Abiogenesis
From Wikipedia, the free encyclopedia
Abiogenesis (
// AY-by-oh-JEN-ə-siss[1]) or
biopoiesis[2] is the natural process of
life arising from non-living matter such as simple
organic compounds.
[3][4][5][6]
The
Earth was formed about
4.54 billion years ago. The earliest undisputed evidence of
life on Earth dates at least from 3.5 billion years ago,
[7][8][9] during the
Eoarchean Era after sufficient
crust had solidified following the earlier molten
Hadean Eon. There are
microbial mat fossils found in 3.48 billion-year-old
sandstone discovered in
Western Australia.
[10][11] Other early physical evidence for
life on Earth is
biogenic graphite in 3.7 billion-year-old
metasedimentary rocks discovered in
Western Greenland.
[12] Nevertheless,
several studies suggest that life on Earth may have started even earlier,
[13] as early as 4.25 billion years ago according to one study,
[14] and 4.4 billion years ago according to another study.
[15] Earth is the only place in the universe known to harbor life.
[16][17] Nonetheless, the exact steps in the abiogenesis process, whether occurring on Earth or elsewhere, remain unknown.
Scientific hypotheses about the origins of life can be divided into
three main stages: the geophysical, the chemical and the biological.
[18] Many approaches investigate how self-replicating
molecules or their components came into existence. On the assumption that life originated spontaneously on Earth, the
Miller–Urey experiment and similar experiments demonstrated that most
amino acids, basic chemicals of life, can be
racemically synthesized in conditions which were intended to be similar to those of the
early Earth.
Several mechanisms have been investigated, including lightning and
radiation. Other approaches ("metabolism first" hypotheses) focus on
understanding how
catalysis in chemical systems in the early Earth might have provided the precursor molecules necessary for self-replication.
[19][20]
Early geophysical conditions
Based on recent
computer model studies, the
complex organic molecules necessary for
life may have formed in the protoplanetary disk of
dust grains surrounding the
Sun before the formation of the Earth.
[21] According to the computer studies, this same process may also occur around other
stars that acquire
planets.
[21] (Also see
Extraterrestrial organic molecules).
The
Hadean Earth is thought to have had a
secondary atmosphere, formed through
degassing of the rocks that accumulated from
planetesimal impactors. At first, it was thought that the Earth's
atmosphere consisted of hydrides—
methane,
ammonia and
water vapour—and
that life began under such reducing conditions, which are conducive to
the formation of organic molecules. During its formation, the Earth lost
a significant part of its initial mass, with a nucleus of the heavier
rocky elements of the
protoplanetary disk remaining.
[22]
However, based on today's volcanic evidence, it is now thought that the
early atmosphere would have probably contained 60% hydrogen, 20% oxygen
(mostly in the form of water vapour), 10%
carbon dioxide, 5 to 7%
hydrogen sulfide,
and smaller amounts of nitrogen, carbon monoxide, free hydrogen,
methane and inert gases. As Earth lacked the gravity to hold any
molecular hydrogen, this component of the atmosphere would have been
rapidly lost during the Hadean period, along with the bulk of the
original inert gases. Solution of carbon dioxide in water is thought to
have made the seas slightly
acidic, with a
pH of about 5.5.
[23] The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory."
[24] It is similar to the mixture of gases released by volcanoes, which still support some abiotic chemistry today.
[24]
Oceans may have
appeared first in the Hadean eon, as soon as two hundred million years (200
Ma) after the Earth was formed, in a hot 100 °C (212 °F)
reducing environment, and the
pH of about 5.8 rose rapidly towards neutral.
[25] This has been supported by the dating of 4.404 Ga-old
zircon crystals from metamorphosed
quartzite of
Mount Narryer in Western Australia, which are evidence that oceans and
continental crust existed within 150
Ma of Earth's formation.
[26]
Despite the likely increased vulcanism and existence of many smaller
tectonic "platelets", it has been suggested that between 4.4 and 4.3
Ga, the Earth was a water world, with little if any continental crust, an extremely
turbulent atmosphere and a hydrosphere subject to high
UV, from a
T Tauri sun,
cosmic radiation and continued
bolide impact.
[27]
The Hadean environment would have been highly hazardous to modern
life. Frequent collisions with large objects, up to 500 kilometres
(310 mi) in diameter, would have been sufficient to sterilise the planet
and vaporise the ocean within a few months of impact, with hot steam
mixed with rock vapour becoming high altitude clouds that would
completely cover 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.
[28]
The earliest biological evidence for life on Earth
The earliest life on
Earth existed before 3.5 billion years ago,
[7][8][9] during the
Eoarchean Era when sufficient
crust had solidified following the molten
Hadean Eon. The earliest possible physical evidence for
life on Earth is
biogenic graphite in 3.7 billion-year-old
metasedimentary rocks discovered in
Western Greenland[12] and
microbial mat fossils found in 3.48 billion-year-old
sandstone discovered in
Western Australia.
[10][11] At Strelley Pool, in the
Pilbarra Region of Western Australia compelling evidence from a
pyrite bearing sandstone, a fossilised beach, rounded tubular cells oxidisd
sulfur by
photosynthesis in the absence of oxygen have been found.
[29] Gustaf Arrhenius of the
Scripps Institute of Oceanography using a
mass spectrometer
has identified what appears to be, on the basis of biogenic carbon
isotopes, evidence of early life, found in rocks from Akilia Island,
near
Isua, Greenland, dating to 3.85 billion years old.
[30]
Between 3.8 and 4.1
Ga, changes in the orbits of the
gaseous giant planets may have caused a
late heavy bombardment[31]
that pockmarked the Moon and the other inner planets (Mercury, Mars,
and presumably Earth and Venus). This would likely have repeatedly
sterilized the planet, had life appeared before that time.
[24]
Geologically, the Hadean Earth would have been far more active than at
any other time in its history. Studies of meteorites suggests that
radioactive isotopes such as
aluminium-26 with a
half-life of 7.17×10
5 years, and
potassium-40 with a half-life of 1.250×10
9 years, isotopes mainly produced in supernovae, were much more common.
[32] Coupled with internal heating as a result of
gravitational sorting between the core and the mantle there would have been a great deal of
mantle convection, with the probable result of many more smaller and very active
tectonic plates than in modern times.
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. A study by
Maher and Stevenson 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.
[33]
Further evidence of the early appearance of life comes from the
Isua supercrustal belt in Western Greenland and from similar formations in the nearby
Akilia Island.
Isotopic fingerprints typical of life, preserved in the sediments, have
been used to suggest that life existed on the planet already by 3.85
billion years ago.
[34]
Conceptual history
John Desmond Bernal has identified a number of "outstanding
difficulties in accounts of the origin of life". Earlier theories, he
suggests, such as spontaneous generation were based upon an explanation
that life was continuously created as a result of chance events.
[35]
Spontaneous generation
Belief in the present ongoing
spontaneous generation of certain forms of
life from non-living matter goes back to
Aristotle
and ancient Greek philosophy and continued to have support in Western
scholarship until the 19th century. This belief was paired with a belief
in heterogenesis,
i.e., that one form of life derived from a different form (
e.g. bees from flowers).
[36]
Classical notions of spontaneous generation, which can be considered
under the modern term abiogenesis, held that certain complex, living
organisms are generated by decaying organic substances. According to
Aristotle, it was a readily observable truth that
aphids arise from the dew which falls on plants,
flies from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so on.
[37] In the 17th century, such assumptions started to be questioned. In 1646,
Sir Thomas Browne published his
Pseudodoxia Epidemica (subtitled
Enquiries into Very many Received Tenets, and Commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His contemporary,
Alexander Ross
erroneously refuted him, stating: "To question this (i.e., spontaneous
generation) is to question reason, sense and experience. If he doubts of
this let him go to
Egypt, and there he will find the fields swarming with mice, begot of the mud of
Nylus, to the great calamity of the inhabitants."
[38]
In 1665,
Robert Hooke published the first drawings of a microorganism. Hooke was followed in 1676 by
Anton van Leeuwenhoek, who drew and described microorganisms that are now thought to have been
protozoa and
bacteria.
[39]
Many felt the existence of microorganisms was evidence in support of
spontaneous generation, since microorganisms seemed too simplistic for
sexual reproduction, and
asexual reproduction through
cell division
had not yet been observed. Van Leeuwenhoek took issue with the ideas
common at the time that fleas and lice could spontaneously result from
putrefaction,
and that frogs could likewise arise from slime. Using a broad range of
experiments ranging from sealed and open meat incubation and the close
study of insect reproduction, by the 1680s he became convinced that
spontaneous generation was incorrect.
[40]
The first experimental evidence against spontaneous generation came in 1668 when
Francesco Redi showed that no
maggots
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
biogenesis: 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 Spallanzani demonstrated that
microbes were present in the air, and could be killed by boiling. In 1861,
Louis Pasteur
performed a series of experiments that demonstrated that organisms such
as bacteria and fungi do not spontaneously appear in sterile,
nutrient-rich media, but only invade them from outside.
The origin of the terms biogenesis and abiogenesis
The term
biogenesis is usually credited to either
Henry Bastian or to
Thomas Henry Huxley.
[41] Bastian used the term (around 1869) in an unpublished exchange with
John Tyndall to mean
life-origination or commencement. In 1870, Huxley, as new president of the
British Association for the Advancement of Science, delivered an address entitled
Biogenesis and Abiogenesis.
[42] In it he introduced the term
biogenesis (with an opposite meaning to Bastian) and also introduced the term
abiogenesis:
- And thus the hypothesis that living matter always arises by the
agency of pre-existing living matter, took definite shape; and had,
henceforward, a right to be considered and a claim to be refuted, in
each particular case, before the production of living matter in any
other way could be admitted by careful reasoners. It will be necessary
for me to refer to this hypothesis so frequently, that, to save
circumlocution, I shall call it the hypothesis of Biogenesis; and I
shall term the contrary doctrine–that living matter may be produced by
not living matter–the hypothesis of Abiogenesis.[42]
Subsequently, in the preface to Bastian's 1871 book,
The Modes of Origin of Lowest Organisms,
[43] the author refers to the possible confusion with Huxley's usage and he explicitly renounced his own meaning:
- A word of explanation seems necessary with regard to the
introduction of the new term archebiosis. I had originally, in
unpublished writings, adopted the word biogenesis to express the same
meaning—viz, life-origination or commencement.
-
- But in the mean time the word biogenesis has been made use of, quite
independently, by a distinguished biologist [Huxley], who wished to
make it bear a totally different meaning. He also introduced the term
abiogenesis. I have been informed, however, on the best authority, that
neither of these words can—with any regard to the language from which
they are derived—be supposed to bear the meanings which have of late
been publicly assigned to them. Wishing to avoid all needless confusion,
I therefore renounced the use of the word biogenesis, and being, for
the reason just given, unable to adopt the other term, I was compelled
to introduce a new word, in order to designate the process by which
living matter is supposed to come into being, independently of
pre-existing living matter.[43]
Alternatives to chance: biogenesis
The belief that spontaneous self-ordering of spontaneous generation
is impossible led to an alternative. By the middle of the 19th century,
the theory of
biogenesis
had accumulated so much evidential support, due to the work of Louis
Pasteur and others, that the alternative theory of spontaneous
generation had been effectively disproven.
Pasteur and Darwin
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."
[44][45]
One alternative was that life's origins on Earth had come from
somewhere else in the Universe. Periodically resurrected (see
Panspermia, above) Bernal demonstrates that this approach "is equivalent
in the last resort to asserting the operation of metaphysical,
spiritual entities... it turns on the argument of creation by design by a
creator or demiurge".
[46]
Such a theory, Bernal demonstrated was unscientific and a number of
scientists defined life as a result of an inner "life force", which in
the late 19th century was championed by
Henri Bergson.
The concept of evolution proposed by Charles Darwin put an end to these metaphysical theologies. In a letter to
Joseph Dalton Hooker on 1 February 1871,
[47] Charles Darwin
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."
[48]
In other words, the presence of life itself makes the search for the
spontaneous origin of life dependent on the artificial production of
organic compounds in the sterile conditions of the laboratory.
"Primordial soup" hypothesis
No new notable research or theory on the subject appeared until 1924, when
Alexander Oparin
reasoned that atmospheric oxygen prevents the synthesis of certain
organic compounds that are necessary building blocks for the evolution
of life. In his book
The Origin of Life,
[49][50]
Oparin proposed that the "spontaneous generation of life" that had been
attacked by Louis Pasteur did in fact occur once, but was now
impossible because the conditions found on the early Earth had changed,
and preexisting organisms would immediately consume any spontaneously
generated organism. Oparin argued that a "primeval soup" of organic
molecules could be created in an oxygenless atmosphere through the
action of sunlight. These would combine in ever more complex ways until
they formed
coacervate droplets. These droplets would "
grow" by fusion with other droplets, and "
reproduce" through fission into daughter droplets, and so have a primitive
metabolism
in which those factors which promote "cell integrity" survive, and
those that do not become extinct. Many modern theories of the origin of
life still take Oparin's ideas as a starting point.
Robert Shapiro has summarized the "primordial soup" theory of Oparin and Haldane in its "mature form" as follows:
[51]
- The early Earth had a chemically reducing atmosphere.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
- These compounds accumulated in a "soup", which may have been concentrated at various locations (shorelines, oceanic vents etc.).
- By further transformation, more complex organic polymers – and ultimately life – developed in the soup.
Around the same time,
J. B. S. Haldane
suggested that the Earth's prebiotic oceans—different from their modern
counterparts—would have formed a "hot dilute soup" in which organic
compounds could have formed.
J.D. Bernal, a pioneer in
x-ray crystallography, called this idea
biopoiesis or
biopoesis, the process of living matter evolving from self-replicating but nonliving molecules,
[52][53] and proposed that biopoiesis passes through a number of intermediate stages.
One of the most important pieces of experimental support for the "soup" theory came in 1952. A graduate student,
Stanley Miller, and his professor,
Harold Urey,
performed an experiment that demonstrated how organic molecules could
have spontaneously formed from inorganic precursors, under conditions
like those posited by the Oparin-Haldane Hypothesis. The now-famous "
Miller–Urey experiment" used a highly reduced mixture of gases—methane, ammonia and
hydrogen—to form basic organic
monomers, such as amino acids.
[54]
This provided direct experimental support for the second point of the
"soup" theory, and it is around the remaining two points of the theory
that much of the debate now centers. In the Miller–Urey experiment, a
mixture of water, hydrogen, methane, and ammonia was cycled through an
apparatus that delivered electrical sparks to the mixture. After one
week, it was found that about 10% to 15% of the carbon in the system was
now in the form of a
racemic mixture of organic compounds, including amino acids, which are the building blocks of
proteins.
Bernal shows that based upon this and subsequent work there is no
difficulty in principle in forming most of the molecules which we
recognise as the basic molecules of life from their inorganic
precursors. The underlying hypothesis held by Oparin, Haldane, Bernal,
Miller and Urey, for instance, was that multiple conditions on the
primeval Earth favored chemical reactions that synthesized the same set
of complex organic compounds from such simple precursors. A 2011
reanalysis of the saved vials containing the original extracts that
resulted from the Miller and Urey experiments, using current and more
advanced analytical equipment and technology, has uncovered more
biochemicals than originally discovered in the 1950s. One of the more
important findings was 23 amino acids, far more than the five originally
found.
[55]
However Bernal rightly shows that "it is not enough to explain the
formation of such molecules, what is necessary" he says "..is a
physical-chemical explanation of the origins of these molecules that
suggests the presence of suitable sources and sinks for free energy".
[56]
Proteinoid microspheres
In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal,
Sidney W. Fox in the 1950s and 1960s, studied the spontaneous formation of
peptide
structures under conditions that might plausibly have existed early in
Earth's history. He demonstrated that amino acids could spontaneously
form small chains called peptides. In one of his experiments, he allowed
amino acids to dry out as if puddled in a warm, dry spot in prebiotic
conditions. He found that, as they dried, the amino acids formed long,
often cross-linked, thread-like, submicroscopic
polypeptide molecules now named "
proteinoid microspheres".
[57]
In another experiment using a similar method to set suitable
conditions for life to form, Fox collected volcanic material from a
cinder cone in
Hawaii.
He discovered that the temperature was over 100 °C (212 °F) just 4
inches (100 mm) beneath the surface of the cinder cone, and suggested
that this might have been the environment in which life was
created—molecules could have formed and then been washed through the
loose volcanic ash and into the sea. He placed lumps of lava over amino
acids derived from methane, ammonia and water, sterilized all materials,
and baked the lava over the amino acids for a few hours in a glass
oven. A brown, sticky substance formed over the surface and when the
lava was drenched in sterilized water a thick, brown liquid leached out.
It turned out that the amino acids had combined to form
proteinoids,
and the proteinoids had combined to form small globules that Fox called
"microspheres". His proteinoids were not cells, although they formed
clumps and chains reminiscent of
cyanobacteria, but they contained no functional
nucleic acids
or any encoded information. Based upon such experiments, Colin S.
Pittendrigh stated in December 1967 that "laboratories will be creating a
living cell within ten years," a remark that reflected the typical
contemporary levels of innocence of the complexity of cell structures.
[58]
More recent theories
Bernal in 1967 identified three different sorts of difficulties in the abiogenetic origins of life
[59]
* Stage 1: he saw as the origins of organic molecules, and
this is now fairly well understood. The necessity of a source and sink
of energy, and the necessity of a fluid medium has been much studied
(see above).
* Stage 2: he saw as the necessity to explain how organic
monomers became ordered into biologically active polymers. Once again
there is the necessity of sources and sinks for this process. The
discovery of alkaline vents and the similarity with the "proton pump"
found as the basis of biological life has begun to provide evidence for
this. The second problem foreseen by Bernal was the origin of
replication. The work with the RNA world is specifically intended to
find answers to this problem.
* Stage 3: he saw was the most difficult. This was the
discovery of methods by which biological reactions were incorporated
behind cell walls. Modern work on the self organising capacities by
which cell membranes self-assemble, and the work on micropores in
various substrates as a half-way house towards the development of
independent free-living cells is ongoing research designed to answer
this problem.
[60][61]
Current models
There is still no "standard model" of the origin of life. Most
currently accepted models draw at least some elements from the framework
laid out by
Alexander Oparin (in 1924) and
John Haldane (in 1925), who postulated the molecular or chemical evolution theory of life.
[62]
According to them, the first molecules constituting the earliest cells
"were synthesized under natural conditions by a slow process of
molecular evolution, and these molecules then organized into the first
molecular system with properties with biological order."
[62] Oparin and Haldane suggested that the atmosphere of the early Earth may have been
chemically reducing in nature, composed primarily of methane (CH
4), ammonia (NH
3), water (H
2O),
hydrogen sulfide (H
2S),
carbon dioxide (CO
2) or
carbon monoxide (CO), and
phosphate (PO
43-), with molecular
oxygen (O
2) and
ozone (O
3)
either rare or absent, however, the current scientific model is an
atmosphere that contained 60% hydrogen, 20% oxygen (mostly in the form
of water vapor), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and
smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and
inert gases.
[63][64] In the atmosphere proposed by Oparin and Haldane, electrical activity can catalyze the creation of certain basic small
molecules (
monomers) of life, such as amino acids. This was demonstrated in the
Miller–Urey experiment by
Stanley L. Miller and
Harold C. Urey reported in 1953.
John Desmond Bernal coined the term
biopoiesis in 1949 to refer to the origin of life,
[65] and suggested that it occurred in three "stages": 1) the origin of biological
monomers; 2) the origin of biological
polymers; and 3) the evolution from molecules to cells. He suggested that
evolution commenced between stage 1 and 2.
[66]
The chemical processes that took place on the early Earth are called
chemical evolution. Both
Manfred Eigen and
Sol Spiegelman demonstrated that
evolution, including replication, variation, and
natural selection, can occur in populations of molecules as well as in organisms.
[24] Spiegelman took advantage of natural selection to synthesize
Spiegelman's Monster,
which had a genome with just 218 bases. Eigen built on Spiegelman's
work and produced a similar system with just 48 or 54 nucleotides.
[67]
Chemical evolution was followed by the initiation of
biological evolution, which led to the first cells.
[24] 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
focused on chemosynthesis of polymers. However, some researchers are
working in this field, notably
Steen Rasmussen and
Jack Szostak. Others have argued that a
"top-down approach" is more feasible. One such approach, successfully attempted by
Craig Venter and others at
The Institute for Genomic Research,
involves engineering existing prokaryotic cells with progressively
fewer genes, attempting to discern at which point the most minimal
requirements for life were reached.
[68][69]
Chemical origin of organic molecules
The
elements, except for
hydrogen, ultimately derive from
stellar nucleosynthesis. Complex molecules, including organic molecules, form naturally both in space and on planets.
[70] 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, redox coupling, or electrical discharges) (e.g. Miller's experiments)
- Extraterrestrial origins – formation of organic molecules in interstellar dust clouds and rained down on planets.[71][72][73] (See pseudo-panspermia)
Estimates of these sources suggest that the
heavy bombardment before 3.5
Ga within the early atmosphere made available quantities of organics comparable to those produced by other energy sources.
[74][75]
A
cladogram demonstrating extreme thermophilic bacteria and archaea at the base of the tree of life
It has been estimated that the
Late Heavy Bombardment
may also have effectively sterilised the Earth's surface to a depth of
tens of metres. If life evolved deeper than this, it would have also
been shielded from the early high levels of
ultraviolet radiation from the
T Tauri
stage of the sun's evolution. Simulations of geothermically heated
oceanic crust yield far more organics than those found in the
Miller-Urey experiments (see below). In the deep hydrothermal vents,
Everett Shock has found "there is an enormous thermodynamic drive to
form organic compounds, as seawater and hydrothermal fluids, which are
far from equilibrium, mix and move towards a more stable state".
[76]
Shock has found that the available energy is maximised at around 100 –
150 degrees Celsius, precisely the temperatures at which the
hyperthermophilic bacteria and
archaea have been found, at the base of the tree of life closest to the
Last Universal Common Ancestor.
[77]
Chemical synthesis
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. Palasek showed that self-assembly of RNA molecules
can occur spontaneously due to physical factors in hydrothermal vents.
[78] Virus self-assembly within host cells has implications for the study of the origin of life,
[79] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.
[80][81]
Multiple sources of energy were available for chemical reactions on the early Earth. For example, heat (such as from
geothermal processes) is a standard energy source for chemistry. Other examples include
sunlight and electrical discharges (
lightning), among others.
[24]
Unfavorable reactions can also be driven by highly favorable ones, as
in the case of iron-sulfur chemistry. For example, this was probably
important for
carbon fixation (the conversion of carbon from its inorganic form to an organic one).
[note 1]
Carbon fixation via iron-sulfur chemistry is highly favorable, and
occurs at neutral pH and 100 °C (212 °F). Iron-sulfur surfaces, which
are abundant near
hydrothermal vents, are also capable of producing small amounts of amino acids and other biological metabolites.
[24]
Formamide
produces all four ribonucleotides and other biological molecules when
warmed in the presence of various terrestrial minerals. Formamide is
ubiquitous in the universe, produced by the reaction of water and HCN (
hydrogen cyanide).
It has several advantages as a prebiotic precursor, including the
ability to easily become concentrated through the evaporation of water.
[82][83] Although HCN is poisonous, it only affects
aerobic
organisms (eukaryotes and aerobic bacteria). It can play roles in other
chemical processes as well, such as the synthesis of the amino acid
glycine.
[24]
In 1961, it was shown that the nucleic acid purine base
adenine can be formed by heating aqueous
ammonium cyanide solutions.
[84] Other pathways for synthesizing bases from inorganic materials were also reported.
[85] Leslie Orgel
and colleagues have shown that freezing temperatures are advantageous
for the synthesis of purines, due to the concentrating effect for key
precursors such as
hydrogen cyanide.
[86] Research by
Stanley Miller
and colleagues suggested that while adenine and guanine require
freezing conditions for synthesis, cytosine and uracil may require
boiling temperatures.
[87]
Research by the Miller group notes the formation of seven different
amino acids and 11 types of nucleobases in ice when ammonia and
cyanide were left in a freezer from 1972 to 1997.
[88][89] Other work demonstrated the formation of s-
triazines (alternative
nucleobases),
pyrimidines
(including cytosine and uracil), and adenine from urea solutions
subjected to freeze-thaw cycles under a reductive atmosphere (with spark
discharges as an energy source).
[90] The explanation given for the unusual speed of these reactions at such a low temperature is
eutectic freezing.
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.
At the time of the
Miller–Urey experiment, scientific consensus was that the
early Earth had a
reducing atmosphere with compounds relatively rich in
hydrogen and poor in
oxygen (e.g.,
CH
4 and
NH
3 as opposed to
CO
2 and
NO
2). However, current scientific consensus describes the primitive atmosphere as either weakly reducing or neutral
[91][92] (see also
Oxygen catastrophe).
Such an atmosphere would diminish both the amount and variety of amino
acids that could be produced, although studies that include iron and
carbonate minerals (thought to be present in early oceans) in the
experimental conditions have again produced a diverse array of amino
acids.
[91] Other scientific research has focused on two other potential reducing environments: outer space and deep-sea thermal vents.
[93][94][95]
The spontaneous formation of complex
polymers
from abiotically generated monomers under the conditions posited by the
"soup" theory is not at all a straightforward process. Besides the
necessary basic organic monomers, compounds that would have prohibited
the formation of polymers were formed in high concentration during the
Miller–Urey and Oró experiments.
[96]
The Miller–Urey experiment, for example, produces many substances that
would react with the amino acids or terminate their coupling into
peptide chains.
[97]
Autocatalysis
Autocatalysts
are substances that catalyze the production of themselves, and
therefore are simple molecular replicators. The simplest
self-replicating chemical systems are autocatalytic, and typically
contain three components: two precursors that join together to form a
product molecule, and the product molecule itself. The product molecule
catalyzes the reaction by providing a complementary template which binds
to the precursors, thus bringing them together. Such systems have been
demonstrated both in biological macromolecules and in small organic
molecules.
[98][99] Systems that do not proceed by template mechanisms, such as the self-reproduction of
micelles and
vesicles, have also been observed.
[99]
In 1993, Stuart Kauffman proposed that life initially arose as autocatalytic chemical networks.
[100] British ethologist Richard Dawkins wrote about
autocatalysis as a potential explanation for the origin of life in his 2004 book
The Ancestor's Tale.
[101] In his book, Dawkins cites experiments performed by
Julius Rebek and his colleagues at the
Scripps Research Institute in
California in which they combined
amino adenosine and
pentafluorophenyl esters
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 selection.
[citation needed]
In the early 1970s,
Manfred Eigen and
Peter Schuster examined the transient stages between the molecular chaos and a self-replicating
hypercycle in a prebiotic soup.
[102] In a hypercycle, the
information storing system (possibly RNA) produces an
enzyme,
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
quasispecies,
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
ribozymes, capable of catalyzing their own chemical reactions.
[103]
The hypercycle theory requires the existence of complex biochemicals
such as nucleotides which are not formed under the conditions proposed
by the Miller–Urey experiment.
Geoffrey W. Hoffmann,
a student of Eigen, contributed to the concept of life involving both
replication and metabolism emerging from catalytic noise. His
contributions included showing that an early sloppy translation
machinery can be stable against an error catastrophe of the type that
had been envisaged as problematical by Leslie Orgel ("Orgel's paradox")
[104][105]
and calculations regarding the occurrence of a set of required
catalytic activities together with the exclusion of catalytic activities
that would be disruptive.
[106]
Homochirality
Homochirality refers to the geometric property of some materials that are composed of
chiral units. Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands.
Living organisms use molecules that have the same chirality ("handedness"): with some exceptions,
amino acids are left-handed while
nucleotides and
sugars
are right-handed. Chiral molecules can be synthesized, but in the
absence of a chiral source or a chiral catalyst, they are formed in a
50/50 mixture of both
enantiomers. This is called a
racemic
mixture. Known mechanisms for the production of non-racemic mixtures
from racemic starting materials include: asymmetric physical laws, such
as the
electroweak interaction;
asymmetric environments, such as those caused by circularly polarized
light, quartz crystals, or the Earth's rotation; and statistical
fluctuations during racemic synthesis.
[107]
Once established, chirality would be selected for.
[108] A small
enantiomeric excess can be amplified into a large one by
asymmetric autocatalysis, such as in the
Soai reaction.
[109]
In asymmetric autocatalysis, the catalyst is a chiral molecule, which
means that a chiral molecule is catalysing its own production. An
initial enantiomeric excess, such as can be produced by polarized light,
then allows the more abundant enantiomer to outcompete the other.
[110]
Clark has suggested that homochirality may have started in
outer space, as the studies of the amino acids on the
Murchison meteorite
showed L-alanine to be more than twice as frequent as its D form, and
L-glutamic acid was more than three times prevalent than its D
counterpart. Various chiral crystal surfaces can also act as sites for
possible concentration and assembly of chiral monomer units into
macromolecules.
[111]
Compounds found on meteorites suggest that the chirality of life
derives from abiogenic synthesis, since amino acids from meteorites show
a left-handed bias, whereas sugars show a predominantly right-handed
bias, the same as found in living organisms.
[112]
Reproduction, Duplication and the RNA world
Atomic structure of the ribosome 30S Subunit from
Thermus thermophilus.
[113] Proteins are shown in blue and the single RNA chain in orange.
The
RNA world hypothesis
describes an early Earth with self-replicating and catalytic RNA but no
DNA or proteins. It is generally accepted that current life on Earth
descends from an RNA world,
[114] although RNA-based life may not have been the first life to exist.
[115][116]
This conclusion is drawn from many independent lines of evidence, such
as the observations that RNA is central to the translation process and
that small RNAs can catalyze all of the chemical groups and information
transfers required for life.
[116][117] The structure of the
ribosome has been called the "smoking gun," as it showed that the ribosome is a
ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the
active site where peptide bond formation is catalyzed.
[115] The concept of the RNA world was first proposed in the 1960s by
Francis Crick,
Leslie Orgel, and
Carl Woese, and the term was coined by
Walter Gilbert in 1986.
[116][118]
Possible precursors for the evolution of protein synthesis include a
mechanism to synthesize short peptide cofactors or from a mechanism for
the duplication of RNA. It is likely that the ancestral ribosome was
composed entirely of RNA, although some roles have since been taken over
by proteins. Major remaining questions on this topic include
identifying the selective force for the evolution of the
ribosome and determining how the
genetic code arose.
[119]
Eugene Koonin
said, "Despite considerable experimental and theoretical effort, no
compelling scenarios currently exist for the origin of replication and
translation, the key processes that together comprise the core of
biological systems and the apparent pre-requisite of biological
evolution. The RNA World concept might offer the best chance for the
resolution of this conundrum but so far cannot adequately account for
the emergence of an efficient RNA replicase or the translation system.
The MWO (Ed.: "many worlds in one"
[120]) version of the cosmological model of
eternal inflation could suggest a way out of this conundrum because, in an infinite
multiverse
with a finite number of distinct macroscopic histories (each repeated
an infinite number of times), emergence of even highly complex systems
by chance is not just possible but inevitable."
[120]
RNA synthesis and replication
The RNA world has spurred scientists to try to determine if RNA
molecules could have spontaneously formed that were capable of
catalyzing their own replication.
[121][122][123] Evidences suggest chemical conditions (including the presence of
boron,
molybdenum and
oxygen) for initially producing RNA molecules may have been better on the planet
Mars than those on the planet
Earth.
[121][122] If so, life-suitable molecules, originating on Mars, may have later migrated to Earth via
meteor ejections.
[121][122]
A number of hypotheses of modes of formation have been put forward.
As of 1994, there were difficulties in the abiotic synthesis of the
nucleotides
cytosine and
uracil.
[124] Subsequent research has shown possible routes of synthesis; for example,
formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals.
[82][83] Early cell membranes could have formed spontaneously from
proteinoids,
which are protein-like molecules produced when amino acid solutions are
heated while in the correct concentration in aqueous solution. These
are seen to form micro-spheres which are observed to behave similarly to
membrane-enclosed compartments. Other possibilities include systems of
chemical reactions that take place within
clay substrates or on the surface of
pyrite rocks.
Factors supportive of an important role for RNA in early life include
its ability to act both to store information and to catalyze chemical
reactions (as a
ribozyme); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of
DNA)
in modern organisms; and the ease of chemical synthesis of at least the
components of the molecule under the conditions that approximated the
early Earth. Relatively short RNA molecules have been artificially
produced in labs, which are capable of replication.
[125] Such replicase RNA, which functions as both code and catalyst provides its own 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 conditions were present,
Darwinian selection would favour the proliferation of such self-catalysing structures, to which further functionalities could be added.
[126] Lincoln and Joyce have identified RNA systems capable of self-sustained replication.
[127]
The systems, which include two ribozymes that catalyze each other's
synthesis, replicated with doubling time of about one hour, and were
subject to natural selection.
[128] In evolutionary competition experiments, this led to the emergence of new systems which replicated more efficiently.
[115] This was the first demonstration of evolutionary adaptation occurring in a molecular genetic system.
[128]
Life can be considered to have emerged when RNA chains began to
express the basic conditions necessary for natural selection to operate
as conceived by Darwin: heritability, variation of type, and competition
for limited resources. Fitness of an RNA replicator (its per capita
rate of increase) would likely be a function of adaptive capacities that
were intrinsic (in the sense that they were determined by the
nucleotide sequence) and the availability of resources.
[129][130]
The three primary adaptive capacities may have been (1) the capacity to
replicate with moderate fidelity (giving rise to both heritability and
variation of type), (2) the capacity to avoid decay, and (3) the
capacity to acquire and process resources.
[129][130]
These capacities would have been determined initially by the folded
configurations of the RNA replicators that, in turn, would be encoded in
their individual nucleotide sequences. Competitive success among
different replicators would have depended on the relative values of
these adaptive capacities.
Pre-RNA world
It is possible that a different type of
nucleic acid, such as
PNA,
TNA or
GNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later.
[131][132]
Larralde et al., say that "the generally accepted prebiotic synthesis
of ribose, the formose reaction, yields numerous sugars without any
selectivity."
[133]
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.
[134]
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 using
nitrogenous or oxygenous chemistries.
John Sutherland
has 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.
[135]
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.
[136]
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.
[137] James Ferris's studies have shown that clay minerals of
montmorillonite
will catalyze the formation of RNA in aqueous solution, by joining
activated mono RNA nucleotides to join together to form longer chains.
[138]
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.
Protocells
The three main structures phospholipids form spontaneously in solution: the
liposome (a closed bilayer), the micelle and the bilayer
A
protocell is self-organized, endogenously ordered, spherical collection of
lipids proposed as a stepping-stone to the origin of life.
[139] A central question in
evolution
is how simple protocells first arose and began the competitive process
that drove the evolution of life. Although a functional protocell has
not yet been achieved in a laboratory setting, the goal appears well
within reach.
[140][141][142]
Self-assembled vesicles are essential components of primitive cells.
[139] The
second law of thermodynamics requires that the universe move in a direction in which disorder (or
entropy) increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate
life processes from non-living matter.
[143] Researchers Irene A. Chen and
Jack W. Szostak
(Nobel Prize in Physiology or Medicine 2009) amongst others,
demonstrated that simple physicochemical properties of elementary
protocells can give rise to essential cellular behaviors, including
primitive forms of Darwinian competition and energy storage. Such
cooperative interactions between the membrane and encapsulated contents
could greatly simplify the transition from replicating molecules to true
cells.
[141]
Furthermore, competition for membrane molecules would favor stabilized
membranes, suggesting a selective advantage for the evolution of
cross-linked fatty acids and even the
phospholipids of today.
[141] This
micro-encapsulation allowed for
metabolism within the membrane, exchange of small molecules and prevention of passage of large substances across it.
[144] The main advantages of encapsulation include increased
solubility of the cargo and storing
energy in the form of a
chemical gradient.
A 2012 study led by Armen Mulkidjanian of Germany's University of
Osnabrück, suggests that inland pools of condensed and cooled geothermal
vapour have the ideal characteristics for the origin of life.
[145] Scientists discovered in 2002 that by adding a
montmorillonite clay to a solution of fatty acid
micelles (lipid spheres), the clay sped up the rate of
vesicles formation 100-fold.
[142] So this one mineral can get precursors (
nucleotides) to spontaneously assemble into RNA and membrane precursors to assemble into membrane.
Another protocell model is the
Jeewanu. First synthesized in 1963 from simple minerals and basic organics while exposed to
sunlight, it is still reported to have some metabolic capabilities, the presence of
semipermeable membrane,
amino acids,
phospholipids,
carbohydrates and RNA-like molecules.
[146][147] However, the nature and properties of the Jeewanu remains to be clarified.
Origin of biological metabolism
Laboratory research suggests that metabolism-like reactions could
have occurred naturally in early oceans, before the first organisms
evolved.
[19][20] The findings suggests that
metabolism
predates the origin of life and evolved through the chemical conditions
that prevailed in the worlds earliest oceans. Reconstructions in
laboratories show that some of these reactions can produce RNA, and some
others resemble two essential reaction cascades of metabolism:
glycolysis and the
pentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids and lipids.
[19]
Following are some observed discoveries and related hypotheses.
Iron-sulfur world
Another possible answer to the polymerization conundrum was provided in the 1980s by
Günter Wächtershäuser, encouraged and supported by
Karl R. Popper,
[148][149][150] in his
iron–sulfur world theory.
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
ultraviolet irradiation), "Wächtershäuser systems" come with a built-in source of energy,
sulfides of
iron and other minerals (e.g. pyrite). The energy released from
redox reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of
oligomers and polymers. 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.
[19][20] The experiment produced a relatively small yield of
dipeptides (0.4% to 12.4%) and a smaller yield of
tripeptides (0.10%) although under the same conditions, dipeptides were quickly broken down.
[151]
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. The centrality of the
Krebs cycle to energy production in
aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, including amino acids and
nucleotides, suggests that it was one of the first parts of the metabolism to evolve.
[152] Somewhat in agreement with these notions,
Mike Russell
has proposed that "the purpose of life is to hydrogenate carbon
dioxide" (as part of a "metabolism-first", rather than a
"genetics-first", scenario).
[153][154]
Physicist Jeremy England of MIT has proposed that thermodynamically,
life was bound to eventually arrive, as based on established physics, he
mathematically indicates "that when a group of atoms is driven by an
external source of energy (like the sun or chemical fuel) and surrounded
by a heat bath (like the ocean or atmosphere), it will often gradually
restructure itself in order to dissipate increasingly more energy. This
could mean that under certain conditions, matter inexorably acquires the
key physical attribute associated with life.".
[155][156]
One of the earliest incarnations of this idea was put forward in 1924
with Alexander Oparin's notion of primitive self-replicating
vesicles which predated the discovery of the structure of DNA. Variants in the 1980s and 1990s include
Günter Wächtershäuser's
iron-sulfur world theory and models introduced by
Christian de Duve based on the chemistry of
thioesters.
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 Dyson in the early 1980s and
Stuart Kauffman's notion of collectively
autocatalytic sets, discussed later in that decade.
Leslie Orgel 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/FeS
2 or some other mineral."
[157]
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-CoA
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.
Zn-World hypothesis
The Zn-World (zinc world) theory of Armen Mulkidjanian
[158]
is an extension of Wächtershäuser's pyrite hypothesis. Wächtershäuser
based his theory of the initial chemical processes leading to
informational molecules (i.e. RNA, peptides) on a regular mesh of
electric charges at the surface of
pyrite
that may have made the primeval polymerization thermodynamically more
favourable by attracting reactants and arranging them appropriately
relative to each other.
[159] The Zn-World theory specifies and differentiates further.
[158][160] Hydrothermal fluids rich in H
2S
interacting with cold primordial ocean (or "Darwin pond") water leads
to the precipitation of metal sulfide particles.
Oceanic vent systems
and other hydrothermal systems have a zonal structure reflected in
ancient
volcanogenic massive sulfide deposits (VMS) of hydrothermal origin. They reach many kilometers in diameter and date back to the
Archean eon. Most abundant are
pyrite (FeS
2),
chalcopyrite (CuFeS
2), and
sphalerite (ZnS), with additions of
galena (PbS) and
alabandite
(MnS). ZnS and MnS have a unique ability to store radiation energy,
e.g. provided by UV light. Since during the relevant time window of the
origins of replicating molecules the primordial atmospheric pressure was
high enough (>100 bar) to precipitate near the Earth's surface and
UV irradiation was 10 to 100 times more intense than now, the unique
photosynthetic properties mediated by ZnS provided just the right energy
conditions to energize the synthesis of informational and metabolic
molecules and the selection of photostable nucleobases.
The Zn-World theory has been further filled out with experimental and
theoretical evidence for the ionic constitution of the interior of the
first proto-cells before
Archea,
Eubacteria and
Proto-Eukarya evolved. Archibald Maccallum noted the resemblance of organismal fluids such as blood, lymph to seawater;
[161]
however, the inorganic composition of all cells differ from that of
modern sea water, which led Mulkidjanian and colleagues to reconstruct
the "hatcheries" of the first cells combining geochemical analysis with
phylogenomic scrutiny of the inorganic ion requirements of universal
components of modern cells. The authors conclude that ubiquitous, and by
inference primordial, proteins and functional systems show affinity to
and functional requirement for K
+, Zn
2+, Mn
2+, and
phosphate.
Geochemical reconstruction shows that the ionic composition conducive
to the origin of cells could not have existed in what we today call
marine settings but is compatible with emissions of vapor-dominated
zones of what we today call inland geothermal systems. Under the anoxic,
CO
2-dominated primordial atmosphere, the chemistry of water
condensates and exhalations near geothermal fields would resemble the
internal milieu of modern cells. Therefore, the precellular stages of
evolution may have taken place in shallow "Darwin-ponds" lined with
porous
silicate minerals mixed with metal sulfides and enriched in K
+, Zn
2+, and phosphorus compounds.
[162][163]
Deep sea vent hypothesis
The deep sea vent, or alkaline
hydrothermal vent, theory for the origin of life on Earth posits that life may have begun at submarine hydrothermal vents,
[164] where hydrogen-rich fluids emerge from below the sea floor, as a result of
serpentinization of ultra-
mafic olivine
with sea water and a pH 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 theory). These are highly
exothermic reactions.
[note 2]
Michael Russell demonstrated that alkaline vents created an abiogenic
proton-motive force chemiosmotic gradient,
[165]
in which conditions are ideal for an abiogenic hatchery for life. Their
microscopic compartments "provide a natural means of concentrating
organic molecules", composed of iron-sulfur minerals such as
mackinawite, endowed these mineral cells with the catalytic properties envisaged by
Günter Wächtershäuser.
[152] This movement of ions across the membrane depends on a combination of two factors:
- Diffusion force caused by concentration gradient – all particles including ions tend to diffuse from higher concentration to lower.
- Electrostatic force caused by electrical potential gradient – cations like protons H+ tend to diffuse down the electrical potential, anions in the opposite direction.
These two gradients taken together can be expressed as an
electrochemical gradient,
providing energy for abiogenic synthesis. The proton-motive force (PMF)
can be described as the measure of the potential energy stored as a
combination of proton and voltage gradients across a membrane
(differences in proton concentration and electrical potential).
Nobel laureate Szostak suggested that geothermal activity provides
greater opportunities for the origination of life in open lakes where
there is a buildup of minerals. In 2010, based on spectral analysis of
sea and hot mineral water as well as cactus juice, Ignat Ignatov and
Oleg Mosin demonstrated that life may have predominantly originated in
hot mineral water. The hot mineral water that contains bicarbonate and
calcium ions has the most optimal range.
[166]
This is similar case as the origin of life in hydrothermal vents, but
with bicarbonate and calcium ions in hot water. This water has a pH of
9–11 and is possible to have the reactions in sea water. According to
Nobel winner
Melvin Calvin,
certain reactions of condensation-dehydration of amino acids and
nucleotides in individual blocks of peptides and nucleic acids can take
place in the primary hydrosphere with pH 9-11 at a later evolutionary
stage.
[167]
Some of these compounds like hydrocyanic acid (HCN) have been proven in
the experiments of Miller. This is the environment in which the
stromatolites have been created. David Ward described the formation of
stromatolites in hot mineral water at the Yellowstone National Park.
Stromatolites have lived in hot mineral water and in proximity to areas
with volcanic activity.
[168] Processes have evolved in the sea near geysers of hot mineral water. In 2011 Tadashi Sugawara created a protocell in hot water.
[169]
Thermosynthesis
Today's bioenergetic process of
fermentation
is carried out by either the aforementioned citric acid cycle or the
Acetyl-CoA pathway, both of which have been connected to the primordial
iron-sulfur world. In a different approach, the thermosynthesis
hypothesis considers the bioenergetic process of
chemiosmosis, which plays an essential role in
cellular respiration and
photosynthesis, more basal than fermentation: the
ATP synthase enzyme, which sustains
chemiosmosis, is proposed as the currently extant enzyme most closely related to the first metabolic process.
[170][171]
First, life needed an energy source to bring about the condensation
reaction that yielded the peptide bonds of proteins and the
phosphodiester bonds of RNA. 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 after a
temperature change it was released by thermal unfolding.
The energy source under the thermosynthesis hypothesis was thermal cycling, the result of suspension of protocells in a
convection current, as is plausible in a volcanic hot spring; the convection accounts for the
self-organization 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.
This protein also synthesized a library of many proteins, of which only a
minute fraction had thermosynthesis capabilities. As proposed by
Dyson,
[172]
it propagated functionally: it made daughters with similar
capabilities, but it did not copy itself. Functioning daughters
consisted of different amino acid sequences.
Whereas the iron-sulfur world identifies a circular pathway as the
most simple—and therefore assumes the existence of enzymes—the
thermosynthesis hypothesis 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,
[173]
rather than a regular enzyme's mechanism, which decreases the free
energy. The RNA world also implies the existence of several enzymes. It
has been claimed that the emergence of cyclic systems of protein
catalysts is implausible.
[174]
Other models of abiogenesis
Clay hypothesis
A model for the origin of life based on clay was forwarded by A.
Graham Cairns-Smith in 1985 and explored as a plausible illustration by several scientists.
[175] The
Clay hypothesis
postulates that complex organic molecules arose gradually on a
pre-existing, non-organic replication platform of silicate crystals in
solution.
Cairns-Smith is a trenchant critic of other models of chemical evolution.
[176] However, he admits that like many models of the origin of life, his own also has its shortcomings.
In 2007, Kahr and colleagues reported their experiments that tested
the idea that crystals can act as a source of transferable information,
using crystals of
potassium hydrogen phthalate.
"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 new crystals and found that the
imperfections in the mother crystals were reproduced in the daughters,
but the daughter crystals also had many additional imperfections. For
gene-like behavior to be observed, the quantity of inheritance of these
imperfections should have exceeded that of the mutations in the
successive generations, but it did not. Thus Kahr concluded that the
crystals, "were not faithful enough to store and transfer information
from one generation to the next".
[177][178]
Gold's "deep-hot biosphere" model
In the 1970s,
Thomas Gold
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
nanobes (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks
[179] 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 5 kilometres (3.1 mi) below the surface,
[179] in the form of
extremophile archaea, 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 system
would lend significant credence to this theory. Thomas Gold 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 the flow of such food is due to out-gassing of primordial
methane from the
Earth's mantle;
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.
Exogenesis is related to, but not the same as, the notion of
panspermia.
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 hypothesis is scant, but it finds support in studies of
Martian meteorites found in Antarctica and in studies of
extremophile microbes' survival in outer space.
[180][181][182][183][184][185][186]
On 24 January 2014, NASA reported that
current studies on the planet
Mars by the
Curiosity and
Opportunity rovers will now be searching for evidence of ancient life, including a
biosphere based on
autotrophic,
chemotrophic and/or
chemolithoautotrophic microorganisms, as well as ancient water, including
fluvio-lacustrine environments (
plains related to ancient
rivers or
lakes) that may have been
habitable.
[187][188][189][190] The search for evidence of
habitability,
taphonomy (related to
fossils), and
organic carbon on the planet
Mars is now a primary
NASA objective.
[187]
Methane is one of the simplest organic compounds
An
organic compound is any member of a large class of gaseous, liquid, or solid chemicals whose
molecules contain
carbon. Carbon is the
fourth most abundant element in the universe by mass after
hydrogen,
helium, and
oxygen.
[191] Carbon is abundant in the Sun, stars, comets, and in the
atmospheres of most planets.
[192] Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in
molecular clouds and
circumstellar envelopes, and chemically evolve after reactions are initiated mostly by
ionizing radiation.
[70][193] Based on
computer model studies, the complex organic molecules necessary for life may have formed on
dust grains in the
protoplanetary disk surrounding the Sun before the formation of the Earth.
[21] According to the computer studies, this same process may also occur around other
stars that acquire
planets.
[21]
Observations suggest that the majority of organic compounds introduced on Earth by
interstellar dust particles are considered principal agents in the formation of complex molecules, thanks to their peculiar
surface-catalytic activities.
[194][195] Studies reported in 2008, based on
12C/
13C
isotopic ratios of organic compounds found in the
Murchison meteorite, suggested that the RNA component
uracil and related molecules, including
xanthine, were formed extraterrestrially.
[196][197] On 8 August 2011, a report based on NASA studies of
meteorites found on Earth was published suggesting DNA components (
adenine,
guanine and related organic molecules) were made in
outer space.
[194][198][199][200] Scientists also found that the
cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed
aromatic-
aliphatic structure") that could be created naturally, and rapidly, by
stars.
[201][202][203]
A scientist who suggested that these compounds may have been related to
the development of life on Earth said that "If this is the case, life
on Earth may have had an easier time getting started as these organics
can serve as basic ingredients for life."
[201]
Glycolaldehyde, the first example of an interstellar
sugar
molecule, was detected in the star-forming region near the center of
our galaxy. It was discovered in 2000 by Jes Jørgensen and Jan M.
Hollis.
[204]
Then, on 29 August 2012, the same team reported the detection of
glycolaldehyde in a distant star system. The molecule was found around
the
protostellar binary
IRAS 16293-2422 400 light years from Earth.
[205][206][207] Glycolaldehyde is needed to form
ribonucleic acid (
RNA), which is similar in function to
DNA.
These findings suggest that complex organic molecules may form in
stellar systems prior to the formation of planets, eventually arriving
on young planets early in their formation.
[208] Because sugars are associated with both
metabolism and the
genetic code,
two of the most basic aspects of life, it is thought the discovery of
extraterrestrial sugar increases the likelihood that life may exist
elsewhere in our galaxy.
[204]
NASA
announced in 2009 that scientists had identified another fundamental
chemical building block of life in a comet for the first time,
glycine, an amino acid, which was detected in material ejected from
Comet Wild-2 in 2004 and grabbed by NASA's
Stardust
probe. Glycine has been detected in meteorites before. 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."
[209] Comets are encrusted with outer layers of dark material, thought to be a
tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by
ionizing radiation.
It is possible that a rain of material from comets could have brought
significant quantities of such complex organic molecules to Earth.
[210][211] Amino acids which were formed extraterrestrially may also have arrived on Earth via comets.
[24] It is estimated that during the
Late Heavy Bombardment, meteorites may have delivered up to five million tons of biogenic elements to Earth per year.
[24]
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the visible
universe, and are considered a likely constituent of the
primordial sea.
[212][213][214] PAHs, along with
fullerenes (or "
buckyballs"), have been recently detected in nebulae.
[215][216]
On 3 April 2013, NASA reported that complex
organic chemicals could arise on
Titan, a moon of
Saturn, based on studies simulating the
atmosphere of Titan.
[217]
Lipid world
The
lipid world theory postulates that the first self-replicating object was lipid-like.
[218][219] It is known that
phospholipids form
lipid bilayers
in water while under agitation – the same structure as in cell
membranes. These molecules were not present on early Earth, but 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.
Studies on vesicles from
potentially
prebiotic
amphiphiles have so far been limited to systems containing one or two
types of amphiphiles. This in contrast to the output of simulated
prebiotic chemical reactions, which typically produce very heterogeneous
mixtures of compounds.
[220]
Within the hypothesis of a lipid bilayer membrane composed of a mixture
of various distinct amphiphilic compounds there is the opportunity of a
huge number of theoretically possible combinations in the arrangements
of these amphiphiles in the membrane. Among all these potential
combinations, a specific local arrangement of the membrane would have
favored the constitution of an
hypercycle,
[221][222] according to the terminology by Manfred Eigen, actually a positive
feedback composed of two mutual
catalysts
represented by a membrane site and a specific compound trapped in the
vesicle. Such site/compound pairs are transmissible to the daughter
vesicles leading to the emergence of distinct
lineages of vesicles which would have allowed Darwinian
natural selection.
[223]
Polyphosphates
A problem in 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
polyphosphates.
[224][225] 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 phosphate (
apatite),
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.
[226]
PAH world hypothesis
Polycyclic aromatic hydrocarbons (PAHs) are known to be abundant in the
universe,
[212][213][214] including in the
interstellar medium, in
comets, and in
meteorites, and are some of the most complex molecules so far found in space.
[192]
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 PAHs in a
nebula.
[227] In 2010, another team also detected PAHs, along with
fullerenes (or "
buckyballs"), in nebulae.
[228] The use of PAHs has also been proposed as a precursor to the
RNA world in the
PAH world hypothesis.
[citation needed] The
Spitzer Space Telescope
has 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. In September 2012, NASA scientists reported that PAHs, subjected to
interstellar medium (ISM) conditions, are transformed, through
hydrogenation,
oxygenation and
hydroxylation, to more complex
organics – "a step along the path toward amino acids and
nucleotides, the raw materials of
proteins and DNA, respectively".
[229][230] Further, as a result of these transformations, the PAHs lose their
spectroscopic signature which could be one of the reasons "for the lack of PAH detection in
interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of
protoplanetary disks."
[229][230]
On 21 February 2014,
NASA announced a
greatly upgraded database[192] for tracking PAHs in the
universe. According to scientists, more than 20% of the
carbon in the universe may be associated with PAHs, possible
starting materials for the formation of
life. PAHs seem to have been formed shortly after the
Big Bang, are widespread throughout the universe,
[212][213][214] and are associated with
new stars and
exoplanets.
[192]
Radioactive beach hypothesis
Zachary Adam claims that tidal processes that occurred during a time
when the moon was much closer may have concentrated grains of
uranium
and other radioactive elements at the high-water mark on primordial
beaches, where they may have been responsible for generating life's
building blocks.
[231] According to computer models reported in
Astrobiology,
[232] a deposit of such radioactive materials could show the same
self-sustaining nuclear reaction as that found in the
Oklo uranium ore seam in
Gabon. Such radioactive beach sand might have provided sufficient energy to generate organic molecules, such as amino acids and
sugars from
acetonitrile in water. Radioactive
monazite material also has released soluble
phosphate
into the regions between sand-grains, making it biologically
"accessible". Thus amino acids, sugars, and soluble phosphates might
have been produced simultaneously, according to Adam. Radioactive
actinides,
left behind in some concentration by the reaction, might have formed
part of organo-metallic complexes. These complexes could have been
important early
catalysts to living processes.
John Parnell has suggested that such a process could provide part of
the "crucible of life" in the early stages of any early wet rocky
planet, so long as the planet is large enough to have generated a system
of
plate tectonics
which brings radioactive minerals to the surface. As the early Earth is
thought to have had many smaller plates, it might have provided a
suitable environment for such processes.
[233]
Ultraviolet and temperature-assisted replication model
From a thermodynamic perspective of the origin of life springs the
ultraviolet and temperature-assisted replication (UVTAR) model. Karo
Michaelian points out that any model for the origin of life must take
into account the fact that life is an irreversible thermodynamic process
which arises and persists because it produces
entropy.
Entropy production is not incidental to the process of life, but rather
the fundamental reason for its existence. Present day life augments the
entropy production of Earth by catalysing the water cycle through
evapotranspiration.
[234][235]
Michaelian argues that if the thermodynamic function of life today is
to produce entropy through coupling with the water cycle, then this
probably was its function at its very beginnings. It turns out that both
RNA and DNA when in water solution are very strong absorbers and
extremely rapid dissipaters of ultraviolet light within the 200–300 nm
wavelength range, which is that part of the sun's spectrum that could
have penetrated the dense prebiotic atmosphere.
[236]
have shown that the amount of ultraviolet (UV) light reaching the
Earth's surface in the Archean eon could have been up to 31 orders of
magnitude greater than it is today at 260 nm where RNA and DNA absorb
most strongly. Absorption and dissipation of UV light by the organic
molecules at the Archean ocean surface would have significantly
increased the temperature of the surface and led to enhanced evaporation
and thus to have augmented the primitive water cycle. Since absorption
and dissipation of high energy photons is an entropy producing process,
argues that non-equilbrium abiogenic synthesis of RNA and DNA utilizing
UV light would have been thermodynamically favored.
[137]
A simple mechanism that could explain the replication of RNA and DNA
without resort to the use of enzymes could also be provided within the
same thermodynamic framework by assuming that life arose when the
temperature of the primitive seas had cooled to somewhat below the
denaturing temperature of RNA or DNA (based on the ratio of
18O/
16O found in cherts of the Barberton greenstone belt of South Africa of about 3.5 to 3.2
Ga., surface temperatures are predicted to have been around 70±15 °C,
[237] close to RNA or DNA
denaturing
(uncoiling and separation) temperatures. During the night, the surface
water temperature would drop below the denaturing temperature and single
strand RNA/DNA could act as a template for the formation of double
strand RNA/DNA. During the daylight hours, RNA and DNA would absorb UV
light and convert this directly to heat the ocean surface, thereby
raising the local temperature enough to allow for denaturing of RNA and
DNA. The copying process would have been repeated with each diurnal
cycle.
[238][239] Such a temperature assisted mechanism of replication bears similarity to
polymerase chain reaction
(PCR), a routine laboratory procedure employed to multiply DNA
segments. Michaelian suggests that the traditional origin of life
research, that expects to describe the emergence of life from
near-equilibrium conditions, is erroneous and that non-equilibrium
conditions must be considered, in particular, the importance of entropy
production to the emergence of life.
Since denaturation would be most probable in the late afternoon when
the Archean sea surface temperature would be highest, and since late
afternoon submarine sunlight is somewhat
circularly polarized, the homochirality of the organic molecules of life can also be explained within the proposed thermodynamic framework.
[240][241]
Multiple genesis
Different forms of life with variable origin processes may have appeared quasi-simultaneously in the early
history of Earth.
[242] 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
analogous to organisms of the current life tree. Hartman
[243] 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 biopoiesis, 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 Margulis's
endosymbiotic theory suggests that multiple forms of archea entered into symbiotic relationship to form the eukaryotic cell. The
horizontal transfer of genetic material
between archea 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.
