Origin of Living Systems
Origin of Living
Systems
The fossil record reveals that life existed by 3.8 billion years ago; therefore, the origin of the earliest life form can be estimated at approximately 4 billion years BP. The first living organisms were protocells, autonomous membrane-bound units with a complex functional organization that permitted the essential activity of selfreproduction. The primitive chemical systems that we have described lack this essential property. The principal problem in understanding the origin of life is explaining how primitive chemical systems could have become organized into living, autonomous, self-reproducing cells.
As we have seen, a lengthy chemical evolution on the primitive earth produced several molecular components of living forms. In a later stage of evolution, nucleic acids (DNA and RNA) began to behave as simple genetic systems that directed the synthesis of proteins, especially enzymes. However, this conclusion has led to a troublesome chicken-egg paradox: (1) How could nucleic acids have appeared without enzymes to synthesize them? (2) How could enzymes have evolved without nucleic acids to direct their synthesis? These questions are based on a long-accepted dogma that only proteins could act as enzymes. Startling evidence presented in the 1980s indicates that RNA in some instances has catalytic activity.
Catalytic RNA (ribozymes) can mediate processing of messenger RNA , and can catalyze formation of peptide bonds. Strong evidence suggests that translation of mRNA by ribosomes is catalyzed by their RNA, not protein, content.
Therefore the earliest enzymes could have been RNA, and the earliest self-replicating molecules could have been RNA. Investigators are now calling this stage the “RNA world.” Nonetheless, proteins have several important advantages over RNA as catalysts, and DNA is a more stable carrier of genetic information than RNA. The first protocells containing protein enzymes and DNA should have had a selective advantage over those with only RNA.
Once this stage of organization was reached, natural selection would have acted on these primitive self-replicating systems. This point was critical. Before this stage, biogenesis was shaped by the favorable environmental conditions on the primitive earth and by the nature of the reacting elements themselves. When selfreplicating systems became responsive to the forces of natural selection, they began to evolve. The more rapidly replicating and more successful systems were favored, and they replicated even faster. In short, the most efficient forms survived. Evolution of the genetic code and fully directed protein synthesis followed. The system now meets the requirements for being the common ancestor of all living organisms.
The fossil record reveals that life existed by 3.8 billion years ago; therefore, the origin of the earliest life form can be estimated at approximately 4 billion years BP. The first living organisms were protocells, autonomous membrane-bound units with a complex functional organization that permitted the essential activity of selfreproduction. The primitive chemical systems that we have described lack this essential property. The principal problem in understanding the origin of life is explaining how primitive chemical systems could have become organized into living, autonomous, self-reproducing cells.
As we have seen, a lengthy chemical evolution on the primitive earth produced several molecular components of living forms. In a later stage of evolution, nucleic acids (DNA and RNA) began to behave as simple genetic systems that directed the synthesis of proteins, especially enzymes. However, this conclusion has led to a troublesome chicken-egg paradox: (1) How could nucleic acids have appeared without enzymes to synthesize them? (2) How could enzymes have evolved without nucleic acids to direct their synthesis? These questions are based on a long-accepted dogma that only proteins could act as enzymes. Startling evidence presented in the 1980s indicates that RNA in some instances has catalytic activity.
Catalytic RNA (ribozymes) can mediate processing of messenger RNA , and can catalyze formation of peptide bonds. Strong evidence suggests that translation of mRNA by ribosomes is catalyzed by their RNA, not protein, content.
Therefore the earliest enzymes could have been RNA, and the earliest self-replicating molecules could have been RNA. Investigators are now calling this stage the “RNA world.” Nonetheless, proteins have several important advantages over RNA as catalysts, and DNA is a more stable carrier of genetic information than RNA. The first protocells containing protein enzymes and DNA should have had a selective advantage over those with only RNA.
Once this stage of organization was reached, natural selection would have acted on these primitive self-replicating systems. This point was critical. Before this stage, biogenesis was shaped by the favorable environmental conditions on the primitive earth and by the nature of the reacting elements themselves. When selfreplicating systems became responsive to the forces of natural selection, they began to evolve. The more rapidly replicating and more successful systems were favored, and they replicated even faster. In short, the most efficient forms survived. Evolution of the genetic code and fully directed protein synthesis followed. The system now meets the requirements for being the common ancestor of all living organisms.
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