20: The Origins of Life
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- 20.1: Introduction
- It is nearly universally accepted that there was a time, however brief or long, when the earth was a lifeless planet. Given that the cell is the basic unit of life, and that to be alive is to possess all of the properties of life, any cell biology textbook would be remiss without addressing the questions of when and how the first cells appeared on our planet. Abiogenesis is the origin of life from non-living matter. Of course describing abiogenesis is no longer possible by observation!
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- 20.2: Thinking about Life's Origins- A Short Summary of a Long History
- By all accounts, the earth must have been a very unpleasant place soon after its formation! For that reason, the period from 4.8 to 4.0 billion years ago is called the Hadean Eon, after Hades, the hell of the ancient Greeks! Until recently, geological, geochemical and fossil evidence suggested that life arose between 3.8 and 4.1 billion years ago.
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- 20.3: Formation of Organic Molecules in an Earthly Reducing Atmosphere
- A prerequisite to the prebiotic chemical experimentation is a source of organic molecules. Just as life requires energy (to do anything and everything!), converting inorganic molecules into organic molecules requires an input of free energy. As we have seen, most living things today get free energy by oxidizing nutrients or directly from the sun by photosynthesis. Recall that in fact all the chemical energy sustaining life today ultimately comes from the sun.
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- 20.4: Origins of Organic Molecules in a Non-Reducing Atmosphere
- A prebiotic non-reducing atmosphere is based on several assumptions: The early earth would have had insufficient gravity to hold H2 and other light gasses; thus “outgassing” would have resulted in a loss of H2 and other reducing agents from the atmosphere. Geological evidence suggests that the earth’s oceans and crust formed early in the Hadean Eon, just a few hundred million years after formation of the planet.
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- 20.5: Origins of Life Chemistries in an RNA World
- In the tidal pool scenario, with its feel of ‘best-fit’ with origins of life in a reducing environment, the energy for polymer formation from organic monomers came from an overheated earth environment. In that scenario, we considered the possibility that chains of nucleotides might have been synthesized, and then even replicated to form populations of nucleic acids.
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- 20.6: Molecules Talk- Selecting Molecular Communication and Complexity
- In terms of prebiotic chemical evolution, selection by definition would have favored protective accumulation of longer-lived molecular aggregates. Over time, the same selective imperatives would create webs of such aggregates, increasing the range and specificity of molecular interactions in a challenging environment.
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- 20.7: A Summary and Some Conclusions
- Our consideration of how life began on earth was intentionally placed at the end of this textbook, after we tried to get a handle on how cells work. Clearly any understanding of life origins scenarios is very much a matter of informed, if divergent speculations. Alternative notions for the origins of life entertained here all address events that presaged life under ‘best-guess’ hypothetical conditions.
Thumbnail: 3D structure of a hammerhead ribozyme. (CC BY-SA 3.0; William G. Scott via Wikipedia )