早期地球的环境变化和生命的化学进化

生命起源是当代最大的科学疑谜之一,也是历来人类普遍关注的一个焦点。在地球上最早的生物出现之前,有机物经历了漫长而复杂的化学进化过程,称为生命的化学进化。地球上生命的化学进化与非生物部分的早期演化过程,是密切地相互关联、相互作用并相互制约的。文章着重阐述与生命的化学关系最为密切的冥古宙和太古宙的地球演化历史,指出这两个阶段所形成的还原性原始大气和古海洋条件在生命的化学进行中起了极其重要的作用,并且从宇宙形成、太阳系演化和地球环境早期演化的角度,探讨地球生命的化学进化历程;以地球形成初期发生了一系列复杂的有机化学反应过程,由无机分子生成生物小分子,再进一步生成生物大分子,直至最后产生原始细胞。此外,文章评述当前国际上最流行的生命化学进化学说,对早期地球的化学进化是发生在地球表面的原始海洋、粘土矿物、火山喷发等,或是来源于地球之外的宇宙空间进行了综合的阐述。     

Impact constraints on the environment for chemical evolution and the continuity of life

The Moon and the Earth were bombarded heavily by planetesimals and asteroids that were capable of interfering with chemical evolution and the origin of life. In this paper, we explore the frequency of giant terrestrial impacts able to stop prebiotic chemistry in the probable regions of chemical evolution. The limited time available between impacts disruptive to prebiotic chemistry at the time of the oldest evidence of life suggests the need for a rapid process for chemical evolution of life. The classical hypothesis for the origin of life through the slow accumulation of prebiotic reactants in the primordial soup in the entire ocean may not be consistent with constraints imposed by the impact history of Earth. On the other hand, rapid chemical evolution in cloud systems and lakes or other shallow evaporating water bodies would have been possible because reactants could have been concentrated and polymerized rapidly in this environment. Thus, life probably could have originated near the surface between frequent surface sterilizing impacts. There may not have been continuity of life depending on sunlight because there is evidence that life, existing as early as 3.8 Gyr ago, may have been destroyed by giant impacts. The first such organisms on Earth where probably not the ancestors of present life.     

Life Began When Evolution Began: A Lipidic Vesicle-Based Scenario

The research on the origin of life, as such, seems to have reached an impasse as a clear and universal scientific definition of life is probably impossible. On the contrary, the research on the origin of evolution may provide a clue. But it is necessary to identify the minimum requirements that allowed evolution to emerge on early Earth. The classical approach, the ‘RNA world hypothesis’ is one way, but an alternative based on nonlinear dynamics dealing with far-from-equilibrium self-organization and dissipative structures can also be proposed. The conditions on early Earth, near deep-sea hydrothermal sites, were favorable to the emergence of dissipative structures such as vesicles with bilayer membranes composed of a mixture of amphiphilic and hydrophobic molecules. Experimentally these vesicles are able to self-reproduce but not to evolve. A plausible scenario for the emergence of a positive feedback process giving them the capability of evolving on early Earth is suggested. The possibilities offered by such a process are described in regard to specific characteristics of extant biological organisms and leads for future research in the field are suggested.     

THE EARLY SOLAR SYSTEM

Life arose on an early Earth which was the product of the conditions present, and processes operating, during formation of the solar system. The formation and early state of the solar system are reviewed in order to better understand the nature of the early Earth, and to constrain the conditions present during the origin and early evolution of life on this planet.     

Setting the stage: the history, chemistry, and geobiology behind RNA

No community-accepted scientific methods are available today to guide studies on what role RNA played in the origin and early evolution of life on Earth. Further, a definition-theory for life is needed to develop hypotheses relating to the "RNA First" model for the origin of life. Four approaches are currently at various stages of development of such a definition-theory to guide these studies. These are (a) paleogenetics, in which inferences about the structure of past life are drawn from the structure of present life; (b) prebiotic chemistry, in which hypotheses with experimental support are sought that get RNA from organic and inorganic species possibly present on early Earth; (c) exploration, hoping to encounter life independent of terran life, which might contain RNA; and (d) synthetic biology, in which laboratories attempt to reproduce biological behavior with unnatural chemical systems.     

The early evolution of life: solution to Darwin's dilemma

Recent studies of Precambrian fossils indicate that life on Earth originated earlier than assumed, microscopic life was prevalent in the Precambrian Eon, the tempo and mode of evolution during the Precambrian period were different from other periods, and that only the Precambrian fossil record can be used as evidence of early life. Implications for future research include directing the search for the origin of life away from the geological record, modification of hypotheses about molecular change, use of Precambrian microfossils in dating younger geological units, and progress in defining the nature of major events in early evolution.     

Life as a planetary phenomenon

The success of recent spacecraft from the U.S.A. and the U.S.S.R. has given us a wealth of new data about the planets in our solar system. We can now develop a much better rationale for the reasons that abundant life is only found on our planet. Mars, smaller and more distant from the Sun, may nevertheless hold clues to the early development of Earth's atmosphere. The origin of life on Mars early in that planet's history cannot be ruled out. Titan offers a contemporary example of extremely primitive conditions, where chemical reactions resembling those that preceded the development of life on Earth may be occurring today. Venus and Jupiter illustrate the need for a planet to be the right size and the right distance from the sun if chemical evolution leading to the origin of life is to occur.     

In the Beginning was a Mutualism - On the Origin of Translation

The origin of translation is critical for understanding the evolution of life, including the origins of life. The canonical genetic code is one of the most dominant aspects of life on this planet, while the origin of heredity is one of the key evolutionary transitions in living world. Why the translation apparatus evolved is one of the enduring mysteries of molecular biology. Assuming the hypothesis, that during the emergence of life evolution had to first involve autocatalytic systems which only subsequently acquired the capacity of genetic heredity, we propose and discuss possible mechanisms, basic aspects of the emergence and subsequent molecular evolution of translation and ribosomes, as well as enzymes as we know them today. It is possible, in this sense, to view the ribosome as a digital-to-analogue information converter. The proposed mechanism is based on the abilities and tendencies of short RNA and polypeptides to fold and to catalyse biochemical reactions. The proposed mechanism is in concordance with the hypothesis of a possible chemical co-evolution of RNA and proteins in the origin of the genetic code or even more generally at the early evolution of life on Earth. The possible abundance and availability of monomers at prebiotic conditions are considered in the mechanism. The hypothesis that early polypeptides were folding on the RNA scaffold is also considered and mutualism in molecular evolutionary development of RNA and peptides is favoured.     

Stephen Jay Gould,les procaryotes et leur évolution dans le contexte géologique

Stephen Jay Guld, the prokaryotes and their evolution in a geological context. Stephen Jay Gould (Full House, Harmony Books, New York, 1996) emphasised the importance of the bacterial (prokaryote) world right from the beginnings of the history of life, up to the present day and, even into the future. Moreover, he suggested that the various forms of life on the planet today represent a diversification, or increase in complexity, from these simple organisms, rather than a directed evolution towards complexity. On the scale of evolution, the oldest fossils, dating back to almost 3.5 billion years ago, comprise simple unicellular organisms, i.e. prokaryotes that occur adjacent to the left wall of complexity. As far as can be ascertained from the fossil record, this early life had already exploited all the possible ramifications of diversification within an anaerobic environment and within the habitats available on the early Earth. Further evolution from this stage involved the development of organisms undertaking oxygenic photosynthesis that opportunistically invaded the newly formed, sunlight-bathed, shallow water continental platforms. To cite this article: F. Westall, C. R. Palevol 2 (2003).     

Origins for Everyone

The origin of life on Earth remains a mystery, but the question can still be approached with scientific rigor. Identifying life??s origins requires the definition of life itself, which has been described as a self-sustaining system capable of Darwinian evolution, although it's also possible that there is no good scientific definition. All known living systems contain linear strings of information based on DNA, a molecule that makes Darwinian evolution possible through replication and mutation. This review explains the scientific concepts and issues underlying the origin of life, possible mechanisms of origins, and the features of living systems that can arguably be viewed as an inevitable consequence of the earliest molecules.     

Prebiotic materials from on and off the early Earth

One of the greatest puzzles of all time is how did life arise? It has been universally presumed that life arose in a soup rich in carbon compounds, but from where did these organic molecules come? In this article, I will review proposed terrestrial sources of prebiotic organic molecules, such as Miller-Urey synthesis (including how they would depend on the oxidation state of the atmosphere) and hydrothermal vents and also input from space. While the former is perhaps better known and more commonly taught in school, we now know that comet and asteroid dust deliver tons of organics to the Earth every day, therefore this flux of reduced carbon from space probably also played a role in making the Earth habitable. We will compare and contrast the types and abundances of organics from on and off the Earth given standard assumptions. Perhaps each process provided specific compounds (amino acids, sugars, amphiphiles) that were directly related to the origin or early evolution of life. In any case, whether planetary, nebular or interstellar, we will consider how one might attempt to distinguish between abiotic organic molecules from actual signs of life as part of a robotic search for life in the Solar System.     

Prebiotic Synthesis of Glycine from Ethanolamine in Simulated Archean Alkaline Hydrothermal Vents

Submarine hydrothermal vents are generally considered as the likely habitats for the origin and evolution of early life on Earth. In recent years, a novel hydrothermal system in Archean subseafloor has been proposed. In this model, highly alkaline and high temperature hydrothermal fluids were generated in basalt-hosted hydrothermal vents, where H₂ and CO₂ could be abundantly provided. These extreme conditions could have played an irreplaceable role in the early evolution of life. Nevertheless, sufficient information has not yet been obtained for the abiotic synthesis of amino acids, which are indispensable components of life, at high temperature and alkaline condition. This study aims to propose a new method for the synthesis of glycine in simulated Archean submarine alkaline vent systems. We investigated the formation of glycine from ethanolamine under conditions of high temperature (80–160 °C) and highly alkaline solutions (pH = 9.70). Experiments were performed in an anaerobic environment under mild pressure (0.1–8.0 MPa) at the same time. The results suggested that the formation of glycine from ethanolamine occurred rapidly and efficiently in the presence of metal powders, and was favored by high temperatures and high pressures. The experiment provides a new pathway for prebiotic glycine formation and points out the phenomenal influence of high-temperature alkaline hydrothermal vents in origin of life in the early ocean.     

Could life have arisen in the primitive atmosphere?

Summary A recently proposed model for the origin of prebiotic progenitors of life in particles suspended in a primitive, specially organized atmosphere is considered critically. It is concluded that the physical and chemical framework of the new hypothesis conflicts with the conditions necessary for the evolution of the progenitors of life in the atmosphere of the early Earth. Therefore this model seems not to be a reasonable alternative to the Oparin thesis.     

Strukturiert erhaltene Fossilien aus dem Archaikum von Südafrika

All organic remains known from the Archean are in such a poor state of preservation, that their biogenicity has been repeatedly doubted. Structures of unquestionable organismic origin have been recently detected in cherts of the Onverwacht group in South Africa. The finds are preserved in a detailed three-dimensional condition. A radiometric age of more than 3 350 mio. y. for the stratum is indicated. With this date, the finds represent the oldest certain evidences of life on earth. Moreover the well-preserved details yield information on the principles of structure and growth of a primeval organism. The body is interpreted as a ramificational system of tiny droplike subunits and appears to be constructed according to the principle of consequent homonomy. It seems possible, that the finds represent an initial form of organismic evolution. With this statement and with the data presently known from the early tellural evolution, it seems possible and credible that life originated on Earth.     

The evolution of prebiological self-organization: Probable colloid-chemical evolution of first prokaryotic cells

This is an attempt to analyse the mechanisms of self-assembly in the course of the origin and early evolution of life on the Earth. A special attention is paid to the investigation of transient stages between the physico-chemical and biological bases of self-assembly, including experimental models and paleontological results. The theory of coacervate-in-coacervate is discussed from the point of view of evolution of first procaryotic cells. Many of the high developed structures of the contemporary cells, such as ribosomes, chromosomes, lipid membranes, some other organelles etc., are claimed to posses a rudimentary polyionic coacervate character.     

The evolution of prebiological self-organization: probable colloid-chemical evolution of first prokaryotic cells

This is an attempt to analyse the mechanisms of self-assembly in the course of the origin and early evolution of life on the Earth. A special attention is paid to the investigation of transient stages between the physico-chemical and biological bases of self-assembly, including experimental models and paleontological results. The theory of coacervate-in-coacervate is discussed from the point of view of evolution of first procaryotic cells. Many of the high developed structures of the contemporary cells, such as ribosomes, chromosomes, lipid membranes, some other organelles etc., are claimed to posses a rudimentary polyionic coacervate character.     

NASA's Exobiology Program

The goal of NASA's Exobiology Progam is to understand the origin, evolution, and distribution of life, and life-related molecules, on Earth and throughout the universe. Emphasis is focused on determining how the rate and direction of these processes were affected by the chemical and physical environment of the evolving planet, as well as by planetary, solar, and astrophysical phenomena. This is accomplished by a multi-disciplinary program of research conducted by over 60 principal investigators in both NASA and university laboratories. Major program thrusts are in the following research areas: biogenic elements; chemical evolution; origin of life; organic geochemistry; evolution of higher life forms; solar system exploration; and the search for extraterrestrial intelligence (SETT).     

An overlooked riddle of life's origins: energy-dependent nucleic acid unzipping

The imposing progress in understanding contemporary life forms on Earth and in manipulating them has not been matched by a comparable progress in understanding the origins of life. This paper argues that a crucial problem of unzipping of the double helix molecule of nucleic acid during its replication has been underrated, if not plainly overlooked, in the theories of life's origin and evolution. A model is presented of how evolution may have solved the problem in its early phase. Similar to several previous models, the model envisages the existence of a protocell, in which osmotic disbalance is being created by accumulation of synthetic products resulting in expansion and division of the protocell. Novel in the model is the presence in the protocell of a double-stranded nucleic acid, with each of its two strands being affixed by its 3'-terminus to the opposite sides of the membrane of a protocell. In the course of the protocell expansion, osmotic force is utilized to pull the two strands longitudinally in opposite directions, unzipping the helix and partitioning the strands between the two daughter protocells. The model is also being used as a background for arguments of why life need operate in cycles. Many formal models of life's origin and evolution have not taken into account the fact that logical possibility does not equal thermodynamic feasibility. A system of self-replication has to consist of both replicators and replicants.     

Origins of life: A comparison of theories and application to Mars

The field of study that deals with the origins of life does not have a consensus for a theory of life's origin. An analysis of the range of theories offered shows that they share some common features that may be reliable predictors when considering the possible origins of life on another planet. The fundamental datum dealing with the origins of life is that life appeared early in the history of the Earth, probably before 3.5 Ga and possibly before 3.8 Ga. What might be called the standard theory (the Oparin-Haldane theory) posits the production of organic molecules on the early Earth followed by chemical reactions that produced increased organic complexity leading eventually to organic life capable of reproduction, mutation, and selection using organic material as nutrients. A distinct class of other theories (panspermia theories) suggests that life was carried to Earth from elsewhere — these theories receive some support from recent work on planetary impact processes. Other alternatives to the standard model suggest that life arose as an inorganic (clay) form and/or that the initial energy source was not organic material but chemical energy or sunlight. We find that the entire range of current theories suggests that liquid water is the quintessential environmental criterion for both the origin and sustenance of life. It is therefore of interest that during the time that life appeared on Earth we have evidence for liquid water present on the surface of Mars.     

Abiogenic synthesis of prebiotic matter for the Earth’s biosphere as a stage of self-organization on an astrophysical and paleontological time scale

Existing data suggest that an early circumstellar preplanetary disk was the most likely location for primary abiogenic synthesis of prebiotic organic matter from simple molecules along with the “RNA world” and the origin of life. This paper discusses the stages of self-organization that have resulted in the Earth’s modern biosphere, and the relationships between astrophysical and paleontological events in evolution.