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PRECAMBRIAN

4.6 billion to 3.5 billion years ago

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It is not known how life originated--there are no time machines and fossils of pre-cells either do not exist or are difficult to interpret. Even if life is generated someday in a lab through a series of chemical reactions, there would be no proof that the first living things evolved through the same sequence of chemical reactions. As a result, science will never be able to prove how life started. Scientists can, however, study what scenarios are possible given the conditions of the early earth and determine if the characteristics of modern cells offer support for any of the models. The question boils down to this: over the course of a few hundred million years, could the sum of all the chemical reactions which occurred on the early earth (in its oceans, its continually flooded tidal zones, its subsurface, its volcanic vents, and even in the material which was bombarding it from space) produce complex aggregates of molecules which achieve the level of complexity of the most minimal forms of life? Obviously, answering this question is complicated by the facts that scientists are just beginning to appreciate the wide array of organic molecules which can be produced in the absence of life, the conditions of the early earth, the contribution to the chemistry of the early earth made by molecules found in comets and meteors, etc. Although it is easy enough to study the simplest cells alive today, these cells are more than 3.5 billion years removed from the first cells and they should not be considered as models for the simplest living things.

 

Although science can not at present answer the question of whether life evolved (and may never be able to do so), there are simpler questions which can be asked and possibly answered.

1) Is it possible that organic molecules (those complex molecules of living things) arose from simple inorganic molecules in the absence of life? Yes.


Organic molecules were once thought to exist only in living things and to have possess an "animism" or "vitalism" which could not arise without life (Joyce, 1998). Vitalists once argued that organic molecules could never be generated in a lab or, for that matter, anywhere outside a living organism. They were proven wrong. Organic molecules can be formed without life in labs and organic molecules have been detected in meteorites, comets, and several bodies of our solar system (such as the Jupiter moons Callisto and Ganymede). In other words, the organic molecules which are the "the stuff of life" can be found without life.
Experiments have demonstrated that the simple organic molecules which life depends on can be synthesized using only the gases of the primitive earth's atmosphere and a source of energy. There would have been plenty of energy in a primitive earth-the heat of a semimolten planet, lightning, solar radiation unfiltered by an ozone layer, etc. By simply mixing inorganic molecules and energy, the following organic molecules have been produced: all the amino acids found in living things (in addition to amino acids not found in living things), all essential sugars, triphosphate nucleotide precursors needed for the synthesis of the DNA and RNA, aldehydes, carboxylic acids, and others. These organic molecules synthesized in the absence of life could incorporate a large percentage of the available carbon.

2) Could these small organic building blocks have joined to form larger biomolecules in the absence of life? Yes.

In living organisms, small organic molecules (monomers) can not only function alone, they can bind to each other to form long chains (polymers). Can monomers join to form polymers in the absence of life? Yes. Not only can this occur in solution, there are a number of catalysts which can speed these reactions. Certain mineral surfaces (feldspar, calcite, zeolites, clays) provide sites where small organic molecules can fuse to form larger ones. The small molecules (RNA nucleotides, amino acids) absorb onto these surfaces and, since they are in close enough proximity to each other and in the right orientation, they can bond to form chains. Small proteins of over 200 amino acids have been produced and short strands of DNA and RNA (up to 50 nucleotides long). Because minerals help to catalyze polymerization reactions, the rocks of early earth could have been covered with chains of at least tens of monomer units (Joyce, 1998).

3) Can molecules replicate themselves in the absence of life? Yes, to some degree.


Short RNA and DNA molecules can serve as templates and replicate themselves. One RNA molecule has actually shown itself not only to be a template of its own replication, but a catalyst of the replication of other RNA molecules as well (Green, 1992; Doudna, 1991). In 1996, a small protein (based on a protein found in yeast) was observed to replicate itself (Lee, 1996; Kauffman, 1996). Self-replicating peptides based on coiled motifs have been observed (Ghosh, 2004). Amines and esters can combine to form an amide which then serves as a template for other amines and esters to do the same. There are a number of organic molecules which are not found in living things which have been shown to replicate (especially vinyl homopolymers and copolymers) (Orgel, 1992; Rebek, 1994). Yeast, fungi, and mammals possess unrelated prion proteins which can generate fibers under certain conditions (Chernoff, 2004).
Some experiments have tried to simulate "natural selection" acting on self-replicating molecules. Molecules continue to replicate themselves but mutations occur as the rounds of replication continue. Not only can the replication continue, apparently indefinitely in the right conditions, mutations arise which allow the molecules to replicate faster (so that they have an advantage over the original molecules used). Although no molecule has yet been generated which can replicate itself well enough to serve as a model for the first genetic molecule, the search for self-replicating molecules is a relatively recent endeavor.

4) Could organic molecules form membranous balls? Could these precells perform some activities that cells perform? Yes.


Living cells are surrounded by lipid cell membranes. Although lipids can form in the absence of life, could such lipids spontaneously form cell membrane-like structures? Yes. Lipids in solution form layers that are similar in structure and function to those of cell membranes. In solution, lipids can form spheres known as micelles, coarcervates, and microspheres. Simulations of the formation of organic molecules in the interstellar ices of comets (using UV light) form organic molecules that self-assemble into such vesicles. These vesicles can accumulate organic molecules inside themselves, increase in size, and even split once they reach a certain size. If enzymes (proteins which speed chemical reactions) are in these droplets, chemical reactions can occur. If they contain the enzyme RNA polymerase, RNA nucleotides are taken from the environment and assembled into RNA chains (Zimmer, 1995). Organic molecules gathered from meteorites have been found to form these membranous balls in water.