Clay and the Origins of Life


Clay and the Origins of Life
Liz Vrolyk,1, July 2000

Introduction and Context

To discuss the origins of life, one must first decide on a definition of life itself. Many scientists have adopted NASA's definition; an organism is considered to be alive if it is a "self-sustained chemical system capable of undergoing Darwinian evolution," implying that it must have what we will call genetic material and it must be possible for the genetic material to replicate and mutate.2 Currently, many chemists, biologists, physicists, astronomers, and geologists who are researching the origins of life believe that RNA molecules were capable of performing both enzymatic activity and self-replication, defining what is known as the "RNA world" scenario.

Today, proteins are built through the use of both transfer and messenger RNAs; these proteins then can act as enzymes to catalyze a variety of chemical reactions, including the formation of nucleotides. According to the supporters of the RNA world concept, there would be no protein enzymes in existence to form the first nucleotides or catalyze the first formation of RNA strands because these proteins are only formed by RNA (the chicken and the egg problem). Therefore, the phosphate, sugar, and base groups must have bonded through abiotic (non biological) means. Thus far, scientists have been unable to bind the phosphate to the sugar and base group using conditions that were thought to have been present on the early earth; this is a very active, current field of research.3 However, once a full nucleotide unit has been formed, scientists have been able to make extended chains of RNA, of up to 50 units (mers) in length, which could then fold to perform catalytic activity, accelerating the ease of synthesis for future molecules and initiating the RNA world.4

The Structure and Importance of Clay

If free nucleotides are combined in solution, they do not react at all. Therefore, many scientists have been searching for what types of activating groups and inorganic catalysts must have been involved in the polymer bonding process. Dr. Ferris, of Rensselaer Polytechnic Institute in Troy, New York, has discovered one inorganic material which facilitates this reaction: montmorillonite clay. The particular structure of this clay serves to provide a medium in which the individual activated RNA units combine to form larger chains.5

Most of what we call "dirt" is a mixture of decomposed organic material and inorganic material; leaves, bark, and insects with rocks interspersed, for example. Clay forms in a dramatically different manner, however. When rocks, specifically feldspar, weather by chemical and physical means, they dissolve into their various elements and compounds. These molecules often form into mainly organized layers as they settle based on charge and weight and other considerations. This is basically how the clays form their layering and crystal structure which is based on the charges of the individual molecules.

Montmorillonite Clay

In the case of montmorillonite clay, the aluminum (Al+3) and silicon (Si+4) which have dissolved out of volcanic rock along with oxygen and hydrogen form a three layer crystal structure. Composed of a silica layer, an alumina layer, and another silica layer, these three-layer crystals have a negative charge on their faces. Many of these layers are stacked on top of one another to form larger crystals. Water and various positively charged ions associate between the layers. While the individual three layer crystals are very rigid, the whole multi-layer structure can be quite fluid, depending on the size and strength of the molecules sandwiched in between the crystal layers. The larger the ion in the inner-layer, the larger the crystal becomes. The surface between the layers is negatively charged, so it attracts a large concentration of calcium (Ca+2), sodium (Na+), magnesium (Mg+2), copper (Cu+2), iron (Fe+2) and other positively charged ions. While these ions are in the fluid layer between the crystal layers, they are not specifically bonded to the crystal; they form a positively charged layer between the negatively charged surfaces of the crystals.

The phosphate groups of individual nucleotides, the polymers of which make up long strands of RNA, are negatively charged. Theoretically, they are attracted to the positive pool in the fluid layer of the montmorillonite clay. Entering into the layer, they repel the negative charges on the surfaces of the aluminate layers, naturally positioning themselves in the middle of the positive layer and splitting the distance between the two negative surfaces. So, the negative aluminate layer is loosely bonded to a positive ion which is associated with nucleotide units.

The importance of this set up is reflected in the reactions which then occur between the nucleotides. The positively charged ion pool allows the nucleotides to be held in a such a position with such a strength that they have the opportunity to form bonds between the monomers. Then, these two bonded nucleotides have an attraction to the positive layer which is about twice as strong as the attachment strength of just one unit alone. As more and more nucleotides bond together, this linear relationship continues. Therefore, because the longer strands are held to the clay more firmly than are the individual nucleotides or shorter strands, it is possible that two longer strands would have a higher probability of bonding with each other because of increased contact.

Another aspect of the montmorillonite clay which favors synthesis of long strands of RNA is the huge amount of surface area for this surface chemistry to happen upon. Because all of the inner layers as well as the outer surfaces are available for hydration, the dry ground-up clay swells to fifteen times its volume when exposed to water.6

Conclusion and Outlook

Montmorillonite clay is so far the only mineral found to catalyze the synthesis of RNA polymers (with at least 10 nucleotides) from their individual units. Research is currently progressing on how the phosphate group initially bonds with the ribose-base and if there are any other alternate minerals or clays which serve the same function.
1Assisted by Billie Jean Marks and Dr. James P. Ferris, Rensselaer Polytechnic Institute and the New York Center for Studies on the Origins of Life
2Geoffrey Zubay. Origins of Life on the Earth and in the Cosmos. (San Francisco, Academic Press, 2000) 168.
3William J. Hagan. "Priming the Pre-RNA World." Presentation July 27, 2000 at the New York Center for Studies on the Origins of Life.
4Dr. James P. Ferris. "Montmorillonite Catalysis of RNA Oligomer Formation in Aqueous Solution. A Model for the Prebiotic Formation of RNA." Journal of the American Chemical Society. 1993, 155, 12270-12275.
5Kristen Rybij. The "RNA World" and the Polymerization of RNA Using Clay and Mineral Catalysis, Presentation July 13, 2000.
6Odom, Dr. I. E. "Sodium Montmorillonite and Calcium Montmorillonite: Properties and Uses." Technical Data Sheet IC-1001 28 Nov.90W. American Colloid Company Industrial Chemical Division.

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Center for Studies of Origins of Life, Rensselaer Polytechnic Inst., Troy, NY 12180
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