JTW's Evolutionary Origins - Author: Wood, John T.

Metabolic Vs. Genetic Approaches to the Origin of Life

Iris Fry, in her 2000 historical and scientific overview- The Emergence of Life, notes that within the OOL field there is,

"a basic division of principle between the 'genetic approach,' which claims that the first [life] to emerge was a genetic system, and the 'metabolic approach,' according to which the first living entitiy was an autocatalytic metabolic cycle."

In essence, this is another statement of the old classic, the "chicken or egg" problem.

The Metabolic Tradition

The Russian Aleksandr Ivanovich Oparin (1894-1980), being influenced by the rapid progress occurring within the emerging field of biochemistry in the 1920's and 30's, was primarily focused on chemical transformations that may have occurred prior to the establishment of life.

The novelty of Oparin's contribution lie in that he gave a detailed proposal of the means by which a anaerobic heterotroph could have acquired metabolic pathways, in a manner that was consistent with the known chemical, and geophysical knowledge of his time.

Miller, Schopf, and Lazcano identify the following benchmark contributions made by Oparin:

• The hypothesis that heterotrophs and anaerobic fermentation were primordial;
• The proposal of a reducing atomsphere for the prebiotic synthesis of organic compounds;
• The postulated transition from heterotrophy to autotrophy and;
• The considerable detail to which these concepts were addressed.

Throughout his lifework, Oparin emphasized that living systems are complex metabolic enitities that must be considered as a whole and whose essence can not be reduced to simple replication/ reproduction.

Life was seen to be characterized by the ability to carry out and sustain metabolic transformations.

For Oparin, the "first organisms" were the result of coagulation processes within a colloidal solution (prebiotic broth) leading to the formation of coacervates - condensed polymers that produce enclosed structures whose interiors are isolated from the rest of the solution.

Coacervates are known to absorb substances from the surrounding solution) that could absorb organic molecules to their interiors thereby concentrating the compounds and effecting chemical transformations.

In this manner, Oparin hypothesized that coacervates would serve as enzymatic catalysts for the initiation and maintainence of a metabolic cycle.

As the coacervates continued to grow they would eventually fragment into smaller pieces.

Provided that each fragment contained a critical subset of the original coacervate components necessary for maintaining the original reaction network, an imperfect reproduction process would occur.

Daughter coacervates would undergo subsequent selection processes based upon their robustness and adaptability to fluctuating chemical circumstances.

Oparin believed that his colloidal coacervates were the intermediate stage between inanimate chemistry and living biochemistry.

With the discovery of the structure of DNA in 1953, Oparin sought to integrate the emergence of genetic material into a pre-established metabolic context.

It was he who initiated the "protein first" school of thought, his colloidal coacervates being the precursors to emergence of proteins proper.

The Genetic Tradition

In 1914 and the years soon after, Leonard Thompson Troland, inspired by the work of the geneticist T.H. Morgan, published a series of papers in which he attempted to use a "single physio-chemical conception" to explain "all the fundamental mysteries of vital behavior."

Troland believed that the actual Mendelian He suggested the notion of "genetic enzymes" capable of both autocatalysis and heterocatalysis within the same molecule.

This molecule would originally have formed in the primordial ocean by a fortuitous low probability event.

In support of this notion, Wachtershauser (1992) notes the following: "...low-propensity ignitions seem to be the very chemical stuff of biochemical evolution."

Upon synthesis of the first molecule, amplification by autocatalysis would have lead to a population of molecules.

In 1926 Hermann J. Muller (1890-1967), the 1946 Nobel Prize winner in Physiology or Medicine "for the discovery of the production of mutations by means of X-ray irradiation", added the notion of mutability to a proposed "gene" endowed with the capacity for autocatalysis and heterocatalysis.

The Italian geneticist Guido Pontecorvo (1907-1999), or 'Ponte' as he was commonly known, remembers the following:

"I had the extraordinary luck to be a research student under H.J. Muller, one of the founders of genetics, when he was in Edinburgh (1937-1939). His idea of 1926 - the gene as the basis of life - imbued me thoroughly. That idea was that, given a material like that of the chromosomes, with the three basic properties of duplicating itself, occasionally changing and yet duplicating in the mutated form, evolution by natural selection would automatically follow. When Muller stated this in 1926 at an important meeting it was taken as a joke..."

By the 1940's this was no longer the case.

While most scientists at that time believed that the hereditary material was in the form of proteins, Troland, in 1917, suggested that nucleic acids might serve in this role.

It was not until the 40's that nucleic acids were definitively demonstrated to be the hereditary material of the chromosomes and genetics.

In 1953, James D. Watson and Francis Crick elucidated the structure of DNA and over the next decade, were able to deduce the organization of the triplet code.

Crick soon suggested the possibility that RNA could be the first biomolecule.

Sixty-five years after Troland's suggestion, in 1982, Thomas Cech published the first paper detailing the action of a ribozyme and in 1986 Walter Gilbert coined the now famous phrase "RNA World".

What was once a long dormat concept had attained a new life and vigor.

It was J.B.S. Haldane who is most often credited with bringing the "genetic tradition" into the mainstream of science.

Haldane, being a geneticist, proposed that "the first living or half-living things" were molecular structures capable of replication.

Haldane understood that replication was only part of a dynamically cooridinated and cooperative whole, but for him the emergence of molecules capable of replication (from the prebiotic broth) was the intermediate stage between inanimate chemistry and living biochemistry.

Although we now believe the bacteriophage (and viruses in general) to be parasites that evolved after cellular life was established, Haldane saw the bacteriophage as an ideal phylogentic model of the intermediate stage of life's origin.

Viruses can reproduce only within the context of a cellular host.

Haldane likened the pre-biotic broth to the cytoplasm of a bacterium.

The broth would serve as the original host, nourishing the emergent viruses on their evolutionary way to cellularization and the acquisition of metabolism.

Haldane speculated that life remained "in the virus stage for many millions of years before a suitable assemblage of elementary units was brought together in the first cell."

Haldane's approach was to set a trend within the OOL field.

Although early prebiotic research and experimentation was initially focused on the production of metabolites under prebiotic conditions, with the emergence of molecular biology, the "gene conception" was destined to gain prominence.

The so called "gene conception" postulated that the origins of life was coincident with the emergence of the first replicating molecules.

By the early 1970's, with Sol Spiegelman and Manfred Eigen's seminal publications on the selection of replicating nucleic acids, the field was to be dominated by the concepts of the genetic tradition.

The "Double Origins" Hypothesis

In 1985, the physicist Freeman Dyson published a book entitled "Origins of Life" in which he proposed his "Double Origins" hypothesis.

Dyson based his hypothesis on work of OOL researchers up to that point in time.

He noticed that "the study of pre-biotic evolution divides itself into three main stages which one may label geophysical, chemical, and biological."

While the geochemical and chemical stages have solid theoretical and experimental foundations, the biological stage is less firmly established.

It is at the biological stage that the distinction between the "metabolic approach" vs. the "genetic approach" finds its greatest importance.

Replication Vs Reproduction

Replication is the act of producing copies or replicas of a particular structure.

Reproduction is the act of assembling copies or replicas into whole systems that resemble the parent system.

On Questions of Structure and Function

Schrodinger's Questions:

• What is the physical structure of the molecules which are duplicated when chromosomes divide?
• How is the process of duplication to be understood?
• How do these molecules retain their individuality form generation to generation?
• How do they succeed in controlling the metabolism of cells?
• How do they create the organization that is visible in the structure and function of higher organisms?

Timeline of OOL field

• 1668 Francesco Redi - Controlled Experiment with meat maggots
• 1860 Louis Pasteur - Refutes Spontaneous Generation
• 1865 Gregor Mendel - Mendelian Genetics
• 1871 Charles Darwin - Prebiotic "Pond"
• 1917 Leonard Troland - "genetic enzymes"
• 1924 A.I. Oparin - Metabolic Tradition / Prebiotic "Broth"
• 1926 H.J. Muller - "The Gene as the Basis of Life"
• 1929 J.B.S. Haldane - Genetic Tradition / Prebiotic Broth
• 1941? Max Delbruck - Bacteriophage as the "Hydrogen Atom of Biology"
• 1943 Erwin Schrodinger - "What is Life?"
• 1948 John Von Neumann - "Hardware/Software" distinction
• 1953 Harold Urey/Stanely Miller - Experimental Prebiotic Chemistry
• 1953 Watson/Crick - "A Structure for Deoxyribose Nucleic Acid"
• 1950's J.D. Bernal - Surface Adsorption
• 1966 Evans - Reductive Citric Acid Cycle
• 1970's Manfred Eigen/Leslie Orgel - Autocatalytic Hypercycles / Stripped down RNA Replication
• 1980's Cairns-Smith - "Genetic Takeover"
• 1982 Thomas Cech - Ribozymes
• 1986 Walter Gilbert - RNA World
• 1988 Gunter Wachtershauser - Iron Sulfur World

Oparin-Haldane Hypothesis

• Reducing Atmosphere ---->Primordial Soup
• CH₄, NH₃, H₂, H₂O --^{[Energy Sources]}--> Organic Monomers
• Organic Monomers --^{[Energy Sources]}--> Organic Polymers

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