Abiogenesis: Definition, Theory & Evidence | Biology Notes

In this article, we will discuss about:- 1. Abiogenesis Definition 2. Abiogenesis Theory & 3. Evidence for Abiogenesis

1. Abiogenesis Definition

Abiogenesis is the creation of organic molecules by forces other than living organisms. While organisms can create carbon-carbon bonds relatively easily thanks to enzymes, to do so otherwise requires large inputs of energy. Early in the history of science, this fact was used to dispute evolution, as it could not be conceived how organic molecules could be produced from non-organic sources. The theory of abiogenesis as an evolutionary theory was given much credit when Stanley Miller conducted his famous experiment trying to prove the inorganic beginning of life.

Miller combined various gases that were though to exist in the earliest stages of Earth. These gases were combined in a chamber, and shocked with large amount of electricity for weeks at a time. After the trial, Miller would analyze the samples. He found that the molecules had begun the process of combining into more advanced molecules. Miller theorized that over billions of years, these molecules could combine into self-replicating versions, such as RNA and DNA. Further laboratory experiments confirmed these findings in later decades. Several very precise experiments have provided sufficient evidence that many of the molecular structures of cells could be created from inorganic solutions with an input of energy. Polypeptides (proteins) and RNA have both been synthesized in this way.

The synthesis of both proteins and RNA in the laboratory is a crucial piece of evidence for abiogenesis theory. It is though that the abiogenesis of these molecules could lead to self-replicating RNA molecules. Both proteins and RNA molecules are known to act as catalysts. These molecules, produced by abiogenesis, could catalyze important reactions that could lead to the replication of RNA and the production of complexes such as ribosome, which translate proteins from RNA messages. The formation of these two molecules through abiogenesis proves that the first steps in abiogenesis theory could have taken place. Due to the large amount of energy used, some scientists argue that abiogenesis theory does not consider the amount of lightning and other energy sources in the early atmosphere.

2. Abiogenesis Theory

Abiogenesis theory is the theory that all life started from inorganic molecules, which recombined in different ways due to energy input. These different forms eventually formed a self-replicating molecule, which may have used the other molecules produced by abiogenesis to start creating the basic structures of life, such as the cell.

Just as populations change over time in the evolution of organisms, the evolution of molecules involves the changing of molecules over time. Scientist speculate that the first self-replicating molecules were probably RNA molecules. Some RNA molecules have a known ability to catalyze the formation of new RNA molecules, as seen in the ribosomes of nearly all creatures on Earth. One of these early RNA molecules formed just right, so that it produced an RNA molecule that was identical to it. The concentration of this molecule in the prebiotic soup increased drastically, and the molecule further interacted with itself and some proteins formed around it, also through abiogenesis.

Eventually, the RNA molecule acquired mutations that allowed it to synthesize a protein that would produce more RNA. Other mutations caused proteins to be created that synthesized strands of DNA from RNA. Thus, the genome of the modern organism was born. Over millions of years of evolutionary history, changes slowly accumulated in these molecules, giving rise to the complexity of life we see today. Various scientist that study abiogenesis theory argue over the exact point that abiogenesis switches to biogenesis. Similar arguments can be seen in the case of whether or not viruses constitute living organisms. Abiogenesis, by definition, is simply the creation of organic molecules from inorganic sources. It does not necessarily imply where life begins.

3. Evidence for Abiogenesis

i) Oparin-Haldane Theory:

Alexander Oparin and J.B.S. Haldane were the first scientists to independently propose a chemical-based theory for the evolution of life in the 1920’s. They both hypothesized that Earth’s early atmosphere had mostly ammonia, water, hydrogen sulfide, carbon dioxide or carbon monoxide and phosphate and very little oxygen and ozone. Chemically, this means the atmosphere was reducing, capable of accepting electrons and forming new molecules. Under these primitive conditions, they thought, the effect energy sources such as lightning, ultraviolet radiation or even the shock from an impact could form organic molecules like amino acids. Oparin and Haldane’s theory is also known as the Primordial Soup theory.

The Primordial Soup theory goes on to propose that these organic molecules may have concentrated in certain locations like thermal vents in the ocean or shorelines and underwent further transformation to more complex organic molecules, eventually resulting in life. The work of Oparin and Haldane laid the foundation for other scientists to create laboratory experiments to test these hypotheses.

ii) The Miller-Urey Experiment:

In 1953, chemists Harold Urey and Stanley Miller designed an experimental apparatus which duplicated the atmospheric conditions on Earth proposed by Oparin and Haldane. Urey and Miller filled a chamber with warm water, water vapor, methane, ammonia and molecular hydrogen and then introduced pulses of electrical sparks into the chamber. After one week, they analyzed the material in the chamber and found a variety or organic molecules including amino acids, verifying this aspect of the Primordial Soup theory.

The image above illustrates the experimental setup used by Miller and Urey in 1953 to show that simple molecules could have been created on Earth about 3.5 billion years ago, using only the gases in the primitive atmosphere and electricity from lighting.

The current research and debate about abiogenesis now centers around how and why organic molecules may have accumulated in certain areas on the early Earth and how these molecules increased in complexity and eventually became self-replicating life. Astrobiology, the scientific field that looks for extraterrestrial life, has provided an additional information that supports abiogenesis. For example, studies on Saturn’s moon Titan have shown that its’ atmosphere has no oxygen and that there are organic molecules present.