Some of the steps leading to the synthesis of DNA and RNA can be duplicated in the laboratory; others cannot. We have no problem creating amino acids, life's most basic building block. As was first demonstrated by University of Chicago chemists Stanley Miller and Harold Urey in a famous experiment conducted in 1952, researchers can even produce chains of amino acids under laboratory conditions. In a scene reminiscent of some Frankenstein movie, they created the building blocks of life for the first time—in a test tube. But it has turned out that the challenge of making amino acids in the lab is trivial compared to the far more difficult proposition of creating DNA artificially. The problem is that complex molecules such as DNA (and RNA) cannot simply be assembled in a glass jar by combining various chemicals. Such organic molecules also tend to break down when heated, which suggests that their first formation must have taken place in an environment with moderate, rather than hot, temperatures. How then might these elusive but necessary components of life have arisen on the young Earth?
One scenario that may have led to the formation of DNA was beautifully described by Nobel laureate Christian de Duve in his 1995 book Vital Dust. De Duve notes that amino acids either would have been brought down to the surface of the young Earth by comets and asteroids from space or would have been created on the planet's surface by chemical reactions. De Duve paints the following picture of our planet more than 4 billion years ago.
Brought down by rainfall and by comets and meteorites, the products of these chemical re-shufflings progressively formed an organic blanket around the lifeless surface of our newly condensed planet. Everything became coated with a carbon-rich film, openly exposed to the impacts of falling celestial bodies, the shocks of earthquakes, the fumes and fires of volcanic eruptions, the vagaries of climate, and daily baths of ultraviolet radiation. Rivers and streams carried these materials down to the sea where the materials accumulated until the primitive oceans reached the consistency of hot dilute soup, to quote a famous line from the British geneticist J.B.S. Haldane. In rapidly evaporating inland lakes and lagoons, the soup thickened to a rich puree. In some areas, it seeped into the inner depths of the Earth, violently gushing back as steamy geysers and boiling underwater jets. All these exposures and churning induced many chemical modifications and interactions among the original components showered from the skies.
De Duve maintains a long-held belief that the progression from abiotic to biotic was as follows: Amino acids formed in space and on Earth; these next combined to form primitive proteins, which then somehow united to form early life. The crucial step is the formation of proteins, themselves composed of amino acids joined together by chemical bonding. Why? Because formation of the critical building blocks, the nucleic acids, would require enzymes to catalyze the necessary chemical reactions. Most chemical reactions are reversible; sodium and chlorine, for example, combine to form salt under certain conditions and tear apart (or dissolve) under others. Enzymes mediate chemical reactions, which are necessary to join many complex protein pieces together into larger units such as amino acids, and all biological enzymes are proteins.
The need to have proteins already present in order to assemble the molecules whose job it is to assemble proteins in the first place has seemed an in tractable "chicken and egg" problem. But recently an elegant solution to this apparent paradox has been proposed. What if one of the nucleic acids—in this case, RNA—could act both as the factory building proteins and as the catalyst necessary to favor the important chemical reactions? According to this new model, the early pathway to life may have seen the formation of RNA prior to the formation of protein. In this view, RNA itself acted as the enzymatic catalyst necessary for any further progression toward the ultimate and quintessential component of life, DNA. Francis Crick first suggested this in 1957. Information flows only from the nucleic acids to proteins, he noted, never in the opposite direction, and thus the nucleic acids had to precede protein formation. This point of view was confirmed by the Nobel Prize-winning discovery of Thomas Cech and Sidney Altman that RNA can indeed act as the enzyme necessary for catalytic activity. These RNA enzymes, which were named ribozymes, led to the concept of the "RNA world," where RNA molecules on the early Earth carried out the steps necessary to produce the building blocks of true life, preceding the formation of the first true DNA.
Once RNA has been synthesized, the path toward life is open because RNA can eventually produce DNA. Thus, how the first RNA came into existence— under what conditions, and in what environments—became the central problem confronting chemists. As de Duve notes, "We must now face the chemical problems raised by the abiotic synthesis of an RNA molecule. These problems are far from trivial." The abiotic synthesis of RNA remains the most enigmatic step in the evolution of the first life, for no one has yet succeeded in creating RNA.
Once RNA was created, the leap from RNA to DNA would have been more straightforward. RNA serves as a template for DNA. Yet many mysteries remain: Did it happen once or many times? Was this most vital ingredient of life created over and over and each time snuffed out by another gigantic meteor impact? Or did this essential breakthrough happen just once on Earth and then spread across the planet with its infectious, replicating behavior?
This model of life's origin—from macromolecules to RNA to an "RNA world" to DNA—has not gone unchallenged. Another possibility is that the cradle of life was clay or pyrite crystals. The faces of these flat minerals and crystals could have presented microscopic regions where early organic molecules accumulated. This model suggests the following progression: from clay (mineral) crystals to crystal growth, followed by an "organic takeover" (where purely inorganic molecules are replaced by carbon-based molecules), allowing the formation of organic macromolecules that in turn led to DNA and cells. As envisioned by R. Cairnes, the earliest life would have had several characteristics: It could evolve; it was "low-tech," with few genes (sites on the DNA molecule that code for the formation of specific proteins) and little specialization; and it was made of geochemicals, arising from condensation reactions on solid surfaces, from either pyrite or iron sulfide membranes.
Both of these ideas about the first development of life have at their crux the need to bring various chemical components together somehow and then, from these aggregates, assemble very complex molecules. In the RNA model, the various chemicals assemble in liquid; in the second model, a mineral template becomes an assembly site. There is as yet no consensus on which of these alternatives is correct—or even on whether they are the only alternatives.
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