The Earth is estimated to be a little over 4.5 billion years old and organic life is estimated to have existed on Earth for much of that time, with recently discovered fossils of microorganisms dating back to between 3.8 and 4.3 billion years.
The origin of life, also known as abiogenesis, describes how living organisms first evolved from inorganic matter! The exact process by which this occurred is not yet known, though there are several theories.
We will cover these theories, including some that have been disproven and others that are currently being studied experimentally!
Origin of Life Theories in Biology
Studying the origin of life requires understanding how and why certain chemical reactions within inorganic matter could be capable of producing life. This is a cross-disciplinary study, encompassing portions of biology, chemistry, and physics. In the following, we will look at several theories on the origin of life, what is required for life, and what Charles Darwin's thoughts were on the origin of life.
Life Arose Spontaneously
The, now disproven, spontaneous theory states that life was produced from inorganic matter, such as mud, in the absence of any reactions. This was a very early theory on the origin of life, championed by Aristotle. Hundreds of years ago, experiments were conducted to determine the validity of this theory.
An experiment in 1668 by a scientist named Francesco Redi (1626-1697) utilized raw meat in order to determine the origin of maggots. He placed raw meat into two jars- one sealed and one unsealed (Fig. 1). He then waited a week and found that no maggots were produced from the sealed jar, but that maggots were found in the unsealed jar. Flies had access to the unsealed jar, thus revealing that maggots were produced from the eggs of flies. Several different experiments by other scientists followed, all providing further evidence that the theory was incorrect.
Figure 1: Francesco Redi's experiment testing the spontaneous theory of life's origin. Source: Wiki Commons
The Panspermia Theory of the Origin of Life
The panspermia theory, also known as the extraterrestrial theory, states that life may have begun elsewhere in the universe and brought to Earth via microorganisms present in a meteor or some other method of transportation. Panspermia means “all seeds” or “seeds everywhere”, implying that the seeds of life are present throughout the universe. Three kinds of panspermia have been proposed:
Litho: life is transported throughout space when an impact on one planet causes rocks to be sent into space, moving to other planets in different solar systems.
Ballistic: life is transported throughout space when an impact on one planet causes rocks to be sent into space, moving to other planets within the same solar system (Fig. 2).
Directed: life is spread to other planets and solar systems intentionally by extraterrestrial civilizations with more advanced technology.
Figure 2: The panspermia theory suggests that life arrived on Earth from elsewhere, potentially from ejected debris following the impact of a meteor on another planet. Source: Wiki Commons
Life Began in Ice
It is believed that, billions of years ago, much of the Earth was frozen and covered in thick ice. Such cold temperatures could have allowed for many of the compounds implicated in the formation of life to maintain stability. This stability could have then allowed these compounds to exist for much longer periods of time than under warmer conditions, potentially allowing for reactions to occur and life to emerge.
Life Began in Clay
The clay theory suggests that self-replicating crystals of clay may have given way to the formation of life by trapping molecules and enhancing their ability to replicate and catalyze reactions. This could have resulted in further replication, the emergence of life, and evolution to complex organisms. The theory was first proposed by a scientist named Alexander Graham Cairns-Smith in the 1960s.
The “RNA world” Theory of the Origin of Life
The “RNA world” theory states that life may have begun with molecules of RNA (Ribonucleic acid), which are able to perform self-replication and catalyze reactions. Over time, these molecules evolved to become more complex. This theory has some evidence to support it.
Like DNA, RNA can contain and process genetic information, but it can also catalyze reactions, which DNA cannot. Thus, RNA may have had the ability to independently support organic life. RNA could also, theoretically, pass genetic information down to the following generations and evolve. While the term “RNA world” was first coined by an American scientist named Walter Gilbert in 1986, the theory was originally presented by American biologist Alexander Rich in 1962. The "RNA world" theory is the most widely accepted theory on the origin of life today.
The Oparin-Haldane Theory of the Origin of Life
In 1924 a Russian scientist named Alexander Oparin and, in 1929, an English scientist named J. B. S. Haldane, proposed that life arose from inorganic matter mixed with other compounds (known as the primordial soup) under an oxygen-deprived (reducing) atmosphere and gradually evolved into more complex organisms over time. This is known as the Oparin-Haldane or heterotrophic theory of the origin of life. They suggested that this inorganic matter may have experienced reactions caused by lightning that resulted in the formation of amino acids and other important building blocks for the formation of life, thus creating the primordial soup. Reactions within this primordial soup could have then allowed for the formation of molecules of greater complexity, such as proteins, and eventually evolution into complex organisms.
The Miller-Urey Experiment
The Miller-Urey experiment was conducted in 1952 in order to test the validity of the Oparin-Haldane theory. The experiment, conducted by chemists Stanley Miller under the supervision of Harold Urey, revealed that the spontaneous production of organic molecules was possible under the oxygen-deprived conditions Oparin and Haldane suggested Earth may have been under at the time of life's origin.
They ended up building an enclosed environment containing heated water and various compounds (which may have been present in the Earth’s atmosphere at the time), and sent electrical currents through it, in order to initiate reactions through simulated lightning (Fig. 3). After a week, they discovered that organic molecules, such as amino acids and lipids, had formed. Although no complex molecules had formed, this experiment did show that such conditions could result in the formation of organic molecules.
Figure 3: Diagram of the Miller-Urey experiment. Source: Wikipedia
Unfortunately, it is now believed that the Earth’s atmosphere was not oxygen deprived at the time life emerged, thus rendering the results of Miller-Urey’s experiment questionable in regard to its applicability to life's origins and casting doubt on Oparin-Haldane’s theory, at least the idea that oxygen deprivation played a role. However, since it was shown that organic molecules could indeed form from abiotic matter, this suggests that some portions of the Oparin-Haldane theory could be correct.
Life Began in Deep-Sea Vents
The deep-sea vents theory involves deep-sea hydrothermal vents, which are geologic structures that spew molecules that have abundant hydrogen. Billions of years ago, these molecules may have then clumped together and experienced chemical reactions, which may have resulted in the emergence of life. The world's oldest fossils, containing microorganisms and dating to between 3.8 and 4.3 billion years old, were discovered in hydrothermal vents in Quebec in 2017.
Timeline of Origin of Life Theories
As you can see above, there have been numerous theories on the origin of life. The following (Fig. 4) is a brief timeline looking at the timeline of some of the theories on the origin of life.
Figure 4: A timeline of theories on the origin of life. Source: Susanna O’Brien
Darwin’s Theory of the Origin of Life
While Charles Darwin’s theory of evolution, published in his book On the Origin of Species in 1859, is the scientist’s most famous contribution to science, he also had some ideas concerning the origin of life on Earth. Darwin never published his thoughts on the origin of life in any of his books, but he briefly discussed his ideas on the subject in a letter to naturalist Joseph Dalton Hooker in 1871. In a mere four paragraphs, he proposed that life likely began in a warm pond in which a protein was formed due to chemical processes and, eventually, evolved into more complex organisms.
This theory is, therefore, somewhat similar to the primordial soup theory proposed by Oparin and Haldane. Unfortunately, Darwin’s thoughts were limited by the scientific knowledge of the time, as it was not yet known that proteins were comprised of amino acids, and little was known about genetic processes or cellular structures. Despite this, his thoughts were very prescient, and a high density of chemicals in a small pond would, indeed, require a source of energy, such as light or heat, to drive reactions.
Origin of Life Theories - Key takeaways
- The origin of life, also known as abiogenesis, describes how living organisms first evolved from inorganic matter.
- The, now disproven, spontaneous theory states that life was produced from inorganic matter, such as mud, in the absence of any reactions.
- The panspermia theory, also known as the extraterrestrial theory, states that life may have begun elsewhere in the universe and brought to Earth via microorganisms present in a meteor or some other method of transportation. There are three kinds of panspermia- litho, ballistic, and directed.
- The “RNA world” theory states that life may have begun with molecules of RNA (Ribonucleic acid), which are able to perform self-replication and catalyze reactions. Over time, these molecules evolved to become more complex. The "RNA world" theory is the most widely accepted theory on the origin of life today.
- The Oparin-Haldane theory states that life arose from inorganic matter mixed with other compounds (known as the primordial soup) under an oxygen-deprived (reducing) atmosphere and gradually evolved into more complex organisms over time.
- The Miller-Urey experiment was conducted in 1952 in order to test the validity of the Oparin-Haldane theory.
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