Astrobiology - The "Metabolism First" Model of the Origin of Life and Other Hypotheses
The third article in the series explores the “metabolism first" model and other origin of life theories, some being slight modifications of the two main models discussed in the series.
Metabolism before Nucleic Acids
Some scientists believe that the steps to create a self-replication molecule, as posited by the “RNA world" hypothesis would be too complex and impossible to sustain. They also argue that before such an orderly process as self-replication would arise, it would be more likely that chemical reactions involving the abundant inorganic compounds present on primitive Earth would have predominated.
At the end of the 1980s, Günther Wächtershäuser, a German biochemist, proposed the “metabolism first" hypothesis. He observed that iron pyrite (also known as iron disulfide), or fool’s gold, is able to catalyze biochemical reactions and the presence of iron pyrite on the early Earth would have quickly led to a proliferation of chemical reactions that take place in living things today. Indeed, many enzymes present in modern cells contain iron pyrite clusters within their structure, attesting to the importance of fool’s gold. The “metabolism first" hypothesis essentially posits that the precursor to life was not a nucleic acid-based chain that replicated itself, but a structure that metabolized inorganic compounds on the surface of iron pyrite, most likely around small deep-sea hydrothermal vents.
Of Hydrothermal Vents and Fatty Acids
The proponents of the “metabolism first" model were the first to propose that life originated around deep-sea hydrothermal vents. Since these structures are located well below the surface of the water, any nascent life would be protected from the damaging UV rays of the sun; at this point in earth’s history, there was no ozone layer to protect life. Also, while the intense bombardments experienced by Earth during its early process of formation had subsided somewhat, bodies from space would still hit the planet, obliterating anything on the surface; life located deep in the sea would be protected from these bombardments.
Furthermore, while all scientists agree that water is extremely necessary for the origin of life, there has been the objection that life originating in the oceans would seem highly unlikely because the concentrations of the building blocks necessary to build the molecules would be far too dilute. Concentrating everything near hydrothermal vents would make chemical collisions far more likely. Finally, hydrothermal vents release copious amounts of chemical substances and dissolved gases and facilitate thermal cycling; these conditions provide for very convenient sources of energy. Interestingly, DNA research attempting to shed light on the most recent common ancestor of all living things seems to point to a microorganism that lived in an aquatic environment, at extremely high temperatures. This organism would have been perfectly adapted to a hydrothermal vent habitat.
Again, however, the issue of the first cell needs to be tackled. Life today can indeed be characterized by a specific set of biochemical reactions; but these chemical reactions need to be separated from the environment surrounding them or they would be of no use to an organism. Some scientists argue that the proteinoid proto-cell is not very similar to a modern cell membrane, composed primarily of lipids or fats. What these scientists claim is that primitive fatty acid molecules would have been present on Earth, and very well could have been present in the vicinity of hydrothermal vents. Fatty acids, when in solution, spontaneously form “bubbles", called micelles. These could have very well acted as primitive membranes, enclosing the first primitive biochemical reactions.
There are a number of other hypotheses, many slight modifications of either the “gene first" or the “metabolism first" hypotheses. Some hinge on details regarding either how replication of a molecule like a nucleic acid started, while others hinge on where life started exactly. An example of the former is the “clay hypothesis", put forward by A. Graham Cairns-Smith in the mid 1980s, who was troubled by the absence of “scaffolding" for the early replication of molecules. He proposed that the first organic molecules needed some kind of platform, or template, to aid their replication; the platform was clay, or silicate crystals in solution. Scientists have researched the idea of crystals as templates and found that there may be some basis to the idea. As an example of the latter, there is the “deep biosphere" model of Thomas Gold, proposed in the 1970s, which states that life may have originated several kilometers below Earth’s surface and could have taken advantage of reactions between minerals and gases.
Finally, there are some scientists who subscribe to the idea that life, or at least organic molecules, arose somewhere in space and found their way to Earth by some means, usually an asteroid or a comet. The theories of panspermia and exogenesis are found in this category. Organic material is actually relatively common in outer space, having been found on rocks originating from Mars and on comets. Researchers have taken water, ammonia, carbon monoxide and methanol, have frozen this mixture and then have exposed it to UV radiation; these conditions would be similar to those found in extraterrestrial environments. Organic molecules were created and these molecules, when hydrated, formed bubbles. The simple fact that organic molecules (the basic building blocks of life as we know it, such as proteins, sugars and fatty acids) is found in outer space, though, does not provide the definitive answer to the question as to exactly how these organic molecules arose in the first place.
At this point, a number of scientists allow for the possibility that perhaps more than one of these “models" is true. In other words, perhaps life arose through a combination of mechanisms, or perhaps life “tried" to get started along different pathways. We may never know which one was ultimately the successful pathway that led to life as we know it. The important thing is that mechanisms are being discovered in which the basic building blocks of life are created and linked together by physical and chemical means, by processes that very well could have operated on early Earth. To wrap up the series, we’ll look at what these origin of life theories mean for the search for extraterrestrial life and what kind of clues could indicate that life may have been or may be present on other planets.
Life on Earth (Audesirk et al., 2006)
New Views on an Old Planet (Van Andel, 2000)
The Changing Earth: Exploring Geology and Evolution (Monroe and Wicander, 1997)
Seven Clues to the Origin of Life: A Scientific Detective Story (Cairns-Smith, 2008)
Images courtesy of Wikipedia
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