地球生命的起源

Abstract

宇宙创生于大爆炸, 在其后的恒星形成和星系演化过程中各种生命所需的元素不断合成. 45.6亿年前, 太阳系在银河系的宜居带形成并开始演化, 与此同时, 地球在太阳系的宜居带上开始朝宜居行星演化. 在地球的前生命演化阶段水的海洋已经形成, 且拥有丰富的简单有机碳化合物、具催化功能的过渡金属化合物及携带能量的氢气、硫化氢和甲烷等. 上述各种物质聚集在近地球表面高能、专门化的、封闭的微小地球化学单元中逐渐演化为初步具有生化功能的单元, 最终脱离地球化学反应空间的约束而形成能够独立进行能量代谢和遗传物质合成的生命个体. 从能量和地球化学的角度看, 将氢气/甲烷合成为有机质的生化过程在早期和现代地球上均不难 发生, 这也是生命自养起源说的基本假设. 地球早期还原大气可经由太阳辐射形成简单有机分子, 而陨石和彗星也可将星际有机分子输送到地球, 因此地球早期的海洋可能是充满简单有机分子和营养盐的环境, 此环境支持生命异养起源假说. 从前生命有机化学演化到第一个细胞的形成可能只需要很短的时间. 虽然目前已知的地球上最古老的微生物化石是35亿年前形成的叠层石, 但在地球的海洋形成之时, 即38.5亿年前, 生命就可能已经存在于地球上了.

Origin of life is one of the greatest mysteries for mankind. Life is the most complex outcome of the 13.7 billion years evolution of our Universe. Human has just recently found that life has experienced a long coevolution with its earth environments through the deep geological time. At the beginning of its planetary evolution, Earth lost its highly reduced atmosphere that contained relatively high of hydrogen, ammornia and methane and turned to a secondary atmosphere contained high of carbon dioxide, dinitrogen and water by differentiation. In the first 700 million years with a hot, global climatic temperature, the strong reactions between the water-carbon dioxide atmosphere and the ultramafic crust produced abundant secondary earth materials including clay minerals and carbonates. Those clay minerals migh have catalyzed the indigenous synthesis of organic matter for the prebiotic evolution toward life; while the deposition of massive carbonates sequestrated the atmospheric carbon dioxide that significantly weakened the greenhouse effect and led to the formation of the earliest oceans only ca. 150 million years after Earth’s formation. Though the Earth environments before the Later Heavy Bombardment at ca. 4.1–3.8 billion years was already suitable for life to start, the oldest lithological evidence of life is around 3.71 billion years old. The current controversies on the origin of life include two basic questions: hot or cold, gene first or metabolism first. The Hot Origin theory is supported by the observed interactions between hot water and mafic rocks in producing small organic molecules; the Cold Origin theory insists that the cold temperature is essential for the inorganic synthesis of amino acids and the stability of primary genetic materials. On the other side, astronomists prefer that the exogenous input of organic carbon, water and transition metals by comets and meteorites might be a significant contribution to the prebiotic evolution of life in the early time. Life is a star material. All the cosmic evolutionary stages contributed to the emergence and evolution of life on Earth. The Big Bang produced hydrogen, the main sequence evolution of stars cooked carbon, oxygen, nitrogen, sulfur, phosphorus, silicon and iron, and even supernova, the dramatic ending of massive stars, produced various heavy radioactive isotopes that powers the highly active geodynamic processes of Earth’s interior. Hydrogen, nitrogen, sulfur, phosphorus, silicon and some metals find their usefulness in biochemistry by having chemical reactions with water-carbon based chemical systems. The various extreme conditions on Earth, such as high or low temperatures, strong photo or ionic radiations, high or low pH values, lacking of nutrients, etc., provide constraints for life on the other planets/moons of our solar system, or on the increasing numbers of Earth-like exoplents. In the observable universe, water is the best chemical solvent for life, and only carbon can make the backbone for life. The unique physical properties of water and incomparable biochemical properties of carbon, combined with their high cosmic abundances, strongly suggest life as we can understand should be common in the Universe; and our understanding of extremophiles and the evolutionary pathway of life on Earth suggest that at least microbial life should be common in the Universe.