Chemical evolution. Archaean and Proterozoic

The theory of biochemical evolution was advanced by A. I. Oparin (1924) and J. Haldane (1929): life arose under the conditions of the ancient Earth from inorganic substances through slow chemical evolution. The Earth formed about 4.5 billion years ago; the first traces of life are at least 3.5 billion years old. Stages: 1) abiogenic synthesis of simple organic substances (amino acids, nitrogen bases, sugars) – in the early atmosphere (water vapour, carbon dioxide, nitrogen, methane, ammonia and hydrogen in varying mixtures) under lightning, ultraviolet light and volcanic heat; 2) their joining in water into polymers (polypeptides, polynucleotides); 3) formation from polymers of coacervate drops (A. Oparin) – drops separated from water that exchange substances with their surroundings – or microspheres; 4) protobionts – structures with a membrane-like surface and catalytic properties; 5) nucleic acids and proteins became linked, giving template synthesis and heredity – the first cells. Experiments: S. Miller (1953) obtained amino acids from a gas mixture with an electric spark; J. Oró synthesised adenine; S. Fox obtained protein-like proteinoids from amino acids. Modern hypotheses: the “RNA world” (RNA can both store hereditary information and act as a catalyst – a ribozyme), and synthesis at hydrothermal vents on the sea floor. The first cells were anaerobic heterotrophs (feeding on ready organic matter); then autotrophs – photosynthesising cyanobacteria – released oxygen and changed the atmosphere; an ozone layer formed; aerobic respiration and eukaryotes appeared. Eukaryotic organelles (mitochondria, chloroplasts) are thought to result from symbiogenesis – one cell taking in another and living together (L. Margulis). Many stages of this theory have been tested by experiment, but exactly how life began is still a field of research.