UNDER CONSTRUCTION!
Before we begin, some discussion is helpful. Click on a question below to find the answer:
What is the Darwinian Polymer Model?
How can one determine the origin of life, it occurred so long ago?
What should any origin of life model include?
Does this model say anything about "Intelligent Design"?
Is the Darwinian Polymer Model scientific?
The sections below provide a brief summary of the DPM. In the future, it will be possible to click section headings for a more detailed discussion and references. For a (currently) more coherent outline of the Darwinian Polymer Model, please read the "Story" chapter of Darwin's Dream by Brian Pontius (2008) on Google books.
The Darwinian Polymer Model (DPM)
Time and Place
The path to life started very early (well over 4 billion years ago), and away from large bodies of standing water. The temperature was the boiling point of water. Life started at or very near the Earth's surface.
The DPM proposes that life started at the boiling point of water primarily because it is believed that a stable temperature was an essential condition for creating early life, and this was one of the few ways that a constant temperature could be achieved on a large scale. While the scientific literature is filled with different opinions as to the temperature of the early Earth, a relatively hot Earth (>100 degrees Celsius) is within the range of suggested temperatures.
Temperature stability was achieved by a process of boiling, evaporation and subsequent precipitation at the Earth's surface. This process slowly cooled the Earth and kept the Earth's surface temperature relatively constant. It was during this time that the first molecules that led to living things formed.
Energy Source
The first energy source for living things was polyphosphate, which formed naturally beneath the Earth's surface at high temperature and pressure.
An energy source was needed to create the organization present in living things. This energy source was polyphosphate, which formed beneath the Earth's surface at high temperature and pressure. This process was similar to the way that low-energy hydrocarbons form energy-rich materials such as coal, oil and natural gas.
Once this polyphosphate was expelled to the Earth's surface, it became available as a source of energy to drive the chemical reactions that led to Darwinian Polymers. The DPM assumes that one or more very large sites of polyphosphate accumulation were present on the early Earth's surface. Because water did not accumulate on the Earth's surface (because it was too hot), polyphosphate could accumulate and react with other chemicals in addition to water. Some high-energy phosphate also may have formed in a reaction in which solar energy was absorbed by free phosphate.
First Polymers
The first genetic material was a heterogeneous collection of phosphate polymers formed by small organic compounds that had two sites capable of reacting with polyphosphate.
Initially, the polyphosphate was broken down by many different small chemicals, including water. Most reactions were irrelevant to the origin of life. But occasionally, chemicals with two sites that could react with polyphosphate did so. This led to the formation of many new polymer types which incorporated these small chemicals in long chains, separated by the phosphates originally derived from polyphosphate. These initial polymers were largely heterogeneous and non-functional.
Replication and Evolution of Darwinian Polymers
As Darwinian Polymers underwent many cycles of synthesis and degradation, they became chemically similar and accumulated. The chemicals that formed better-accumulating polymers (and the polymers made from these chemicals) accumulated the most.
Some units of the newly-generated polymer species incorporated more readily into polymers than others. These units may have been incorporated faster, been more stable, or incorporated better for other reasons. Chemicals that were not readily incorporated into stable polymers diffused away, while the polymers themselves remained nearer their sites of synthesis.
Thus, new polymers evolved that accumulated better. This process was not initially strictly Darwinian, as chemistry played a direct role in creating better accumulating genetic material.
The movement (diffusion) of small chemicals into and out of regions of polymer synthesis, coupled with the transient trapping of polymer-forming chemicals in the slower-diffusing polymers, led to regions of high local concentrations of particular types of polymer-forming subunits near regions of polymer synthesis. In this way, the chemicals near sites of polymer synthesis became concentrated in the types of chemicals that most readily formed polymers.
Over time, the chemicals useful for polymer accumulation (and the polymers made from these chemicals) greatly accumulated near sites rich in polyphosphate. These polymers were relatively simple in sequence (homopolymers or simple alternating copolymers). Ribonucleotide polymer subunits began to predominate because ribonucleotide-containing polymers replicated and accumulated best under the conditions found on the early Earth (100 C).
Simple Metabolism
As the polymers accumulated, they began to act as crude catalysts.
The polymers present near sites of polymer synthesis began to alter the types of chemicals nearby by acting as crude catalysts. That is, the many similar sites along the polymers acted as weak catalysts, creating new chemicals. If these new chemicals helped in polymer accumulation, the catalytic polymers that made them preferentially accumulated.
Cooperation and Linkage
Functional and physical linkages formed between different polymer types.
Polymers that formed functional linkages occasionally also formed physical linkages. Some polymers linked end-to-end or formed simple, double-stranded, heteropolymer hybrids. The polymers with both a functional and a physical linkage accumulated more efficiently and began to predominate.
This stage marks the beginning of useful polymer complexity (in contrast to the random polymer complexity that existed at earlier stages of the DPM).
Polymer Caps
As the Darwinian Polymers became more stable, some catalyzed the formation of small molecules that capped polymer length. These "capped" Darwinian Polymers replicated and accumulated better.
As Darwinian Polymers became more stable, small chemical evolved that could not be extended and therefore capped polymer length. These "caps" prevented the polymers from growing so long and stable that they could not replicate efficiently. Such capped Darwinian Polymers preferentially accumulated.
Polymer caps, Cap Polymers and Proto-Translation
The transfer of polymer caps between Darwinian Polymers led to proto-translation.
Some of the polymer caps could chemically attack the caps on other polymers. This allowed for a new class of "cap" polymer to form that did not have a phosphate backbone. Some members of this new class of polymer helped the Darwinian Polymers accumulate. This new class of polymer (the cap polymer or proto-protein) eventually evolved into the modern protein. The ability of the proto-proteins to facilitate Darwinian Polymer replication drove the development of the modern genetic code and genome sequence complexity.
SUMMARY
The Darwinian Polymer Model proposes the following:
Life began at (or very near) the Earth's surface well over four billion years ago
The temperature was the boiling point of water
Polyphosphate and other compounds that formed underground were deposited on the Earth's surface by volcanic action
Small carbon, oxygen and nitrogen based chemicals became covalently linked to high energy phosphate, thereby facilitating additional chemistry
Darwinian polymers formed, accumulated, and evolved to accumulate better
Darwinian polymers that accumulated better became less disordered over time. Homopolymers, copolymers and simple heteropolymers formed.
Darwinian Polymers catalyzed chemical reactions, creating a crude metabolism
Darwinian Polymers evolved ribonucleotide-like subunits
Darwinian Polymers formed more stable functional and physical linkages
Some Darwinian Polymers became capped at their ends. Some of these caps were transferable to caps on other Darwinian Polymers
The transfer of polymer caps (forming cap polymers) led to a crude proto-protein synthesis and proto-translation
Functional cap polymers (proto-proteins) drove the development of a more efficient translation machinery and an efficient genetic code
The Darwinian Polymers and proto-proteins simultaneously evolved greater functionality and sequence complexity. This process was driven by the increased functionality of the proto-proteins to facilitate Darwinian Polymer accumulation
As genetic complexity increased, the temperature dropped, and water accumulated: DNA and cellular life evolved
