Chemical evolution and the origin of life.

During the last three decades major advances have been made in our understanding of the formation of carbon compounds in the universe and of the occurence of processes of chemical evolution. 1) Carbon and other biogenic elements (C,H,N,O,S and P) are some of the most abundant in the universe. 2) The interstellar medium has been found to contain a diversity of molecules of these elements. 3) Some of these molecules have also been found in comets which are considered the most primordial bodies of the solar system. 4) The atmospheres of the outer planets and their satellites, for example, Titan, are actively involved in the formation of organic compounds which are the precursors of biochemical molecules. 5) Some of these biochemical molecules, such as amino acids, purines and pyrimidines, have been found in carbonaceous chondrites. 6) Laboratory experiments have shown that most of the monomers and oligomers necessary for life can be synthesized under hypothesized but plausible primitive Earth conditions from compounds found in the above cosmic bodies. 7) It appears that the primitive Earth had the necessary and sufficient conditions to allow the chemical synthesis of biomacromolecules and to permit the processes required for the emergence of life on our planet. 8) It is unlikely that the emergence of life occurred in any other body of the solar system, although the examination of the Jovian satellite Europa may provide important clues about the constraints of this evolutionary process. Some of the fundamental principles of chemical evolution are briefly discussed.     

Cosmic radiation and evolution of life on earth: roles of environment, adaptation and selection.

The role of ionizing radiation in general, and cosmic radiation in particular, in the evolution of organisms on the earth by adaptation and natural selection is considered in a series of questions: (1) Are there times during the evolution of the earth and of life when genetic material could be exposed to heavy ion radiation? (2) Throughout the course of chemical and biological evolution on the earth, what fraction of environmental mutagenesis could be attributable to cosmic and/or solar ionizing radiation? (3) Is ionizing radiation an agent of adaptation or selection, or both? (4) What can the cladistics of the evolution of genetic repair tell us about the global history of genotoxic selection pressures? (5) How much genetic diversity can be attributed to the selection of radiation-damage repair processes?     

Exobiology revisited

The term “Exobiology” was introduced about 25 years ago, at a time when intensive discussions were under way concerning plans for the biological exploration of Mars. The search for life on Mars was to be a critical test of the concept of chemical evolution- not an end in itself. After the Viking mission, when it became apparent that prospects were dim for the discovery of extraterrestrial life within our solar system, many people concluded that this new field of endeavor would soon expire. Quite the contrary, over the past decade, the field has broadened considerably into a multidisciplinary approach to understanding the circumstances that led to the origin of life and the interplay between the evolution of this planet and its biota.     

Emergence of adaptable systems and evolution of a translation device

An over-all organizational framework for the origin of life is outlined and attemps for realization are given. Evolution can be described as a process resulting in an increase of “knowledge” where knowledge is the number of carriers of genetic information discarded, on the average, until the evolutionary state under consideration is reached. A model for the evolution of a translation device, a crucial event in the origin of life, is described in detail. Aggregates of short polynucleotide strands in a hairpin conformation play a major role in this model. Experimental evidence for the selectivity of aggregation supports the idea of aggregates as error filters. Chromatographic separation as selection process during chemical evolution supports the model of the early translation device leading to the origin of the genetic code.     

Life in the universe: foundation for exciting multidisciplinary science activities for middle and elementary school classes.

Young students find extra-terrestrial life one of the most intriguing of all topics. A project funded by the National Science Foundation and NASA, and administered by the SETI Institute, is underway to devise science lessons for grades 3-9 that draw upon this fascination. The lessons are designed by teachers and persons with long experience at curriculum design, tested in classrooms, revised and retested. Six guides, each containing some 6-10 science lessons, will be finished by summer, 1994. The theme Life in the Universe lends itself naturally to integrated treatment of facts and concepts from many scientific disciplines. The lessons for two completed guides span the origin of planet systems, evolution of complex life, chemical makeup of life, astronomy, spectroscopy, continental drift, mathematics and SETI (Search for Extra-Terrestrial Intelligence). All lessons are hands-on, interesting, and successful.     

Life in the solar system.

Life, defined as a chemical system capable of transferring its molecular information via self-replication and also capable of evolving, must develop within a liquid to take advantage of the diffusion of complex molecules. On Earth, life probably originated from the evolution of reduced organic molecules in liquid water. Organic matter might have been formed in the primitive Earth's atmosphere or near hydrothermal vents. A large fraction of prebiotic organic molecules might have been brought by extraterrestrial-meteoritic and cometary dust grains decelerated by the atmosphere. Any celestial body harboring permanent liquid water may therefore accumulate the ingredients that generated life on the primitive Earth. The possibility that life might have evolved on early Mars when water existed on the surface marks it as a prime candidate in a search for bacterial life beyond the Earth. Europa has an icy carapace. However, cryovolcanic flows at the surface point to a possible water subsurface region which might harbor a basic life form. The atmosphere and surface components of Titan are also of interest to exobiology for insight into a hydrocarbon-rich chemically evolving world. One-handed complex molecules and preferential isotopic fractionation of carbon, common to all terrestrial life forms, can be used as basic indicators when searching for life beyond the Earth.     

Comets and life in the Universe.

The notion that comets supplied the primitive Earth with the requisite chemical species for the process of chemical evolution, which is widely held to have led to the origin of life on Earth, has now gained considerable intellectual momentum since its first formulation in 1961. In fact, in the fall of 1991, the University of Wisconsin-Eau Claire hosted a well attended scientific meeting devoted solely to the discussion of this topic, entitled Comets and the Origin and Evolution of Life [see Special Issue of Origins of Life, P.J. Thomas (ed), vol. 21(5-6)]. As a result of the above meeting, the recently completed COSPAR/World Space Congress Symposium on Extraterrestrial Organic Chemistry and the Origins of Life, and numerous independent reports, the role of comets in the Earth's biogenesis has been thoroughly addressed in the literature. At this time, in light of a few recent findings, we present here a concise review of this topic together with a brief discussion of the possible role of cometary material in the origin of life elsewhere in the Universe.     

Life as a set of matter transformation cycles: ecological attributes of life.

An approach to searching for extraterrestrial life on the base of "autotroph" concept of the origin of life is presented in the paper. According to this concept the origin of life took place in three stages. The first stage was developed inside the global geochemical cycle in which the turnover of different chemical transformations was implemented by solar radiation and/or heat energy of bowels of the Earth. At the second stage, after the autocatalytic systems have emerged these systems evolved as a result of "natural selection" by autocatalysis parameters up to emergence of special inheritance systems that drastically improved the autocatalysis parameters. The best in terms of autocatalysis parameters were the autocatalysis systems based on phase-separated particles where complex structures can form not only on the basis of covalent interactions. Such autocatalysis systems can emerge only in liquid in a certain range of temperatures and pressures. At this stage the geochemical cycle complicated involving new substances. At the third stage the evolution involved improvement of inheritance systems resulting in formation of the modern type of genetic apparatus. This concept formed the basis to consider approaches to experimental modeling of major aspects of the origin of life and to outlining some general features of life that can extend the sensitive horizon of searching for extraterrestrial life.     

Life on Mars? I. The chemical environment.

The origin of life at its abiotic evolutionary stage, requires a combination of constituents and environmental conditions that enable the synthesis of complex replicating macromolecules from simpler monomeric molecules. It is very likely that the early stages of this evolutionary process have been spontaneous, rapid and widespread on the surface of the primitive Earth, resulting in the formation of quite sophisticated living organisms within less than a billion years. To what extent did such conditions prevail on Mars? Two companion-papers (Life on Mars? I and II) will review and discuss the available information related to the chemical, physical and environmental conditions on Mars and assess it from the perspective of potential exobiological evolution.     

Exobiological exploration of Mars.

Of all the other planets in the solar system, Mars remains the most promising for further elucidating concepts about chemical evolution and the origin of life. Strategies were developed to pursue three exobiological objectives for Mars exploration: determining the abundance and distribution of the biogenic elements and organic compounds, detecting evidence of an ancient biota on Mars, and determining whether indigenous organisms exist anywhere on the planet. The three strategies are quite similar and, in fact, share the same sequence of phases. In the first phase, each requires global reconnaissance and remote sensing by orbiters to select sites of interest for detailed in situ analyses. In the second phase, lander missions are conducted to characterize the chemical and physical properties of the selected sites. The third phase involves conducting "critical" experiments at sites whose properties make them particularly attractive for exobiology. These critical experiments would include, for example, identification of organics, detection of fossils, and detection of extant life. The fourth phase is the detailed analysis of samples returned from these sites in Earth-based laboratories to confirm and extend previous discoveries. Finally, in the fifth phase, human exploration is needed to establish the geological settings for the earlier findings or to discover and explore sites that are not accessible to robotic spacecraft.     

Life on Mars? II. Physical restrictions.

The primary physical factors important to life's evolution on a planet include its temperature, pressure and radiation regimes. Temperature and pressure regulate the presence and duration of liquid water on the surface of Mars. The prolonged presence of liquid water is essential for the evolution and sustained presence of life on a planet. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that early mars possessed a dense atmosphere of CO2 (> or = 1 bar) and sufficient water to cut large channels across its surface. The degree to which either of these postulates is true correlates with the suitability of Mars for life's evolution. Although radiation can destroy living systems, the high fluxes of UV radiation on the martian surface do not necessarily stop the origin and early evolution of life. The probability for life to have arisen and evolved to a significant degree on Mars, based on the postulated ranges of early martian physical factors, is almost solely related to the probability of liquid water existing on the planet for at least hundreds of millions to billions of years.     

Heavy-ion induced genetic changes and evolution processes.

On Moon and Mars, there will be more galactic cosmic rays and higher radiation doses than on earth. Our experimental studies showed that heavy ion radiation can effectively cause mutation and chromosome aberrations and that high-LET heavy-ion induced mutants can be irreversible. Chromosome translocations and deletions are common in cells irradiated by heavy particles, and ionizing radiations are effective in causing hyperploidy. The importance of the genetic changes in the evolution of life is an interesting question. Through evolution, there is an increase of DNA content in cells from lower forms of life to higher organisms. The DNA content, however, reached a plateau in vertebrates. By increasing DNA content, there can be an increase of information in the cell. For a given DNA content, the quality of information can be changed by rearranging the DNA. Because radiation can cause hyperploidy, an increase of DNA content in cells, and can induce DNA rearrangement, it is likely that the evolution of life on Mars will be effected by its radiation environment. A simple analysis shows that the radiation level on Mars may cause a mutation frequency comparable to that of the spontaneous mutation rate on Earth. To the extent that mutation plays a role in adaptation, radiation alone on Mars may thus provide sufficient mutation for the evolution of life.     

金星火山与气候探测任务

金星火山和气候探测任务（Venus Volcano Imaging and Climate Explorer,VOICE）聚焦金星火山与热演化历史、水与板块运动、内部结构和动力学、气候演化和生命信息探索等重大科学问题,提出采用极化合成孔径雷达（Polarimetric Synthetic Aperture Radar,PolSAR） 、下视与临边结合的微波辐射探测仪（Microwave Radiometric Sounder,MWRS）和紫外–可见–近红外多光谱成像仪（Ultraviolet-Visible-Near Infrared Multispectral Imager,UVN-MSI）等三个先进的有效载荷,在350 km圆轨道上对金星全球表面和大气联合探测。 PolSAR将对金星全球表面进行高分辨多极化雷达成像;MWRS将对金星全球云下大气的热力结构和化学组成,云中可能的宜居环境及与生命相关大气成分进行探测;UVN-MSI则实现大气全貌成像、表面光谱成像和闪电检测。通过多种先进探测载荷和技术手段的结合,VOICE任务将揭示金星构造热演化历史和超温室效应机理,探索其宜居性和生命信息。VOICE任务的实施将实现国际金星研究探索中许多“零”的突破,为理解行星宜居性和太阳系演化提供极为关键的观测支持,对提升中国在国际深空探测与空间科学研究中的地位产生重大影响。      

代谢起源进化的分子模拟研究

新陈代谢为生命提供了物质和能量基础,与生命起源和进化密切相关.然而,由于缺乏化石证据,代谢的起源及其影响生物进化的分子机制等重要问题尚待解决.近年来,网络扩张算法等分子模拟方法的出现为解决这些问题提供了新途径.本文综述了近年来代谢起源进化的分子模拟研究,以期为相关领域学科发展提供新思路.     

The seeding of life by comets.

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earth's atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.     

The atmospheric greenhouse effect and climates on various planets

The greenhouse effect of the planetary atmospheres is considered and its evolution as a result of variations in the chemical composition and in gas abundances of the atmospheres as well as in the chemical composition, size distribution and concentration of aerosol components. A computer modelling gave the values of the greenhouse effect of the atmospheres of the Earth, Mars, Venus, Jupiter, and Titan. It is shown that the atmospheric greenhouse effect plays a decisive role in the formation of the planetary climates and that it has substantially changed in the process of the planetary evolution. The greenhouse effect mechanism has always been and still is a major factor of the mean global planetary climate.     

Hypotheses on the appearance of life on Earth (review).

It is generally accepted within the natural sciences that life emerged on Earth by a kind of proto-Darwinian evolution from molecular assemblies that were predominantly formed from the various constituents of the primitive atmosphere and hydrosphere. Evolutionary stages under discussion are: the self-organization of spontaneously formed biomolecules into early precursors of life (protobionts), their stepwise evolution via (postulated) protocells to (postulated) progenotes and the Darwinian evolution from progenotes to the three kingdoms of contemporary organisms (archaebacteria, eubacteria and eukaryotes). Considerable discrepancies between scientists have arisen because all evolutionary stages from prebiotic molecules to progenotes are entirely hypothetical and so are the postulated environmental conditions. We can only theorize that all those environmental conditions that allow the existence of the various forms of contemporary life might have allowed also the development of their precursors. Because of all these difficulties the hypothesis that life came to our planet from a remote place of our universe (panspermia) has been revived. But experimental evidence only supports the view that spores can--under favorable circumstances--survive a relatively short journey within our solar system (interplanetary transfer of life). It is extremely unlikely that spores can survive a journey of hundreds or thousands of years through interstellar space.     

火星生命探测中一种潜在的生物标志物磷酸盐

地外生命探索是国际上广泛关注的深空探测重要目标之一.中国第一个火星探测器天问一号成功发射,开启了对火星表面形貌、生命迹象等进行科学探索的旅程.作为太阳系中与地球最为相似的星球,火星带给人类无穷的遐想.火星上是否存在生命,未来人类是否可以移民火星,磷作为重要的生命元素,在生命的整个进化过程具有不可替代的作用.磷酸盐可以作...     

Theoretical studies on the space CIV experiment over China

Recently the Active Experiment Working Group, Center for Space Science and Applied Research, has proposed a chemical release experiment program. The initial scientific object of the program is to study the critical ionization velocity (CIV) mechanism at low and middle latitudes over China. Beside of the development of the chemical module system, theoretical studies on the CIV phenomena of barium or strontium released over China are studied. We found that the angle between the release velocity and the ambient magnetic field over China is smaller than that in the same latitude region in North America and is advantageous for studying CIV phenomena and the evolution of ionized cloud along the magnetic lines. We also used a numerical simulation to simulate the evolution of chemical release clouds under various release conditions. For 1 kg barium and V-B angle equal to 60 degrees, we have a better configuration of ions cloud which can easily distinguish the ions created by CIV phenomena from that by the UV of the sunlight.     

Exponential evolution: implications for intelligent extraterrestrial life.

Some measures of biologic complexity, including maximal levels of brain development, are exponential functions of time through intervals of 10(6) to 10(9) yrs. Biological interactions apparently stimulate evolution but physical conditions determine the time required to achieve a given level of complexity. Trends in brain evolution suggest that other organisms could attain human levels within approximately 10(7) yrs. The number (N) and longevity (L) terms in appropriate modifications of the Drake Equation, together with trends in the evolution of biological complexity on Earth, could provide rough estimates of the prevalence of life forms at specified levels of complexity within the Galaxy. If life occurs throughout the cosmos, exponential evolutionary processes imply that higher intelligence will soon (10(9) yrs) become more prevalent than it now is. Changes in the physical universe become less rapid as time increases from the Big Bang. Changes in biological complexity may be most rapid at such later times. This lends a unique and symmetrical importance to early and late universal times.