Molecular Biology

To what Extent does the Rna World Hypothesis Stands True



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Metabolism or Replicators?

There has been a debate over the origin of life that has been widely ignored in the mainstream of science. Today’s accepted theory is that of the ‘RNA world’ and it lacks the answers to many critical questions. The RNA theory comes in two forms. The first is the theory that life started with the ribozyme, which was the first self-replicating RNA molecule. From this, all earth life eventually evolved. The other RNA world theory says that RNA came about before DNA and proteins but wasn’t necessarily the replicating molecule involved in the origin of life. These theories usually have an ancient RNA like molecule that was a replicator instead. Any RNA world theory has the same problems that make the theory hard to save. Any big molecule like RNA takes a vast number of smaller molecules to bond in exactly the right way. There would be too many short connections that would terminate instead of grow, bad connections, defective units, etc. Any of these things could happen, and would ruin the replication of the molecule. Also, there are thousands of likely nucleotide arrangements besides the ones we actually see and millions of stable organic molecules that aren’t nucleotides. In Robert Shapiro’s words, “An indifferent nature would theoretically combine units at random, producing an immense variety of short, terminated chains, rather than the much longer one of uniform backbone geometry needed to support replicator and catalytic functions. The probability of this latter process succeeding is so vanishingly small that its happening even once anywhere in the visible universe would count as a piece of good luck.” (Shapiro 131). Fortunately, we need not rely on theories that assume the production of a large, complicated molecule. These theories are called metabolism first theories, and deal only with small molecules that were abundant at the time of life’s forming. I think that these theories offer a simpler, more likely, and therefore, a better solution to the mystery of the origin of life.

These theories all have a few things on common. There are 5 common requirements for a metabolism first theory of life to work, as outlined in Shapiro’s article. He talks about the need for a boundary, an energy source, coupling or linkage, the formation of a chemical network, and reproduction. If all these things can happen, it is likely possible to form metabolic pre-life without any big complex molecule, these molecules: RNA, DNA, and proteins form later under Darwinian evolution, according to this theory. Quickly, I’ll explain these 5 criteria, and by doing so, the basic idea of how a theory like this would work.

First, a boundary is needed. This is anything that would separate an area from the outside environment. If a boundary exists, than the entropy of that bound area can decrease while the entropy outside of that boundary is increasing even more. If this happens, then the second law of thermodynamics is still obeyed. Many options exist for natural boundaries such as iron sulfide membranes, rock surfaces, or small ponds. Also, as Shapiro points out, “in support of this idea, David W. Dreamer, of the University of California, Santa Cruz, has observed membrane like structures in meteorites.” (Shapiro 132).

Second, there must be an energy source present. Obviously, if the chemicals in the boundary have no way of interacting with the outside environment, metabolism can’t exist. If there are substances surrounding the network, it can use these for redox reactions, which all life lives on.

Third, coupling is necessary for life to form this way. Coupling is achieved when a reaction happens that produces energy, and the energy is used to make something else happen. In this context it is assumed that reactions with the same intermediate step and primitive catalysts would suffice.

Fourth, a chemical network must form. For this to happen, a chain of chemical reactions that work in a circle and use outside molecules or catalysts has to form. This is something like A converts to B converts to C converts to D which converts back to A. If a simple circular chemical reaction can happen, then the system can start to evolve. Side reactions can form more similar circular chains, more A can be produced, the cycle can get more complex, and it can start to develop its own, more efficient catalysts.

Finally, the fifth condition is the network must grow and reproduce. Growing means to gain material faster than it loses it. There are many ways a network can fail this condition, but it isn’t absurd to think it could succeed as well. Assuming there is enough chemical material around to keep replenishing the network, and the reaction doesn’t produce anything that leaves the network, it can grow efficiently. As for reproduction, this can happen when a system grows outside of its compartment and spills into another compartment, making two independently functioning compartments or more. Once this occurs, the independent sources can undergo Darwinian evolution by competing for different resources, letting only the strongest and most efficient survive.

Kauffman talks about this view in his article, “‘What is life?’ was Schrodinger right?” He has a view similar to Shapiro where he gives a detailed account of how these systems come into being. He says that it makes more biological sense for autocatalytic systems to arise prior to RNA or other large molecules, and that life can function as an autocatalytic system, without a real need for DNA. His idea is that every molecule can catalyze some other reaction, so if there is a high enough concentration of random molecules in some closed off area; they can start to randomly catalyze each other’s formation. In this way, the system can turn into an autocatalytic system where, like in Shapiro’s 4th condition, a chain of molecules forms that catalyze each others own formations, therefore starting the conditions that will lead to life. Kauffman is unique in this approach because he has modeled what this might actually look like on a computer, and shown that theoretically, given enough random molecules in a space, autocatalysis can randomly start to happen. This gives more evidence to the validity of a metabolism first origin of life hypothesis, because in Kauffman’s models, no DNA or replicator is needed.

In light of the evidence against the RNA world, the metabolism first hypothesis seems much more likely. Because there can be simulations on a computer of such a thing actually working, this is a scenario that should be looked at more and given some thought. The majority of science today believes in some kind of RNA world origin of life hypothesis, but this hypothesis obviously runs into trouble when it becomes apparent that any replicator just randomly forming is extremely unlikely. If we have come up with a theory that makes more sense statistically, even if it is less pretty, we should give it a chance, and see if this could really be the origin of life on earth. If we end up discovering that it is closer to the truth, we will have taken a gigantic leap forward to discovering what is crucial to life as we know it, and therefore we will be closer to obtaining a well educated answer on what life is. 


Sources:

1."At Home in the Universe. The Search for Laws of Self-Organization and Complexity."
by Stuart Kauffman. Viking. 1995

2. Small Molecule Interactions were Central to the Origin of Life. Robert Shapiro

Department of Chemistry, New York University New York, New York 10003–6688 USA

3. A Universal Definition of Life: Autonomy and Open-Ended Evolution

Ruiz-Mirazo, Kepa; Peretó, Juli; Moreno, Alvaro

Origins of Life and Evolution of the Biosphere, v. 34, Issue 3, p. 323-346 (2004).

4. Cleland. 2010. Class notes and her new book were used for the content of this paper.

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