How did photosynthesis start?
Measurements of the distribution of radioisotopes led to the conclusion that the earth was formed about 4.6 billion years ago. The earliest indica-tors of life on earth are fossils of bacteria-like structures, estimated to be 3.5 billion years old. There was no oxygen in the atmosphere when life on earth commenced. This is concluded from the fact that in very early sedi-ment rocks iron is present as Fe2+ . Mineral iron is oxidized to Fe3+ in the presence of oxygen. According to our present knowledge, the earth’s atmosphere initially contained components such as carbon dioxide, molec-ular hydrogen, methane, ammonia, prussic acid, and water.
In 1922 the Russian scientist Alexander Oparin presented the interesting hypothesis that organic compounds were formed spontaneously in the early atmosphere by the input of energy (e.g., in the form of ultraviolet radia-tion (there was no protective ozone layer), electrical discharges (lightning), or volcanic heat). It was further postulated that these organic compounds accumulated in ancient seas and became the constituents of early forms of life. In 1953 the American scientists Stanley Miller and Harold Urey sub-stantiated this hypothesis by simulating the postulated prebiotic synthesis of organic substances. They exposed a gaseous mixture of components present in the early atmosphere, consisting of H2O, CH4, NH3 and H2 to electrical discharges for about a week at 80°C. Amino acids (such as gly-cine and alanine) and other carboxylic acids (such as formic, acetic, lactic, and succinic acid) were found in the condensate of this experiment. Other investigators added substances such as CO2, HCN, and formaldehyde to the gaseous mixture, and these experiments showed that many components of living cells (e.g., carbohydrates, fatty acids, tetrapyrroles, and the nucleo bases adenine, guanine, cytosine, and uracil) were formed spontaneously by exposing a postulated early atmosphere to electric or thermal energy.
It is assumed that the organic substances formed by the abiotic proc-esses accumulated in the ancient seas, lakes, and pools over a long period of time prior to the emergence of life on earth. There was no oxygen to oxi-dize the compounds that had accumulated and no bacteria or other organ-isms to degrade them. Alexander Oparin speculated that a “primordial” soup was formed in this way, providing the building material for the origin of life. Since oxygen was not yet present, the first organisms must have been anaerobes.
It is widely assumed now that early organisms on this planet generated the energy for their subsistence by chemolithotrophic metabolism, for exam-ple, by the reaction:
FeS + H2 → FeS + H2 (ΔG0’ = -42 kJ/mol)
It seems likely that already at a very early stage of evolution the catalysis of this reaction was coupled to the generation of a proton motive force across the cellular membrane, yielding the energy for the synthesis of ATP by a primitive ATP synthase.Archaebacteria, which are able to live anaerobically under extreme environmental conditions (e.g., near hot springs in the deep sea), and which are regarded as the closest relatives of the earliest organisms on earth, are able to produce ATP via the preceding reaction. It was probably a breakthrough for the propa-gation of life on earth when organisms evolved that were able to utilize the energy of the sun as a source for biomolecule synthesis, which occurred at a very early stage in evolution. The now widely distributed purple bacteria and green sulfur bacteria may be regarded as relics from an early period in the evolution of photosynthesis.
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