History of research into the origin of life

The history of research into the origin of life encompasses theories about how life began, from ancient times with the philosophy of Aristotle through to the Miller-Urey experiment in 1952.

Panspermia

Main article: Panspermia

Panspermia is the hypothesis that life exists throughout the universe, distributed by meteoroids, asteroids, comets^[1] and planetoids.^[2] It does not attempt to explain how life originated, but shifts the origin to another heavenly body. The advantage is that life is not required to have formed on each planet it occurs on, but rather in a single location, and then spread across the galaxy to other star systems via cometary or meteorite impact.^[3] Evidence for this is scant, but it finds some support in studies of Martian meteorites found in Antarctica and of extremophile microbes' survival in outer space tests.^[4][5][6][7] Terrestrial bacteria, particularly Deinococcus radiodurans, highly resistant to environmental hazards, could survive for at least three years in outer space, based on studies on the International Space Station.^[8][9]

An extreme speculation is that the biochemistry of life could have begun as early as 17 My (million years) after the Big Bang, during a supposedly habitable epoch, and that life may exist throughout the universe.^[10][11] Carl Zimmer has speculated that the chemical conditions, including boron, molybdenum and oxygen needed to create RNA, may have been better on early Mars than on early Earth.^[12][13][14] If so, life-suitable molecules originating on Mars would have later migrated to Earth via meteor ejections.

Spontaneous generation

General acceptance until 19th century

Main article: Spontaneous generation

Antonie van Leeuwenhoek

Traditional religion attributed the origin of life to deities who created the natural world. Spontaneous generation, the first naturalistic theory of abiogenesis, goes back to Aristotle and ancient Greek philosophy, and continued to have support in Western scholarship until the 19th century.^[15] The theory held that "lower" animals are generated by decaying organic substances. Aristotle stated that, for example, aphids arise from dew on plants, flies from putrid matter, mice from dirty hay, and crocodiles from rotting sunken logs.^[16] The basic idea was that life was continuously created as a result of chance events.^[17] In the 17th century, people began to question spontaneous generation, in works like Thomas Browne's Pseudodoxia Epidemica. His contemporary, Alexander Ross, erroneously rebutted him.^[18][19] In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria.^[20] Many felt their existence supported spontaneous generation, since they seemed too simplistic for sexual reproduction, and asexual reproduction: cell division had not yet been observed. Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect.^[21] In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs.^[22] In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could be killed by boiling. In 1861, Louis Pasteur's experiments demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but could only appear by invasion from without.^{[citation needed]}

Considered disproven in 19th century

Charles Darwin in 1879

Main article: Biogenesis

By the middle of the 19th century, biogenesis was supported by so much evidence that spontaneous generation had been effectively disproven. Pasteur remarked, about an 1864 finding of his, "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment."^[23][24] This gave a mechanism by which life diversified from a few simple organisms to a variety of complex forms. Today, scientists agree that all current life descends from earlier life, which has become progressively more complex and diverse through Charles Darwin's mechanism of evolution by natural selection.

Darwin wrote to J.D. Hooker on 29 March 1863 stating that "It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter". In On the Origin of Species, he had referred to life having been "created", by which he "really meant 'appeared' by some wholly unknown process", but had soon regretted using the Old Testament term "creation".^[25]

Oparin: Primordial soup hypothesis

Main article: Primordial soup

There is no single generally accepted model for the origin of life. Scientists have proposed several plausible hypotheses which share some common elements. While differing in details, these hypotheses are based on the framework laid out by Alexander Oparin (in 1924) and John Haldane (in 1929), that the first molecules constituting the earliest cells

. . . were synthesized under natural conditions by a slow process of molecular evolution, and these molecules then organized into the first molecular system with properties with biological order".^[26]

Oparin and Haldane suggested that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH₄), ammonia (NH₃), water (H₂O), hydrogen sulfide (H₂S), carbon dioxide (CO₂) or carbon monoxide (CO), and phosphate (PO₄³⁻), with molecular oxygen (O₂) and ozone (O₃) either rare or absent. According to later models, the atmosphere in the late Hadean period consisted largely of nitrogen (N₂) and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen (H₂), and sulfur compounds;^[27] while it did lack molecular oxygen and ozone,^[28] it was not as chemically reducing as Oparin and Haldane supposed.

No new notable research or hypothesis on the subject appeared until 1924, when Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for life. In his book The Origin of Life,^[29][30] he proposed (echoing Darwin) that the "spontaneous generation of life" that had been attacked by Pasteur did, in fact, occur once, but was now impossible because the conditions found on the early Earth had changed, and preexisting organisms would immediately consume any spontaneously generated organism. Oparin argued that a "primeval soup" of organic molecules could be created in an oxygenless atmosphere through the action of sunlight. These would combine in ever more complex ways until they formed coacervate droplets. These droplets would "grow" by fusion with other droplets, and "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which factors that promote "cell integrity" survive, and those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.

About this time, Haldane suggested that the Earth's prebiotic oceans (quite different from their modern counterparts) would have formed a "hot dilute soup" in which organic compounds could have formed. Bernal called this idea biopoiesis or biopoesis, the process of living matter evolving from self-replicating but non-living molecules,^[17][31] and proposed that biopoiesis passes through a number of intermediate stages.

Robert Shapiro has summarized the "primordial soup" theory of Oparin and Haldane in its "mature form" as follows:^[32]

1. The early Earth had a chemically reducing atmosphere.
2. This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
3. These compounds accumulated in a "soup" that may have concentrated at various locations (shorelines, oceanic vents etc.).
4. By further transformation, more complex organic polymers—and ultimately life—developed in the soup.

John Bernal

John Bernal showed that based upon this and subsequent work there is no difficulty in principle in forming most of the molecules we recognize as the necessary molecules for life from their inorganic precursors. The underlying hypothesis held by Oparin, Haldane, Bernal, Miller and Urey, for instance, was that multiple conditions on the primeval Earth favoured chemical reactions that synthesized the same set of complex organic compounds from such simple precursors. Bernal coined the term biopoiesis in 1949 to refer to the origin of life.^[33] In 1967, he suggested that it occurred in three "stages":

1. the origin of biological monomers
2. the origin of biological polymers
3. the evolution from molecules to cells

Bernal suggested that evolution commenced between stages 1 and 2. Bernal regarded the third stage, in which biological reactions were incorporated behind a cell's boundary, as the most difficult. Modern work on the way that cell membranes self-assemble, and the work on micropores in various substrates, may be a key step towards understanding the development of independent free-living cells.^[34][35][36]

Miller–Urey experiment

Stanley Miller

Main article: Miller–Urey experiment

In 1952, Stanley Miller and 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 Miller–Urey experiment used a highly reducing mixture of gases—methane, ammonia, and hydrogen, as well as water vapor—to form simple organic monomers such as amino acids.^[37] The mixture of gases was cycled through an apparatus that delivered electrical sparks to the mixture. After one week, it was found that about 10% to 15% of the carbon in the system was then in the form of a racemic mixture of organic compounds, including amino acids, which are the building blocks of proteins. 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 centers. A 2011 reanalysis of the saved vials has uncovered more biochemicals than originally discovered in the 1950s, including 23 amino acids, not just five.^[38] 2020 studies suggest that the primeval atmosphere of the Earth was much different than the conditions used in the Miller-Urey studies.^[39][40]

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