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Frontiers in Microbiology - II. Microbes and the Origin of Life

The history of microbes is long indeed. Their appearance on earth predates that of visible creatures by about 3 billion years. Trying to determine exactly when the first microbes arose is complicated. Traces of life from so long ago are subtle and difficult to interpret. So called biosignatures are traces of life left behind in rocks by ancient microbes. They may be microscopic features found inside the rocks or data derived from isotopic ratios. Unfortunately, other nonbiological processes can mimic biosignatures. If a particular biosignature is found in an area that had conditions favorable to life, such as a shallow sea, then that observation strengthens the case that the biosignature arose from microbes and not through a nonbiological process. This means that a good understanding of geology is essential to interpret putative ancient microbial fossils.

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One type of biosignature is called a stromatolite. Stromatolites are layered dome-shaped formations produced by ancient bacterial colonies. Some stromatolites from northwestern Australia have been dated to 3.5 billion years ago. Other evidence suggests that microbes arose even before that time. In 1996 a meteorite from Mars dated to 3.9 billion years ago was reported to contain evidence of microbes. Further analysis suggested that most of this evidence could be accounted for by other natural processes. Today, the 3.9 billion year estimate from the meteorite data has been largely discredited (Simpson, 2003).

Regardless of exactly when the first microbes arose, they evolved out of what is commonly referred to as the primordial soup. According to this view, as the new earth began to cool, organic compounds were created through energy supplied by lightening, radiation from the sun, and the Earth’s own heat. As local concentrations of these organic molecules increased, they began to polymerize. Eventually, they became autocatalytic and replicated themselves. Experimental support for this view was provided by Stanley Miller, a graduate student working in the laboratory of Harold Urey at the University of Chicago in 1953 (Miller, 1953).

In Miller’s experiment, he heated water in a flask, creating water vapor. This was his model of the primordial ocean. The top of the flask contained methane, ammonia, and hydrogen. These gases, along with the water vapor, were his atmosphere. He then subjected the gases to a continuous electric discharge (his lightning). As the gases interacted, they formed a variety of water-soluble organic compounds, including amino acids.

This ingenious, yet simple, experiment demonstrated that organic molecules could be spontaneously created under conditions thought to be similar to those of early earth. This idea was given further support in 1970 when extraterrestrial amino acids were found in a meteorite. This discovery demonstrated that chemical reactions similar to those created by Miller occurred on the meteorite parent body early in the history of the solar system.

Impacts from colliding asteroids and comets would have added some organic material to the primordial soup. Nevertheless, the soup would contain just a dilute concentration of organic molecules. How could this organic broth develop into a living cell? To create life as we know it, these small organic molecules would have to form larger polymeric molecules and acquire the ability to replicate themselves. The dilute conditions of the soup would not be thermodynamically favorable to polymerization. Scientists speculate that clays and metal cations functioned as chemical catalysts to stimulate these reactions. This scenario would be plausible if the earth’s surface had cooled and temperatures were low enough to allow weak noncovalent bonds characteristic of adsorption to form (Bada and Lazcana, 2002). This may have been the case; scientists believe that 3 billion years ago the sun burned less brightly than it does today.

Although DNA is the genetic material in living cells, RNA is a simpler molecule and was more likely the first type of self-replicating polymer to form. This notion is supported by the discovery of various types of catalytic RNAs found in cells living today. At the same time that RNA polymers were forming, other organic molecules were forming colloidal suspensions. Such suspensions can spontaneously form spherical structures called coacervates. Coacervates are small, cell-sized bodies with diameters of between 1 and 100 microns. They are bounded by an arrangement of small organic molecules that resembles the outer membrane of a cell. The first cells are thought to have arisen after a self-replicating polymer such as RNA became enclosed within a coacervate.

Within the past decade, some scientists have proposed that the first life on earth was characterized by a series of self-sustaining chemical reactions based on simple monomeric compounds derived from carbon monoxide and carbon dioxide. If true, this theory would describe a form of life unlike that which we know today. Furthermore, such organisms would be difficult, if not impossible, to study since they left behind no hereditary material to serve as an historical record. This “metabolist theory” remains mostly speculation. The chemical reactions it describes have not been shown to be autocatalytic or to take place under prebiotic conditions. The metabolist and RNA theories of life are not mutually exclusive. If the chemistry described by the metabolist theory did occur, it could have enriched the prebiotic soup in which RNA-based life was brewing.

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