In the natural sciences,
abiogenesis, or "
chemical evolution", is the study of how
life on EarthLife on Earth: A Natural History by David Attenborough is a groundbreaking television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...
could have arisen from inanimate matter. It should not be confused with
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
, which is the study of how groups of living things change over time.
Amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
s, often called "the building blocks of life", can form via natural chemical reactions unrelated to life, as demonstrated in the Miller–Urey experiment, which involved simulating the conditions of the early Earth. In all living things, these amino acids are organized into
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s, and the construction of these proteins is mediated by
nucleic acidA nucleic acid is a macromolecule composed of chains of monomeric nucleotides. In biochemistry these molecules carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid and ribonucleic acid . Nucleic acids are universal in living things, as...
s. Thus the question of how life on Earth originated is a question of how the first nucleic acids arose.
The first living things on Earth are thought to be single cell
prokaryoteThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles...
s. The oldest ancient fossil microbe-like objects are dated to be 3.5 Ga (billion years old), just a few hundred million years younger than Earth itself. By 2.4 Ga, the ratio of stable isotopes of
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
,
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
and
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
shows the action of living things on inorganic minerals and sediments and molecular biomarkers indicate
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
, demonstrating that life on Earth was widespread by this time.
On the other hand, the exact sequence of chemical events that led to the first nucleic acids is not known. Several hypotheses about early life have been proposed, most notably the
iron-sulfur world theoryThe iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich chemist and patent lawyer, involving forms of iron and sulfur...
(
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
without
geneticsGenetics, , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding...
) and the
RNA world hypothesisThe RNA world hypothesis proposes that a world filled with life based on ribonucleic acid predates the current world of life based on deoxyribonucleic acid . RNA, which can both store information like DNA and act as an enzyme, may have supported cellular or pre-cellular life...
(RNA life-forms).
Spontaneous generation
Until the early 19th century, people generally believed in the ongoing
spontaneous generationSpontaneous generation or Equivocal generation is an obsolete theory regarding the origin of life from inanimate matter, which held that this process was a commonplace and everyday occurrence, as distinguished from univocal generation, or reproduction from parent...
of certain forms of
lifeLife is a characteristic that distinguishes objects that have self-sustaining biological processes from those that do not—either because such functions have ceased , or else because they lack such functions and are classified as "inanimate."In biology, the science of living organisms, "life"...
from non-living matter. This was paired with heterogenesis, the belief that one form of life derives from a different form (
e.g. bees from flowers). Classical notions of abiogenesis, now more precisely known as
spontaneous generation, held that certain complex, living
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s are generated by decaying organic substances. According to
AristotleAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
it was a readily observable truth that
aphidAphids, also known as plant lice , are small plant-eating insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...
s arise from the dew which falls on plants,
fleaFlea is the common name for insects of the order Siphonaptera which are wingless insects whose mouthparts are adapted for piercing skin and sucking blood....
s from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so on.
In the 17th century, such assumptions started to be questioned; for example, in 1646, Sir Thomas Browne published his
Pseudodoxia EpidemicaSir Thomas Browne's vast work refuting the common errors and superstitions of his age, Pseudodoxia Epidemica, first appeared in 1646 and went through five subsequent editions, the last revision occurring in 1672. Also known as Vulgar Errors, derived from its full title, Pseudodoxia Epidemica or...
(subtitled
Enquiries into Very many Received Tenets, and Commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His conclusions were not widely accepted. For example, his contemporary,
Alexander RossAlexander Ross was a prolific Scottish writer and controversialist.-Life:He was born in Aberdeen, and entered King's College, Aberdeen, in 1604. About 1616 he succeeded Thomas Parker in the mastership of the free school at Southampton, an appointment which he owed to Edward Seymour, 1st Earl of...
wrote: "To question this (i.e., spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to
EgyptEgypt , officially the Arab Republic of Egypt, is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Western Asia...
, and there he will find the fields swarming with mice, begot of the mud of
NylusThe Nile is a major north-flowing river in Africa, generally regarded as the longest river in the world....
, to the great calamity of the inhabitants."
In 1665,
Robert HookeRobert Hooke, FRS was an English natural philosopher, architect and polymath who played an important role in the scientific revolution, through both experimental and theoretical work....
published the first drawings of a microorganism. Hooke was followed in 1676 by
Anton van LeeuwenhoekAntonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, the Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...
, who drew and described microorganisms that are now thought to have been
protozoaProtozoa or Cornelius protozoans Protozoa or Cornelius protozoans Protozoa or Cornelius protozoans (from Greek πρῶτον proton "first" and ζῷα zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes....
and
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for
sexual reproductionSexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
, and
asexual reproductionAsexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes...
through
cell divisionMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
had not yet been observed.
The first solid evidence against spontaneous generation came in 1668 from
Francesco RediFrancesco Redi was an Italian physician.He is most well-known for his series of experiments, published in 1668 as Esperienze Intorno alla Generazione degl'Insetti which is regarded as one of the first steps in refuting "spontaneous generation" - a theory also known as Aristotelian...
, who proved that no
maggotMaggot is the common name of the larval phase of development in insects of the order Diptera . Sometimes the word is used to denote the larval stage of any insects.-Fishing:...
s appeared in meat when flies were prevented from laying eggs. It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be
biogenesisBiogenesis is the process of lifeforms producing other lifeforms, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Generatio spontanea:...
: that every living thing came from a pre-existing living thing (
omne vivum ex ovo, Latin for "every living thing from an egg").
In 1768,
Lazzaro SpallanzaniLazzaro Spallanzani was an Italian biologist and physiologist who made important contributions to the experimental study of bodily functions, animal reproduction, and essentially discovered echolocation...
demonstrated that microbes were present in the air, and could be killed by boiling. In 1861,
Louis PasteurLouis Pasteur was a French chemist and microbiologist born in Dole. He is best known for his remarkable breakthroughs in the causes and preventions of disease. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies. His experiments supported the germ...
performed a series of experiments which demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media.
Pasteur and Darwin
By the middle of the 19th century, the theory of
biogenesisBiogenesis is the process of lifeforms producing other lifeforms, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Generatio spontanea:...
had accumulated so much evidential support, due to the work of
Louis PasteurLouis Pasteur was a French chemist and microbiologist born in Dole. He is best known for his remarkable breakthroughs in the causes and preventions of disease. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies. His experiments supported the germ...
and others, that the alternative theory of spontaneous generation had been effectively disproven. Pasteur himself remarked, after a definitive finding in 1864, "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment."
The collapse of spontaneous generation, however, left a vacuum of scientific thought on the question of how life
had first arisen.
In a letter to
Joseph Dalton HookerSir Joseph Dalton Hooker OM, GCSI, CB, MD, FRS was one of the greatest British botanists and explorers of the 19th century. Hooker was one of the founders of geographical botany, and Charles Darwin's closest friend...
on February 1, 1871,
Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
addressed the question, suggesting that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes". He went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." In other words, the presence of life itself makes the search for the origin of life dependent on the sterile conditions of the laboratory.
"Primordial soup" theory
No new notable research or theory on the subject appeared until 1924, when Alexander Oparin (Aleksandr I. Oparin) reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for the evolution of life. In his
The Origin of Life, Oparin proposed that the "spontaneous generation of life" that had been attacked by
Louis PasteurLouis Pasteur was a French chemist and microbiologist born in Dole. He is best known for his remarkable breakthroughs in the causes and preventions of disease. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies. His experiments supported the germ...
, did in fact occur once, but was now impossible because the conditions found in the early earth had changed, and the presence of living organisms would immediately consume any spontaneously generated organism. Oparin argued that a "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine in ever-more complex fashions until they formed
coacervateA coacervate is a tiny spherical droplet of assorted organic molecules which is held together by hydrophobic forces from a surrounding liquid....
droplets. These droplets would "
growThe term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells".-Cell populations:Cell populations go through a type of...
" by fusion with other droplets, and "
reproduceReproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction...
" through fission into daughter droplets, and so have a primitive
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
in which those factors which promote "cell integrity" survive, those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.
Around the same time,
J. B. S. HaldaneJohn Burdon Sanderson Haldane FRS , known as Jack , was a British-born geneticist and evolutionary biologist...
suggested that the Earth's pre-biotic oceans–very different from their modern counterparts–would have formed a "hot dilute soup" in which organic compounds could have formed. This idea was called
biopoiesis or
biopoesis, the process of living matter evolving from self-replicating but nonliving molecules.
Early conditions
Morse and MacKenzie have suggested that
oceans may have appearedThe question of the origin of water on Earth, or more accurately put, the question of why there is clearly more water on the Earth than on the other planets of the Solar System, has not been clarified. There are various popular theories as to how the world's oceans were formed over the past 4.6...
first in the
HadeanThe Hadean is the geologic eon before the Archean. It started at Earth's formation about 4.6 billion years ago , and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld," referring to the...
eon, as soon as two hundred million years (200 Ma) after the Earth was formed, in a hot
reducingRedox describes all chemical reactions in which atoms have their oxidation number changed....
environment, and that the
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
of about 5.8 rose rapidly towards neutral. This has been supported by Wilde who has pushed the date of the
zirconZircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1-x4x-y...
crystals found in the metamorphosed
quartziteQuartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts...
of
Mount NarryerThe Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6...
in Western Australia, previously thought to be 4.1–4.2 Ga, to 4.404 Ga. This means that oceans and
continental crustThe continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
existed within 150 Ma of Earth's formation.
Despite this, the
HadeanThe Hadean is the geologic eon before the Archean. It started at Earth's formation about 4.6 billion years ago , and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld," referring to the...
environment was one highly hazardous to life. Frequent collisions with large objects, up to in diameter, would have been sufficient to vaporise the ocean within a few months of impact, with hot steam mixed with rock vapour leading to high altitude clouds completely covering the planet. After a few months the height of these clouds would have begun to decrease but the cloud base would still have been elevated for about the next thousand years. After that, it would have begun to rain at low altitude. For another two thousand years rains would slowly have drawn down the height of the clouds, returning the oceans to their original depth only 3,000 years after the impact event.
Between 3.8 and 4.1 Ga, changes in the orbits of the
gaseous giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune...
planets may have caused a
late heavy bombardmentThe Late Heavy Bombardment is a period of time approximately 3.8 to 4.1 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
that pockmarked the moon and other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have sterilized the planet had life appeared before that time.
By examining the time interval between such devastating environmental events, the time interval when life might first have come into existence can be found for different early environments. The study by Maher and Stephenson shows that if the deep marine hydrothermal setting provides a suitable site for the origin of life, abiogenesis could have happened as early as 4.0 to 4.2 Ga, whereas if it occurred at the surface of the earth abiogenesis could only have occurred between 3.7 and 4.0 Ga.
Other research suggests a colder start to life. Work by
Leslie OrgelLeslie Eleazer Orgel FRS was a British chemist.Born in London, England, Orgel received his B.A. in chemistry with first class honours from Oxford University in 1949...
and colleagues on the synthesis of purines has shown that freezing temperatures are advantageous, due to the concentrating effect for key precursors such as
HCNHydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. Hydrogen cyanide is a colorless, extremely poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C...
.
Research by
Stanley MillerStanley Lloyd Miller was an American chemist and biologist who is known for his studies into the origin of life, particularly the Miller-Urey experiment which demonstrated that organic compounds can be created by fairly simple physical processes from inorganic substances...
and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Based on this research, Miller suggested a beginning of life involving freezing conditions and exploding meteorites. A new article in
Discover MagazineDiscover is a science magazine that publishes articles about science for a general audience. The monthly magazine was launched in October 1980 by Time Inc. It was sold to Family Media, the owners of Health, in 1987. Walt Disney Company bought the magazine when Family Media went out of business in...
points to research by the Miller group indicating the formation of seven different amino acids and 11 types of nucleobases in ice when
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
and
cyanideA cyanide is any chemical compound that contains the cyano group , which consists of a carbon atom triple-bonded to a nitrogen atom. Inorganic cyanides are hydrogen cyanide salts in which cyanide is generally the anion CN
-. Organic compounds that have a -C≡N functional group bonded to...
were left in a freezer from 1972–1997. This article also describes research by Christof Biebricher showing the formation of RNA molecules 400 bases long under freezing conditions using an RNA template, a single-strand chain of RNA that guides the formation of a new strand of RNA. As that new RNA strand grows, it adheres to the template. The explanation given for the unusual speed of these reactions at such a low temperature is
eutectic freezingThe melting point of a mixture of two or more solids depends on the relative proportions of its ingredients. A eutectic or eutectic mixture is a mixture at such proportions that the melting point is a local temperature minimum, which means that all the constituents crystallize simultaneously at...
. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often.
Evidence of the early appearance of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in the nearby
Akilia IslandAkilia Island is an island in southwestern Greenland, about 22 kilometers south of Nuuk , at . Akilia is the location of a rock formation that has been proposed to contain the oldest known sedimentary rocks on Earth,...
s. Carbon entering into rock formations has a ratio of
Carbon-13Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by NMR spectroscopy :...
(
13C) to
Carbon-12Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
(
12C) of about −5.5 (in units of δ
13C), where because of a preferential biotic uptake of
12C,
biomassBiomass, a renewable energy source, is biological material derived from living, or recently living organisms, such as wood, waste, and alcohol fuels. Biomass is commonly plant matter grown to generate electricity or produce heat. For example, forest residues , yard clippings and wood chips may be...
has a δ
13C of between −20 and −30. These isotopic fingerprints are preserved in the sediments, and Mojzis has used this technique to suggest that life existed on the planet already by 3.85 billion years ago. Lazcano and Miller (1994) suggest that the rapidity of the evolution of life is dictated by the rate of recirculating water through mid-ocean submarine vents. Complete recirculation takes 10 million years, thus any organic compounds produced by then would be altered or destroyed by temperatures exceeding . They estimate that the development of a 100 kilobase genome of a DNA/protein primitive
heterotrophA Heterotroph is an organism that uses organic carbon for growth. This contrasts with autotrophs, such as plants, which are able to directly use sources of energy, such as light to produce organic substrates from inorganic carbon dioxide.- Ecology :Heterotrophs are known as consumers in food...
into a 7000 gene filamentous
cyanobacteriumCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
would have required only 7 Ma.
Current models
There is no truly "standard model" of the origin of life. Most currently accepted models draw at least some elements from the framework laid out by the Oparin-Haldane hypothesis. Under that umbrella, however, are a wide array of disparate discoveries and conjectures such as the following, listed in a rough order of postulated emergence:
- Some theorists suggest that the atmosphere of the early Earth may have been chemically reducing
Redox describes all chemical reactions in which atoms have their oxidation number changed....
in nature, composed primary of methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
(CH4), ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
(NH3), waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
(H2O), hydrogen sulfideHydrogen sulfide is the chemical compound with the formula H
2S. This colorless, toxic and flammable gas is partially responsible for the foul odor of rotten eggs and flatulence....
(H2S), carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
(CO2) or carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
(CO), and phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
(PO43-), with molecular oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
(O2) and ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
(O3) either rare or absent.
- In such a reducing atmosphere, electrical activity can catalyze the creation of certain basic small molecule
A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
s (monomerA monomer is a small molecule that may become chemically bonded to other monomers to form a polymer...
s) of life, such as amino acids. This was demonstrated in the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
- Phospholipid
Phospholipids are a class of lipids and are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
s (of an appropriate length) can spontaneously form lipid bilayerA lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and...
s, a basic component of the cell membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
.
- A fundamental question is about the nature of the first self-replicating molecule. Since replication is accomplished in modern cells through the cooperative action of proteins and nucleic acids, the major schools of thought about how the process originated can be broadly classified as "proteins first" and "nucleic acids first".
- The principal thrust of the "nucleic acids first" argument is as follows:
- The polymer
A polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
ization of nucleotideNucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides play central roles in metabolism...
s into random RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
molecules might have resulted in self-replicating ribozymeA ribozyme is an RNA molecule that catalyzes a chemical reaction...
s (RNA world hypothesisThe RNA world hypothesis proposes that a world filled with life based on ribonucleic acid predates the current world of life based on deoxyribonucleic acid . RNA, which can both store information like DNA and act as an enzyme, may have supported cellular or pre-cellular life...
)
- Selection
In the context of evolution, certain traits or alleles of a species may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the succeeding generation than...
pressures for catalytic efficiency and diversity might have resulted in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. The first ribosomeRibosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
might have been created by such a process, resulting in more prevalent protein synthesis.
- Synthesized proteins might then outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer, relegating nucleic acids to their modern use, predominantly as a carrier of genomic information.
As of 2009, no one has yet synthesized a "protocell" using basic components which would have the necessary properties of life (the so-called
"bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers are working in this field, notably
Steen RasmussenSteen Rasmussen was born in Elsinore, Denmark, in 1955. He is an Artificial Life scientist who has published numerous reviews and reports in the Journal, Artificial Life. He coined the term, complex systems dogma, which alludes to the presupposition that simple lower-level elements can give rise to...
at
Los Alamos National LaboratoryLos Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security, LLC , located in Los Alamos, New Mexico...
and Jack Szostak at
Harvard UniversityHarvard University is a private university located in Cambridge, Massachusetts and a member of the Ivy League. Founded in 1636 by the colonial Massachusetts legislature, Harvard is the oldest institution of higher learning in the United States and currently comprises ten separate academic units...
. Others have argued that a
"top-down approach" is more feasible. One such approach, attempted by
Craig VenterJohn Craig Venter is an American biologist and entrepreneur. Venter founded The Institute for Genomic Research and the J. Craig Venter Institute, now working at the latter to create synthetic biological organisms for environmental change and to document genetic diversity in the world's oceans...
and others at
The Institute for Genomic ResearchThe Institute for Genomic Research was a non-profit genomics research institute founded in 1992 by Craig Venter in Rockville, Maryland, United States. It is now a part of the J. Craig Venter Institute.-History:...
, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist John Desmond Bernal coined the term Biopoesis for this process, and suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life.
- Stage 1: The origin of biological monomers
- Stage 2: The origin of biological polymers
- Stage 3: The evolution from molecules to cell
Bernal suggested that
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
may have commenced early, some time between Stage 1 and 2.
Origin of organic molecules
There are two possible sources of organic molecules on the early Earth:
- Terrestrial origins – organic synthesis driven by impact shocks or by other energy sources (such as ultraviolet light or electrical discharges) (eg.Miller's experiments)
- Extraterrestrial origins – delivery by objects (eg carbonaceous chondrites) or gravitational attraction of organic molecules or primitive life-forms from space
Recently, estimates of these sources suggest that the
heavy bombardmentThe Late Heavy Bombardment is a period of time approximately 3.8 to 4.1 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
before 3.5 Ga within the early atmosphere made available quantities of organics comparable to those produced by other energy sources.
"Soup" theory today: Miller's experiment and subsequent work
Biochemist Robert Shapiro has summarized the "Primordial Soup" theory of Oparin and Haldane in its "mature form" as follows:
- The early Earth had a chemically reducing atmosphere, as discussed above.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomer
A monomer is a small molecule that may become chemically bonded to other monomers to form a polymer...
s").
- These compounds accumulated in a "soup".
- By further transformation, more complex organic polymer
A polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
s— and ultimately life— developed in the soup.
Regarding the reducing atmosphere
Whether the mixture of gases used in the Miller–Urey experiment truly reflects the atmospheric content of
early EarthThe "early Earth" is a term usually defined as Earth's first billion years, or gigayear. On the geologic time scale, the "early Earth" comprises all of the Hadean eon , as well as the Eoarchean and part of the Paleoarchean eras of the Archean eon.This period of Earth's history, being its earliest,...
is a controversial topic. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case. (See
Oxygen catastropheThe Great Oxidation Event was a major environmental change around associated with the appearance of free oxygen in the atmosphere....
).
Regarding monomer formation
One of the most important pieces of experimental support for the "soup" theory came in 1953. A graduate student,
Stanley MillerStanley Lloyd Miller was an American chemist and biologist who is known for his studies into the origin of life, particularly the Miller-Urey experiment which demonstrated that organic compounds can be created by fairly simple physical processes from inorganic substances...
, and his professor,
Harold UreyHarold Clayton Urey was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 and later led him to theories of planetary evolution.-Biography:...
, 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–
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
, ammonia and
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
–to form basic organic
monomerA monomer is a small molecule that may become chemically bonded to other monomers to form a polymer...
s, such as amino acids. This provided direct experimental support for the second point of the "soup" theory as described above, and it is around the remaining three points of the theory that much of the debate now centers.
Apart from the Miller–Urey experiment, described above, the next most important step in research on prebiotic organic synthesis was the demonstration by John Oró that the nucleic acid purine base, adenine, was formed by heating aqueous ammonium cyanide solutions. In support of abiogenisis in eutectic ice (see above), more recent work demonstrated the formation of s-
triazineA triazine is one of three organic chemicals, isomeric with each other, whose empirical formula is
333.- Structure :...
s (alternative
nucleobaseNucleobases are the parts of DNA and RNA that may be involved in pairing . The main ones are cytosine, guanine, adenine , thymine and uracil , abbreviated as C, G, A, T, and U, respectively. They are usually simply called bases in genetics...
s),
pyrimidinePyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
s (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).
Regarding monomer accumulation
The "soup" theory relies on the assumption proposed by Darwin (see above) that in an environment with no pre-existing life, organic molecules may have accumulated and provided an environment for
chemical evolutionIn the natural sciences, abiogenesis, or "chemical evolution", is the study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution, which is the study of how groups of living things change over time...
.
Regarding further transformation
The spontaneous formation of complex
polymerA polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
s 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.
More fundamentally, it can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment
hydrolysisHydrolysis is a chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions, which may go on to participate in further reactions. It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization...
of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers.
The deep sea vent theory
The deep sea vent, or
hydrothermal ventA hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots....
, theory for the origin of life on Earth posits that life may have begun at submarine hydrothermal vents, where hydrogen-rich fluids emerge from below the sea floor and interface with carbon dioxide-rich ocean water. Sustained chemical energy in such systems is derived from redox reactions, in which electron donors, such as molecular hydrogen, react with electron acceptors, such as carbon dioxide (see
iron-sulfur world theoryThe iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich chemist and patent lawyer, involving forms of iron and sulfur...
).
Fox's experiments
In the 1950s and 1960s,
Sidney W. FoxSidney Walter Fox was a Los Angeles-born biochemist responsible for unique discoveries in the autosynthesis of protocells.-Professor:In 1943 Fox was granted his first academic position at Iowa State College....
studied the spontaneous formation of
peptidePeptides are short polymers formed from the linking, in a defined order, of α-amino acids. The link between one amino acid residue and the next is called an amide bond or a peptide bond....
structures under conditions that might plausibly have existed early in Earth's history. He demonstrated that amino acids could spontaneously form small peptides. These amino acids and small peptides could be encouraged to form closed spherical membranes, called protenoid microspheres, which show many of the basic characteristics of 'life'.
Eigen's hypothesis
In the early 1970s the problem of the origin of life was approached by
Manfred EigenManfred Eigen is a German biophysicist who won the 1967 Nobel Prize in Chemistry for work on measuring fast chemical reactions.-Career:...
and
Peter SchusterPeter K. Schuster is a renowned biophysicist, known for his work with the German Nobel Laureate Manfred Eigen in developing the quasispecies model...
of the
Max Planck Institute for Biophysical ChemistryThe Max Planck Institute for Biophysical Chemistry in Göttingen is a research institute of the Max Planck Society. Currently, 730 people work at the Institute, 370 of them are scientists....
. They examined the transient stages between the molecular chaos and a self-replicating
hypercycleHypercycle may refer to:* Hypercycle , a line of equal distance in hyperbolic geometry* Hypercycle , a kind of reaction network prominent in a theory of the self-organization of matter, see also Manfred Eigen...
in a prebiotic soup.
In a hypercycle, the
information storing systemInformation as a concept has many meanings, from everyday usage to technical settings. The concept of information is closely related to notions of constraint, communication, control, data, form, instruction, knowledge, meaning, mental stimulus, pattern, perception, and representation.The English...
(possibly
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
) produces an
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create
quasispeciesThe quasispecies model is a description of the process of the Darwinian evolution of certain self-replicating entities within the framework of physical chemistry...
, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances, forms itself into
ribozymeA ribozyme is an RNA molecule that catalyzes a chemical reaction...
s, capable of catalyzing their own chemical reactions. However, these reactions are limited to self-excisions (in which a longer RNA molecule becomes shorter), and much rarer small additions that are incapable of coding for any useful protein. The hypercycle theory is further degraded since the hypothetical RNA would require the existence of complex biochemicals such as nucleotides which are not formed under the conditions proposed by the Miller–Urey experiment.
Wächtershäuser's hypothesis
Another possible answer to this polymerization conundrum was provided in 1980s by the German chemist
Günter WächtershäuserGünter Wächtershäuser , a German chemist turned patent lawyer, is mainly known for his work on the origin of life, and in particular his iron-sulfur world theory, a theory that life on Earth had hydrothermal origins...
, in his
iron-sulfur world theoryThe iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich chemist and patent lawyer, involving forms of iron and sulfur...
. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.
In contrast to the classical Miller experiments, which depend on external sources of energy (such as simulated lightning or UV irradiation), "Wächtershäuser systems" come with a built-in source of energy,
sulfideA sulfide is a chemical compound containing sulfur in its lowest oxidation number of −2.- Properties :...
s of
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
and other minerals (e.g. pyrite). The energy released from
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of
oligomerIn chemistry, an oligomer consists of a less than five number of monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Many oils are oligomeric, such as liquid paraffin. Plasticizers are oligomeric esters widely used to soften...
s and
polymerA polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
s. It is therefore hypothesized that such systems may be able to evolve into autocatalytic sets of self-replicating, metabolically active entities that would predate the life forms known today.
The experiment produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of
tripeptideA tripeptide is a peptide consisting of three amino acids joined by peptide bonds.Examples of tripeptides are:*Isoleucine-Proline-Proline These Lactotripeptides are responsible for a blood pressure lowering effect...
s (0.10%) but the authors also noted that: "under these same conditions dipeptides hydrolysed rapidly."
Radioactive beach hypothesis
Zachary Adam at the
University of WashingtonUniversity of Washington, founded in 1861, is a public research university in Seattle, Washington, United States. UW is the largest university in the northwestern United States and one of the oldest public universities on the west coast. The university has three campuses, with its flagship campus...
, Seattle, claims that stronger tidal processes from a much closer moon may have concentrated grains of
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
and other radioactive elements at the high water mark on primordial beaches where they may have been responsible for generating life's building blocks. According to computer models reported in
AstrobiologyFor the International Journal of Astrobiology, see International Journal of Astrobiology.Astrobiology is a peer-reviewed scientific journal specialising in the origin, evolution, distribution and future of life across the universe. It is a multidisciplinary journal, drawing together a wide range...
, a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the
OkloOklo is a region near the town of Franceville, in the Haut-Ogooué province of the Central African state of Gabon.The discovery in September 1972 of several natural nuclear fission reactors in the uranium mines situated there has fired the imagination and aroused the curiosity of...
uranium ore seam in
GabonGabon is a country in west central Africa sharing borders with the Gulf of Guinea to the west, Equatorial Guinea to the northwest, and Cameroon to the north, with the Republic of the Congo curving around the east and south. Its size is almost 270,000 km² with an estimated population...
. Such radioactive beach sand provides sufficient energy to generate organic molecules, such as
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
s and
sugarSugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many...
s from
acetonitrileAcetonitrile is the chemical compound with formula CH3CN. This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture. It is widely used as a polar aprotic solvent in synthetic chemistry, and as a medium-polarity solvent in...
in water. Radioactive
monaziteMonazite is a reddish-brown phosphate mineral containing rare earth metals and is an important source of thorium, lanthanum, and cerium. It occurs usually in small isolated crystals...
also releases soluble
phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
into regions between sand-grains, making it biologically "accessible". Thus amino acids, sugars and soluble phosphates can all be simultaneously produced, according to Adam. Radioactive
actinideThe actinoid or actinide series encompasses the 15 chemical elements that lie between actinium and lawrencium included on the periodic table, with atomic numbers 89 - 103...
s, then in greater concentrations, could have formed part of organo-metallic complexes. These complexes could have been important early catalysts to living processes.
John Parnell of the
University of AberdeenThe University of Aberdeen is an ancient university founded in 1495, in Aberdeen, Scotland. It is the fifth oldest university in what is now the United Kingdom, and in the wider English-speaking world....
suggests that such a process could provide part of the "crucible of life" on any early wet rocky planet, so long as the planet is large enough to have generated a system of
plate tectonicsPlate tectonics is a theory which describes the large scale motions of Earth's lithosphere...
which brings radioactive minerals to the surface. As the early Earth is believed to have had many smaller "platelets" it would provide a suitable environment for such processes.
Models to explain homochirality
Some process in chemical evolution must account for the origin of
homochiralityHomochirality is a term used to refer to a group of molecules that possess the same sense of chirality. Molecules involved are not necessarily the same compound, but similar groups are arranged in the same way around a central atom. In biology homochirality is found inside living organisms...
, i.e. all building blocks in living organisms having the same "handedness" (amino acids being left-handed, nucleic acid sugars (
riboseRibose, primarily occurring as
D-ribose, is an organic compound that occurs widely in nature. It is an aldopentose, that is a monosaccharide containing five carbon atoms that, in its acyclic form, has an aldehyde functional group at one end. This species predominantly exists in the...
and
deoxyriboseDeoxyribose, also known as
D-Deoxyribose and 2-deoxyribose, is an aldopentose — a monosaccharide containing five carbon atoms, and including an aldehyde functional group in its linear structure...
) being right-handed, and chiral phosphoglycerides). Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst are formed in a 50/50 mixture of both enantiomers. This is called a
racemicIn chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule...
mixture. Clark has suggested that homochirality may have started in space, as the studies of the amino acids on the
Murchison meteoriteThe Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....
showed L-alanine to be more than twice as frequent as its D form, and L-glutamic acid was more than 3 times prevalent than its D counterpart. It is suggested that polarised light has the power to destroy one
enantiomerIn stereochemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are "non-superposable" , much as one's left and right hands are "the same" but opposite....
within the proto-planetary disk. Noyes showed that
beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted. In the case of electron emission, it is referred to as beta minus , while in the case of a positron emission as beta plus...
caused the breakdown of D-
leucineLeucine is an α-amino acid with the chemical formula HO
2CCHCH
2CH
2. It is an essential amino acid, which means that humans cannot synthesise it. Its codons are UUA, UUG, CUU, CUC, CUA, and CUG. With a hydrocarbon side chain, leucine is classified as a hydrophobic...
, in a
racemicIn chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule...
mixture, and that the presence of
14CCarbon-14,
14C, or radiocarbon, is a radioactive isotope of carbon discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, though its existence had been suggested already in 1934 by Franz Kurie. Its nucleus contains...
, present in larger amounts in organic chemicals in the early Earth environment, could have been the cause. Robert M. Hazen reports upon experiments conducted in which various chiral crystal surfaces act as sites for possible concentration and assembly of chiral monomer units into macromolecules. Once established, chirality would be selected for. Work with organic compounds found on meteorites tends to suggest that chirality is a characteristic of abiogenic synthesis, as amino acids show a left-handed bias, whereas sugars show a predominantly right-handed bias.
Self-organization and replication
While features of self-organization and self-replication are often considered the hallmark of living systems, there are many instances of abiotic molecules exhibiting such characteristics under proper conditions. For example Martin and Russel show that physical compartmentation by cell membranes from the environment and self-organization of self-contained
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
reactions are the most conserved attributes of living things, and they argue therefore that inorganic matter with such attributes would be life's most likely last common ancestor.
Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.
From organic molecules to protocells
The question "How do simple organic molecules form a protocell?" is largely unanswered but there are many hypotheses. Some of these postulate the early appearance of nucleic acids ("
geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
s-first") whereas others postulate the evolution of biochemical reactions and pathways first ("
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.
"Genes first" models: the RNA world
The
RNA world hypothesisThe RNA world hypothesis proposes that a world filled with life based on ribonucleic acid predates the current world of life based on deoxyribonucleic acid . RNA, which can both store information like DNA and act as an enzyme, may have supported cellular or pre-cellular life...
describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. This has spurred scientists to try to determine if relatively short
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. A number of hypotheses of modes of formation have been put forward. Early cell membranes could have formed spontaneously from
proteinoidProteinoids, or thermal proteins, are protein-like molecules formed inorganically from amino acids. Some theories of abiogenesis propose that proteinoids were a precursor to the first living cells....
s, protein-like molecules that are produced when amino acid solutions are heated–when present at the correct concentration in aqueous solution, these form microspheres which are observed to behave similarly to membrane-enclosed compartments. Other possibilities include systems of chemical reactions taking place within
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
substrates or on the surface of
pyriteThe mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS
2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold due to its resemblance to gold...
rocks. Factors supportive of an important role for RNA in early life include its ability to act both to store information and catalyse chemical reactions (as a
ribozymeA ribozyme is an RNA molecule that catalyzes a chemical reaction...
); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
) in modern organisms; and the ease of chemical synthesis of at least the components of the molecule under conditions approximating the early Earth. Relatively short RNA molecules which can duplicate others have been artificially produced in the lab. Such replicase RNA, which functions as both code and catalyst provides a template upon which copying can occur. Jack Szostak has shown that certain catalytic RNAs can, indeed, join smaller RNA sequences together, creating the potential, in the right conditions for self-replication. If these were present, Darwinian selection would favour the proliferation of such self-catalysing structures, to which further functionalities could be added. Lincoln and Joyce identified an RNA enzyme capable of self sustained replication.
Researchers have pointed out difficulties for the abiotic synthesis of nucleotides from
cytosineCytosine is one of the four main bases found in DNA and RNA. It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
and
uracilUracil is a common and naturally occurring pyrimidine derivative. Originally discovered in 1900, it was isolated by hydrolysis of yeast nuclein that was found in bovine thymus and spleen, herring sperm, and wheat germ...
. Cytosine has a
half-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
of 19 days at and 17,000 years in freezing water. Larralde et al., say that "the generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity." and they conclude that their "results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability." The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.
A slightly different version of the RNA-world hypothesis is that a different type of
nucleic acidA nucleic acid is a macromolecule composed of chains of monomeric nucleotides. In biochemistry these molecules carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid and ribonucleic acid . Nucleic acids are universal in living things, as...
, such as PNA, TNA or GNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later. Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by going against the dogma that nitrogenous and oxygenous chemistries should be avoided. In a series of publications, The Sutherland Group at the School of Chemistry, University of Manchester have demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60 % or greater. This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry. In 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerise into RNA. This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates. James Ferris's studies have shown that clay minerals of
montmorilloniteMontmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central...
will catalyze the formation of RNA in aqueous solution, by joining activated mono RNA nucleotides to join together to form longer chains. Although these chains have random sequences, the possibility that one sequence began to non-randomly increase its frequency by increasing the speed of its catalysis is possible to "kick start" biochemical evolution.
"Metabolism first" models
Several models reject the idea of the self-replication of a "naked-gene" and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication.
Iron-sulfur world
One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin's notion of primitive self-replicating
vesiclesA vesicle is a bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell. Vesicles form naturally because of the properties of lipid membranes ...
which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include
Günter WächtershäuserGünter Wächtershäuser , a German chemist turned patent lawyer, is mainly known for his work on the origin of life, and in particular his iron-sulfur world theory, a theory that life on Earth had hydrothermal origins...
's
iron-sulfur world theoryThe iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich chemist and patent lawyer, involving forms of iron and sulfur...
and models introduced by
Christian de DuveChristian René de Duve is an internationally acclaimed cytologist and biochemist. De Duve was born in Thames Ditton, Surrey, Great Britain, as a son of Belgian immigrants. They returned to Belgium in 1920...
based on the chemistry of
thioesterThioesters are compounds resulting from the bonding of sulfur with an acyl group with the general formula R-S-CO-R. They are the product of esterification between a carboxylic acid and a thiol ....
s. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by
Freeman DysonFreeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, and nuclear engineering. Dyson is a member of the Board of Sponsors of . Dyson lives in Princeton, New Jersey, as he has for over fifty...
in the early 1980s and
Stuart KauffmanStuart Alan Kauffman is an American theoretical biologist and complex systems researcher concerning the origin of life on Earth...
's notion of collectively
autocatalytic setAn autocatalytic set is a collection of entities, each of which can be created catalytically by other entities within the set, such that as a whole, the set is able to catalyze its own production. In this way the set as a whole is said to be autocatalytic...
s, discussed later in that decade.
However, the idea that a closed metabolic cycle, such as the reductive
citric acid cycleThe citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or more rarely, the Szent-Györgyi-Krebs cycle — is a series of enzyme-catalysed chemical reactions, which is of central importance in all living cells that use oxygen as part of cellular respiration...
, could form spontaneously (proposed by Günter Wächtershäuser) remains debated. In an article entitled "Self-Organizing Biochemical Cycles", the late
Leslie OrgelLeslie Eleazer Orgel FRS was a British chemist.Born in London, England, Orgel received his B.A. in chemistry with first class honours from Oxford University in 1949...
summarized his analysis of the proposal by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral." It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the "open"
acetyl-CoAAcetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main use is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester between coenzyme A and acetic...
pathway (another one of the five recognised ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organisation on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase harbours mixed nickel-iron-sulfur clusters in its reaction centers and catalyses the formation of acetyl-CoA (which may be regarded as a modern form of acetyl-thiol) in a single step.
Thermosynthesis world
Today’s bioenergetic process of
fermentationFermentation may refer to: the or* Fermentation , the process of energy production in a cell under anaerobic conditions...
is related to the just mentioned citric acid cycle or the Acetyl-CoA pathway that have been connected to the primordial iron-sulfur world. In a different approach, today’s bioenergetic process of
chemiosmosisChemiosmosis is the diffusion of ions across a selectively-permeable membrane. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....
, which plays an essential role in
cellular respirationCellular respiration is one of the key ways a cell gains useful energy. It is the set of the metabolic reactions and processes that take place in organisms' cells to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products...
and
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
, is considered as more fundamental than fermentation: in Anthonie Muller’s “
thermosynthesisThermosynthesis is a theoretical mechanism proposed by Anthonie Muller for biological use of the free energy in a temperature gradient to drive energetically uphill anabolic reactions. It makes use of this thermal gradient, or the dissipative structure of convection in this gradient, to drive a...
world” the
ATP SynthaseAn ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate from adenosine diphosphate and inorganic phosphate by using some form of energy...
enzyme that sustains chemiosmosis is proposed as today’s enzyme that is the closest connected to the first metabolic process.
First life needed an energy source to bring about the condensation reaction that yielded the peptide bonds of proteins and the phosphodiester bonds of
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
. In a generalization and thermal variation of the binding change mechanism of today’s ATP Synthase, the “First Protein” would have bound substrates (peptides, phosphate, nucleosides, RNA ‘monomers’) and condensed them to a reaction product that remained bound until it after a temperature change was released upon a thermal unfolding.
The energy source of the thermosynthesis world was thermal cycling, the result of suspension of the protocell in a
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
current, as is plausible in a volcanic hot spring; the convection accounts for the
self-organizationSelf-organization is a process of attraction and repulsion in which the internal organization of a system, normally an open system, increases in complexity without being guided or managed by an outside source...
and dissipative structure required in any origin of life model. The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis.
By phosphorylating cell membrane lipids, this ‘First Protein’ gave a selective advantage to the lipid protocell that contained the protein. In the beginning this First Protein also synthesized a library with many proteins, of which only a minute fraction had thermosynthesis capabilities. Just as proposed by
DysonFreeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, and nuclear engineering. Dyson is a member of the Board of Sponsors of . Dyson lives in Princeton, New Jersey, as he has for over fifty...
for the first proteins, the First Protein propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Functioning daughters consisted of different amino acid sequences.
Over a long time, RNA sequences where selected among the at first randomly synthesized RNAs by the criterion of speed and efficiency increase of First Protein synthesis, for instance by the creation of RNA that functioned as messenger RNA.. Transfer RNA and ribosomal RNA, or, even more generally, all the components of the RNA World were also generated and selected. The thermosynthesis world therefore in theory accounts for the origin of the genetic machinery.
Whereas the iron-sulfur world identifies a circular pathway as the most simple—and therefore assumes the existence of enzymes— the thermosynthesis world does not even invoke a pathway, and does not assume the existence of regular enzymes: ATP Synthase’s binding change mechanism resembles a physical adsorption process that yields free energy, rather than a regular enzyme’s mechanism, which decreases the free energy. The RNA World also implies the existence of several enzymes. But even the emergence of a single enzyme by chance is implausible. The thermosynthesis world is therefore more simple, and thus more plausible, than the iron-sulfur and RNA worlds.
Possible role of bubbles
Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through
evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
. While bubbles composed mostly of water burst quickly, water containing
amphiphilesAmphiphile is a term describing a chemical compound possessing both hydrophilic and lipophilic properties. Such a compound is called amphiphilic or amphipathic. This forms the basis for a number of areas of research in chemistry and biochemistry, notably that of lipid polymorphism...
forms much more stable bubbles, lending more time to the particular bubble to perform these crucial reactions.
Amphiphiles are oily compounds containing a hydrophilic head on one or both ends of a hydrophobic molecule. Some amphiphiles have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane. If a
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
would increase the integrity of its parent bubble, that bubble had an advantage, and was placed at the top of the
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the 'experiment' into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multicellular organisms could be achieved.
Similarly, bubbles formed entirely out of protein-like molecules, called
microsphereMicrosphere is a term used for small spherical particles, with diameters in the micrometer range . Microspheres are sometimes referred to as microparticles....
s, will form spontaneously under the right conditions. But they are not a likely precursor to the modern cell membrane, as cell membranes are composed primarily of lipid compounds rather than amino-acid compounds (for types of membrane spheres associated with abiogenesis, see protobionts,
micelleA micelle is an aggregate of surfactant molecules dispersed in a liquid colloid...
,
coacervateA coacervate is a tiny spherical droplet of assorted organic molecules which is held together by hydrophobic forces from a surrounding liquid....
).
A recent model by Fernando and Rowe suggests that the enclosure of an autocatalytic non-enzymatic metabolism within protocells may have been one way of avoiding the side-reaction problem that is typical of metabolism first models.
Autocatalysis
In 1993 Stuart Kauffman proposed that life initially arose as autocatalytic chemical networks.
BritishThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
ethologist
Richard DawkinsClinton Richard Dawkins, FRS, FRSL is a British ethologist, zoologist, Neo-Darwinian evolutionary biologist and theorist and a popular science author....
wrote about
autocatalysisA single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the catalyst for that reaction....
as a potential explanation for the origin of life in his 2004 book
The Ancestor's TaleThe Ancestor's Tale is a 2004 popular science book by Richard Dawkins, with contributions from Dawkins' research assistant Yan Wong. It follows the path of humans backwards through evolutionary history, meeting humanity's cousins as they converge on common ancestors...
. Autocatalysts are substances which catalyze the production of themselves, and therefore have the property of being a simple molecular replicator. In his book, Dawkins cites experiments performed by
Julius RebekJulius Rebek, Jr. is a Hungarian-born American chemist and expert on molecular self-assembly.Rebek was born in Beregszasz , Hungary in 1944 and lived in Austria from 1945 to 1949. In 1949 he and his family immigrated to the United States and settled in Kansas. Rebek graduated from the University...
and his colleagues at the Scripps Research Institute in
CaliforniaCalifornia is the most populous state in the United States, and the third largest by area. California is the second most populous sub-national entity in the Americas, behind only São Paulo, Brazil...
in which they combined amino adenosine and pentafluorophenyl ester with the autocatalyst amino adenosine triacid ester (AATE). One system from the experiment contained variants of AATE which catalysed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
.
Clay theory
A model for the origin of life based on
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
was forwarded by A.
Graham Cairns-SmithAlexander Graham Cairns-Smith is an organic chemist and molecular biologist at the University of Glasgow, most famous for his controversial 1985 book, Seven Clues to the Origin of Life...
of the
University of GlasgowThe University of Glasgow is the fourth-oldest surviving university in the English-speaking world and one of Scotland's four ancient universities...
in 1985 and explored as a plausible illustration by several other scientists, including
Richard DawkinsClinton Richard Dawkins, FRS, FRSL is a British ethologist, zoologist, Neo-Darwinian evolutionary biologist and theorist and a popular science author....
. Clay theory postulates that complex organic molecules arose gradually on a pre-existing, non-organic replication platform—silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then
exaptedExaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behavior...
to serve the replication of organic molecules independently of their silicate "launch stage".
Cairns-Smith is a staunch critic of other models of chemical evolution. However, he admits, that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).
In 2007, Kahr and colleagues reported their experiments to examine the idea that crystals can act as a source of transferable information, using crystals of
potassium hydrogen phthalatePotassium hydrogen phthalate, often called simply KHP, is a white or colorless, ionic solid that is the monopotassium salt of phthalic acid. The hydrogen is slightly acidic, and it is often used as a primary standard for acid-base titrations because it is solid and air-stable, making it easy to...
. "Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. They then examined the distribution of imperfections in the crystal system and found that the imperfections in the mother crystals were indeed reproduced in the daughters. The daughter crystals had many additional imperfections. For a gene-like behavior the additional imperfections should be much less than the parent ones, thus Kahr concludes that the crystals "were not faithful enough to store and transfer information from one generation to the next".
Gold's "Deep-hot biosphere" model
In the 1970s,
Thomas GoldThomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...
proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. The discovery in the late 1990s of
nanobeNanobes are tiny filamental structures first found in some rocks and sediments. Some hypothesize that they are the smallest form of life, 1/10th the size of the smallest known bacteria...
s (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks might be seen as lending support to Gold's theory.
It is now reasonably well established that microbial life is plentiful at shallow depths in the Earth, up to below the surface, in the form of
extremophile archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, rather than the better-known eubacteria (which live in more accessible conditions). It is claimed that discovery of microbial life below the surface of another body in our
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
would lend significant credence to this theory.
Thomas GoldThomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...
also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold's theory is that flow of food is due to out-gassing of primordial
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
from the
Earth's mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks.
"Primitive" extraterrestrial life
An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on a nearby planet (Mars). (Note that exogenesis is related to, but not the same as, the notion of
panspermiaPanspermia is the hypothesis that "seeds" of life exist already all over the Universe, that life on Earth may have originated through these "seeds", and that they may deliver or have delivered life to other habitable bodies....
). A supporter of this theory was
Francis CrickFrancis Harry Compton Crick OM FRS , was a British molecular biologist, physicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson. He, James D...
.
Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating.
CometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s are encrusted by outer layers of dark material, thought to be a
tarTar is modified resin produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. It is a viscous black liquid. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving and...
-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light. It is supposed that a rain of material from
cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s could have brought significant quantities of such complex organic molecules to Earth.
An alternative but related hypothesis, proposed to explain the presence of life on Earth so soon after the planet had cooled down, with apparently very little time for prebiotic evolution, is that life formed first on early
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
. Due to its smaller size Mars cooled before Earth (a difference of hundreds of millions of years), allowing prebiotic processes there while Earth was still too hot. Life was then transported to the cooled Earth when crustal material was blasted off Mars by asteroid and comet impacts. Mars continued to cool faster and eventually became hostile to the continued evolution or even existence of life (it lost its atmosphere due to low volcanism); Earth is following the same fate as Mars, but at a slower rate.
Neither hypothesis actually answers the question of how life first originated, but merely shifts it to another planet or a comet. However, the advantage of an extraterrestrial origin of primitive life is that life is not required to have evolved on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence to support the plausibility of the concept is scant, but it finds support in recent study of Martian meteorites found in Antarctica and in studies of
extremophileAn extremophile is an organism that thrives in and even may require physically or geochemically extreme conditions that are detrimental to the majority of life on Earth...
microbes. Additional support comes from a recent discovery of a bacterial ecosytem whose energy source is radioactivity.
A recent experiment led by Jason Dworkin subjected a frozen mixture of water,
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with formula CH3OH . It is toxic: drinking 10 ml will cause blindness, and as little as 100 ml will cause death...
,
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
and
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
to UV radiation, mimicking conditions found in an extraterrestrial environment. This combination yielded large amounts of organic material that self-organised to form bubbles when immersed in water. Dworkin considered these bubbles to resemble cell membranes that enclose and concentrate the chemistry of life, separating their interior from the outside world.
The bubbles produced in these experiments were between , or about the size of red blood cells. Remarkably, the bubbles fluoresced, or glowed, when exposed to UV light. Absorbing UV and converting it into visible light in this way was considered one possible way of providing energy to a primitive cell. If such bubbles played a role in the origin of life, the fluorescence could have been a precursor to primitive
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
. Such fluorescence also provides the benefit of acting as a sunscreen, diffusing any damage that otherwise would be inflicted by UV radiation. Such a protective function would have been vital for life on the early Earth, since the
ozone layerThe ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 93-99% of the sun's high frequency ultraviolet light, which is potentially damaging to life on earth. Over 91% of the ozone in Earth's atmosphere is present here...
, which blocks out the sun's most destructive UV rays, did not form until after photosynthetic life began to produce oxygen.
Extraterrestrial Amino Acids Seeding Earth
Another possibility is that amino acids which were formed extra-terrestrially arrived on Earth via comets. In 2009 it was announced by the US space agency NASA that scientists have identified one of the fundamental chemical buildings blocks of life in a comet for the first time: Glycine, an amino acid, was detected in the material ejected from Comet Wild-2 in 2004 and grabbed by Nasa's Stardust probe. Tiny grains, just a few thousandths or a millimetre in size, were collected from the comet and returned to Earth in 2006 in a sealed capsule, and distributed among the world's leading astro-biology labs. NASA said in a statement that it took sometime for the investigating team, led by Dr Jamie Elsila, to convince itself that the glycine signature found in Stardust's sample bay was genuine and not just Earthly contamination. Glycine has been detected in meteorites before and there are also observations in interstellar gas clouds claimed for telescopes, but the Stardust find is described as a first in cometary material. It is known that prior to the emergence of life on Earth, the early solar system's planets were regularly bombarded by comets. Dr. Carl Pilcher, who leads Nasa's Astrobiology Institute commented that "The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the Universe may be common rather than rare."
Lipid World
This theory postulates that the first self-replicating object was lipid-like. It is known that phospholipids form bilayers in water while under agitation– the same structure as in cell membranes. These molecules were not present on early earth, however other amphiphilic long chain molecules also form membranes. Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favorably. Still, no biochemical mechanism has been offered to support the Lipid World theory.
Polyphosphates
The problem with most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of
polyphosphatePolyphosphates are anionic phosphate polymers linked between hydroxyl groups and hydrogen atoms. The polymerization that takes place is known as a condensation reaction. Phosphate chemical bonds are typically high-energy covalent bonds, which means that energy is available upon breaking such bonds...
s. Polyphosphates are formed by polymerization of ordinary monophosphate ions PO
4−3. Several mechanisms for such polymerization have been suggested. Polyphosphates cause polymerization of amino acids into peptides . They are also logical precursors in the synthesis of such key biochemical compounds as ATP. A key issue seems to be that calcium reacts with soluble phosphate to form insoluble
calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions.It is the principal form of calcium found in bovine milk...
(
apatiteApatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH
−, F
−, Cl
− or...
), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate.
There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early earth.
PAH world hypothesis
Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of polycyclic aromatic hydrocarbons (PAH's) in a
nebulaA nebula is an interstellar cloud of dust, hydrogen gas, helium gas and plasma...
. Those are the most complex molecules so far found in space. The use of PAH's has also been proposed as a precursor to the RNA world in the
PAH world hypothesisThe PAH world hypothesis is a biological hypothesis that proposes that the use of polycyclic aromatic hydrocarbons was a means for a pre-RNA World basis for the origin of life...
. The
Spitzer Space TelescopeThe Spitzer Space Telescope is an infrared space observatory launched in 2003...
has recently detected a star, HH 46-IR, which is forming by a process similar to that by which the sun formed. In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. PAHs have also been found all over the surface of galaxy M81, which is 12 million light years away from the Earth, confirming their widespread distribution in space.
Multiple genesis
Different forms of life may have appeared quasi-simultaneously in the early history of Earth. The other forms may be extinct, leaving distinctive fossils through their different biochemistry (e.g., using arsenic instead of phosphorus), survive as extremophiles, or simply be unnoticed through their being
analogousTwo structures in biology are said to be analogous if they perform the same or similar function by a similar mechanism but evolved separately...
to organisms of the current life tree. Hartman for example combines a number of theories together, by proposing that:
The first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids. If biosynthesis recapitulates biopoesis, then the synthesis of amino acids preceded the synthesis of the purine and pyrimidine bases. Furthermore the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides.
Lynn MargulisLynn Margulis is an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst...
's
endosymbiotic theoryThe endosymbiotic theory concerns the origins of mitochondria and plastids , which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts...
suggests that multiple forms of bacteria entered into symbiotic relationship to form the eucaryotic cell. The horizontal transfer of genetic material between bacteria promotes such symbiotic relationships, and thus many separate organisms may have contributed to building what has been recognised as the Last Universal Common Ancestor (LUCA) of modern organisms.
James LovelockJames Ephraim Lovelock, CH, CBE, FRS is an independent scientist, author, researcher, environmentalist, and futurist who lives in Devon, England...
's Gaia theory, proposes that such bacterial symbiosis establishes the environment as a system produced by and supportive of life. His arguments strongly weaken the case for life having evolved elsewhere in the solar system.
See also
- Astrochemistry
Astrochemistry, the overlap of the disciplines of astronomy and chemistry, is the study of the abundance and reactions of chemical elements and molecules in space, and their interaction with radiation. The word astrochemistry can refer to both the Solar System, and the interstellar medium...
- Autocatalytic reactions and order creation
- Biogenesis
Biogenesis is the process of lifeforms producing other lifeforms, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Generatio spontanea:...
- Common descent
A group of organisms is said to have common descent if they have a common ancestor. In modern biology, it is generally accepted that all living organisms on Earth are descended from a common ancestor or ancestral gene pool....
- Drake equation
The Drake equation is a famous result in the fields of exobiology and the search for extraterrestrial intelligence ....
- Entropy and life
Much writing has been devoted to entropy and life. Research concerning the relationship between the thermodynamic quantity entropy and the evolution of life began in around the turn of the 20th century...
- History of Earth
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation to the present day during the last 4.54 billion years. Nearly all branches of natural science have contributed to the understanding of the main events of the...
- List of independent discoveries#20th century
- List of publications in biology#Origin of life
- Mediocrity principle
The mediocrity principle is the notion in philosophy of science that there is nothing special about humans or the Earth. It is a Copernican principle, used either as an heuristic about Earth's position or a philosophical statement about the place of humanity...
- Origin of the world's oceans
- Mimivirus
Mimivirus is a viral genus containing a single identified species named Acanthamoeba polyphaga mimivirus . Or is a group of phylogenetically related large viruses. In colloquial speech, APMV is more commonly referred to as just “mimivirus”...
- Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...
- Rare Earth hypothesis
In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...
- Shadow biosphere
A shadow biosphere is a postulated microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life...
- Thermosynthesis
Thermosynthesis is a theoretical mechanism proposed by Anthonie Muller for biological use of the free energy in a temperature gradient to drive energetically uphill anabolic reactions. It makes use of this thermal gradient, or the dissipative structure of convection in this gradient, to drive a...
- Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
Further reading
- Buehler, Lukas K. (2000–2005) The physico-chemical basis of life, http://www.whatislife.com/about.html accessed 27 October 2005. (Cited on p. 108). (Cited on p. 108).
External links