Primordial soup

Primordial soup, also known as prebiotic soup and Haldane soup, is the hypothetical set of conditions present on the Earth around 3.7 to 4.0 billion years ago. It is an aspect of the heterotrophic theory (also known as the Oparin–Haldane hypothesis) concerning the origin of life, first proposed by Alexander Oparin in 1924, and J. B. S. Haldane in 1929.

As formulated by Oparin, in the primitive Earth's surface layers, carbon, hydrogen, water vapour, and ammonia reacted to form the first organic compounds. The concept of a primordial soup gained credence in 1953 when the Miller–Urey experiment used a highly reduced mixture of gases—methane, ammonia and hydrogen—to form basic organic monomers, such as amino acids.

Historical background

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:

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 (outside that of deity as a causal agent) 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 a 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 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".

Although Darwin did not speak explicitly about the origin of life in On the Origin of Species, he did mention a "warm little pond" in a letter to Joseph Dalton Hooker dated February 1, 1871:

Heterotrophic theory

A coherent scientific argument was introduced by 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 J. B. S. Haldane independently arrived at a 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:

According to the theory, organic compounds essential for life forms were synthesized in the primitive Earth under prebiotic conditions. The mixture of inorganic and organic compounds with water on the primitive Earth became the prebiotic or primordial soup. There, life originated and the first forms of life were able to use the organic molecules to survive and reproduce. Today the theory is variously known as the heterotrophic theory, 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 the prebiotic soup, which may have been concentrated at places such as shorelines and oceanic vents.
By further transformation, more complex organic polymers – and ultimately life – developed in the soup.

Oparin's theory

thumb|120px|right|Alexander Oparin

Alexander Oparin first postulated his theory (in the Russian language) 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. he modified the chemical composition of the primordial environment as strictly reducing, consisting of methane, ammonia, free hydrogen and water vapour—excluding oxygen.

Soda lakes are considered as environments that conserve and/or mimic ancient life conditions

and as "a recreated model of late Precambrian ocean chemistry" — that is, the "soda lake" environment that prepared the great explosion of life during the Cambrian.

Haldane's theory

thumb|right|121px|J.B.S. Haldane

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.

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."

Unanswered questions

Though Oparin and Haldane presented a convincing theory for the origin of life, there are some natural phenomena that their work fails to explain. It is understood, based on the heterotrophic theory, that at the time life was generated, the atmosphere was strongly reducing. However, evidence suggests that the atmosphere was likely not nearly reducing enough to support this. The availability of highly reduced compounds such as NH<sub>3</sub> and CH<sub>4</sub> was limited, there was likely not enough of them to support heterotrophic redox and life.

Another complication with the heterotrophic theory exists due to the selective chirality of biological molecules. Chirality refers to the lack of symmetry in biological molecules and which orientation they prefer. For instance, amino acids exist predominantly in the L conformation and sugars prefer the D conformation. Biological molecules are highly specific in which enantiomer they prefer. Because of this unique fact, scientists feel that the correct theory of the origin of life should explain this selective chirality.

The heterotrophic theory is highly specific and includes details about the conditions of early metabolism. However, in doing this, it is unable to provide a grounds for evolution and the distinction between bacteria, archaea, and eucarya. How did organisms that utilize the same type of metabolism become so highly differentiated? This is another unanswered question we are left with if the heterotrophic theory is true.

Finally, as the name implies, the heterotrophic theory indicates that early life on earth consisted entirely of heterotrophs. A condition of heterotrophic metabolism, is that the energetic substrate is not produced by the same organism that consumes it. Because of this, heterotrophy works well in tandem with other species that replenish the depleted substrate. However, if all early life was heterotrophic, there would be no way to regenerate the metabolite needed for energy production. The heterotrophic theory fails to explain this key fallacy.

Monomer formation

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. 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 Darwinian dynamic

The evolution of living systems by natural selection that presumably emerged in the primordial soup, and certain nonliving physical order-generating systems, were proposed to obey a common fundamental principle that was termed the Darwinian dynamic. The basic conditions necessary for natural selection to operate as conceived by Darwin are variation of type, heritability and competition for limited resources. These conditions can apply to short replicating RNA molecules that were presumably present in the primordial soup, and such RNA molecules have been proposed to have preceded the emergence of more complex life (see RNA world). The basic processes of natural selection applicable to short replicating RNA molecules were shown to have the same form and content as equations that govern the emergence of macroscopic order in nonliving systems maintained far from thermodynamic equilibrium.