Abiogenesis

Abiogenesis is the theory that life on Earth originally arose through natural processes from simple chemical compounds and other nonliving matter. This theory supposes that the earliest lifeforms were extremely simplistic and only evolved into more complex organisms over a long period of time. In essence, abiogenesis is a hypothetical explanation of how nonliving organic molecules may be capable of forming the most basic building blocks of life under the right circumstances. According to its proponents, the conditions necessary for abiogenesis to take place only existed for a relatively short period of Earth’s primordial history when the chemical composition of the atmosphere was ideal for it to occur. As a result, abiogenesis is no longer an active scientific phenomenon on Earth. While most modern scientists generally accept abiogenesis as a likely explanation of how life first formed on Earth, it is important to note that key unanswered questions remain as to how exactly nonliving organic matter can yield the fundamental building blocks of organic life. Because of this, abiogenesis is viewed as a theory rather than an established scientific fact.

Background

The theory of abiogenesis has historic roots that stretch back to antiquity and early scientists’ belief in what was known as spontaneous generation. Spontaneous generation was the idea that it was possible for complex living organisms to form from nonliving organic matter. Among the most important proponents of spontaneous generation was the Greek philosopher and polymath Aristotle. Like other scientists who supported the theory of spontaneous generation, Aristotle believed that plant and animal reproduction was not entirely dependent on sexual and parthenogenetic methods. Rather, he and other likeminded scientists argued that reproduction could also occur spontaneously. Seventeenth-century Flemish chemist Jean Baptist van Helmont also subscribed to the theory of spontaneous generation. He argued that certain non-living ingredients could, when used in the correct combinations, lead to the spontaneous generation of living animals like mice and scorpions.

Despite its longstanding popularity in the early scientific community, the theory of spontaneous generation did not ultimately stand the test of time. Later researchers like Italian physician Francesco Redi and French biologist Louis Pasteur eventually refuted spontaneous generation, using more advanced tools and techniques to uncover empirical evidence that it was impossible for nonliving matter to spontaneously generate living organisms. Scientists next developed the theory of biogenesis, which was based on the premise that life arises through similar existing lifeforms. In the nineteenth century, English biologist and anthropologist Thomas Henry Huxley coined the terms “biogenesis” and “abiogenesis.” According to his definitions, biogenesis referred to the process through which life arose from similar life, while abiogenesis referred to the traditional idea of spontaneous generation. While biogenesis explained how new lifeforms were created by already existing lifeforms, it offered little insight into how life on Earth initially came into existence. Trying to explain this eventually led to a different definition of abiogenesis.

Russian biochemist Aleksandr Oparin and British scientist J.B.S. Haldane both conducted independent studies in the 1920s that set into motion the emergence of the modern understanding of abiogenesis. In short, Oparin and Haldane concluded that it was possible for organic molecules to form from abiogenic materials under the right circumstances. The next important step came when American chemists Harold C. Urey and Stanley Miller successfully tested the Oparin-Haldane theory in 1953. Through their combined efforts, Oparin-Haldane and Urey-Miller proved that it was theoretically possible for life to be formed from nonliving materials and, in doing so, established the contemporary scientific understanding of abiogenesis.

Overview

Abiogenesis is a natural process through which living organisms are thought to have arisen from nonliving organic matter. In this process, simple elements combine to form compounds that gradually become more structured and varied over time. At some point, these compounds evolve and link to create even more complex molecules, including amino acids. Amino acids are the critical building blocks of the proteins that form the foundation of all organic processes. It is possible that the amino acids derived from linked organic compounds eventually combined to create protein chains that subsequently became self-replicating, thus allowing for the formation of simple lifeforms.

The Oparin-Haldane theory explains in greater detail how abiogenesis may have been responsible for the emergence of life on Earth. Both men hypothesized that it was possible for organic molecules to be formed from nonliving matter with exposure to an external energy source. In the case of primordial Earth, this energy source would have been ultraviolet radiation. They also theorized that such a phenomenon would additionally require the existence of a reducing atmosphere—one that has very low free oxygen level—that contains a significant amount of ammonia, carbon dioxide, water vapor, and other gases. Believing that such conditions probably existed during the early stages of Earth’s development, Oparin and Haldane agreed that abiogenesis was the most likely explanation for how life first formed. Further, they both also postulated that the earliest lifeforms initially appeared in primitive Earth’s warm oceans and acquired the nutrients necessary for survival from preexisting compounds rather than generating nutrients from inorganic material or through processes like photosynthesis. All of this formed the basis of the modern scientific understanding of abiogenesis.

The Miller-Urey experiment put the Oparin-Haldane theory to the test. To definitively determine whether the theory of abiogenesis was realistically plausible, Miller and Urey conducted an experiment in which they sought to recreate the conditions that allegedly allowed for the emergence of life through abiogenesis. To do this, the pair created a simulated environment composed of warm water mixed with gases like water vapor, ammonia, methane, and molecular hydrogen. They also pulsed the atmosphere of their miniature environment with electrical discharges to simulate lightning. In this way, Miller and Urey created a working model of early Earth that included simulations of the primitive ocean, the prebiotic atmosphere, and heat as provided in the form of lightning. After allowing their experiment to carry on for just a single week, the two confirmed the formation of amino acids and other organic molecules. In doing so, they proved that there was at least some scientific merit to the Oparin-Haldane theory of abiogenesis.

Unresolved Questions

Although it successfully proved that it was possible for the fundamental building blocks of life to be formed from nonliving materials under the right circumstances, the Miller-Urey test did not provide definitive answers to all the questions involved in the theory of abiogenesis. Perhaps the biggest remaining question was how exactly the simple organic compounds formed through abiogenesis eventually yielded living cells. In the years since the Miller-Urey experiment, scientists have developed three leading theories on how this might happen. These include the replication first theory, the metabolism first theory, and the ribonucleic acid (RNA) world theory. The replication first theory holds that the amino acids and other organic materials formed through abiogenesis gradually grew more complex until they came to include deoxyribonucleic acid (DNA) segments capable of self-replication. This would subsequently allow for the development of cell behavior and metabolism. The metabolism first theory posits that early organic molecules eventually became capable of sustaining themselves by integrating and altering substances they encountered in the surrounding environment, thus developing into proto-cells and only then achieving the ability to self-replicate. The RNA world theory suggests that organic molecules first became precursor RNA segments capable of producing copies of DNA molecules, which would then allow them to simultaneously develop both cell behavior and metabolism. Despite continued research into this matter, no simulation has yet managed to successfully demonstrate which of these theories might accurately explain how simple organic compounds transform into living cells.

There are other unanswered questions about how the complex molecules that result from the simple organic compounds created via biogenesis eventually lead to the emergence of actual lifeforms. The first problem is that scientists have yet to arrive at a detailed theoretical path explaining this phenomenon. Scientists have also thus far failed to conduct any experiments that successfully demonstrate that it is possible for molecules more complex than amino acids to be formed through biogenesis. Another problem is that there is no known mechanism that would allow RNA building blocks to create the purine/pyrimidine bases found in full RNA. Finally, a lack of scientific consensus exists on how molecules capable of metabolizing and/or self-replicating could eventually evolve into actual lifeforms.

One other factor that complicates scientists’ ability to understand abiogenesis is that the conditions necessary for it to occur in nature no longer exist on Earth. Atmospheric conditions have changed significantly over time and, as a result, abiogenesis cannot naturally occur today. In fact, most scientists agree that abiogenesis probably only took place once in Earth’s history. Some, however, believe that it could have happened more than once, with phosphate-based lifeforms eventually gaining an evolutionary advantage over other types of lifeforms that might have been created through abiogenesis and ultimately becoming Earth’s dominant lifeforms.

Finally, some debate exists about the validity of the Miller-Urey experiment’s results. Since the time the test was completed, many scientists have come to believe that the atmosphere of primordial Earth may have been different than originally imagined. These scientists question the long-prevailing opinion that early Earth had a reducing atmosphere rich in ammonia and methane. If this was not the case, then the Miller-Urey experiment, which recreated the supposed atmosphere of primordial Earth as it was then imagined, would not have produced valid results. That being said, other similar experiments performed since the Miller-Urey experiment have confirmed that it is possible for organic building blocks to form from nonliving matter.

Alternative Theories

Because the Oparin-Haldane theory of abiogenesis does not offer a complete and proven explanation of how life first formed on Earth, scientists have also developed other theories about the possible origins of life. It is important to note, however, that none of these alternative theories is backed by more hard evidence than the Oparin-Haldane theory. As such, no one theory can definitively be deemed correct.

Some alternative theories about the origins of life on Earth are similar to the abiogenesis theory but suggest that such a process specifically occurred in geothermal vents in the depths of the sea or somewhere within Earth’s crust. The community clay theory suggests that the earliest molecules capable of generating life formed on clay. Specifically, proponents of the community clay theory believe that clay minerals might have arranged organic molecules into organized patterns that made it possible for life to begin. Another theory posits that ice that covered Earth three billion years ago could have protected the organic compounds in the water below and helped them survive long enough to be able to produce life. Yet another theory holds that the generation of life may have been the result of interactions between smaller molecules and the cycles of reactions that resulted from such interactions. Finally, there is the panspermia theory. Unlike the Oparin-Haldane theory and the other theories scientists have put forward, the panspermia theory is based on the idea that life did not actually originate on Earth at all. Instead, those who support the panspermia theory believe that complex organic compounds or simple lifeforms like viruses first arrived on Earth from outer space via meteorites or comets that collided with the planet. This theory is lent some support because scientists know Earth was heavily pelted by space debris during the Hadean Eon around 4 to 4.6 billion years ago. If correct, the panspermia theory would mean that none of the living things found on Earth is native to the planet.

Bibliography

“Abiogenesis.” Bartleby, 2022, www.bartleby.com/subject/science/biology/concepts/abiogenesis. Accessed 23 Feb. 2022.

“Abiogenesis.” Biology Online, 2022, www.biologyonline.com/dictionary/abiogenesis. Accessed 23 Feb. 2022.

“Abiogenesis.” Bio-Medicine, 2022, www.bio-medicine.org/biology-definition/Abiogenesis. Accessed 23 Feb. 2022.

Childers, Tim. “Is Life a Gamble? Scientist Models Universe to Find Out.” Space.com, 23 Apr. 2020, www.space.com/origin-of-life-rna-universe-model.html. Accessed 23 Feb. 2022.

Gardiner, Aisling. “Abiogenesis: A Brief History on the Origins of Life on Earth.” Young Scientists Journal, 30 Apr. 2020, ysjournal.com/abiogenesis-a-brief-history-on-the-origins-of-life-on-earth. Accessed 23 Feb. 2022.

“Hypothesis about the Origins of Life.” Khan Academy, 2022, www.khanacademy.org/science/ap-biology/natural-selection/origins-of-life-on-earth/a/hypotheses-about-the-origins-of-life. Accessed 23 Feb. 2022.

“Origins of Life.” Lumen, 2022, courses.lumenlearning.com/boundless-microbiology/chapter/origins-of-life. Accessed 23 Feb. 2022.

Reginald, Davey. "What Is the Theory of Abiogenesis?" News in Medical Life Sciences, 1 Feb. 2023, www.news-medical.net/life-sciences/What-is-the-Theory-of-Abiogenesis.aspx. Accessed 18 Nov. 2024.