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ORIGIN OF LIFE 123 Warden AJ (1971) Manganese Mineralisation in the New Hebrides New Hebrides Condominium Geological Survey New Hebrides: British Service ORIGIN OF LIFE J Bailey, Anglo-Australian Observatory and Australian Centre for Astrobiology, Sydney, Australia ß 2005, Elsevier Ltd All Rights Reserved Introduction Establishing the nature of the processes by which life originated is one of the most fundamental unsolved scientific problems Fifty years of study have enabled us to assemble many pieces of the puzzle but have also left a number of critical gaps in our understanding Even the issues of when and where life originated are still poorly constrained, but life probably appeared on Earth between about 4.2 Ga and 3.5 Ga ago There is good evidence that the current DNA–RNA–protein basis for life developed from a stage known as the ‘RNA world’ in which RNA performed the information-storage and catalytic roles currently performed by DNA and proteins Direct formation of the RNA world from prebiotic chemistry seems unlikely, so much current research focuses on possible precursors to the RNA world Various suggestions for pre-RNA worlds have been made, but none has yet proved entirely satisfactory A number of terrestrial and extraterrestrial processes can produce prebiotic organic molecules, but there are still problems in obtaining the right molecules in sufficient quantities Development of Ideas on the Origin of Life The idea that all life on Earth has a common origin became well established only in the twentieth century Early ideas on the evolution of life were most clearly expressed by Lamarck, who described the process as one of progression from simpler to more complex and advanced forms (see Evolution) The observation that both simple and complex organisms are currently present therefore required that simple organisms are appearing even today from inorganic matter by a process of ‘spontaneous generation’ (a concept that has its origins in antiquity) Thus, the ideas of transformism (evolution) and spontaneous generation were closely linked in the early nineteenth century and put Lamarck and his followers in conflict with the religious and scientific establishment, who argued for the immutability of species, which were divinely created in their current forms Lamarck’s ideas were criticized by scientists such as Cuvier (see Famous Geologists: Cuvier) (in a celebrated debate with Geoffroy Saint-Hilaire in 1830) and later by Pasteur, who carried out his famous ‘swan-necked flask’ experiments to discredit the idea of spontaneous generation Darwin’s theory of evolution by natural selection opened the way for the modern view of the origin of life (see Famous Geologists: Darwin) In Darwin’s theory there is not necessarily a ladder of progress from simple to more complex forms Simple organisms can be as evolutionarily successful as complex ones This allowed the idea that all life, simple and complex, had a single origin in the distant past While the subject of the origin of life is hardly mentioned in Darwin’s published writings, the following quote (from a letter to his botanist friend Joseph Hooker) gives us an inkling of his thoughts on the subject at that time: If (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phos phoric salts, light, heat, electricity present, that a protein compound was chemically formed, ready to undergo still more complex changes Oparin and Haldane in the early twentieth century developed the idea that chemical reactions on the early Earth could have led to the production of a range of organic compounds, forming a ‘primordial soup’ in which the required building blocks for life would have been present But it was not until 1953 that these ideas received experimental support Stanley Miller, then a graduate student working in the laboratory of Harold Urey, set up his famous experiment in which electrical discharges were passed through a mixture of gases (methane, ammonia, hydrogen, and water vapour) simulating a thunderstorm on the primitive Earth The experiment produced a mixture of several amino acids, the building blocks of proteins Miller speculated that this was how organic compounds had been made on the early Earth In the same year, Crick and Watson published their structure for DNA, the first step in elucidating the fundamental molecular basis of life These two discoveries meant that we had both a plausible way of generating simple organic building blocks and an understanding of the macromolecules on which
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