One of life's biggest mysteries is how life itself came to be, the Oparin-Haldane hypothesis sets out to answer that question.
The Oparin-Haldane Hypothesis: definition
The Oparin-Haldane Hypothesis is a theory concerning how the Origin of Life on Earth came to be. In 1924 Russian scientist Aleksandr Oparin and in 1929 English scientist J. B. S. Haldane independently proposed new theories for the origins of life - what we now refer to as the Oparin-Haldane Hypothesis. They suggested life emerged from a series of step by step reactions between inorganic matter driven by a large energy input.1 These reactions initially produced the 'building blocks' of life (e.g., amino acids and nucleotides), then more and more complex molecules until primitive life forms arose.
The Oparin-Haldane Hypothesis proposes early life evolved through the process of abiogenesis.1
Biogenesis: Creating life from other pre-existing life. Historically Biogenesis refers to the idea that only life can create life.
Abiogenesis: Creating life from non-life. Abiogenesis refers to the idea that life could have evolved from inorganic matter or non-living substances.
Abiogenesis vs spontaneous generation
Abiogenesis and spontaneous generation are often used interchangeably - though they shouldn't be.1
Historically, Thomas Henry Huxley coined the terms biogenesis and abiogenesis, using them to refer to life arising from 'similar life' and from 'non-life' respectively. These terms were brought to life when the theory of spontaneous generation had yet to be completely disproved. Initially, abiogenesis and spontaneous generation went hand in hand, though since the proposal of the Oparin-Haldane Hypothesis their paths have strayed.
In modern-day terms, abiogenesis refers to the creation of very simple life from non-living matter. Spontaneous generation, however, refers to the disproven theory that complex life arises "spontaneously" and "continuously" from non-living matter.
Spontaneous generation was originally proposed by the Greek philosopher Aristotle after witnessing life come to be in, what he thought was, the absence of sexual reproduction.2 Since its proposal 'recipes' for maggots and mice have been put forward where putting aside a piece of meat or bread under the right conditions creates life.
Later experiments (Fig. 1) have shown these 'recipes' only work when there is already life present in the ingredients.
Figure 1: Francesco Redi disproved the theory of spontaneous generation through his experiments on maggots. Source: LumenLearning.com
The Oparin-Haldane Hypothesis and the origin of life
Both Oparin and Haldane believed life could have arisen through the abiogenesis of non-living materials subjected to an external energy source, though their ideas on exactly how this occurred differ slightly. Their theories outlined the conditions in which this may have happened.
Both Oparin and Haldane theories described :
The presence of a primitive reducing (oxygen-deprived) atmosphere containing ammonia, water vapour and other gases.
Early life forms arising in the oceans.
Early life forms as heterotrophic (they obtained nutrients already available during the primitive conditions).
The Oparin-Haldane Hypothesis described how life may have emerged from the primordial soup.
The origins of life according to Oparin
Oparin believed the earliest life forms developed from coacervates. Coacervates are small liquid droplets made up of 2+ different liquids that when mixed will not form a homogenous solution (a solution where the same share of components are found throughout).3 Coacervates are often produced and held together by the union of oppositely charged or hydrophobic molecules.
Oparin observed how coacervates formed, seemingly of their own accord, and suggested this mechanism is what formed the first pre-cells in the primordial soup. Oparin undertook experiments which proved that critical-for-life metabolic reactions were more efficient when contained in this way, as opposed to the reactants floating freely through aqueous solutions.2
The origins of life according to Haldane
Haldane, at the time of his initial origin of life proposal, was unfamiliar with Oparin's work concerning coacervates. Haldane believed ultraviolet light provided energy for reactions to produce simple organic molecules. According to Haldane, these organic molecules continued to react until they eventually formed the first primitive cells.1
- At its most basic the difference between the Oparin's and Haldane's proposals boils down to when cells formed.
- Oparin proposed pre-cells formed very quickly, held together by electrostatic forces and this proximity drove further complexity of life and molecules.
- Haldane proposed more complex molecules formed first, and then membrane-bound cells evolved.
Evidence supporting the Oparin-Haldane Hypothesis
In 1953 American chemists, Harold C. Urey and Stanley Miller, set out to test the Oparin-Haldane Hypothesis. Miller and Urey attempted to recreate the reducing primordial atmospheric conditions laid out by Oparin and Haldane (Figure 2) by combining four gases:
Water vapor
Methane
Ammonia
Molecular hydrogen
The pair of scientists then stimulated their faux atmosphere with electrical pules to simulate energy provided by lightning, UV rays or hydrothermal vents. After a week, simple organic molecules, including amino acids, were produced by the experiment - proving organic molecules could form under the conditions laid out in the Oparin-Haldane Hypothesis. These finding became known as the Miller-Urey Experiment. The Miller-Urey Experiment was the first evidence that organic molecules could be spontaneously produced from only inorganic molecules.1,4
Figure 2: Diagram of the Miller-Urey Experiment. Source: Wiki Commons.
Weaknesses in the Oparin-Haldane Hypothesis
The Oparin-Haldane Hypothesis has been fundamental in the journey of research into the origins of life on earth. Their proposals kickstarted research into new, finally plausible, areas of chemical evolution with many experiments conducted on the backbone of their theory.
But the proposal laid out in the Oparin-Haldane Hypothesis is wrong.
Weaknesses of the Oparin-Haldane Hypothesis: unlikely coacervates
When Oparin first published his book, Origins of Life, little was known about genetic material. As knowledge about DNA and RNA emerged, coacervates as the first early life forms lost support.5 With the favor tipping towards theories which acknowledges the crucial role of genetic material for Darwinian evolution, such as the most widely supported RNA World Hypothesis.
Over the past couple of decades, observations of membrane-less organelles have been touted by the scientific community. Originally, Oparin's 'coacervate-first theory' had lost steam as membranes seemed so critical to life. However, these recent observations show membrane-less organelles as not only a more dynamic organizational tool but have proved they are essential for functioning life.3
Many scientists now view coacervation as a mechanism aiding other origins of life theories, rather than an explanation for the origin of life itself.6
Weaknesses of the Oparin-Haldane Hypothesis: the primordial atmosphere wasn't reducing
For decades primordial earth was thought to house a methane heavy, reducing atmosphere. This was the atmospheric composition the Oparin-Haldane hypothesis was based on, however, recent geochemical reconstructions have turned this theory on its head. Scientists now believe water vapor, carbon dioxide, and sulfur dioxide were the big players in Earth's early atmosphere. Their analysis showed the primordial atmosphere was actually largely oxidative, not unlike today's.7
What does this mean for the Oparin-Haldane Hypothesis?
Well, today's atmosphere, whilst able to sustain modern life is not a great starting point for life to emerge. The methane-rich, low oxygen atmosphere laid out previously by both Oparin and Haldane, has a much greater (biologic) potential to make the jump from inorganic to organic molecules and the building blocks of life. Some scientists, therefore, suggest this is evidence for the building blocks of life being extraterrestrial in origin.
Others point out that the Miller-Urey Experiment set the ball rolling for many similar experiments over the years. Even though the conditions created specifically in the Miller-Urey Experiment were not accurate, the organic molecules we consider the building blocks and precursors for life on earth have been produced in a large variety of potential conditions since. Therefore it is still likely the core component of the Oparin-Haldane Hypothesis still rings true: chemical evolution is at the origin of life.
Whilst Oparin and Haldane may have got some of the specifics wrong, the proposed philosophy of a stepwise transition from simple inorganic molecules to organic molecules has been adopted by many other origin of life on earth theories. This philosophy even underpins the beginning stages of the most widely believed origin of life on earth theory: The RNA World Hypothesis.
Steps for the origin of life and the Oparin-Haldane Hypothesis.
Even though the initial conditions laid out in the Oparin-Haldane hypothesis, are no longer supported through geochemical evidence, this theory still plays a crucial role in the steps required for the origin of life on earth:
- Simple inorganic molecules are mixing and abundant in the primordial soup. Chemical reactions occur, synthesizing more complex, organic molecules that make up the building blocks of life (e.g., amino acids and nucleotides).
- Longer polymers form. Depending on the theory these polymers could be RNA strands, DNA strands, or early proteins.
- Proto-cells or Pre-cells form. In early protocells, organic molecules are grouped together to more efficiently carry out metabolic processes. Depending on the origin of life theory you believe, membranes may have formed before or after this clustering.
Oparin-Haldane Hypothesis - Key takeaways
- The Oparin-Haldane hypothesis is a theory that was proposed by Russian and British scientists independently in 1924 and 1929 respectively.
- The Oparin-Haldane hypothesis is an origin of life on earth theory detailing a plausible method of abiogenesis.
- The Oparin-Haldane Hypothesis outlines three key steps in the evolution and origin of early life on earth: 1.) Simple inorganic molecules in the primordial soup mixed to form more complex, organic molecules. 2.) Longer polymers formed from the organic molecules. 3.) Proto-cells formed as concentrations of organic molecules carrying out early metabolic processes.
- Recent evidence has shown that the earth's primordial atmosphere was unlikely to be reducing as previously thought.
- The Miller-Urey Experiment provided the first evidence that organic molecules can form from inorganic molecules under the right conditions and in the presence of an energy source.
References
- Kara Rogers, Abiogenesis, Encyclopedia Britannica, 2022.
- Matt Simon, Fantastically Wrong: Why People Once Thought Mice Grew Out of Wheat and Sweaty Shirts., Wired, 2014.
- Emanuele Astoricchio et al, The Wide World of Coacervates: From the Sea to Neurodegeneration, Trends in Biochemical Sciences, 2020.
- Tony Hyman et al, In Retrospect: The Origin of Life, Nature, 2021.
- Stanley L. Miller et al, Oparin’s ‘‘Origin of Life’’: Sixty Years Later, Journal of Molecular Evolution, 1997.
- Basusree Ghosh et al, Can coacervation unify disparate hypotheses in the origin of cellular life? Current Opinion in Colloid & Interface Science, 2021.
- Dustin Trail et al, The oxidation state of Hadean magmas and implications for early Earth’s atmosphere, Nature, 2011.
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