Monday, July 23, 2007

Glimpsing the Origin of life

A look at an interesting theory

Theories regarding the ancient elemental substances that gave rise to the origin of life as we know it remain clouded in the shrouded mystery of time. Even the infinite evolutionary permutations and adaptation of life itself have, as one effect of success, brushed away many of the “footprints” that lead back to this far off ancient epoch.

Even so, here and there, scattered throughout the interactive biochemical and physiological reactions of the myriads of structural and functional elements, there are hints and glimpses of just how present life may have once been like. In fact, some of these clues may provide the beginnings of a general theory, of not just evolutionary development, but of the foundation of possible mechanisms that allowed for life to become life.

Realizing that the quintessential core information source of present life; DNA and its associated complex protein creations are far too complex and co-dependent to have provided the first building blocks for life, scientists have delved deeper into the biochemical cauldron for simpler structures that may have opened the doors to life.

These structures or molecular compounds would need to fulfill a primal requirement in order to be considered a viable precursor. It would have to be able to self replicate itself- thread a line of information through time- in some sustainable fashion. The implications here are huge. This simple, yet possibly improbable event would open the gateway to the process of evolutionary dynamics if enough time could pass.

Obviously, one primary candidate which may fulfill this primal role is the RNA molecule. This compound has a variety of blended functions that are DNA and functional protein “like” realizing replicating reactions, information transfer roles, and protein synthesis functions that opens the doors to theories that posit RNA as that primal replicator.

Although an original replicator theory is a very attractive possibility, it does present challenges among which is how such a relatively large molecule could have formed before evolutionary processes began as well as managing to survive in a rich organic primordial soup without being smothered amongst a plethora of chemical reaction. Research continues down this theoretical path.

However, there are other theories that are looking deeper beyond the RNA molecule for a yet simpler and possibly more plausible mechanism. These theories probe the molecular soup of what might have been a primordial earth for chemical reactions with certain life-like properties. These are known as a small molecule or metabolic approach to life origins.

These small molecule theories posit that life can be defined not in a genetic context but by describing it as a thermodynamic phenomenon. Carl Sagan noted that life could be stated as “a localized region that increases in order (decreases in entropy) through cycles driven by an energy flow.”

These approaches may provide a theoretical underpinning to a multiple origins theory for life origins and may provide for a far less improbable scenario for its beginnings. The natural constraints required for these small molecule theories to work are less complex than those necessary for RNA first approaches.

According to Robert Shapiro there are five basic conditions that would allow for the possibility of small molecular life to “congeal” and open the way for more complex forms. These conditions include:

  1. A boundary is needed to separate life from nonlife. Living cells convert energy or radiation to heat. The released heat increases the entropy of the environment, compensating for the decrease in living systems. The boundary maintains this division of the world into pockets of life and the nonliving environment in which they must sustain themselves.

  2. An energy source is needed to drive the organization process. …the transformations [producing energy] that are involved are called redox reactions. They entail the transfer of electrons from an electron rich (or reduced) substance to an electron poor (or oxidized) one.

  3. A coupling mechanism must link the release of energy to the organization process that produces and sustains life. Everyday, in our own cells, each of us degrades ponds of ATP. The energy released by this reaction serves to drive the processes necessary for our biochemistry that would otherwise proceed too slowly or not at all. Linkage is achieved when the reactions share a common intermediate, and the process is sped up by the intervention of an enzyme. One assumption of the small molecule approach is that coupled reactions and primitive catalysts sufficient to get life started exist in nature.

  4. A chemical network must be formed to permit adaptation and evolution. Imagine, for example, that an energetically favorable redox reaction of a mineral drives the conversion of an organic chemical A, to another one, B, within a compartment. I call this key transformation a driver reaction, because it serves as the engine that mobilizes the organization process. If B simply reconverts back to A or escapes from the compartment, we would not have a path that leads to increased organization. In contrast, if a multi-step chemical pathway-say, B to C to D to A- reconverts B to A, then the steps in that circular process (or cycle) would be favored to continue operating because they replenish the supply of A, allowing the continued useful discharge of energy by the mineral reaction.

  5. The network must grow and reproduce.”

To date, little validating research has been done to confirm this attractive theory there are some tantalizing studies that support its plausibility. The focus may be to seek out an example of the previously mentioned “driver reaction” which may reveal the theorized connection between this chemistry and sustainability, eventually leading to evolutionary mechanisms.

In essence, origin of life research is still in its infancy, and it is likely many new ideas and concepts will present themselves in a fascinating quest to uncover life’s most basic beginnings.

Ref: Shapiro, Robert. A simpler origin for life.Scientific American 2007. Vol 296, No6

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