\mathrm{H}}_2\mathrm{O}$ does not have the character of an ideal definition. It cannot be viewed as explicating the concept that has historically been associated with the term ‘water’ because that concept encompasses stuff varying widely in chemical and physical composition. The claim that water is ${\mathrm{H}}_2\mathrm{O}$ began as a testable empirical conjecture, and it is now considered so well confirmed that most scientists characterize it as a fact.
用“ ${\mathrm{H}}_2\mathrm{O}$ ”来定义水，算不上一个理想的定义。它并不能解释与“水”这个词相关的历史概念，因为“水”的概念包含了化学和物理成分差异很大的物质。水是 ${\mathrm{H}}_2\mathrm{O}$ 这一说法最初只是一个可检验的经验猜想，现在则被认为是经过了充分验证，大多数科学家将其视为事实。

4.1.4. What is Life?
4.1.4. 什么是生命？

If life is a natural kind, then attempts to define life are fundamentally misguided. Definitions serve only to explain the concepts that we currently associate with terms. Concepts cannot reveal the objecting underlying nature (or lack thereof) of the categories designated by natural kind terms. It is the underlying nature (not the concepts in our heads) that we are interested in when we use a natural kind term. The term ‘water’ means whatever the stuff in streams, lakes, oceans, and everything else that is water has in common. We currently believe that this stuff is ${\mathrm{H}}_2\mathrm{O}$, and our belief is based on a well-confirmed, general scientific theory of matter.
如果生命是一种自然类，那么试图定义生命从根本上就是误导。定义只是为了解释我们目前与术语相关的概念。概念无法揭示自然类术语所指定类别的客观基础性质（或缺乏这种性质）。当我们使用自然类术语时，我们感兴趣的是其基础性质（而不是我们头脑中的概念）。“水”这个词指的是溪流、湖泊、海洋以及其他所有水中物质的共同点。我们目前认为这种物质是 ${\mathrm{H}}_2\mathrm{O}$ ，而我们的信念是基于一个经过充分证实的、关于物质的一般科学理论。

We cannot be absolutely positive that molecular theory is the final word on the nature of matter; conclusive proof is just not possible in science. Our current scientific concept of water as ${\mathrm{H}}_2\mathrm{O}$ represents a vast improvement over earlier concepts based on superficial sensory experience. If we someday discover that molecular theory is wrong, we will change the concept that we associate with ‘water,’ but we will still be talking about the same thing.
我们无法完全肯定分子理论就是关于物质本质的最终定论；科学领域不存在绝对的证据。 我们目前将水理解为由分子构成的概念，相较于过去仅仅基于感官体验的认识，已经有了巨大的进步。 即使未来有一天我们发现分子理论是错误的，我们也会改变对“水”的理解，但我们讨论的依然是同一种物质。

We could define ‘life’ to mean whatever cyanobacteria, hyperthermophilic archaeobacteria, amoeba, mushrooms, palm trees, sea turtles, elephants, humans, and everything else that is alive (on Earth or elsewhere) has in common. No purported definition of ‘life’ can provide a scientifically satisfying answer to the question “What is life?” because no mere analysis using human concepts can reveal the nature of a world that lies beyond them. The best we can do is to construct and empirically test theories about the general nature of living systems. We want theories that settle our dilemmas in classification by explaining puzzle cases:
我们可以尝试给“生命”下一个定义，囊括蓝藻、超嗜热古细菌、变形虫、蘑菇、棕榈树、海龟、大象、人类以及地球内外所有生物的共同特征。但实际上，任何所谓的“生命”定义都无法科学地回答“生命是什么”这个问题，因为人类的概念分析无法揭示超越其认知范围的世界本质。我们所能做的，就是构建关于生命系统一般性质的理论，并通过实验进行检验。我们希望这些理论能够解释那些难以归类的特殊案例，从而解决我们在分类上面临的难题。

• Why things that are alive sometimes lack features that we associate with life?
  为什么生物有时会缺乏我们与生命相关的特征？

• Why things that are non-living sometimes have features that we associate with life?
  为什么非生物有时会具有我们与生命相关的特征？

No scientific theory can be conclusive, but someday we may have a well-confirmed, adequately general theory of life that will allow us to formulate a theoretical identity statement (e.g., “water is ${\mathrm{H}}_2\mathrm{O}$”) providing a scientifically satisfying answer to the question “What is life?”
没有任何科学理论可以做到绝对的结论性，但也许在将来的某一天，我们能够拥有一个关于生命的、经过充分验证的、足够普适的理论，使我们能够提出一个理论上的定义（例如，“水是 ${\mathrm{H}}_2\mathrm{O}$ ”），从而为“生命是什么？”这个问题提供一个在科学上令人满意的答案。

4.1.4.1. Dreams of a General Theory of Life
4.1.4.1. 对生命大一统理论的展望

To formulate a convincing theoretical identity statement for life, we need a general theory of living systems. Our current sample size for such a theory is one (i.e., terrestrial life), which is a problem. Although the morphological diversity of terrestrial life is extraordinary, all known life on Earth is also extraordinarily similar in its biochemistry. With the exception of some viruses, the hereditary material of all known life on Earth is DNA of the same right-handed chirality. Life on Earth utilizes 20 amino acids to construct proteins, and these amino acids are typically of left-handed chirality. These biochemical similarities lead to the conclusion that life on Earth had a single origin.
为了给生命下一个精准又令人信服的定义，我们需要建立一个通用的生命系统理论。然而，我们目前只有一个样本——地球生命，这无疑是个巨大的障碍。虽然地球上的生命形态千差万别，但它们在生物化学层面却惊人地相似。除了某些病毒以外，地球上所有已知生命的遗传物质都是相同右手螺旋结构的 DNA。地球生命都利用 20 种氨基酸来构建蛋白质，而且这些氨基酸通常都是左旋的。这些生物化学上的相似性让我们得出结论：地球上的生命起源于同一个祖先。

Darwinian evolution then explains how this common biochemical framework yielded such an amazing diversity of life. It is difficult to decide which features of terrestrial life are common to all life (wherever it may be found) because the biochemical similarities of all life on Earth can be explained in terms of a single origin. Many biochemical features that strike us as important may derive from mere chemical or physical contingencies present at the time life originated on Earth (Sagan (1974)).
达尔文进化论接着解释了这种共同的生化框架是如何产生如此惊人的生命多样性的。很难确定地球生命的哪些特征是所有生命（无论在哪里发现）所共有的，因为地球上所有生命的生化相似性都可以用单一起源来解释。许多我们认为重要的生化特征可能仅仅源于地球上生命起源时的化学或物理偶然性（Sagan (1974)）。

How can we discriminate the contingent from the essential? It is a bit like trying to come up with a theory of mammals when one can observe only zebras. What features of zebras should one focus upon (e.g., their stripes or mammary glands)? The mammary glands (although present in only some zebras) tells us more about what it means to be a mammal than do the ubiquitous stripes. Without access to living things with a different historical origin, it is difficult to formulate an adequately general theory of living systems. This problem is not unique to life, where it reflects a simple logical point. One cannot generalize from a single example. Biochemical analyses coupled with knowledge of evolution suggests that much of the diversity observed on Earth may be a historical accident. Had the history of the Earth been different, life on Earth would certainly be different. In essence, the common origin of contemporary terrestrial life blinds us to the possibilities for life in general.
我们该如何区分生命的偶然特征和本质特征呢？这就好比试图仅凭观察斑马就建立一个关于哺乳动物的理论。我们应该关注斑马的哪些特征呢？（比如它们的条纹还是乳腺？）虽然只有部分斑马拥有乳腺，但相比于普遍存在的条纹，乳腺更能揭示哺乳动物的本质。如果我们无法接触到具有不同历史起源的生物，就很难建立一个关于生命系统的、充分普适的理论。这个问题并非生命科学所独有，它反映了一个简单的逻辑：我们无法从单一案例中进行概括。结合进化论的生物化学分析表明，地球上观察到的生物多样性很可能只是历史的偶然。如果地球的历史轨迹不同，地球上的生命也必然会截然不同。从本质上讲，当代地球生命共同的起源局限了我们对生命形式的想象力。

Many definitions of life cite sensible properties of terrestrial life, such as metabolism, reproduction, complex hierarchical structure, and self-regulation. But defining life in terms of sensible properties is analogous to defining water as being wet, transparent, tasteless, etc. Reference to sensible properties for water is unable to exclude things that are not water and to include everything that is water (e.g., ice).
许多对生命的定义都引用了地球生命的可感知特性，例如新陈代谢、繁殖、复杂的分层结构和自我调节。但是，根据可感知特性来定义生命类似于将水定义为潮湿、透明、无味等。对水的可感知特性的描述既不能排除非水的东西，也不能涵盖所有水的东西（例如冰）。

4.1.4.2. How to Search for Extraterrestrial Life
4.1.4.2. 如何寻找外星生命

There remains the problem of how to hunt for extraterrestrial life without either a definition of life or a general theory of living systems. One approach is to treat the features that we currently use as tentative criteria for life. The features will then necessarily be inconclusive, where their absence cannot be taken as sufficient for concluding that something is not alive. Therefore, they cannot be viewed as providing operational definitions of life. The purpose of using tentative criteria is not to definitively settle the issue of whether something is alive, but rather to focus attention on possible candidates (i.e., physical systems whose status as living or non-living is genuinely unclear).
一个问题仍然存在：在没有对“生命”的定义，也没有关于生命系统的一般理论的情况下，我们该如何寻找地外生命？一种方法是将我们目前使用的特征作为生命的“暂定标准”。这些标准必然不是绝对的，因为即使缺少这些特征，也不能就此断定某个事物没有生命。因此，它们不能被视为对“生命”的有效定义。使用暂定标准的目的并不是要彻底解决某个事物是否具有生命的问题，而是将注意力集中在可能的候选者身上（即那些生命状态尚不明确的物理系统）。

The criteria should include a wide diversity of features of terrestrial life, where diversity is absolutely crucial when one is looking for extinct extraterrestrial life (e.g., in the martian meteorite ALH84001, or with instrument packages delivered to ancient martian flood plains). Some feature for shaping searches for extraterrestrial life may not even be universal to terrestrial life. For example, features that are common only to life found in certain terrestrial environments may prove more useful for searching for life in analogous extraterrestrial environments than are features that are universal to terrestrial life. Features that are common or non-existent among non-living terrestrial systems may make good criteria for present or past life because the stand out against the background of non-living processes. The chains of chemically pure, single-domain magnetite crystals found in ALH84001 provide a potential example. If it turns out that these chains can only be produced biogenically, then they will provide a good biosignature for life, despite the fact that most terrestrial bacteria do not produce them.
在寻找地外生命时，尤其是在寻找灭绝的地外生命时（比如在火星陨石 ALH84001 中，或者通过探测器在火星远古洪水冲积平原上寻找），我们使用的标准应该包含地球生命的多样性特征，因为多样性至关重要。有些用来寻找地外生命的特征甚至可能不适用于地球生命。例如，只在某些特定地球环境中发现的生命所具有的特征，可能比地球生命普遍具有的特征更有助于在类似的外星环境中寻找生命。那些在非生物地球系统中普遍存在或不存在的特征，可以作为判断现在或过去存在生命的良好标准，因为它们在非生物过程中显得尤为突出。在 ALH84001 陨石中发现的化学纯净的单畴磁铁矿晶体链就是一个很好的例子。 如果事实证明这些化学链只能由生物体产生，那么它们将成为生命存在的有力证据，尽管大多数地球细菌并不会产生它们。

The basic idea behind our strategy for searching for extraterrestrial life is to employ empirically well-founded (albeit provisional) criteria that increase the probability of recognizing extraterrestrial life while minimizing the chances of being misled by inadequate definitions. This is similar in spirit to suggestions that in situ searches for extraterrestrial life should rely when possible on contrasting definitions of life. The important point is to include the potential boundaries of our current concept of life. It is only in this way that we can move beyond our geocentric ideas and recognize genuinely weird extraterrestrial life, should we be fortunate enough to encounter it.
我们搜寻地外生命的策略，其基本思路是采用有可靠经验基础（尽管是暂时的）的标准。这些标准能够提高我们识别地外生命的概率，同时最大限度地减少因定义不足而被误导的可能性。这和一些建议的思路相似，这些建议认为，对地外生命的实地探测应该尽可能地依赖于对生命的对比性定义。重要的是要考虑到我们目前对生命概念的潜在边界。只有这样，我们才能超越以地球为中心的观念，并在足够幸运的情况下，识别出真正奇特的地外生命。

4.1.5. Requirements and Limits for Life in the Context of Exoplanets (McKay 2014)
4.1.5. 系外行星环境下的生命限制条件（McKay, 2014）¹ ¹此处数字上标对应参考文献列表，格式自拟。

4.1.5.1. Limits to Life
## 生存的局限性

There are two somewhat different approaches to the question of the limits to life:
关于生命体存在的限制，有两种略微不同的研究方法：

• to determine the requirements for life
  确定生命体存在的必要条件

• to determine the extreme environments in which adapted organisms (i.e., extremophiles) can survive.
  确定适应性生物体（即极端微生物）可以生存的极端环境。

The requirements for life on Earth broadly include four items: (1) energy, (2) carbon, (3) liquid water, and (4) various other elements. In our Solar System, it is the occurrence of liquid water that appears to limit the occurrence of habitable environments, and it appears to be the case for exoplanetary systems as well.
地球上生命体存在的必要条件大体包括四个方面：（1）能量，（2）碳，（3）液态水，以及（4）其他各种元素。在我们的太阳系中，液态水的存在似乎限制了宜居环境的出现，而系外行星系统似乎也是如此。

The first requirement arises from basic thermodynamic considerations, where it is clear that life requires a source of energy. Life on Earth uses only one energy source: that associated with the transfer of electrons by chemical reactions of reduction and oxidation to power metabolism and growth. Methane-producing microbes use the reaction of $\mathrm{C}{\mathrm{O}}_2$ with ${\mathrm{H}}_2$ to produce $\mathrm{C}{\mathrm{H}}_4$. Photosynthetic organisms use a light-absorbing protein (e.g., chlorophyll, bacteriochlorophylls, and bacteriorhodopsin) to convert photon energy to the energy of an electron which then completes a redox reaction. This occurs in the mitochondria in of most eukaryotes and in the cell membrane of prokaryotic cells. Although life can detect and generate other energy sources (including magnetic, kinetic, gravitational, thermal gradient, and electrostatic), none of these is used for metabolic energy.
生命首先需要能量来源，这一点从基本的热力学角度就能明白。地球上的生命都利用同一种能量来源：通过氧化还原反应传递电子来驱动新陈代谢和生长。例如，产甲烷菌利用 $\mathrm{C}{\mathrm{O}}_2$ 和 ${\mathrm{H}}_2$ 反应生成 $\mathrm{C}{\mathrm{H}}_4$ 。而光合生物则利用光吸收蛋白（如叶绿素、细菌叶绿素和细菌视紫红质）将光能转化为电子能量，进而完成氧化还原反应。这一过程发生在大多数真核生物的线粒体和原核生物的细胞膜上。虽然生命可以感知和产生其他能量形式（包括磁能、动能、重力势能、热能和静电能），但这些能量都不能被生命用来维持新陈代谢。

Carbon has the dominant role as the backbone molecule for Earth life and is widespread in the Solar System. However, carbon may not be that useful as an indicator because the Earth is significantly depleted in carbon compared with the outer Solar System. The vast majority of the carbon on Earth is stored in sedimentary rocks within the crust. However, light carbon-containing molecules (e.g., $\mathrm{C}{\mathrm{O}}_2$, $\mathrm{C}\mathrm{O}$, and $\mathrm{C}{\mathrm{H}}_4$) are volatile, which means adequate carbon is present at the surface of the terrestrial planets.
碳是地球生命骨架分子的主角，并且在太阳系中广泛存在。 然而，碳可能不是一个很好的指标，因为与太阳系外部相比，地球上的碳含量明显偏低。 地球上绝大多数的碳都储存在地壳内的沉积岩中。 然而，轻质含碳分子（例如， $\mathrm{C}{\mathrm{O}}_2$ 、 $\mathrm{C}\mathrm{O}$ 和 $\mathrm{C}{\mathrm{H}}_4$ ）是挥发性的，这意味着在地球行星的表面存在着充足的碳。

Life on Earth uses a vast array of the elements available at the surface. However this does not prove that these elements are absolute requirements for carbon-based life in general. The elements $\mathrm{N}$, $\mathrm{S}$, and $\mathrm{P}$ are probably the leading candidates for the status of required elements. If liquid water and biologically available nitrogen are present, then phosphorous, potassium, sodium, sulfur, and calcium might come next, as these are the most abundant elements in bacteria. However, there is no definitive list and habitability can only be confirmed by showing inhabitation.
地球上的生命利用了地表存在的各种元素。但这并不能证明这些元素是碳基生命存在的绝对必要条件。元素 $\mathrm{N}$ 、 $\mathrm{S}$ 和 $\mathrm{P}$ 很可能是生命必需元素的最佳候选者。如果存在液态水和生物可利用的氮，那么磷、钾、钠、硫和钙可能紧随其后，因为它们是细菌中含量最丰富的元素。然而，目前还没有一个明确的清单，宜居性只能通过证明存在生命来确认。

4.1.5.2. Strategy for Exoplanets
4.1.5.2. 系外行星策略 #

The second approach to the requirement for life is based on the abilities of extremophiles in a range of environmental factors (see Table 4.1) It may seem logical to focus on primary production because without that there cannot be an ecosystem. However, its is possible that photochemical processes in an exoplanet atmosphere play the roe of primary production as has been suggested for Titan. The most important parameter for Earth-like life is the presence of liquid water, which directly depends on pressure and temperature. Temperature is key because of its influence on liquid water and it can be directly estimated from orbital and climate models of exoplanetary systems.
寻找生命迹象的第二种方法是根据极端微生物在各种环境因素下的生存能力（见表4.1）。将主要生产作为关注焦点看似合理，因为没有它，生态系统就无从谈起。然而，系外行星大气中的光化学过程可能也起着类似初级生产的作用，正如人们对土卫六所推测的那样。对于类地生命而言，最重要的参数是液态水的存在，这直接取决于压力和温度。温度之所以关键，是因为它会影响液态水，而且可以通过系外行星系统的轨道和气候模型直接估算出来。

Table 4.1 Ecological limits to life; taken from McKay (2014).
表 4.1 生命存在的生态限制；摘自 McKay (2014)。#

Parameter 参数	Limit 限值	Note 注释
Lower temperature 温度下限	$\sim -15{}^{\circ }\mathrm{C}$	Limited by liquid water associated with thin films or saline solutions 受限于与薄膜或盐溶液相关的液态水
Upper temperature 温度上限	$122{}^{\circ }\mathrm{C}$	Solubility of lipids in water, protein stability 脂类在水中的溶解度，蛋白质稳定性
Maximum pressure 最大压力	$1100\mathrm{a}\mathrm{t}\mathrm{m}$
Low light 弱光	$\sim 5\times {10}^{-6}$ direct solar flux on Earth $\sim 5\times {10}^{-6}$ 到达地球的太阳光通量	Algae under ice and deep sea 冰层和深海中的藻类
pH	$0-12.5$
Salinity 盐度	Saturated $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$ 饱和 $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$	Depends on the salt and temperature 取决于盐和温度
Water activity 水分活度	0.6 (0.8)	Yeast and molds (bacteria) 酵母菌和霉菌（细菌）
UV Radiation 紫外线辐射	$\ge 1000\mathrm{J}/{\mathrm{m}}^2$ or $50\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{y}/\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}$ $\ge 1000\mathrm{J}/{\mathrm{m}}^2$ 或 $50\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{y}/\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}$	D. radiodurans growth with continuous dose D. radiodurans在持续辐射下的生长情况

• Temperature, Cold limit: Many organisms can grow and reproduce at temperatures well below the freezing point of pure water because their intracellular material contains salts and other solutes that lower the freezing point of the solution. The snow algae Chlamydomanas nivalis thrives in liquid water associated with snow (coloring it red), but the algae are the beneficiaries of external processes that melt the snow.
  温度，低温极限：许多生物可以在远低于纯水冰点的温度下生长和繁殖，因为它们的细胞内物质含有盐和其他溶质，这些物质会降低溶液的冰点。雪藻Chlamydomanas nivalis在雪中与液态水共存（并使其呈现红色），但藻类的生长得益于融化积雪的外部过程。

• Temperature, Hot limit: As water is heated (and maintained as a liquid under pressure), the dielectric constant and the polarity of the liquid decreases sharply. This significantly changes the characteristics of water as a solvent and it interaction with dissolved biomolecules (in particular lipids, but also proteins and nucleic acids). Organisms that can survive the highest temperatures are archaea, as their membrane lipids are held together with ether bonds.
  温度，高温极限：随着水被加热（并在压力下保持液态），其介电常数和极性会急剧下降。这会显著改变水作为溶剂的特性及其与溶解的生物分子（特别是脂质，还有蛋白质和核酸）的相互作用。能够在最高温度下生存的生物是古细菌，因为它们的膜脂是由醚键连接在一起的。

• Water, Dry limit: On worlds where the temperature is within the appropriate range (for Earth-like life), life may be limited by the availability of water (i.e., low water activity), where we only have to look at Mars for an example. In dry environments on Earth, photosynthetic cyanobacteria and lichens survive in several dry deserts. Endolithic cyanobacteria live just below the surface of halite rock in the dry core of the Atacama Desert.
  **水与干旱极限** 在一个温度适宜生命存在的星球上（比如类似地球的星球），生命的存活会受到水资源供应的限制，比如水活度较低的环境。火星就是一个很好的例子。 不过，即使在地球上一些极度干旱的沙漠中，仍然存活着一些顽强的生命形式，例如进行光合作用的蓝藻和地衣。还有一种叫做内生蓝藻的生物，它们生活在阿塔卡马沙漠腹地干旱的岩盐岩石表面之下。

• Energy: Energy for life can come from chemical redox couples generated by geothermal processes or light from the central star. Geothermal flux can arise from (i) the planet cooling off from its gravitational heat of formation, (ii) decay of long-lived radioactive elements, or (iii) tidal heating for a close-orbiting world or moon. On Earth only a tiny fraction of the geothermal heat is converted into chemical energy, whereas about half the solar flux occurs at wavelengths that are usable for photosynthesis. Life is able to use light at very low levels, as red macroalgae on deep seamounts grows at low light levels.
  **能量：**生命的能量来源可以是地热过程产生的化学氧化还原对，也可以是来自中心恒星的光能。地热通量可能来自以下几个方面：(1) 行星形成时的重力热逐渐冷却，(2) 长寿命放射性元素的衰变，或者 (3) 对于靠近恒星运行的行星或卫星来说，潮汐力也会产生热量。在地球上，只有一小部分地热能转化为化学能，而大约一半的太阳光辐射处于可用于光合作用的波长范围内。即使在光线非常微弱的情况下，生命体也能够利用光能，例如生长在深海热液喷口附近的红色宏观藻类。

• UV and Radiation: Complex lifeforms (e.g., humans) are sensitive to radiation but the does that can be tolerated by many microorganisms is astonishingly high. A well-studied soil heterotroph with high radiation tolerance is Deinococcus radiodurans, which can withstand an acute dose of radiation due to adaptation to dehydration stress.
  **紫外线和辐射**：像人类这样的复杂生命形式对辐射很敏感，但许多微生物所能承受的辐射剂量却高得惊人。耐辐射奇球菌（*Deinococcus radiodurans*）是一种经过充分研究的土壤异养生物，它对辐射具有极高的耐受性，能够承受急性辐射剂量，这得益于它对脱水胁迫的适应能力。

4.2. Evolution: a Defining Feature of Life
## 4.2 生命的定义性特征：进化

4.2.1. From Lamarck to Darwin to the Central Dogma
**4.2.1 从拉马克到达尔文，再到中心法则**

The basic notion of evolution is that inherited changes in populations of organisms result in expressed differences over time. These differences are at the gene level (i.e., genotype) and/or expression of the gene into an identifiable characteristic (i.e., phenotype). The important underlying fact of evolution is that all organism share a common ancestor. We see this in the universal nature of the genetic code and in the unity of biochemistry:
进化的基本理念认为，生物群体中可遗传的变异会随着时间的推移导致生物性状出现差异。这些差异体现在基因层面（即*基因型*）和/或基因表达成的可识别特征（即*表型*）。进化论的一个重要基础是所有生物都拥有一个共同祖先。这一点从普遍存在的遗传密码和*生物化学的统一性*可见一斑：

• all organisms share the same biochemical tools to translate the universal information code from genes to proteins,
  所有生物都使用相同的生化工具将通用的信息代码从基因翻译成蛋白质，

• all proteins are composed of the same twenty essential amino acids, and
  所有蛋白质都由相同的二十种必需氨基酸组成，以及

• all organisms derive energy from metabolic, catalytic, and biosynthetic processes from the same high energy organic compounds (e.g., adenosine triphosphate (ATP)).
  所有生物都从相同的高能有机化合物（例如三磷酸腺苷 (ATP)）的代谢、催化和生物合成过程中获取能量。

Darwin’s On the Origin of Species describes his theory of evolution using evidence that included an ancient Earth, which geologists at the time (in 1859) believed to be in the millions of years. Darwin also took the extinction of species to ba a real phenomenon since fossils existed that were without living representatives. Since different species showed close phenotypic similarities, he argued that existing organisms descended from other organisms including extinct groups.
达尔文在其著作《物种起源》中阐述了他的进化论。这本书于 1859 年问世，当时的地质学家认为地球的年龄已有数百万年，达尔文将此作为证据之一来支持他的理论。他还将物种灭绝视为一个真实存在的现象，因为已经发现的化石中，有一些物种并没有现存的后代。由于不同物种之间存在着密切的表型相似性，达尔文认为现存的生物是起源于其他生物，包括那些已经灭绝的物种。

The key to his evolutionary theory was that inherited characteristics of organisms can change through time and that these changes occurred gradually and without discontinuities. Jean-Baptiste Lamarck recognized (in 1809) a similar principle of evolution and offered an explanation generally referred to as inheritance of acquired characters. Lamarck meant that the variation in characteristics or adaptations seen in organisms were acquired in response to the environment. Classic examples include the long neck of the giraffe as an adaptation for foraging tender foliage on treetops, or the use of long legs by some aquatic fowl to venture into deep waters in search of prey. Under Darwinian principles they were not acquired as a response to the needs of the environments.
他进化论的核心在于，生物的遗传特征会随着时间推移而发生改变，而且这种改变是逐渐发生的，不存在断层。让-巴蒂斯特·拉马克早在 1809 年就意识到了类似的进化原则，并提出了一种解释，通常被称为获得性遗传。拉马克认为，生物体特征的变异或适应性是为应对环境而获得的。经典的例子包括长颈鹿为了吃到树梢上的嫩叶而进化出长脖子，以及一些水禽为了到深水中觅食而进化出长腿。但根据达尔文的理论，这些特征并不是为了应对环境需求而获得的。

One of Darwin’s major contributions was his explanation of how and why organisms change over time and how they acquire characteristics useful for living in different environments. Darwin referred to the mechanism for character changes through time as natural selection. Natural selection is based on the idea of the struggle for existence (i.e., survival of the fittest) in populations where there are more individuals of each species than can survive. A variation in any characteristic of an individual that gives an advantage in surviving (and reproducing) will be “naturally selected” since the new trait will be preferentially inherited by subsequent generations (see Fig. 4.3 or Khan Academy for more detail). Darwin differed from Lamarck by recognizing that character changes that offer a survival advantage and are “naturally selected” originate from a pool of randomly generated character changes that are not directed by environmental conditions.
达尔文的主要贡献之一，就是解释了生物是如何以及为什么随着时间的推移而发生变化，以及它们如何获得适应不同环境生存的特征。达尔文将这种随着时间推移而发生性状改变的机制称为“自然选择”。自然选择基于“生存斗争”（也就是“适者生存”）的理念，即在群体中，每个物种的个体数量都超过了环境所能承载的生存极限。如果一个个体在某个特征上出现了变异，并且这种变异赋予了它在生存（和繁殖）方面的优势，那么这种变异就会被“自然选择”出来，因为这种新的性状会被后代优先继承（更详细的解释请参见图 4.3 或访问可汗学院）。 达尔文与拉马克的不同之处在于：达尔文认为，那些能带来生存优势并被“自然选择”出来的性状改变，最初的来源是一系列随机产生的性状变化，而非环境条件直接导致的。

Note 注释

Darwin proposed his theory without knowing the mechanism for the inheritance of acquired traits. Forty years later, the field of genetics would begin with the recognition that Gregor Mendel’s principle of discrete units of inheritance (i.e., genes) was correct.
达尔文提出他的理论时，并不知道获得性状遗传的机制。四十年后，遗传学领域开始认识到格雷戈尔·孟德尔的离散遗传单位（即基因）原理是正确的。

Fig. 4.3 Diagram of mutation and selection in evolution. Image Credit: Wikimedia Commons:mutation and selection diagram.
图 4.3 演化过程中突变和选择的示意图。图片来源：维基共享资源：突变和选择示意图。#

Mendelian genetics established that phenotypes are transmitted from one generation to another following statistical principles and that these phenotypes reside in simple heritable “characters.” The nature of these heritable characters were unknown to Mendel but their location was confined to chromosomes by 1910 and then to DNA as the genetic material by Hershey & Chase (1952). This immediately led to the discovery by Watson & Crick (1953) of the double-helix structure of DNA. Shortly afterwards the concept of the genetic code was developed with the understanding that a gene is primarily a sequence along a section of DNA that codes for a protein using an alphabet composed of the four bases that constitute DNA.
孟德尔遗传学说确立了生物的性状会遵循统计学原理从一代传递到下一代，而这些性状存在于简单的可遗传“因子”中。虽然孟德尔并不知道这些可遗传因子的本质，但到了 1910 年，人们确定它们位于染色体上，而赫希和蔡斯在 1952 年的实验又进一步确定了 DNA 才是遗传物质。在此基础上，沃森和克里克于 1953 年发现了 DNA 的双螺旋结构。不久之后，人们发展出了遗传密码的概念，认识到基因本质上是 DNA 片段上的一串序列，它利用构成 DNA 的四种碱基作为字母表，编码蛋白质。

The steps leading from DNA to a specific protein are referred to as the central dogma: a DNA gene is transcribed to make messenger RNA (mRNA), followed by translation of mRNA into a protein. The exceptions to the central dogma are those genes that specify not proteins, but instead the various classes of RNA that are involved in both transcription and translation (e.g., ribosomal RNA (rRna) and transfer RNA (tRNA)).
从 DNA 到形成特定蛋白质的过程被称为“中心法则”。简单来说，DNA 上的基因先转录成信使 RNA（mRNA），然后再翻译成蛋白质。当然，凡事都有例外，中心法则也不例外。有些基因并不指导蛋白质合成，而是指导参与转录和翻译过程的各类 RNA 的合成，比如核糖体 RNA（rRNA）和转运 RNA（tRNA）。

4.2.2. Evolution at the Molecular Level
4.2.2. 分子水平的进化

The discovery by Watson and Crick opened the doors to studies of evolution at the molecular level and helped develop classification schemes that allow for the evolutionary comparisons of groups of extant organisms. Usually ribosomal RNA genes and ribosomal protein genes are used for evolutionary studies because they have highly conserved sequences (i.e., sequences that are found across all domains of life). Most functional RNA molecules have secondary structure that are associated with their function. Mutations that change the secondary structure of RNA molecules will frequently render them inactive. These conserved sequences must have originated a long time ago in a common ancestor and are fundamentally important to all species.
沃森和克里克的发现为分子水平的进化研究打开了大门，并推动了分类方案的发展，使人们能够对现存生物类群进行进化比较。通常，核糖体 RNA 基因和核糖体蛋白基因被用于进化研究，因为它们具有高度保守的序列（即在所有生命领域都存在的序列）。大多数功能性 RNA 分子都具有与其功能相关的二级结构。改变 RNA 分子二级结构的突变通常会使其失活。这些保守序列一定起源于很久以前的共同祖先，对所有物种都至关重要。

A central concept in evolutionary theory is that a gene coding for a characteristic is subject to mutation (i.e., change) in a random fashion, which in some cases can lead to variability in that characteristic in the next generation.
进化论中的一个核心概念是：**基因**，也就是决定生物特征的遗传单位，会随机发生**突变**（即改变）。这种突变有时会导致下一代生物在该特征上出现差异。

• Mutations come about due to mistakes made during DNA replication, or through external factors such as ionizing radiation or toxic chemicals.
  突变的发生是由于 DNA 复制过程中出现的错误，或者是由电离辐射或有毒化学物质等外部因素造成的。

• Most mutations have little or no effect on the protein product of the gene or the function of the RNA.
  大多数突变对基因的蛋白质产物或 RNA 的功能几乎没有影响。

• Those that involve deletions or insertions can result in structural changes in the transcribed protein or render structures inactive int the ribosomal RNA.
  那些涉及缺失或插入的突变会导致转录蛋白质的结构发生变化，或者使核糖体 RNA 中的结构失活。

• The most lethal mutations are those that damage the genes involved in DNA replication, transcription of DNA into mRNA, or translation of mRNA into a protein.
  最致命的突变是那些损害参与 DNA 复制、DNA 转录成 mRNA 或 mRNA 翻译成蛋白质的基因的突变。

Fig. 4.4 Examples of mutation within the genome. Image Credit: National Human Genome Research Institute.
图 4.4 基因组内突变的例子。图片来源：美国国家人类基因组研究所。

Changing environmental conditions can negatively affect growth and survival (e.g., inducing stresses), which can result in the death of an organism depending on the degree and kind of stress. To survive a lethal stress, the organism must have mutation rates sufficiently high to handle the stress, but not so high as to cause lethal damage to the genome (i.e., the entire set of genes defining the species).
环境的变化会对生物的生长和生存产生负面影响（例如，诱发各种压力），并可能导致生物死亡，死亡的可能性取决于压力的程度和类型。为了在致命压力下生存，生物体必须具备足够高的突变率来应对压力，但同时，突变率又不能过高，否则会导致基因组（即定义该物种的整套基因）受到致命损害。

All extant organisms have a set of conserved genes for repairing mutations or proteins affected by environmental stresses (e.g., starvation, heat, radiation, changes in pH, etc.). Stress genes greatly reduce the number of deleterious mutations, while they are not 100% effective. The same genes can also targe other specific genes for an increased mutation rate under stress conditions, which are called stress-directed adaptive mutations. This mechanism can be observed when bacteria are starving, but undergo increased evolutionary rates of specific genes involved in the metabolism of alternative nutrients for growth.
所有现存生物都拥有一套保守基因，用于修复突变或修复受环境压力（例如饥饿、高温、辐射、pH 值变化等）影响的蛋白质。应激基因可以显著减少有害突变的数量，但它们并非 100% 有效。在应激条件下，相同的基因也可以靶向其他特定基因，提高其突变率，这被称为应激导向的适应性突变。当细菌处于饥饿状态时，可以观察到这种机制，但参与代谢生长所需替代营养素的特定基因的进化速率会提高。

In evolution, adaptation means more than simply being well suited to the environment, where it also involves (in any generation) the selection of one particular genetic change (over many other possibilities) that results in maximum reproductive success. Many incremental steps are involved in evolving complex structures and process, where it would seem that adaptation involves a sequence of coordinated (not random) steps. Until recently, there was no satisfactory mechanism that could account for the evolution of complex structures.
在进化论中，“适应”不仅仅意味着适应环境，它还包括每一代中对特定基因变化的选择（从众多可能性中选择），这种选择最终能带来最大的繁殖成功。进化出复杂的结构和过程需要许多渐进的步骤，而适应似乎就涉及到一系列协调的（而非随机的）步骤。直到最近，还没有一种令人满意的机制可以解释复杂结构是如何进化的。

To kinds of evolutionary change are recognized.
人们认识到两种进化变化。

• Microevolution results in changes at the species level and accounts for the short term variability observed in populations.
  微观演化是指在物种层面上发生的改变，它解释了在种群中观察到的短期变异。

• Macroevolution involves the more substantial changes that over long time result in the development of a new hierarchy, or higher taxa (e.g., genera, families, orders, etc.). It affects the genotypes of individuals within populations and also involves microevolution. It is also invoked as the mechanism that results in the gradual formation of novel complex structures that involve multiple genes.
  宏观演化则涉及更实质性的变化，这些变化经过长时间会导致新的生物分类阶元的产生，或者说更高阶元的产生（例如：属、科、目等）。它不仅影响种群内个体的基因型，还包含了微观演化的过程。宏观演化也被认为是导致新复杂结构逐渐形成的机制，而这些结构往往涉及多个基因。

Development of the eye has provided a classic illustration for gradualism producing increasing complexity and function. There are more than forty different eye structures found in both invertebrates and vertebrates with a range of complexity from light sensitive patches to compound eyes. It was previously believed that these photosensitive organs developed independently along several different branches of the Tree of Life, and a classic example of convergent evolution (i.e., independent evolution of morphologically an/or functionally similar structures). In many cases macroevolution is not totally a gradual set of changes based on mutation and natural selection. There appears to be an common set of genes that instigated the evolution of the eye in a fruit fly, squid, and humans. These genes are called “tool box” genes, and are common to many diverse organisms, which implies that they are inherited from a common ancestor.
眼睛的演化一直是渐进式演化如何导致复杂性和功能性不断增加的经典例证。 无脊椎动物和脊椎动物都拥有四十多种不同的眼睛结构，其复杂程度从简单的光敏点到精巧的复眼，不一而足。过去，人们认为这些感光器官是在生命之树的不同分支上独立演化而来的，是趋同演化（即形态或功能相似的结构独立演化的现象）的典型例子。然而，在许多情况下，宏观进化并非完全基于突变和自然选择的一系列渐进变化。果蝇、乌贼和人类的眼睛演化似乎都源于一组共同的基因。这些基因被称为“工具箱”基因，广泛存在于各种生物体中，这意味着它们是从共同祖先那里遗传下来的。

For example, one group of genes called HOX genes accounts for the incredibly high diversity found in animal body plans. The three principal anatomical plans for wings exemplified in birds, bats, and pterosaurs were also though to be products of convergent evolution.
比如说，一组叫做“同源异型盒基因”的基因，决定了动物身体结构的丰富多样性。鸟类、蝙蝠和翼龙这三种截然不同的翅膀结构，也被认为是趋同进化的结果。

• The bird wing developed from the entire arm.
  鸟类的翅膀是由整条手臂进化而来的。

• The bat wing developed from a hand.
  蝙蝠的翅膀是由手进化而来的。

• The pterosaur wing from a single finger.
  翼龙的翅膀是由一根手指进化而来的。

Similar HOX genes, acting on different sets of genes in birds, bats, and pterosaurs resulted in the evolution of different kinds of wings.
类似的 HOX 基因作用于鸟类、蝙蝠和翼龙的不同基因组，导致了不同类型翅膀的进化。

The profoundly important point is that the origin of diverse body forms of animals and their organs may have more to do with the way multiple genes are expressed and less to do with the number of different kinds of genes. We are learning that a basic set of genes is used in animals in different ways to produce the myriad different body forms, appendages, and organs.
其中最关键的一点是，动物及其器官多样化形态的起源可能更多地与多个基因的表达方式有关，而与不同种类基因的数量关系不大。我们了解到，动物体内的一组基本基因以不同的方式组合，形成了无数种不同的体型、附肢和器官。

The new combination of evolution with developmental biology is called Evo-Devo and is revolutionizing our understanding of macroevolution and embryology. Evo-Devo also offers an explanation for the rapid macroevolutionary changes (called punctuated equilibrium by Eldredge & Gould (1972)) that appear in the fossil record and that cannot be explained by gradualism. An example of punctuated equilibrium is the sudden appearance of diverse animal forms during the Cambrian explosion 550 million years ago (or 550 Ma). Evo-Devo studies indicate that this sudden emergence of highly diverse animal forms was due to the evolution of key regulatory HOX genes in the common ancestor to all Cambrian animals.
进化生物学与发育生物学的最新结合被称为“演化发育生物学”，它正在彻底改变我们对宏观进化和胚胎学的理解。演化发育生物学也为化石记录中出现的快速宏观进化变化（埃尔德里奇和古尔德在 1972 年将其称为“间断平衡”）提供了一种解释，而这种变化是渐进主义无法解释的。间断平衡的一个例子是 5.5 亿年前（或 5.5 亿年前）寒武纪生命大爆发期间各种动物形态的突然出现。演化发育生物学研究表明，这种高度多样化动物形态的突然出现是由于所有寒武纪动物共同祖先中关键调控基因 HOX 基因的进化所致。

Fig. 4.5 The punctuated equilibrium model (top) consists of morphological stability followed by rare bursts of evolutionary change. In contrast, phyletic gradualism (below), is a more gradual, continuous model of evolution, with equilibrium states separated by jump phases. Image Credit: Wikipedia:punctuated equilibrium.
图 4.5 间断平衡模型（上）的特点是形态稳定，偶尔爆发式进化。相比之下，系统渐变论（下）是一种更加渐进、连续的进化模型，平衡状态之间通过跳跃阶段分隔。图片来源：维基百科：间断平衡。#

Computer algorithms have been developed that are inspired by Darwinian evolution and are aptly named genetic algorithms due to how they can modify populations of data over generations. These algorithms mimic the process of natural selection by employing a fitness function, which evaluates data within the domain and modifies it based on a set of rules. The process of Darwinian evolution can be more efficient than a brute force method of sorting (see this blog for an example).
人们从达尔文进化论中汲取灵感，开发出了一种叫做“遗传算法”的计算机算法，这个名字取得非常贴切，因为它就像生物进化那样，能够让数据群体一代代地发生改变。遗传算法模拟了自然选择的过程，它利用一种名为“适应度函数”的机制来评估领域内的数据，并根据一系列规则对其进行修改。相比于暴力排序方法，达尔文进化过程的效率更高（参见这篇博客）。

4.2.3. Mechanisms for Acquiring New Genes
## 获取新基因的机制

Other mechanisms (besides mutation) can effect changes in genes that coordinate cell structures, metabolism, or physiological traits through the sudden acquisition of new genes or incremental changes of individual genes or groups of genes. Theses mechanisms are:
除了突变以外，其他机制也能够通过突然获得新基因，或者单个或一组基因的累积变化，来影响协调细胞结构、代谢或生理特征的基因。这些机制包括：

• fusion of different cells (i.e., endosymbiosis),
  不同细胞的融合（例如，内共生），

• coevolution, and 协同进化，以及

• lateral gene transfer. 基因横向转移。

Symbiosis is any interaction between two organisms in which at least one of the organisms benefits from the relationship. This broad definition includes parasitic associations in which the parasite benefits at the expense of the host, or mutualistic associations in which both organisms benefit. Some symbiotic associations are obligatory, where either the host or symbiont (or both) is unable to live independently.
共生是指两种生物体之间的任何相互作用，其中至少有一种生物体从这种关系中获益。这个广义的定义包括寄生关系，即寄生生物从宿主身上获益，以及互惠关系，即两种生物体都获益。有些共生关系是强制性的，也就是说宿主或共生体（或两者）都不能独立生存。

For instance, the first eukaryote cell (i.e., cell with a nucleus) may have formed by the fusion of Bacteria and Archaea, where the Bacteria contributed the operational genes while the Archaea contributed the informational genes (Rivera & Lake (2004)). Such a fusion would fall into the category of mutualistic symbiosis since both cells benefited from this association. There is evidence that ancient symbioses in eukaryote cells, where their mitochondria (for oxygen respiration) and chloroplasts (for photosynthesis) occurred in specific groups of bacteria (see here for more details).
比如说，第一个真核细胞（也就是有细胞核的细胞）可能是由细菌和古菌融合而成的。在融合过程中，细菌贡献了负责细胞运作的基因，而古菌贡献了负责遗传信息的基因（Rivera & Lake, 2004）。这种融合属于互惠共生关系，因为两种细胞都能从中受益。有证据表明，真核细胞中的线粒体（负责有氧呼吸）和叶绿体（负责光合作用）就分别起源于某些特定的细菌（详见此处）。

The proposed fusion of an archaeum with a bacterium somehow resulted in conditions favorable for evolution to greater complexity, multicellularity, and sexual reproduction. The later acquisition by early eukaryotes of the mitochondria and chloroplast from bacteria allowed eukaryotes to adapt into habitats with abundant light and oxygen (i.e., aerobic environment). Unfortunately, most of the evolutionary steps from the proposed fusion-based “proto-eukaryote” to single-cell eukaryotes are unknown since no known extant organism retains the intermediate characteristics.
古细菌和细菌的融合创造了有利条件，使得生物得以向更复杂的方向进化，最终出现了多细胞生物和有性生殖。后来，早期真核生物从细菌那里获得了线粒体和叶绿体，从而适应了富含光和氧气的环境（即有氧环境）。可惜的是，我们对从这种融合产生的“原始真核生物”进化到单细胞真核生物的具体过程知之甚少，因为现存生物中没有发现保留了中间特征的物种。

The fusion of two cells is not believed to have been a common occurrence in the early life history of organisms, but there are many examples of other forms of symbiosis that are widely distributed in eukaryotes and allow them to live under conditions that would be otherwise impossible. One of the first cases was the symbiosis of an alga (singular of algae) and a fungus to form a lichen, which was recognized and researched in detail by Beatrix Potter (author of Peter Rabbit) well before symbiosis was accepted by the British scientific community.
在生物的早期生命史中，两个细胞的融合并不常见，但其他形式的共生现象却广泛存在于真核生物中，并使它们能够在原本无法生存的环境下生存。其中一个最早的例子是藻类和真菌的共生，它们形成了地衣。有趣的是，早在共生现象被英国科学界接受之前，以创作《彼得兔》而闻名的作家碧翠克斯·波特就对地衣进行了详细的观察和研究。

Parasitism can result in radical changes in the physiology of the host that include mating and feeding behavior, as well as morphological changes. Coevolution is a special kind of symbiosis, in which two kinds of organisms interact in such a way that each exerts a selective pressure on the other. Classical examples include flowering plants and their insect pollinators, or predators with their prey. There may be traits in modern-day dogs that have coevolved due to their interactions with humans (Nagasawa et al. (2015)). Understanding the nature of coevolving ecosystems is one of the most difficult and important challenges in ecology.
寄生（Parasitism）会导致宿主生理机能发生剧烈变化，包括交配和进食行为，以及形态变化。协同进化（Coevolution）是一种特殊的共生关系，其中两种生物以相互施加选择压力。典型的例子包括开花植物和它们的昆虫传粉者，或捕食者和它们的猎物。现代犬类中可能存在一些由于与人类互动而共同进化的特征（Nagasawa et al. (2015)）。理解协同进化生态系统的本质是生态学中最困难和最重要的挑战之一。

Lateral gene transfer is the transfer of DNA from one organism to another such that the recipient is permanently changed in their genetic composition. Genetic exchange can be mediated by
基因横向转移（Lateral gene transfer）是指 DNA 从一个生物体转移到另一个生物体，从而使受体的基因组成发生永久性改变。基因交换可以通过以下方式进行

• cell-cell contact (i.e., conjugation),
  细胞与细胞之间的接触（例如接合），

• viral infection (i.e., transduction), or
  病毒感染（例如，转导），或

• incorporation of DNA from the environment (i.e., transformation).
  从环境中吸收 DNA（例如，转化）。

The recent accumulation of complete genome sequence from representatives of all the domains of life has revealed a universal pattern of lateral gene transfer for acquiring genes or parts of genes. We now know that viruses have played and continue to play a role in the evolution of life through lateral gene transfer (Canchaya et al. (2003)).
近期，通过对来自所有生命领域的代表性物种进行全基因组测序，我们发现了一种普遍存在的横向基因转移模式，生物体可以通过这种模式获得基因或基因片段。我们现在知道，病毒在进化史上一直通过横向基因转移发挥着作用 (Canchaya 等人，2003)。

Note 注释

A virus is defined as an intracellular parasite and is incapable of living without a host cell. While it shares many of the biochemical characteristics of a living cell including nucleic acids and proteins, it cannot reproduce independently, only by infecting a normal cell.
病毒被定义为一种细胞内寄生虫，离开宿主细胞就无法生存。虽然病毒与活细胞一样，具有许多生化特征，包括核酸和蛋白质，但它不能独立繁殖，只能通过感染正常细胞来繁殖。

This is illustrated by the high abundance of bacterial viruses (i.e., bacteriophages or phages) in marine environments, exceeding the bacterial population by an order of magnitude. Viruses are the most abundant biological entity on Earth, yet are poorly understood. It is presumed that the primary role of viruses in the environment is causing death in bacteria (or producing disease in eukaryotes), but their significance as vehicle for transmitting new genes to bacteria in situ is not well understood, although likely extensive. Jiang & Paul (1998) calculated that at the low infection rate of ${10}^{-8}$ per infected bacterial population, viral-mediated gene transfer takes place in Earth’s oceans at the rate of $\sim 2\times {10}^{16}$ per second.
海洋环境中，细菌病毒（也称为噬菌体）数量极多，比细菌本身还要多出一个数量级，这足以说明问题。病毒是地球上数量最庞大的生物实体，但我们对它们的了解却少之又少。人们普遍认为，病毒在环境中的主要作用是导致细菌死亡（或在真核生物中引发疾病），但它们作为基因载体，在原地将新基因传递给细菌的重要性，尽管可能非常巨大，却尚未得到充分认识。Jiang 和 Paul（1998）计算出，即使在每感染细菌群体中只有 ${10}^{-8}$ 的低感染率下，病毒介导的基因转移在地球海洋中仍然以每秒 $\sim 2\times {10}^{16}$ 的速度发生。

How important is lateral gene transfer in evolution? Results from whole genome sequences of bacteria and archaea indicate that lateral gene transfer may be the most important mechanism for acquiring new genes, including those involved in complex and coordinated phenotypes. For example, $\sim 16\mathrm{\%}$ of the genome of Escherichia coli K12 is viral genes. Microorganisms have evolved elaborate mechanisms for incorporation of acquired genes into their chromosome at specific sites. These sites can serve as “pathogenicity islands” if all of the acquired genes are involved in disease production, or they may be “genetic islands,” which align acquired genes in involved in key physiological activities (e.g., magnetotaxis).
**基因横向转移在进化中究竟有多重要？** 对细菌和古细菌全基因组序列的研究结果表明，基因横向转移可能是生物获取新基因的最重要机制，包括那些参与复杂而协调的性状的基因。举个例子，大肠杆菌 K12 的基因组中，很大一部分竟然来自病毒基因。微生物已经进化出精密的机制，能够将获得的基因整合到染色体上的特定位置。如果所有获得的基因都与致病相关，这些位置就成了所谓的“致病岛”。而如果获得的基因与关键生理活动（例如趋磁性）相关，它们就可能形成“基因岛”。

It is very unlikely that the formation of a genome with sufficient information to lead to free-living (self-sufficient) cells could have originated without a mechanism for acquiring “functional” genes from other early cells or communities of interdependent cells or “precells” (Baross & Hoffman (1985)). This is certainly consistent with the fact that all life on Earth is derived from a common ancestral pool of genes based on a universal genetic code. Darwinian evolution would have played an important role in these early stages and selection would have favored specific biochemical and molecular structures and mechanisms over others.
一个基因组如果要储存足够的信息来形成自由生活的（自给自足的）细胞，那它必然需要从其他早期细胞、相互依存的细胞群落或“前细胞”(Baross & Hoffman, 1985)中获取“功能性”基因，否则几乎不可能自行起源。这一点与地球上所有生命都源自一个拥有共同祖先基因库，并基于通用遗传密码的事实完全一致。达尔文进化论在这些早期阶段发挥了重要作用，自然选择会偏爱某些特定的生化和分子结构及机制。

Could this imply that if we started over again by resetting the clock to 4 billion years ago (4 Ga), the resultant life would have the same biochemical and molecular properties (including the same genetic code) as present-day Earth life? If environmental conditions and the starting pool of organic compounds were the same, it is probable that a second genesis would result in biochemistry that would resemble or possibly be indistinguishable from present-day Earth life. In such a scenario, contingency in evolution could result in the selection of organisms and ecosystems significantly different from those found on Earth. Compared to present-day organisms, they would share a similar biochemistry and evolve many or all of the same phenotypes (both structural and functional) albeit possibly with different genotypes.
这是否意味着，如果我们把时间倒回到 40 亿年前（4 Ga），重新开始，那么由此产生的生命形式，会和今天地球上的生命一样，具有相同的生物化学和分子特性（包括相同的遗传密码）？如果环境条件和初始的有机化合物库相同，那么第二次生命起源很有可能会产生与今天地球生命相似，甚至可能无法区分的生物化学特征。在这种情况下，进化的偶然性可能会导致所选择的生物体和生态系统与地球上的生物体和生态系统截然不同。与今天的生物体相比，它们将拥有相似的生物化学，并进化出许多或所有相同的表型（包括结构和功能），尽管基因型可能不同。

4.2.4. Could There be Life without Evolution?
**4.2.4. 生命的存在可以脱离进化吗？**

Many of the definitions of life include the phrase “undergoes Darwinian evolution.” The implication is that phenotypic changes and adaptation are necessary to:
许多对生命的定义都包含“经历达尔文进化”的说法。这意味着表型变化和适应性对于以下方面是必要的：

• exploit unstable environmental conditions,
  利用不稳定的环境条件，

• function more optimally in the environment, and
  在环境中更有效地发挥作用，以及

• provide a mechanism to increase biological complexity.
  提供一种增加生物复杂性的机制。

Evolutionary changes have even been suggested for hypothesized “clay crystal life” of Graham Cairns-Smith, referring to randomly occurring errors in crystal structure during crystal growth as analogous to mutations. Would a self-replicating chemical system capable of chemical transformations in the environment be considered life? If self-replicating chemical compounds are not life, then replication by itself is not sufficient as a defining characteristic of life. The ability to undergo Darwinian evolution is also not sufficient to define life if we consider minerals that are capable of reproducing errors in their crystal structure as equivalent to evolution. It is important to emphasize, that evolution is not simply reproducing mutations (mistakes in clays), but selecting those variants that are functionally more fit.
有人甚至提出，假设中由格雷厄姆·凯恩斯-史密斯提出的“粘土晶体生命”也会发生进化变化。他们认为，晶体生长过程中晶体结构中随机出现的错误类似于突变。那么，一个能够在环境中进行化学转化的自我复制化学系统可以被视为生命吗？如果自我复制的化合物不算生命，那么仅仅依靠复制本身并不足以作为定义生命的特征。如果我们认为矿物能够复制其晶体结构中的错误等同于进化，那么仅仅依靠达尔文进化能力也不足以定义生命。需要强调的是，进化不仅仅是复制突变（粘土中的错误），更重要的是*选择*那些功能更适应环境的变异。

The canonical characteristics of life are an inherent capacity to:
生命的典型特征是其内在的能力：

• adapt to changing environmental conditions, and
  适应不断变化的环境条件，以及

• increase in complexity by multiple mechanisms, but particularly by interactions with other living organisms (including viruses).
  生命体复杂性的增加，是通过多种机制实现的，特别是与其他生物体（包括病毒）的相互作用。

Natural selection is the key to evolution and the main reason why Darwinian evolution persists as a characteristic of many definitions of life. Clays could never evolve an eye or a nose, or adapt behavioral strategies to exclude clays with other characteristics.
自然选择是进化的关键，也是达尔文进化论之所以能够成为许多生命定义的特征的主要原因。黏土永远不可能进化出眼睛或鼻子，也不可能通过调整行为策略来排除具有其他特征的黏土。

The only alternative to evolution for producing diversity would be to have environmental conditions that continuously create different lifeforms, or similar lifeforms with random and frequent “mistakes” made in the synthesis of chemical templates used for replication or metabolism. These lead to traits that gave some selective advantage in an existing community or in exploiting new habitats. This could lead to lifeforms that undergo a form of evolution without a master information macromolecule (e.g., DNA or RNA). It is difficult to imagine such lifeforms being able to “evolve” into complex structures unless other mechanisms, such as symbiosis or cell-cell fusion are available.
要想在不依赖进化的情况下产生多样性，唯一的方法就是让环境条件不断地创造出不同的生命形式，或者是在复制或代谢过程中使用的化学模板合成中出现随机和频繁的“错误”，从而产生类似的生命形式。这些错误会导致某些性状的出现，这些性状在现有群落中或在开发新栖息地时具有一定的选择优势。这可能会导致生命形式在没有主信息大分子（例如 DNA 或 RNA）的情况下经历某种形式的进化。我们很难想象这样的生命形式能够“进化”成复杂的结构，除非有其他的机制，例如共生或细胞融合。

4.2.5. Evolution and Extraterrestrial Life
## 4.2.5. 进化与外星生命

Evolution is the key mechanism for heritable changes to occur in a population. Mutation is not the only mechanism for acquiring new genes. Lateral gene transfer appears to be one of the most important mechanisms and clearly one the earliest for creating diversity and possibly for building genomes with the requisite information to result in free-living cells (as opposed to codependent communities of “precells” that are unable to escape communal life). Lateral gene transfer is also one of the mechanisms to align genes from different sources into complex functional activities. The coevolution between two or more species is also a hallmark of evolution manifested in many ways from insect-planet interactions to the hundreds of species of bacteria involved in the nutrition of ruminant animals (e.g., cows, sheep, goats, etc.).
进化是群体遗传性状发生改变的关键机制。获取新基因的途径并非只有基因突变一种，水平基因转移似乎才是最重要的机制之一，并且显然是生物早期创造物种多样性和构建基因组的关键，这种基因组或许正是构成自由生活的细胞（而非无法摆脱共生生活的“前细胞”群落）所必需的。水平基因转移也是将不同来源的基因整合到复杂功能活动中的机制之一。两个或多个物种之间的协同进化也是进化的一个标志，它以多种方式体现，从昆虫与植物的相互作用到参与反刍动物（如牛、羊等）营养的数百种细菌，都是协同进化的例证。

If the ability to undergo Darwinian evolution is a canonical trait of life no matter how different that lifeform is from Earth life, then are there properties of evolving extraterrestrial organisms that would be detectable as positive signs of life? Evolution provides an organism the opportunity to exploit new and changing environments. One piece of evidence for the probable cosmic ubiquity of evolution is that, on Earth, life occupies all available habitats and even creates new habitats as a consequence of its metabolisms.
如果说，无论地外生命与地球生命有多么不同，只要能够进行达尔文进化，就可以被称之为生命，那么，这些不断进化的地外生物体是否具有一些可以被探测到的，作为生命象征的特征呢？进化赋予了生物体开发新环境和适应不断变化的环境的能力。地球生命占据了所有可用的栖息地，甚至通过自身的新陈代谢创造新的栖息地，这一现象证明了进化在宇宙中可能是普遍存在的。

Another hallmark of evolution is the ability of organisms to coevolve with other organisms and to form permanent (and obligatory) associations. It is highly probable that an inevitable consequence of evolution is the elimination of radically different biochemical lineages of life that may have formed during the earliest period of evolution of life. Extant Earth life is the result of either selection of the most fit lineage or homogenization of some or all fo the different lineages into a common ancestral community that developed into the present three major lineages (domains). All have a common biochemistry based on presumably the most “fit” molecular information strategies and energy yielding pathways among a potpourri of possibilities. One caveat and perhaps verification of the above statement is that some of the deeply rooted archaea exist as remnants of other lineages.
生物进化的另一个标志是生物体能够与其他生物体共同进化，并形成永久的（和强制性的）关联。高度可能的是，进化不可避免的结果是消除了生命早期可能形成的根本不同的生物化学谱系。现存的地球生命要么是选择了最适应环境的谱系，要么是将一些或所有不同的谱系 homogenized 成一个共同的祖先群落，然后发展成现在的三大谱系（域）。它们都拥有基于假定最“合适”的分子信息策略和能量产生途径的共同生物化学，而这些途径是从众多可能性中挑选出来的。需要注意的是，也许可以证明上述说法的是，一些根深蒂固的古细菌是其他谱系的残余。

One of the apparent generalizations that can be made from extant Earth life (and the explanation of a “unity of biochemistry” in all organisms) is that lateral gene transfer is both an ancient and efficient mechanism for rapidly creating diversity and complexity. Lateral gene transfer is also an efficient mechanism for selecting the genes that are most “fit” for specific proteins and transferring them into diverse groups of organisms. The result is both the addition of new genes and the replacement of less-fit genes having a similar function. Natural selection based solely on mutation is not likely and adequate mechanism for evolving complexity.
从现存的地球生命形式中，我们可以明显地观察到一个普遍规律，那就是基因横向转移是一种古老且高效的机制，能够快速地创造生物多样性和复杂性。这也解释了为什么所有生物都拥有相似的生物化学基础。基因横向转移还是一种高效的基因筛选机制，它能够选择最适合特定蛋白质的基因，并将它们转移到不同的生物群体中。这样的结果是，新的基因被添加进来，而功能相似但不那么适合的基因则被取代。仅仅依靠突变的自然选择机制，不太可能充分地进化出复杂的生命形式。

Lateral gene transfer and endosymbiosis are probably the most obvious mechanisms for creating complex genomes that can lead to free-living cells and complex cellular communities in the short geological time available from life’s origins to the establishment of microbial communities at 3.8 Ga. An important implication of the existence of viruses or virus-like entities during the early evolution of cellular organism is that their genomes may have been the source of most genetic innovations, due to their (1) rapid replication rates, (2) high rates of mutation from replication errors, and (3) gene insertions from diverse host cells.
横向基因转移和内共生很可能是生命起源到 38 亿年前微生物群落建立的短暂地质时期内，创造复杂基因组、进而形成自由生活的细胞和复杂细胞群落的最明显机制。病毒或类病毒实体在细胞生物早期进化过程中存在的一个重要意义在于，它们的基因组可能是大多数基因创新的源泉，这是因为它们具有以下特点：(1) 复制速度快；(2) 复制错误导致的高突变率；(3) 来自不同宿主细胞的基因插入。

It is clear that both the individual organism and its community coevolve. Evolution is allowing cells to control their own evolution, where they accept or reject changes in genotypes from newly acquired foreign genes. The source of foreign genes and the kinds of genes most likely to be selected for permanence are largely not known. It is clear that the evolution of a useful trait by one organism frequently means that it is likely to be acquired by other organisms. It appears that biology is transitioning from just genes to a broader emphasis on the cell, communities, and ecosystems.
很明显，生物个体和其所在的生物群落是协同进化的。进化使得细胞能够控制自身的进化方向，它们可以选择接受或拒绝外源基因带来的基因型改变。虽然我们还不清楚外源基因的来源以及哪些基因最有可能被永久选择，但有一点是肯定的：当一个生物进化出一种有益性状时，其他生物很可能也会获得这种性状。看起来，生物学的研究重点正在从基因本身扩展到细胞、生物群落和生态系统。

What are the limits of evolution for Earth life? This is a complex question with many different components. It involves the different biochemistries from carbon chemistry that are not found in extant Earth organisms, but could be better suited for environmental conditions that exist on other planets and moons. The technology exists to design genes and groups of genes that could lead to novel phenotypes suited to exploit new habitats and energy sources (e.g., petroleum eating bacteria). These kinds of studies would be important and perhaps essential in our quest to search for life elsewhere. Another component to the question is where we are going and what will Homo sapiens be like in 10,000 years? This is an integral part of our search for advanced extraterrestrial intelligence, which requires us to imagine our future portrait. One possible outcome will be an increasing ability to control our environment and all that is evolving based on our history in primate evolution. A more recent perspective can be found here.
地球生命的演化极限究竟在哪里？这是一个复杂的问题，包含了许多方面。 首先，地球上现存生物都基于碳基化学，但在其他星球或卫星上，可能存在更适合当地环境的、不同于碳基化学的生命形式。 其次，我们现有的技术已经可以设计出新的基因和基因组，创造出能够适应新环境和利用新能源（比如能“吃”石油的细菌）的全新生物。这类研究对我们探索外星生命至关重要，甚至可以说是不可或缺的。 最后，这个问题还关乎我们人类自身的未来：一万年后，我们会发展成什么样子？这个问题与我们对外星智慧生命的探索息息相关，因为在寻找外星文明的过程中，我们也需要展望自身未来的模样。 一种可能的结果是，我们将越来越有能力控制我们的环境，以及所有基于我们灵长类动物进化史而演变的事物。 想了解更多近期观点，请点击此处。

4.3. Planetary Requirements for Life
4.3 行星的生命要求 #

Earth offers a benign environment because it was endowed with enough volatiles to produce an ocean and a significant atmosphere. Our planet
地球之所以拥有一个宜居的环境，是因为它拥有充足的挥发性物质，足以形成海洋和厚厚的大气层。我们的星球

• is large enough to retain an atmosphere,
  足够大，能够保持大气层，

• can recycle crust via plate tectonics, and
  可以通过板块构造循环地壳，并且

• is far enough away from the asteroid belt that it is not frequently bombarded by projectiles that could cause mass extinctions.
  距离小行星带足够远，不会频繁遭受可能导致大规模灭绝的撞击。

Despite these advantages, there are still questions that astrobiologists ask for how life arises on a planet in general. Such questions are:
尽管有这些优势，天体生物学家仍然对生命如何在一个星球上诞生存在疑问。这些问题包括：

• Does the origin of life require land masses? If land masses are required, were the small, (probably) short-lived island volcanoes sufficient for the origin of life?
  生命的起源是否需要陆地？如果需要陆地，那么那些（可能）短暂存在的小型火山岛是否足以孕育生命？

• Did life’s origin await the growth of larger more stable ‘continents’?
  生命的起源是否要等到更大、更稳定的“大陆”出现？

• Does the origin of life require large oceans?
  生命的起源是否一定需要广阔的海洋？

• Does life require tidal zones to have cycling and pumping of environments that allow molecular self-assembly in lakes or ponds?
  生命是否需要潮汐带对环境进行循环和泵送，以便在湖泊或池塘中进行分子自组装？

• Did life actually start elsewhere and was later transplanted to Earth? Perhaps our ancestors began on Mars and traveled to Earth?
  生命真的起源于其他地方，后来才被移植到地球上的吗？也许我们的祖先起源于火星，然后 travelled to 地球？

• Must life ascend to land in order to develop technology?
  生命必须登陆才能发展出科技吗？

Clearly, there is a wide variety of criteria that a planet must meet to be suitable for life, but many of these criteria are difficult to understand in detail and quantify.
很明显，一个星球要适合生命存在，必须满足各种各样的标准，但其中许多标准难以详细理解和量化。

4.3.1. Biogeochemical Cycles
4.3.1. 生物地球化学循环

Carbon dioxide on our planet cycles between the atmosphere, oceans, life, fossils, other rock on a wide range of timescales. Carbonate rock (the majority of which is located in Earth’s mantle) forms the largest reservoir. Silicate rocks and atmospheric $\mathrm{C}{\mathrm{O}}_2$ can react with each other with the aid of liquid water to form silica and carbonate rocks. In some cases, living organisms are involved, while in other cases they are not.
地球上的二氧化碳在很长一段时间内，于大气、海洋、生物、化石和其他岩石之间循环。碳酸盐岩（其中大部分位于地幔）构成了最大的碳库。硅酸盐岩和大气中的 $\mathrm{C}{\mathrm{O}}_2$ 可以在液态水的帮助下相互反应，形成二氧化硅和碳酸盐岩。在某些情况下，生物体会参与这个过程，而在另一些情况下则不会。

Planets and other photosynthetic organisms remove $\mathrm{C}{\mathrm{O}}_2$ from the atmosphere, where this carbon is first stored in living organisms. Some of the carbon is returned via respiration, while a portion remains isolated for long periods of time as fossils. However, the carbonate-silicate cycle introduces the products of silicate weathering, including calcium $(\mathrm{C}{\mathrm{a}}^{++})$, bicarbonate $(\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-})$, and dissolved silica $(\mathrm{S}\mathrm{i}{\mathrm{O}}_2)$, which are transported by rivers to the ocean.
行星和其他光合生物会从大气中移除 $\mathrm{C}{\mathrm{O}}_2$ ，并将碳最初储存在生物体内。其中一部分碳通过呼吸作用返回大气，而另一部分则作为化石被长期隔离。然而，碳硅酸盐循环会引入硅酸盐风化的产物，包括钙 $(\mathrm{C}{\mathrm{a}}^{++})$ 、碳酸氢盐 $(\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-})$ 和溶解性二氧化硅 $(\mathrm{S}\mathrm{i}{\mathrm{O}}_2)$ ，这些物质通过河流输送到海洋中。

Organisms (e.g., foraminifera) use these products to make shells of calcium carbonate $(\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3)$, while other organisms make shells out of silica. Most of these shells eventually dissolve, but some are buried in sediments on the seafloor. On a lifeless planet, calcium carbonate (i.e., chalk upon precipitation) would be incorporated in ocean sediments through nonbiological chemistry until the mineral reaches a high enough concentration to become saturated in seawater. The combination of silicate weathering plus carbonate precipitation is given in Eqn. (3.76). Thermal processing of buried carbonate rocks returns $\mathrm{C}{\mathrm{O}}_2$ to the atmosphere.
像有孔虫这样的生物利用这些产物制造碳酸钙外壳，而其他生物则利用二氧化硅制造外壳。大多数外壳最终都会溶解，但有些会被埋在海底的沉积物中。在一个没有生命的星球上，碳酸钙（即沉淀形成的白垩）会通过非生物化学过程融入海洋沉积物中，直到其浓度达到足够高，使海水饱和。硅酸盐风化和碳酸盐沉淀的结合如方程式 (3.76) 所示。埋藏的碳酸盐岩经过热加工后，会将[CO2]释放回大气中。

The episodic return of $\mathrm{C}{\mathrm{O}}_2$ to the atmosphere affects the climate, where for the past few million years, the climate cycle on Earth has alternated between ice ages and interglacials (i.e., periods between ice ages). When the climate cools, ice sheets form because the ice is highly reflective (i.e., has a high albedo) and this change leads to further cooling through a positive feedback loop.
大气中 $\mathrm{C}{\mathrm{O}}_2$ 的周期性回归会影响气候。在过去的几百万年里，地球气候一直在冰期和间冰期（即两个冰期之间的时期）之间循环交替。当气候变冷时，由于冰能够高度反射阳光（即具有高反照率），冰盖就会形成，而这又会通过正反馈循环导致进一步降温。

Evidence suggests that most (or all) of Earth’s surface was covered by ice about $700\mathrm{M}\mathrm{y}\mathrm{r}$ ago in a state known as snowball Earth. Significant ice ages (like snowball Earth) may have precipitated large scale changes to the biosphere, where an ice age appears to correlate with both the great oxidation event and the Cambrian explosion. Positive feedbacks can have large effects on habitability, even with small changes in input parameters.
有证据表明，大约 7 亿年前，地球表面大部分（甚至全部）都被冰雪覆盖，这种状态被称为“雪球地球”。像雪球地球这样的大冰期可能引发了生物圈的大规模变化，其中冰期似乎与大氧化事件和寒武纪生命大爆发都存在关联。即使输入参数只有微小的变化，正反馈也会对宜居性产生重大影响。

Fig. 4.6 Illustration of the long-term (geological) carbon cycle fluxes. Shown are the long-term fluxes (in petagrams of carbon/yr) in the GENIE (Grid ENabled Integrated Earth system) model at steady state. In red are sources of $\mathrm{C}{\mathrm{O}}_2$ to the atmosphere or ocean, and in dark blue are sinks of $\mathrm{C}{\mathrm{O}}_2$. Figure Credit: Colbourn, Ridgwell, & Lenton (2015).
图 4.6 展示了长期（地质）碳循环通量。图中显示的是 GENIE（网格化地球系统模型）在稳态下的长期通量（单位为 PgC/年）。红色箭头表示大气或海洋中 $\mathrm{C}{\mathrm{O}}_2$ 的来源，深蓝色箭头表示 $\mathrm{C}{\mathrm{O}}_2$ 的汇。图片来源：Colbourn, Ridgwell, & Lenton (2015)。

However, variations of parameters can damp the atmospheric $\mathrm{C}{\mathrm{O}}_2$ abundance, which results in a negative feedback loop. This process buffers the terrestrial climate on timescales of $100\mathrm{M}\mathrm{y}\mathrm{r}$. Carbon dioxide is an important greenhouse gas, where the reaction rate of $\mathrm{C}{\mathrm{O}}_2$ with silicate rocks can increase exponentially with temperature. But the other phases of the $\mathrm{C}{\mathrm{O}}_2$ cycle are virtually temperature independent. If it gets hot, weathering rates increase and the $\mathrm{C}{\mathrm{O}}_2$ abundance drops. During colder times, the reverse occurs and $\mathrm{C}{\mathrm{O}}_2$ increases. Variations in the atmospheric abundance of $\mathrm{C}{\mathrm{O}}_2$ are thought to have played a major role in maintaining moderate climates on Earth for most of our planet’s history (see Fig. 4.6 for carbon flux rates).
然而，参数的变化会降低大气中 $\mathrm{C}{\mathrm{O}}_2$ 的含量，从而形成负反馈循环。这个过程在 $100\mathrm{M}\mathrm{y}\mathrm{r}$ 的时间尺度上缓冲了地球的气候。二氧化碳是一种重要的温室气体，它与硅酸盐岩石的 $\mathrm{C}{\mathrm{O}}_2$ 反应速率会随着温度的升高呈指数级增长。但 $\mathrm{C}{\mathrm{O}}_2$ 循环的其他阶段实际上与温度无关。如果地球变热，风化速率就会加快， $\mathrm{C}{\mathrm{O}}_2$ 的含量就会下降。在较冷的时期，则会发生相反的情况， $\mathrm{C}{\mathrm{O}}_2$ 的含量会增加。人们认为，大气中 $\mathrm{C}{\mathrm{O}}_2$ 含量的变化在维持地球大部分历史时期温和气候方面发挥了重要作用（碳通量速率见图4.6）。

The carbon cycle also limits the abundance of atmospheric $\mathrm{C}{\mathrm{O}}_2$ on billion-year timescales. Ocean basalts remove $\mathrm{C}{\mathrm{O}}_2$ from seawater at a rate that increases with the $\mathrm{C}{\mathrm{O}}_2$ abundance in seawater, which varies in sync with its atmospheric abundance. These carbonates descend into the Earth’s mantle as plates are subducted. Heating within the Earth’s mantle frees the $\mathrm{C}{\mathrm{O}}_2$, which is subsequently returned to the atmosphere via volcanic activity. This process does not depend on climate and acts to buffer the atmospheric $\mathrm{C}{\mathrm{O}}_2$ abundance rather than climate variations.
碳循环还能在数十亿年的时间尺度上限制大气中二氧化碳的含量。海底玄武岩会吸收海水中的二氧化碳，吸收速率随着海水中二氧化碳含量的增加而加快，而海水中的二氧化碳含量又与其在大气中的含量同步变化。随着板块的俯冲，这些碳酸盐会进入地幔。地幔内部的高温会释放出二氧化碳，然后通过火山活动返回大气层。这个过程不受气候的影响，它的作用是缓冲大气中二氧化碳的含量，而不是气候变化。

Nitrogen is often a limiting nutrient for life on Earth, despite abundant atmospheric ${\mathrm{N}}_2$. Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form in the atmosphere and converted into soluble nitrogen compounds, such as ammonia, nitrate, and nitrogen dioxide (see Fig. 4.7). Few organisms can metabolize nitrogen unless it has been fixed from the atmosphere.
尽管大气中充满了氮气，但氮元素通常是地球上生物生长的一种限制性营养物质。这是因为大气中的氮气分子化学性质非常稳定，难以被生物体直接利用。氮固定指的是将大气中相对惰性的氮气分子转化为可溶性氮化合物的过程，例如氨、硝酸盐和二氧化氮（见图 4.7）。这些可溶性氮化合物才能被大多数生物体吸收利用。除非氮气经过了固定，否则很少有生物能够代谢利用它。

In the anoxic (oxygen poor) prebiotic atmosphere of the young Earth, lightning oxidized ${\mathrm{N}}_2$ via the reaction
在年轻地球的缺氧（贫氧）的原始大气中，闪电通过以下反应氧化了 ${\mathrm{N}}_2$

(4.1)$$\begin{array}{r}{\mathrm{N}}_2+2\mathrm{C}{\mathrm{O}}_2\to 2\mathrm{N}\mathrm{O}+2\mathrm{C}\mathrm{O}.\end{array}$$

The nitric oxide $(\mathrm{N}\mathrm{O})$ was then converted to nitrosyl hydride $(\mathrm{H}\mathrm{N}\mathrm{O})$, which is soluble in water. Lightning thereby provided a modest flux of fixed nitrogen, which may have been important to early life. In an atmosphere containing some methane $(\mathrm{C}{\mathrm{H}}_4)$, nitrogen fixation can progress via $\mathrm{H}\mathrm{C}\mathrm{N}$, which is hydrolyzed in solution to form ammonium $(\mathrm{N}{\mathrm{H}}_4^{+})$.
生成的一氧化氮 $(\mathrm{N}\mathrm{O})$ 随后转化为亚硝酰氢 $(\mathrm{H}\mathrm{N}\mathrm{O})$ ，后者可溶于水。因此，闪电提供了一定量的固定氮，这可能对早期生命非常重要。在含有甲烷 $(\mathrm{C}{\mathrm{H}}_4)$ 的大气中，固氮可以通过 $\mathrm{H}\mathrm{C}\mathrm{N}$ 进行， $\mathrm{H}\mathrm{C}\mathrm{N}$ 在溶液中水解形成铵 $(\mathrm{N}{\mathrm{H}}_4^{+})$ 。

Fig. 4.7 Diagram illustrating the nitrogen cycle, where atmospheric nitrogen enters ecosystems through fixation. Image Credit: Wikipedia:Nitrification.
图 4.7 氮循环示意图，其中大气中的氮通过固氮作用进入生态系统。图片来源：维基百科：硝化作用。

Biological fixation of nitrogen arose when fixed $\mathrm{N}$ became scarce, which may have been prior to the last common ancestor of present-day Earth life, and is strictly an anaerobic process. Biological fixation of nitrogen occurs when anaerobes developed the capability of nitrogen fixation, and $\mathrm{N}{\mathrm{H}}_4^{+}$ probably became the dominant form of combined nitrogen in Earth’s oceans. Once our planet’s atmosphere contained significant quantities of ${\mathrm{O}}_2$, then ammonium ions were oxidized to nitrite $(\mathrm{N}{\mathrm{O}}_2^{-})$ and then converted to nitrate $(\mathrm{N}{\mathrm{O}}_3^{-})$ by bacteria nitrification. The reverse process (nitrates back to molecular nitrogen) is called denitrification.
地球上的生命最初依赖固定态的 $\mathrm{N}$ 生存。但随着 $\mathrm{N}$ 逐渐减少，生物开始进化出固定氮的能力。这种能力很可能出现在地球生命最后的共同祖先出现之前，并且只能在**厌氧**环境下进行。厌氧生物进行的氮固定作用，使得 $\mathrm{N}{\mathrm{H}}_4^{+}$ 很可能成为当时地球海洋中氮元素的主要存在形式。 后来，随着大气中 ${\mathrm{O}}_2$ 的逐渐增多，细菌开始利用 ${\mathrm{O}}_2$ 将铵离子氧化成亚硝酸盐 $(\mathrm{N}{\mathrm{O}}_2^{-})$ ，然后再将其转化为硝酸盐 $(\mathrm{N}{\mathrm{O}}_3^{-})$ ，这一过程被称为**硝化作用**。而将硝酸盐转化回氮气的反向过程则被称为**反硝化作用**。

Note 注释

The term anoxic is used to describe environments without molecular oxygen, while anaerobic refers to organisms that are able to survive without molecular oxygen.
“缺氧”一词用于描述没有分子氧的环境，而“厌氧”指的是能够在没有分子氧的情况下生存的生物。

4.3.2. Gravitational and Magnetic Fields
**4.3.2 重力场和磁场**

Biomechanics depends on material strength, gravity, and other physical properties related to viscosity, which means that organisms do not scale simply scale with size.
生物力学取决于材料强度、重力以及与粘度相关的其他物理性质，这意味着生物体的尺寸变化并不遵循简单的比例关系。

• Larger animals require more support, so elephants need to have more bulky legs than a scaled-up mouse.
  体型更大的动物需要更强的支撑力，因此大象的腿必须比按比例放大的老鼠腿更加粗壮。

• Birds use a fundamentally different mechanism to fly than do insects.
  鸟类飞行的机制与昆虫有着本质的区别。

• Single-celled organisms have little inertia (compared with viscous drag) moving in liquid water, so they use different locomotion strategies than do larger creatures.
  单细胞生物在水中运动时惯性很小（相对于粘滞阻力而言），因此它们采用与大型生物不同的运动策略。

These factors must be accounted when considering planets with a different surface gravity than Earth. Planetary gravity is also important through its effect on escape of atmospheric gases and mountain building.
在考虑表面重力与地球不同的行星时，必须考虑到这些因素。行星重力还通过其对大气逃逸和山脉形成的影响发挥着重要作用。

Earth has a much larger magnetic field than do any of the other terrestrial planets within our Solar System. Are magnetic fields necessary for life? Direct effects of the magnetic field are minor, where a few species are known to use them as aids for navigation (but most organisms do not). Earth’s magnetic field also plays a protective role by stopping charged particles from eroding the atmosphere or reaching the surface.
地球的磁场比太阳系中任何其他类地行星都要强大得多。**磁场是生命存在的必要条件吗？** 磁场对生命的影响其实微乎其微，目前已知的只有少数物种利用磁场来导航，绝大多数生物对磁场并没有什么依赖。不过，地球磁场确实扮演着重要的守护神角色，它可以阻止带电粒子侵蚀大气层或直接抵达地表。

Such particles could have profound direct and indirect effects on life, but they are unlikely to sterilize a planet that has a sufficiently massive atmospheric blanket. During magnetic field reversals, the dipole component of Earth’s magnetic field drops to near zero and yet life persists. A subsurface biosphere would be weakly affected by charged particles hitting the planet’s surface and thus, does not require the protection of either a planetary magnetic field or an atmosphere.
此类粒子可能对生命产生深远的直接和间接影响，但它们不太可能使拥有足够厚的大气层的行星变得毫无生机。在地磁场反转期间，地球磁场的偶极子分量会下降到接近零，但生命依然存在。地下生物圈几乎不会受到撞击地球表面的带电粒子的影响，因此不需要行星磁场或大气的保护。

4.3.3. Can Moonless Planets Host Life?
**4.3.3 没有月亮的行星上可以孕育生命吗？**

The moon’s orbit correlates with many cycles on Earth, where some species of sea turtles lay their eggs on beaches only at full Moon. The nocturnal behavior of various species requires the light of the Moon, and clearly different ecological niches would be available on a moonless planet. Although these factors would affect some aspects of life significantly, they cannot be regarded being necessary (i.e., without a large moon, there is no Earth life) nor sufficient (i.e., there is a large moon, therefore there must be life) for life to exist. However, other effects of the Moon have been suggested to be more essential to life on Earth.
月球的运行周期与地球上的许多自然循环息息相关，例如，某些种类的海龟只在满月时分才会爬上沙滩产卵。许多生物的夜间活动也离不开月光。显然，如果地球没有月亮，生态环境将会截然不同。虽然月球的缺失会对地球生命产生显著影响，但我们不能武断地认为月球是生命存在的**必要条件**（也就是说，没有大卫星，地球上就不会有生命），也不能说它是**充分条件**（也就是说，因为地球有个大卫星，所以就一定会有生命）。不过，月球对地球生命的影响远不止于此，还有其他一些影响可能是生命存在的必要条件。

Fig. 4.8 Diagram illustrating possible changes in obliquity (color-coded) for a moonless Earth. The present-day Earth (with the Moon’s effects included) has an obliquity of ${23.3}^{\circ }$ and spin precession constant of $\sim 51{}^{\prime \prime }/\mathrm{y}\mathrm{r}$, which results in a small obliquity variation (blue). A moonless Earth (with a precession constant $\sim 15-30{}^{\prime \prime }/\mathrm{y}\mathrm{r}$) would have a much larger obliquity variation (up to $\sim {60}^{\circ }$; in red). Image Credit: Jacques Laskar from Laskar, Joutel, & Robutel (1993).
图 4.8 无卫星地球倾角可能变化示意图（颜色编码）。 现今地球（包含月球的影响）的倾角为 ${23.3}^{\circ }$ ，自转进动常数为 $\sim 51{}^{\prime \prime }/\mathrm{y}\mathrm{r}$ ，导致倾角变化很小（蓝色）。 无卫星地球（进动常数为 $\sim 15-30{}^{\prime \prime }/\mathrm{y}\mathrm{r}$ ）的倾角变化幅度要大得多（高达 $\sim {60}^{\circ }$ ；红色）。图片来源：Jacques Laskar，摘自Laskar, Joutel, & Robutel (1993)。

The Moon stabilizes Earth’s obliquity (see Fig. 4.8) and thereby its climate (see Fig. 3.23). In contrast, perturbations from the other planets produce substantial variations in the martian obliquity, which probably are responsible for the patterns observed in Mars’ layered polar terrains. However, planets with
月球稳定了地球的黄赤交角（见图4.8），进而稳定了地球的气候（见图3.23）。相比之下，其他行星的扰动会导致火星的黄赤交角发生剧烈变化，这可能是火星极地层状地貌形成的原因。然而，行星即使

• retrograde spins, 逆行的行星，

• more rapid spin rates, or
  更快的自转速度，或

• those in planetary systems with a different orbital architecture from our own
  那些轨道结构与我们太阳系不同的行星系统中的行星

can have stable obliquities, even without a moon (e.g., Lissauer, Barnes, & Chambers (2012); Barnes et al. (2016); Quarles, Li, & Lissauer (2019); Quarles et al. 2020). The climate of a planet with more ocean and less continental mass than Earth would be less sensitive on planetary obliquity.
没有卫星，也可能拥有稳定的黄赤交角（例如，Lissauer, Barnes, & Chambers (2012)；Barnes et al. (2016)；Quarles, Li, & Lissauer (2019)；Quarles et al. 2020）。与地球相比，如果一颗行星的海洋面积更大、大陆质量更小，那么它的气候对行星黄赤交角的变化就不会那么敏感。

The Moon is the primary body responsible for ocean tides on Earth (in the present era), and it produced substantially larger tides when it was closer to Earth billions of years ago. Tidal zones at the boundaries between sea and land on Earth are subject to cyclic variations in conditions with alternating wet and damp/dry periods. Very productive ecosystems currently exist within tidal zones. organic molecules can be concentrated by evaporation in tide pools and be supplied with repeated addition of nutrients. Life on Earth many have originated in tidal regions, where a large moon is advantageous to the formation of life. Since solar tides are negligible, a moon cannot be viewed as essential to life on this account.
地球上海洋潮汐的产生，主要归功于月球引力。几十亿年前，月球离地球更近，那时的潮汐也比现在更加壮观。在海洋和陆地的交界处，存在着潮间带，这里受潮汐影响，干湿交替，环境变化万千。也正是在这种独特的环境中，孕育出了生机勃勃的生态系统。潮起潮落，不仅为潮间带带来了丰富的营养物质，也使得有机分子在潮水退去后，能够在水洼中浓缩。因此，拥有大型卫星的地球，其潮间带更有利于生命的形成，地球上的生命也很可能就起源于此。当然，太阳也会引发潮汐，但影响甚微，所以说，月球的存在并非生命诞生的绝对必要条件。

The leading model for the Moon’s formation is the giant impact theory. This large impact removed volatile elements and compounds from Earth, where such a devolatilization may have been required for Earth itself to become suitable for life. The video below (from Miki Nakajima) shows the standard giant impact model (Hartmann and Davis 1975; Cameron and Ward 1976). Models of planetary growth suggest that most terrestrial volatiles come from a small fraction of the material composing our planet, so an otherwise Earth-like planet could simply accrete fewer volatile compounds than Earth. Alternatively, a planet could be devolatilized by a nearly head-on, mega-impact that would not loft enough material into orbit to produce a large moon.
关于月球形成的主流模型是大碰撞说。这一巨大的碰撞事件让地球损失了大量的挥发性元素和化合物，而这种去挥发作用可能是地球自身变得宜居的必要条件。下方视频（来自Miki Nakajima）展示了标准的大碰撞模型（Hartmann 和 Davis，1975；Cameron 和 Ward，1976）。行星演化模型表明，地球上的大部分挥发性物质都来自于构成地球的一小部分物质，因此一颗与地球相似的行星可能只是吸积了比地球更少的挥发性化合物。或者，一颗行星也可能因为一次近乎正面的巨大撞击而失去挥发性物质，而这种撞击不会将足够多的物质抛入轨道形成一个大型卫星。

4.3.4. Giant Planets and Life
4.3.4. 巨行星与生命

Giant planets are unlikely abodes for life because any solid surface that they might have would be at extremely high pressure. Living organism occupy essentially all known environments on or below the surface of our planet in which liquid water is present.
巨行星不太可能是生命的居所，因为它们可能存在的任何固体表面都将承受极高的压力。而在地球上，只要有液态水存在，无论是在地表还是地下，几乎所有已知的环境中都存在着生物体。

Despite billions of year of evolution, no known terrestrial organisms have adapted to a purely aerial life cycle. Although, Sagan & Salpeter (1976) hypothesized how an aerial ecosystem could develop on a gas giant. Even though moderate temperate zones occur within giant planet atmospheres, parcels of gas do not remain in place, but are repeatedly mixed downwards by convection into regions that are too hot for organic molecules to survive.
尽管地球生命已经历了数十亿年的演化，但至今仍未发现任何一种陆地生物能够完全适应空中生活。尽管如此，萨根和萨尔皮特早在 1976 年就提出过一个设想：在气态巨行星上，一个空中生态系统究竟是如何形成的？ 虽然巨行星的大气层中存在着温度适宜的区域，但气体团却不会停留在原地，而是会被对流不断向下混合，最终去到温度过高的区域，而有机分子在这种环境下根本无法生存。

Giant planets may harbor habitable moons, where moons of giant planets seem to have a similar potential for habitability as do terrestrial planets of the same mass and distance from their star. Formation circumstances may result in different average compositions, but the distributions of composition os such similarly sized moons and planets probably overlap.
巨行星可能拥有宜居卫星，这些卫星的宜居性似乎与质量和轨道半径与其相当的类地行星相似。虽然形成环境可能导致平均物质组成有所不同，但大小相似的卫星和行星的物质组成分布可能存在重叠。

Tidal interactions between large moons and giant planets are likely to be much more substantial than between stars and planets within the respective habitable zones due to the close proximity of moons to their host planets. This implies that such moons are probably in synchronous orbits. The consequences of such a spin-orbit resonance would be far less than for planets in synchronous rotation within the habitable zones of faint, low-mass, M-dwarf stars.
与恒星和行星在其各自宜居带内的相互作用相比，大型卫星和巨行星之间的潮汐作用可能要强烈得多，这是因为卫星与其宿主行星的距离更近。这意味着此类卫星很可能处于同步轨道上。这种自旋轨道共振的影响远小于处于质量较小、光度较暗的 M 型矮星宜居带内的同步自转行星。

The length of the day on such a moon would be slowed to approximately the moon’s orbital period about its planet. If it was in orbital resonance with another moon, energy from their coupled tidal recession from the planet would be deposited in one or both moons. This energy source could produce subsurface oceans, as is likely the case on Jupiter’s moon Europa.
在这样的卫星上，一天的长度将减慢到与其绕行星运行的周期大致相同。如果它与另一颗卫星处于轨道共振状态，那么它们从行星潮汐衰退中获得的耦合能量将沉积到其中一颗或两颗卫星上。这种能量来源可以产生地下海洋，就像木星的卫星木卫二那样。

Giant planets can also affect the habitability of a terrestrial planet orbiting the same star, where they can destabilize the terrestrial planet’s orbit causing an increase in the terrestrial planet’s eccentricity. The increased eccentricity allows for the planet to collide with another nearby planet or to ejection into interstellar space with a close approach to the host star.
巨行星也会影响围绕同一颗恒星运行的类地行星的宜居性，它们会破坏类地行星轨道的稳定，导致类地行星的偏心率增加。偏心率的增加使得行星有可能与附近的另一颗行星发生碰撞，或者在近距离接近主恒星的情况下被弹出到星际空间。

Giant planets can cause obliquity variations that affect the climate of terrestrial planets (as would be the case for a moonless Earth). Giant planets affect the flux of impactors that bombard terrestrial planets. Such impacts can have a devastating effect on life (as was the case for the dinosaurs).
巨行星会导致地球倾角的变化，从而影响类地行星的气候（就像没有月球的地球一样）。巨行星还会影响撞击类地行星的陨石流。这种撞击会对生命造成毁灭性的影响（就像恐龙灭绝一样）。

Giant planets may be important during the formation of habitable planets. It is likely that giant, vulcan planets migrated through the habitable zone on their way to the final orbit that is very close to their host star. As a result, they may have cleared material from that region to prevent the formation of terrestrial planets large enough to retain an atmosphere.
巨行星可能在宜居行星形成过程中发挥着重要作用。像巨行星这样的类木行星很可能在迁移到靠近其主恒星的最终轨道之前，穿越过宜居带。因此，它们可能清除了该区域的物质，阻止了足以保留大气层的类地行星的形成。

Giant planets that orbit farther from their star may have positive effects on terrestrial planet habitability, where the small fraction of volatiles possessed by Earth were probably diverted from colder regions of the protoplanetary disk by perturbations from Jupiter and Saturn.
巨行星如果距离其恒星较远，可能对类地行星的宜居性产生积极影响，因为地球上拥有的少量挥发性物质很可能是在原行星盘形成过程中，受到木星和土星等巨行星的扰动，从较冷的区域转移而来的。

4.4. How Life Affects Planets
4.4. 生命如何影响行星

Life an the environments where it flourishes on Earth have been closely intertwined for billions of years. The first microbes consumed the most readily available food, and subsequent life had to develop new energy sources to survive. The most productive energy source developed was to harness solar energy via photosynthesis, where the most efficient form of photosynthesis releases oxygen $({\mathrm{O}}_2)$ as a byproduct.
在地球上，生命与其繁荣的环境之间已经密切交织了数十亿年。最早的微生物消耗了最容易获得的食物，随后的生命必须开发新的能源才能生存。而出现的最有效的能量来源是通过光合作用来利用太阳能，其中最高效的光合作用形式会释放氧气 $({\mathrm{O}}_2)$ 作为副产品。

Oxygenic photosynthesis also provides a significant sink for atmospheric carbon dioxide. Microbes living in the digestive systems of cattle excrete methane into the atmosphere. Large forest fires can fill the atmosphere with soot, warming the atmosphere but shielding the surface from solar radiation. The burning of extant and extinct biomass releases gases and particulates into the environment.
产氧光合作用也为大气中的二氧化碳提供了重要的吸收汇。生活在牛消化系统中的微生物会将甲烷排放到大气中。大型森林火灾会使大气中充满烟尘，从而使大气变暖，但同时也会遮蔽太阳辐射到达地表。现存和已灭绝生物量的燃烧会将气体和颗粒物释放到环境中。

The presences of life has profound effects on the surface morphology of Earth. Forests cover large fractions of continental crust, which changes the
生命的存在对地球表面形态产生了深远的影响。森林覆盖了大陆地壳的大部分，这改变了

• albedo (including seasonal variations),
  反照率（包括季节性变化），

• soil composition, and 土壤成分，以及

• the local climate. 当地气候。

Microorganisms also change the soil composition through metabolism. The roots of land plants stabilize soil and reduce erosion, while beavers dam rivers. Humankind has large effects on the surface morphology through building and mining projects, pavement that affects drainage, and altering the composition of the atmosphere.
微生物的新陈代谢也会改变土壤成分。陆地植物的根系可以稳固土壤并减少侵蚀，而海狸则会筑坝拦河。人类通过建筑和采矿项目、影响排水的路面以及改变大气成分，对地表形态产生了巨大的影响。

The second most abundant gas in Earth’s present-day atmosphere is oxygen $({\mathrm{O}}_2)$, where this has not always been the case (see Fig. 3.25). Many gaps exist within the geologic record, but it is clear that the atmosphere of the early Earth was reducing. The oxygenation of Earth’s atmosphere occurred over billions of years, with an especially significant increase $\sim 2.2\mathrm{G}\mathrm{y}\mathrm{r}$ ago.
地球大气中含量第二多的气体是氧气，但这并非地球大气自诞生以来的常态（参见图 3.25）。虽然地质记录中存在许多空白，但早期地球的大气是还原性大气，这一点是毫无疑问的。地球大气的氧合作用经历了数十亿年，其中一次意义重大的增长发生在……前。

Oxygenic photosynthetic bacteria produce ${\mathrm{O}}_2$ as a waste product, while methanogenic organisms release $\mathrm{C}{\mathrm{H}}_4$. Some of this methane rises to the stratosphere and is photodissociated. A portion of the hydrogen formed by dissociation continues moving upwards to the exosphere and subsequently escapes. For the first few billion years, Earth’s atmosphere and crust were sufficiently reducing that they were capable of absorbing the released oxygen. Some processes (e.g., hydrogen escape and long-term sequestration of carbon within the crust and the mantle) diminished the environment’s ability to absorb oxygen, and eventually the ${\mathrm{O}}_2$ abundance in our planet’s atmosphere increased substantially.
产氧光合细菌会产生 ${\mathrm{O}}_2$ 作为废物，而产甲烷生物则释放 $\mathrm{C}{\mathrm{H}}_4$ 。其中一些甲烷会上升到平流层并在那里被光解。光解产生的部分氢气会继续向上移动到外逸层，最终逃逸到太空中。在地球形成的最初几十亿年里，地球的大气和地壳具有很强的还原性，足以吸收释放出来的氧气。然而，一些过程（例如氢气的逃逸以及碳在长期内被地壳和地幔吸收）逐渐削弱了环境吸收氧气的能力，最终导致地球大气中 ${\mathrm{O}}_2$ 的含量大幅增加。

Biological processes play an important role in the carbon cycle. Today’s fossil fuels (e.g., coal, crude oil, and ‘natural gas’ (methane)) were produced from partially decayed planets and many now extinct organisms. Shells of some marine organisms sequester carbon and other elements for long timescales. The simultaneous long-term presence of abundant mutually reactive gases (e.g., oxygen and methane) in a planet’s atmosphere requires a highly non-equilibrium process, such as oxygenic photosynthesis.
生物过程在碳循环中扮演着至关重要的角色。我们今天使用的化石燃料（比如煤炭、石油和“天然气”（甲烷））就来自于远古时代部分腐烂的植物和许多现已灭绝的生物。一些海洋生物的贝壳能够将碳和其他元素长期封存起来。而像氧气和甲烷这样相互反应的气体能够长期大量共存于一颗行星的大气中，则需要像能够产生氧气的光合作用这样非常特殊的非平衡过程。

Life has persisted on Earth for well over three billion years despite a significant increase in the Sun’s luminosity, a large reduction of the amount of escaping geothermal heat, and many other changing conditions on our planet. This endurance suggests that life itself may play a role in regulating the environment to allow for its continued existence. Figure 4.9 illustrates many of the factors that affect a planet’s habitability in which the planet’s biology plays a part of a much larger web.
地球上的生命已经持续了三十多亿年，尽管在此期间太阳的光度显著增加，地球内部逃逸的地热能大幅减少，而且地球上的环境也发生了许多其他变化。这种顽强的生命力表明，生命本身可能在调节环境方面发挥着作用，从而使其能够持续存在。图4.9展示了影响行星宜居性的许多因素，其中行星上的生物是这张复杂网络的一部分。

Fig. 4.9 Factors affecting habitability. This diagram shows currently understood planetary, stellar, and planetary system properties that may impact planetary habitability. Figure Credit: Meadows & Barnes (2018).
图 4.9 影响宜居性的因素。该图展示了目前已知的可能影响行星宜居性的行星、恒星和行星系统属性。图片来源：Meadows & Barnes (2018)。

4.5. Homework
4.5. 课后作业

Problem 1 问题 1

Summarize the attempts to define life and contrast the differences between the four main definitions. Which definition for life is the most popular and why?
总结一下人类尝试定义生命的努力，并对比四种主要定义之间的差异。哪一种生命定义最受欢迎？为什么？

Problem 2 问题 2

What are the parts of a definition? How does a definition guide (bias) astrobiologists in the search for life elsewhere?
定义的组成部分是什么？定义如何引导（或限制）天体生物学家寻找外星生命？

Problem 3 问题 3

There are some ecological limits to life. Summarize these limits and explain how they can be used to make inferences for the potential for life in the Solar System.
生命存在一些生态限制。总结这些限制，并解释如何利用它们推断太阳系中存在生命的可能性。

Problem 4 问题 4

Describe the process of evolution as envisioned by Darwin. How does it proceed at the molecular level?
描述达尔文设想的进化过程。它在分子水平上是如何进行的？

Problem 5 问题 5

There are three primary mechanisms for organisms to acquire new genes. Summarize these mechanisms and propose what environmental conditions that would best suit each mechanism.
生物获得新基因主要有三种机制。总结这些机制，并提出最适合每种机制的环境条件。

Problem 6 问题 6

Are there any alternatives for life to gain complexity without Darwinian evolution? Summarize at least one of the alternative hypotheses.
生命除了通过达尔文进化论获得复杂性之外，还有其他选择吗？总结至少一个其他的假设。

Problem 7 第 7 题

Draw diagrams illustrating main features of the Carbon and Nitrogen cycles. Explain how these cycles help support Earth-like ecosystems.
画出碳循环和氮循环主要特征的示意图。解释这些循环是如何帮助维持类地生态系统的。

Problem 8 第 8 题

Describe how external factors (e.g., Magnetic fields, large moons, and giant planets) may affect the potential for life to arise on an Earth-like planet.
描述外部因素（例如磁场、大型卫星和巨行星）如何影响类地行星上生命起源的可能性。