原始地球에 있어서 化學進化와 原始生命體 誕生

Chemical evolution and first life forms on the primitive earth

The Earth has existed in solid form for 4.6 billion years. Abiotic synthesis of organic molecules had occurred on the primeval Earth in the absence of life : first, simple primitive molecules such as hydrogen cyanide(HCN) and aldehyde(R-CHO) from the primitive atmospheric gas, then such monomers as amino acids or ribose sugar, and eventually polymers such as polypeptides and nucleic acids. The first on billion years after the formation of the Earch are said to be the era of chemical evolution. Fossil evidences discovered in Australia and South Africa suggest that true life forms apperaed on the Earth around 3.6 billion years ago. Cellular life has three basic components of (1) a membrance boundry separating the living system from its environment, (2) metabolism catalized by enzymes, and (3) information-storing genetic system capable of self-replication and transmission to descendants. Three major models of protobionts are proposed : Opariri’s coacervate model, Fox’s proteinoid microsphere, and Goldacre’s lipid bilayer model, which have in common a selectively permeable boundary. These three protobionts are not true life forms but precursors, being simply membrane-bound metabolizing systems lacking a genetic component. With the incorporation of a genetic system, protobionts would have evoled into true life forms, the eubionts. All cellular life today stores its genetic information in the form of triplet codon in the DNA molecules. It has been suggested, however, that earlier life forms had RNA as their information storage system, and the RNA had been replaced later by DNA. RNA is considered fit as genetic system for the primeval life as it has dual function of information storage and catalytic activities. Primordial anaerobic heterotrophs relied on abundant pre-existing organic nutrients. Scarcity of a nutrient favored selection of those that could produced the nutrient from a precursor. By this process, progressively longer and more complex metabolic pathways evoleved. With the appearance of such photosynthetic autotrophs as cyanobacteria(blue green algae) which used CO₂ and hydrogen from water and released oxygen, the originally reducing atmosphere turned to oxidizing atmosphere. After the long, era of prokayotes which lasted nearly two billion years, eukaryotes had arisen from cell wall-less prokaryote with motile plasma membrance around 1.5 billion years ago. DNA was relocated to the cell interior from its attachment site on the plasma membrance and a nuclear envelope developed, forming nucleus and establishing mitotic system. Multicellularity gave organism possibilities for structural and functional spcialization that increased their life expectancy, stability, and efficiency.