ONE of the arguments in favour of a divine power – or god – is that life could not have arisen naturally and needed a touch of the divine to come into being. While Oparin, Haldane and Bernal had argued how organic life forms can arise naturally, experimental proof that this is indeed the case was lacking, though the Urey-Miller experiments in 1953 came quite close to it. A recent experiment by a Stanford group has finally shown that complex molecules akin to what we see on Earth as life can arise naturally.

Once complex molecules can be shown to arise naturally, the theory of evolution takes care of the rest: from simple life forms to homo sapiens. Or us. So, let us look at this history of how life could arise naturally: from the complex chemical soup of the primordial atmosphere to complex organic molecules.

In the 1920s, Alexander Oparin, a Soviet Union biochemist, proposed that natural chemical reactions might be the source of life. His proposition was that in the "primordial soup" of prebiotic Earth, building blocks of life, or complex organic molecules, could have spontaneously formed in the atmosphere of methane, ammonia, and water vapour. These molecules might have gathered into microscopic, gelatinous droplets, which Oparin called "coacervates". These would then absorb nutrients and reproduce themselves.

In 1929, Haldane, a British scientist and member of the Communist Party, independently postulated that life arose from simple organic molecules that, in the presence of ultraviolet light, became increasingly complex, ultimately forming what can be described as living cells. Both showed that there were no fundamental differences between organic and inorganic compounds except their complexity.

Later, in 1949, the British physicist and Marxist JD Bernal proposed that tidal zones, with a wealth of clay minerals, might have been places where such organic chemicals formed. He proposed that clays could absorb organic molecules, bringing together large quantities of these chemicals and facilitating their arrangement into intricate polymers such as proteins and nucleic acids. He helped answer a significant issue as to how the precursors of biological life, which would have been highly diluted in the enormous oceans and rivers, might have accumulated to create something more structured.

In 1953, Stanley Miller, a young PhD student of Harold Urey, showed experimentally what Oparin and Haldane had argued. Miller-Urey added water, methane, ammonia, and hydrogen to a sealed container. Then, the system was subjected to electrical sparks to simulate lightning. From the resulting soup, Miller recognised the essential building blocks of proteins, such as amino acids glycine, alanine, and aspartic acid. The Miller-Urey experiments showed that lightning strikes in oceans, interacting with the then primitive atmosphere of methane, ammonia and hydrogen, could produce life's building blocks.

However, there is one catch. Lightning is too infrequent, and the ocean is too large for this to be a realistic cause of the beginning of life. Recently, researchers from Stanford University in the US have discovered another way to form life's chemical building blocks and a more likely way to do so. Professor Richard Zare and three postdoctoral scholars have shown that micro-lightning between oppositely charged droplets in a primordial atmosphere, a much more frequent event, could produce the same building blocks of life.

The Stanford group conducted an experiment capturing such "micro-lightning" in a primordial soup using high-speed cameras. Even though the tiny flashes of micro-lightning may be hard to see, they still carry a lot of energy. The Stanford researchers found that carbon-nitrogen bonds, the important constituents of the chemical building blocks of proteins and DNA, were formed by such micro-electrical discharges. Their laboratory setup yielded uracil, an essential component of Ribo Nucleic Acid (RNA), hydrogen cyanide, and glycine, the most basic amino acid. Proteins are long polymers of amino acids.

This finding provided an alternative to the idea that infrequent, massive lightning strikes drove prebiotic chemistry. Micro-lightning would have been pervasive in coastal regions and rivers, where organic molecules could have accumulated. In contrast to lightning strikes over large oceanic surfaces, such micro lightning in water sprays could have caused chemical reactions on early Earth.

The Miller-Urey experiment was groundbreaking at the time. Given the appropriate circumstances, the organic molecules required for life could arise naturally. Later studies, however, showed that the atmospheric model of methane and ammonia used had shortcomings. According to geologists then, the early Earth's atmosphere had mostly carbon dioxide and nitrogen. This would have slowed down the efficiency of amino acid formation.

In 1983, Miller repeated the experiment in the modified atmosphere. He created a colourless solution that contained very few amino acids. However, in the early 2000s, Jeffrey Bada, a student of Miller, revisited the original experiment with a more realistic gas mixture and the addition of volcanic minerals. His process now yielded plenty of amino acids. After Miller's death in 2007, Bada inherited Miller's sample jars. He now used advanced techniques that were more than a billion times more sensitive than what Miller could use. He found not 6 but 14 amino acids. Later, Bada used greater airflow in his experiments and found 22 amino acids. Subsequently, he added hydrogen sulfide, which was present in volcanic gases, and found the presence of sulfur-containing amino acids, which are used in protein synthesis.

It was still a fundamental question of how these organic molecules evolved from simple chemistry to self-replicating biology. Studies by Miller, Bada, and Zare demonstrate how the components of life can arise spontaneously. However, it is still not clear how precisely these molecules arranged themselves into functional cells. It has been proposed that deep-sea hydrothermal vents might have provided energy and mineral surfaces for molecules to assemble on. Another possibility is Bernal's clay hypothesis, which postulated that mineral surfaces might have provided the templates for the stabilised formation of molecular structures.

Irrespective of the precise route, the findings show that the emergence of life was a slow process propelled by the forces of chemistry and physics rather than a single creation event; or the involvement of mythical beings. It's possible that deep-sea vents, volcanic eruptions, tidal clays, and micro-lightning all contributed to the formation and early evolution of biological life.

Our planet coalesced from stardust 4.6 billion years ago. Isotopic traces in ancient rocks from comets have shown that carbon-based chemistry, which is life's precursors, was likely 4 billion years ago. We have evidence from fossilised cells embedded in rocks on Earth of the emergence of biological life around 3.5 billion years ago. Due to Oparin, Haldane, Bernal, Urey, Miller, Bada, and now Zare, we know that the chemical units of biological life can arise from the forces of physics and chemistry on Earth.

These findings also help us think beyond Earth. The building blocks of life could thus arise in other worlds with similar environments and forces. Life, probably not identical to that on Earth, could be formed in Earth-like conditions through comparatively simple processes. This idea could now direct the worldwide scientific search for extraterrestrial life, with missions aimed at planets with water, energy sources, and organic chemistry. Such planets are Mars, Jupiter's Moon Europa, and Saturn's Moon Enceladus, all of which have water, energy sources, and therefore could develop organic chemistry and key ingredients for life as we know it.

The more we understand, the more we realise that life's emergence did not require the concept of gods, magic, mythical creators, or intelligent design. All it took was chemistry, persistence, and the inexorable drive in nature for molecules to organise and evolve from simple systems into complex systems. 

There is now the exciting possibility that the universe may be filled with worlds where sparks are still flying, shaping the first steps toward biology. Or life has already evolved into complex life forms. Life is the inevitable outcome of the laws of physics and chemistry. Instead of a single, spectacular event of creation, life on Earth is the result of a billion-year story of cosmic serendipity, combined with the universality of physics and chemistry and the robustness of molecular organisation.