Primordial soup, or prebiotic soup (also sometimes referred as prebiotic broth), is the hypothetical set of conditions present on the Earth around 4.2 to 4.0 billions of years ago. It is a fundamental aspect to the heterotrophic theory of the origin of life, first proposed by Alexander Oparin in 1924, and John Burdon Sanderson Haldane in 1929.
The notion that living beings originated from inanimate materials comes from the Ancient Greeks—the theory known as spontaneous generation. Aristotle in the 4th century BCE gave a proper explanation, writing:
So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs — Aristotle, On the History of Animals, Book V, Part 1
Aristotle also states that it is not only that animals originate from other similar animals, but also that living things do arise and always have arisen from lifeless matter. His theory remained the dominant idea on origin of life from the ancient philosophers to the Renaissance thinkers in various forms. With the birth of modern science, experimental refutations emerged. Italian physician Francesco Redi demonstrated in 1668 that maggots developed from rotten meat only in a jar where flies could enter, but not in closed-lid jar. He concluded that: omne vivum ex vivo (All life comes from life).
The experiment of French chemist Louis Pasteur in 1859 is regarded as the death blow to spontaneous generation. He experimentally showed that organisms (microbes) can not grow in a sterilised water, unless it is exposed to air. The experiment won him the Alhumbert Prize in 1862 from the French Academy of Sciences, and he concluded: Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.
Evolutionary biologists believed that a kind of spontaneous generation, but different from the simple Aristotelian doctrine, must have worked for the emergence of life. French biologist Jean-Baptiste de Lamarck had speculated that the first life form started from non-living materials. "Nature, by means of heat, light, electricity and moisture", he wrote in 1809 in Philosophie Zoologique (The Philosophy of Zoology), "forms direct or spontaneous generation at that extremity of each kingdom of living bodies, where the simplest of these bodies are found."
When English naturalist Charles Darwin introduced the theory of natural selection in his book On the Origin of Species in 1859, his supporters, such as a German zoologist Ernst Haeckel, criticised him for not using his theory to explain the origin of life. Haeckel wrote in 1862: "The chief defect of the Darwinian theory is that it throws no light on the origin of the primitive organism—probably a simple cell—from which all the others have descended. When Darwin assumes a special creative act for this first species, he is not consistent, and, I think, not quite sincere."
It is often said that all the conditions for the first production of a living being are now present, which could ever have been present. But if (and oh what a big if) we could conceive in some warm little pond with all sort of ammonia and phosphoric salts,—light, heat, electricity present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present such matter would be instantly devoured, or absorbed, which would not have been the case before living creatures were formed [...].— Charles Darwin, Letter to Joseph Dalton Hooker on February 1, 1871
A coherent scientific argument was introduced by a Soviet biochemist Alexander Oparin in 1924. According to Oparin, in the primitive Earth's surface, carbon, hydrogen, water vapour, and ammonia reacted to form the first organic compounds. Unbeknownst to Oparin, whose writing was circulated only in Russian, an English scientist John Burdon Sanderson Haldane independently arrived at similar conclusion in 1929. It was Haldane who first used the term "soup" to describe the accumulation of organic material and water in the primitive Earth 
When ultra-violet light acts on a mixture of water, carbon dioxide, and ammonia, a vast variety of organic substances are made, including sugars and apparently some of the materials from which proteins are built up. [...] before the origin of life they must have accumulated till the primitive oceans reached the consistency of hot dilute soup.— J. B. S. Haldane, The Origin of Life
Today the theory is variously known as the "Heterotrophic origin of life theory" or the "Oparin-Haldane hypothesis" Biochemist Robert Shapiro has summarized the basic points of the theory in its "mature form" as follows:
- Early Earth had a chemically reducing atmosphere.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
- These compounds accumulated in a "soup", which may have been concentrated at various locations (shorelines, oceanic vents etc.).
- By further transformation, more complex organic polymers – and ultimately life – developed in the soup.
Alexander Oparin first postulated his theory in Russian in 1924 in a small pamphlet titled Proiskhozhdenie Zhizny (The Origin of Life). According to Oparin, the primitive Earth's surface had a thick red-hot liquid, composed of heavy elements such as carbon (in the form of iron carbide). This nucleus was surrounded by the lightest elements, i.e. gases, such as hydrogen. In the presence of water vapour, carbides reacted with hydrogen to form hydrocarbons. Such hydrocarbons were the first organic molecules. These further combined with oxygen and ammonia to produce hydroxy- and amino-derivatives, such as carbohydrates and proteins. These molecules accumulated on the ocean's surface, becoming gel-like substances and growing in size. They gave rise to primitive organisms (cells), which he called coacervates. In his original theory, Oparin considered oxygen as one of the primordial gases; thus the primordial atmosphere was an oxidising one. However, when he elaborated his theory in 1936 (in a book by the same title, and translated into English in 1938), he modified the chemical composition of the primordial environment as strictly reducing, consisting of methane, ammonia, free hydrogen and water vapour—excluding oxygen.
J.B.S. Haldane independently postulated his primordial soup theory in 1929 in an eight-page article "The origin of life" in The Rationalist Annual. According to Haldane the primitive Earth's atmosphere was essentially reducing, with little or no oxygen. Ultraviolet rays from the Sun induced reactions on a mixture of water, carbon dioxide, and ammonia. Organic substances such as sugars and protein components (amino acids) were synthesised. These molecules "accumulated till the primitive oceans reached the consistency of hot dilute soup." The first reproducing things were created from this soup.
As to the priority over the theory, Haldane accepted that Oparin came first, saying, "I have very little doubt that Professor Oparin has the priority over me."
One of the most important pieces of experimental support for the "soup" theory came in 1953. A graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that demonstrated how organic molecules could have spontaneously formed from inorganic precursors, under conditions like those posited by the Oparin-Haldane Hypothesis. The now-famous "Miller–Urey experiment" used a highly reduced mixture of gases—methane, ammonia and hydrogen—to form basic organic monomers, such as amino acids. This provided direct experimental support for the second point of the "soup" theory, and it is around the remaining two points of the theory that much of the debate now centers.
Apart from the Miller–Urey experiment, the next most important step in research on prebiotic organic synthesis was the demonstration by Joan Oró that the nucleic acid purine base, adenine, was formed by heating aqueous ammonium cyanide solutions. In support of abiogenesis in eutectic ice, more recent work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source).
The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were formed in high concentration during the Miller–Urey and Oró experiments. The Miller experiment, for example, produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.
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