History of research into the origin of life

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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.

Contents

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 about 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

Antonie van Leeuwenhoek Anton van Leeuwenhoek.png
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 Charles Darwin by Julia Margaret Cameron, c. 1868.jpg
Charles Darwin in 1879

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 to 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

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 Miller1999.jpg
Stanley Miller

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]

Related Research Articles

<span class="mw-page-title-main">Astrobiology</span> Science concerned with life in the universe

Astrobiology is a scientific field within the life and environmental sciences that studies the origins, early evolution, distribution, and future of life in the universe by investigating its deterministic conditions and contingent events. As a discipline, astrobiology is founded on the premise that life may exist beyond Earth.

<span class="mw-page-title-main">Hypothetical types of biochemistry</span> Possible alternative biochemicals used by life forms

Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons it may be chemically similar, though it is also possible that there are organisms with quite different chemistries – for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.

<span class="mw-page-title-main">Life</span> Matter with biological processes

Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy transformation, and reproduction. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. Biology is the science that studies life.

<span class="mw-page-title-main">Miller–Urey experiment</span> Chemical experiment that simulated conditions on the early Earth and tested the origin of life

The Miller–Urey experiment (or Miller experiment) was a chemistry experiment carried out in 1952 that simulated the conditions thought at the time to be present in the atmosphere of the early, prebiotic Earth, in order to test the hypothesis of the chemical origin of life under those conditions. The experiment used water (H2O), methane (CH4), ammonia (NH3), hydrogen (H2), and an electric arc (the latter simulating hypothesized lightning).

<span class="mw-page-title-main">Stanley Miller</span> American scientist (1930–2007)

Stanley Lloyd Miller was an American chemist who made landmark experiments in the origin of life by demonstrating that a wide range of vital organic compounds can be synthesized by fairly simple chemical processes from inorganic substances. In 1952 he carried out the Miller–Urey experiment, which showed that complex organic molecules could be synthesised from inorganic precursors. The experiment was widely reported, and provided support for the idea that the chemical evolution of the early Earth had led to the natural synthesis of chemical building blocks of life from inanimate inorganic molecules. He has been described as the "father of prebiotic chemistry".

<span class="mw-page-title-main">Heterotroph</span> Organism that ingests organic carbon for nutrition

A heterotroph is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers. Living organisms that are heterotrophic include all animals and fungi, some bacteria and protists, and many parasitic plants. The term heterotroph arose in microbiology in 1946 as part of a classification of microorganisms based on their type of nutrition. The term is now used in many fields, such as ecology in describing the food chain.

<span class="mw-page-title-main">Tholin</span> Class of molecules formed by ultraviolet irradiation of organic compounds

Tholins are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide, methane or ethane, often in combination with nitrogen or water. Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically nitriles and hydrocarbons and their degraded forms such as amines and phenyls. Tholins do not form naturally on modern-day Earth, but they are found in great abundance on the surfaces of icy bodies in the outer Solar System, and as reddish aerosols in the atmospheres of outer Solar System planets and moons.

Primordial soup, also known as primordial goo, primordial ooze, prebiotic soup and prebiotic broth, 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 concerning the origin of life, first proposed by Alexander Oparin in 1924, and J. B. S. Haldane in 1929.

<span class="mw-page-title-main">Alexander Oparin</span> Soviet biochemist (1894–1980)

Alexander Ivanovich Oparin was a Soviet biochemist notable for his theories about the origin of life, and for his book The Origin of Life. He also studied the biochemistry of material processing by plants and enzyme reactions in plant cells. He showed that many food production processes were based on biocatalysis and developed the foundations for industrial biochemistry in the USSR.

<span class="mw-page-title-main">Joan Oró</span> Spanish biochemist (1923–2004)

Joan Oró i Florensa was a Spanish biochemist, whose research has been of importance in understanding the origin of life. He participated in several NASA missions, including Apollo mission to the Moon and the Viking lander. He received the Oparin Medal, awarded by the International Astrobiology Society for his contributions to the field of origins of life.

A biosignature is any substance – such as an element, isotope, or molecule – or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical or chemical structures and its use of free energy and the production of biomass and wastes. A biosignature can provide evidence for living organisms outside the Earth and can be directly or indirectly detected by searching for their unique byproducts.

<span class="mw-page-title-main">Viking lander biological experiments</span>

In 1976 two identical Viking program landers each carried four types of biological experiments to the surface of Mars. The first successful Mars landers, Viking 1 and Viking 2, then carried out experiments to look for biosignatures of microbial life on Mars. The landers each used a robotic arm to pick up and place soil samples into sealed test containers on the craft.

<span class="mw-page-title-main">Sidney W. Fox</span> American biochemist (1912–1998)

Sidney Walter Fox was a Los Angeles-born biochemist responsible for discoveries on the origins of biological systems. Fox explored the synthesis of amino acids from inorganic molecules, the synthesis of proteinous amino acids and amino acid polymers called "proteinoids" from inorganic molecules and thermal energy, and created what he thought was the world's first protocell out of proteinoids and water. He called these globules "microspheres". Fox believed in the process of abiogenesis where life spontaneously organized itself from the colloquially known "primordial soup;" poolings of various simple organic molecules that existed during the time before life on Earth. He also suggested that his experiments possessed conditions that were similar to those of primordial Earth.

<span class="mw-page-title-main">Coacervate</span>

Coacervate is an aqueous phase rich in macromolecules such as synthetic polymers, proteins or nucleic acids. It forms through liquid-liquid phase separation (LLPS), leading to a dense phase in thermodynamic equilibrium with a dilute phase. The dispersed droplets of dense phase are also called coacervates, micro-coacervates or coacervate droplets. These structures draw a lot of interest because they form spontaneously from aqueous mixtures and provide stable compartmentalization without the need of a membrane.

<span class="mw-page-title-main">PAH world hypothesis</span> Hypothesis about the origin of life

The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons (PAHs), known to be abundant in the universe, including in comets, and assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world. However, as yet, the hypothesis is untested.

<span class="mw-page-title-main">Life on Titan</span> Scientific assessments on the microbial habitability of Titan

Whether there is life on Titan, the largest moon of Saturn, is currently an open question and a topic of scientific assessment and research. Titan is far colder than Earth, but of all the places in the Solar System, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface. Its thick atmosphere is chemically active and rich in carbon compounds. On the surface there are small and large bodies of both liquid methane and ethane, and it is likely that there is a layer of liquid water under its ice shell. Some scientists speculate that these liquid mixes may provide prebiotic chemistry for living cells different from those on Earth.

<span class="mw-page-title-main">Abiogenesis</span> Natural process by which life arises from non-living matter

In biology, abiogenesis or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. Many proposals have been made for different stages of the process.

Paleolightning refers to the remnants of ancient lightning activity studied in fields such as historical geology, geoarchaeology, and fulminology. Paleolightning provides tangible evidence for the study lightning activity in Earth's past and the roles lightning may have played in Earth's history. Some studies have speculated that lightning activity played a crucial role in the development of not only Earth's early atmosphere but also early life. Lightning, a non-biological process, has been found to produce biologically useful material through the oxidation and reduction of inorganic matter. Research on the impact of lightning on Earth's atmosphere continues today, especially with regard to feedback mechanisms of lightning-produced nitrate compounds on atmospheric composition and global average temperatures.

Formamide-based prebiotic chemistry is a reconstruction of the beginnings of life on Earth, assuming that formamide could accumulate in sufficiently high amounts to serve as the building block and reaction medium for the synthesis of the first biogenic molecules.

A scenario is a set of related concepts pertinent to the origin of life (abiogenesis), such as the iron-sulfur world. Many alternative abiogenesis scenarios have been proposed by scientists in a variety of fields from the 1950s onwards in an attempt to explain how the complex mechanisms of life could have come into existence. These include hypothesized ancient environments that might have been favourable for the origin of life, and possible biochemical mechanisms.

References

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  16. ↑ Lennox 2001 , pp. 229–258
  17. 1 2 Bernal 1967
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  24. ↑ Tyndall 1905 , IV, XII (1876), XIII (1878)
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  28. ↑ Kasting 1993 , p. 920
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  30. ↑ Oparin 1953
  31. ↑ Bryson 2004 , pp. 300–302
  32. ↑ Shapiro 1987 , p. 110
  33. ↑ Bernal 1951
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