The origin of life on Earth

Abstract

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.