Oparin-Haldane Hypothesis

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.

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:

1. Water vapor

2. Methane

3. Ammonia

4. 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.

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:

1. 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).
2. Longer polymers form. Depending on the theory these polymers could be RNA strands, DNA strands, or early proteins.
3. 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.