Abiogenesis, or informally the origin of life,
is the natural process by which life has arisen
from non-living matter, such as simple organic
compounds. While the details of this process
are still unknown, the prevailing scientific
hypothesis is that the transition from non-living
to living entities was not a single event,
but a gradual process of increasing complexity
that involved molecular self-replication,
self-assembly, autocatalysis, and the emergence
of cell membranes. Although the occurrence
of abiogenesis is uncontroversial among scientists,
there is no single, generally accepted model
for the origin of life, and this article presents
several principles and hypotheses for how
abiogenesis could have occurred.
Researchers study abiogenesis through a combination
of molecular biology, paleontology, astrobiology,
oceanography, biophysics, geochemistry and
biochemistry, and aim to determine how pre-life
chemical reactions gave rise to life. The
study of abiogenesis can be geophysical, chemical,
or biological, with more recent approaches
attempting a synthesis of all three, as life
arose under conditions that are strikingly
different from those on Earth today. Life
functions through the specialized chemistry
of carbon and water and builds largely upon
four key families of chemicals: lipids (fatty
cell walls), carbohydrates (sugars, cellulose),
amino acids (protein metabolism), and nucleic
acids (self-replicating DNA and RNA). Any
successful theory of abiogenesis must explain
the origins and interactions of these classes
of molecules. Many approaches to abiogenesis
investigate how self-replicating molecules,
or their components, came into existence.
Researchers generally think that current life
on Earth descends from an RNA world, although
RNA-based life may not have been the first
life to have existed.The classic 1952 Miller–Urey
experiment and similar research demonstrated
that most amino acids, the chemical constituents
of the proteins used in all living organisms,
can be synthesized from inorganic compounds
under conditions intended to replicate those
of the early Earth. Scientists have proposed
various external sources of energy that may
have triggered these reactions, including
lightning and radiation. Other approaches
("metabolism-first" hypotheses) focus on understanding
how catalysis in chemical systems on the early
Earth might have provided the precursor molecules
necessary for self-replication. Complex organic
molecules occur in the Solar System and in
interstellar space, and these molecules may
have provided starting material for the development
of life on Earth.The biochemistry of life
may have begun shortly after the Big Bang,
13.8 billion years ago, during a habitable
epoch when the age of the universe was only
10 to 17 million years. The panspermia hypothesis
suggests that microscopic life was distributed
to the early Earth by space dust, meteoroids,
asteroids and other small Solar System bodies
and that life may exist throughout the universe.
The panspermia hypothesis proposes that life
originated outside the Earth, but does not
definitively explain its origin.
Earth remains the only place in the universe
known to harbour life, and fossil evidence
from the Earth informs most studies of abiogenesis.
The age of the Earth is about 4.54 billion
years; the earliest undisputed evidence of
life on Earth dates from at least 3.5 billion
years ago, and possibly as early as the Eoarchean
Era (between 3.6 and 4.0 billion years ago),
after geological crust started to solidify
following the molten Hadean Eon. In May 2017
scientists found possible evidence of early
life on land in 3.48-billion-year-old geyserite
and other related mineral deposits (often
found around hot springs and geysers) uncovered
in the Pilbara Craton of Western Australia.
However, a number of discoveries suggest that
life may have appeared on Earth even earlier.
As of 2017, microfossils, or fossilised microorganisms,
within hydrothermal-vent precipitates dated
from 3.77 to 4.28 billion years old found
in rocks in Quebec may harbour the oldest
record of life on Earth, suggesting life started
soon after ocean formation 4.4 billion years
ago. According to biologist Stephen Blair
Hedges, "If life arose relatively quickly
on Earth … then it could be common in the
universe."
== Early geophysical conditions on Earth ==
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 hydrogen compounds—methane,
ammonia and water vapour—and that life began
under such reducing conditions, which are
conducive to the formation of organic molecules.
According to later models, suggested by study
of ancient minerals, the atmosphere in the
late Hadean period consisted largely of water
vapour, nitrogen and carbon dioxide, with
smaller amounts of carbon monoxide, hydrogen,
and sulfur compounds. 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.
As a consequence, Earth lacked the gravity
to hold any molecular hydrogen in its atmosphere,
and rapidly lost it during the Hadean period,
along with the bulk of the original inert
gases. The solution of carbon dioxide in water
is thought to have made the seas slightly
acidic, giving it a pH of about 5.5. The atmosphere
at the time has been characterized as a "gigantic,
productive outdoor chemical laboratory." It
may have been similar to the mixture of gases
released today by volcanoes, which still support
some abiotic chemistry.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. This has been supported
by the dating of 4.404 Ga-old zircon crystals
from metamorphosed quartzite of Mount Narryer
in the Western Australia Jack Hills of the
Pilbara, which are evidence that oceans and
continental crust existed within 150 Ma of
Earth's formation. Despite the likely increased
volcanism and existence of many smaller tectonic
"platelets," it has been suggested that between
4.4 and 4.3 Ga (billion years), the Earth
was a water world, with little if any continental
crust, an extremely turbulent atmosphere and
a hydrosphere subject to intense ultraviolet
(UV) light, from a T Tauri stage Sun, cosmic
radiation and continued bolide impacts.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 sterilize the planet and vaporize 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.
=== Earliest biological evidence for life
===
The most commonly accepted location of the
root of the tree of life is between a monophyletic
domain Bacteria and a clade formed by Archaea
and Eukaryota of what is referred to as the
"traditional tree of life" based on several
molecular studies starting with C. Woese.
A very small minority of studies have concluded
differently, namely that the root is in the
Domain Bacteria, either in the phylum Firmicutes
or that the phylum Chloroflexi is basal to
a clade with Archaea+Eukaryotes and the rest
of Bacteria as proposed by Thomas Cavalier-Smith.
More recently Peter Ward has established an
alternative view which is rooted in abiotic
RNA synthesis which becomes enclosed within
a capsule and then creates RNA ribozyme replicates.
It is proposed that this then bifurcates between
Dominion Ribosa (RNA life), and after the
loss of ribozymes RNA viruses as Domain Viorea,
and Dominion Terroa, which after creating
a large cell within a lipid wall, creating
DNA the 20 based amino acids and the triplet
code, is established as the last universal
common ancestor or LUCA, of earlier phylogenic
trees.
The earliest life on Earth existed more than
3.5 billion years ago, during the Eoarchean
Era when sufficient crust had solidified following
the molten Hadean Eon. The earliest physical
evidence so far found consists of microfossils
in the Nuvvuagittuq Greenstone Belt of Northern
Quebec, in "banded iron formation" rocks at
least 3.77 billion and possibly 4.28 billion
years old. This finding suggested that there
was almost instant development of life after
oceans were formed. The structure of the microbes
was noted to be similar to bacteria found
near hydrothermal vents in the modern era,
and provided support for the hypothesis that
abiogenesis began near hydrothermal vents.Also
noteworthy is biogenic graphite in 3.7 billion-year-old
metasedimentary rocks from southwestern Greenland
and microbial mat fossils found in 3.48 billion-year-old
sandstone from Western Australia. Evidence
of early life in rocks from Akilia Island,
near the Isua supracrustal belt in southwestern
Greenland, dating to 3.7 billion years ago
have shown biogenic carbon isotopes. In other
parts of the Isua supracrustal belt, graphite
inclusions trapped within garnet crystals
are connected to the other elements of life:
oxygen, nitrogen, and possibly phosphorus
in the form of phosphate, providing further
evidence for life 3.7 billion years ago. At
Strelley Pool, in the Pilbara region of Western
Australia, compelling evidence of early life
was found in pyrite-bearing sandstone in a
fossilized beach, that showed rounded tubular
cells that oxidized sulfur by photosynthesis
in the absence of oxygen. Further research
on zircons from Western Australia in 2015
suggested that life likely existed on Earth
at least 4.1 billion years ago.Traditionally
it was thought that during the period between
4.28 and 3.8 Ga, changes in the orbits of
the giant planets may have caused a heavy
bombardment by asteroids and comets 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.
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 7.17×105 (717 thousand)
years, and potassium-40 with a half-life of
1.250×109 (1.25 billion) years, isotopes
mainly produced in supernovae, were much more
common. Internal heating as a result of gravitational
sorting between the core and the mantle would
have caused a great deal of mantle convection,
with the probable result of many more smaller
and more active tectonic plates than now exist.
The time periods between such devastating
environmental events give time windows for
the possible origin of life in the early environments.
If the deep marine hydrothermal setting was
the site for the origin of life, then abiogenesis
could have happened as early as 4.0 to 4.2
Ga. If the site was at the surface of the
Earth, abiogenesis could only have occurred
between 3.7 and 4.0 Ga.In 2016, a set of 355
genes likely present in the Last Universal
Common Ancestor (LUCA) of all organisms living
on Earth was identified. A total of 6.1 million
prokaryotic protein coding genes from various
phylogenic trees were sequenced, identifying
355 protein clusters from amongst 286,514
protein clusters that were probably common
to LUCA. The results "depict LUCA as anaerobic,
CO2-fixing, H2-dependent with a Wood–Ljungdahl
pathway, N2-fixing and thermophilic. LUCA’s
biochemistry was replete with FeS clusters
and radical reaction mechanisms. Its cofactors
reveal dependence upon transition metals,
flavins, S-adenosyl methionine, coenzyme A,
ferredoxin, molybdopterin, corrins and selenium.
Its genetic code required nucleoside modifications
and S-adenosylmethionine-dependent methylations."
The results depict methanogenic clostridia
as a basal clade in the 355 phylogenies examined,
and suggest that LUCA inhabited an anaerobic
hydrothermal vent setting in a geochemically
active environment rich in H2, CO2 and iron.
M.D. Brazier has shown that early micro-fossils
came from a hot world of gases such as methane,
ammonia, carbon dioxide and hydrogen sulphide,
which are toxic to much current life. Another
analysis of the conventional threefold tree
of life shows thermophilic and hyperthermophilic
bacteria and archaea are closest to the root,
suggesting that life may have evolved in a
hot environment.
== Conceptual history ==
=== 
Spontaneous generation ===
Belief in 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). Classical
notions of spontaneous generation 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 that
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. In the 17th century, people began to
question such assumptions. 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 [spontaneous generation], is to question
Reason, Sense, and Experience: If he doubts
of this, let him go to Ægypt, and there he
will finde the fields swarming with mice begot
of the mud of Nylus, to the great calamity
of the Inhabitants."In 1665, Robert Hooke
published the first drawings of a microorganism.
Hooke was followed in 1676 by Antonie van
Leeuwenhoek, who drew and described microorganisms
that are now thought to have been protozoa
and bacteria. 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 he became,
by the 1680s, convinced that spontaneous generation
was incorrect.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 could only appear
by invasion from without.
The belief that self-ordering by spontaneous
generation was impossible begged for an alternative.
By the middle of the 19th century, the theory
of biogenesis had accumulated so much evidential
support, due to the work of Pasteur and others,
that the alternative theory of spontaneous
generation had been effectively disproven.
John Desmond Bernal, a pioneer in X-ray crystallography,
suggested that earlier theories such as spontaneous
generation were based upon an explanation
that life was continuously created as a result
of chance events.
=== Etymology ===
The term biogenesis is usually credited to
either Henry Charlton Bastian or to Thomas
Henry Huxley. 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. In it he introduced the term
biogenesis (with an opposite meaning to Bastian's)
as well as 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.Subsequently, in
the preface to Bastian's 1871 book, The Modes
of Origin of Lowest Organisms, Bastian referred
to the possible confusion with Huxley's usage
and 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
word 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.
=== Louis Pasteur and Charles Darwin ===
Louis Pasteur remarked, about a finding of
his in 1864 which he considered definitive,
"Never will the doctrine of spontaneous generation
recover from the mortal blow struck by this
simple experiment." One alternative was that
life's origins on Earth had come from somewhere
else in the universe. Periodically resurrected
(see Panspermia, above) Bernal said 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."
Such a theory, Bernal said, was unscientific.
A theory popular around the same time was
that life was the result of an inner "life
force", which in the late 19th century was
championed by Henri Bergson.
The idea of evolution by natural selection
proposed by Charles Darwin put an end to these
metaphysical theologies. In a letter to Joseph
Dalton Hooker on 1 February 1871, Darwin discussed
the suggestion that the original spark of
life may have begun in a "warm little pond,
with all sorts of ammonia and phosphoric salts,
light, heat, electricity, &c., present, that
a proteine 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."
He had written to Hooker in 1863 stating that,
"It is mere rubbish, thinking at present of
the origin of life; one might as well think
of the origin of matter." In On the Origin
of Species, he had referred to life having
been "created", by which he "really meant
'appeared' by some wholly unknown process",
but had soon regretted using the Old Testament
term "creation".
=== "Primordial soup" hypothesis ===
No new notable research or hypothesis 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, 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 factors that promote "cell integrity"
survive, and those that do not become extinct.
Many modern theories of the origin of life
still take Oparin's ideas as a starting point.
Robert Shapiro has summarized the "primordial
soup" theory of Oparin and J. B. S. Haldane
in its "mature form" as follows:
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" that
may have concentrated at various locations
(shorelines, oceanic vents etc.).
By further transformation, more complex organic
polymers – and ultimately life – developed
in the soup.About this time, Haldane suggested
that the Earth's prebiotic oceans (quite different
from their modern counterparts) would have
formed a "hot dilute soup" in which organic
compounds could have formed. Bernal called
this idea biopoiesis or biopoesis, the process
of living matter evolving from self-replicating
but non-living molecules, 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.
Stanley L. Miller and Harold C. 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 reducing mixture of gases—methane,
ammonia, and hydrogen, as well as water vapour—to
form simple organic monomers such as amino
acids. The mixture of gases was cycled through
an apparatus that delivered electrical sparks
to the mixture. After one week, it was found
that about 10% to 15% of the carbon in the
system was then in the form of a racemic mixture
of organic compounds, including amino acids,
which are the building blocks of proteins.
This provided direct experimental support
for the second point of the "soup" theory,
and it is around the remaining two points
of the theory that much of the debate now
centres.
Bernal showed that based upon this and subsequent
work there is no difficulty in principle in
forming most of the molecules we recognize
as the necessary molecules for life from their
inorganic precursors. The underlying hypothesis
held by Oparin, Haldane, Bernal, Miller and
Urey, for instance, was that multiple conditions
on the primeval Earth favoured chemical reactions
that synthesized the same set of complex organic
compounds from such simple precursors. 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. However, Bernal
said that "it is not enough to explain the
formation of such molecules, what is necessary,
is a physical-chemical explanation of the
origins of these molecules that suggests the
presence of suitable sources and sinks for
free energy."More recent studies, in October
2017, support the notion that life may have
begun right after the Earth was formed as
RNA molecules emerging from "warm little ponds".
=== 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
(small chains of amino acids) under conditions
that might plausibly have existed early in
Earth's history. 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".In another experiment
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 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.
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 naivety
about the complexity of cell structures.
== Current models ==
There is no single, generally accepted model
for the origin of life. Scientists have proposed
several plausible hypotheses, which share
some common elements. While differing in the
details, these hypotheses are based on the
framework laid out by Alexander Oparin (in
1924) and by J. B. S. Haldane (in 1925), who
postulated the molecular or chemical evolution
theory of life. 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". Oparin and Haldane suggested
that the atmosphere of the early Earth may
have been chemically reducing in nature, composed
primarily of methane (CH4), ammonia (NH3),
water (H2O), hydrogen sulfide (H2S), carbon
dioxide (CO2) or carbon monoxide (CO), and
phosphate (PO43−), with molecular oxygen
(O2) and ozone (O3) either rare or absent.
According to later models, the atmosphere
in the late Hadean period consisted largely
of nitrogen (N2) and carbon dioxide, with
smaller amounts of carbon monoxide, hydrogen
(H2), and sulfur compounds; while it did lack
molecular oxygen and ozone, it was not as
chemically reducing as Oparin and Haldane
supposed. In the atmosphere proposed by Oparin
and Haldane, electrical activity can produce
certain small molecules (monomers) of life,
such as amino acids. The Miller–Urey experiment
reported in 1953 demonstrated this.
Bernal coined the term biopoiesis in 1949
to refer to the origin of life. In 1967, he
suggested that it occurred in three "stages":
the origin of biological monomers
the origin of biological polymers
the evolution from molecules to cellsBernal
suggested that evolution commenced between
stages 1 and 2. Bernal regarded the third
stage—discovering methods by which biological
reactions were incorporated behind a cell's
boundary—as the most difficult. Modern work
on the way that cell membranes self-assemble,
and the work on micropores in various substrates
may be a halfway house towards the development
of independent free-living cells.The chemical
processes that took place on the early Earth
are called chemical evolution. Since the end
of the nineteenth century, 'evolutive abiogenesis'
means increasing complexity and evolution
of matter from inert to living state. 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. Spiegelman
took advantage of natural selection to synthesize
the Spiegelman Monster, which had a genome
with just 218 nucleotide bases, having deconstructively
evolved from a 4500-base bacterial RNA. Eigen
built on Spiegelman's work and produced a
similar system further degraded to just 48
or 54 nucleotides—the minimum required for
the binding of the replication enzyme.Following
on from chemical evolution came the initiation
of biological evolution, which led to the
first cells. No one has yet synthesized a
"protocell" using simple components with the
necessary properties of life (the so-called
"bottom-up-approach"). Without such a proof-of-principle,
explanations have tended to focus on chemosynthesis.
However, some researchers work in this field,
notably Steen Rasmussen and Jack W. Szostak.
Others have argued that a "top-down approach"
is more feasible. One such approach, successfully
attempted by Craig Venter and others at J.
Craig Venter Institute, involves engineering
existing prokaryotic cells with progressively
fewer genes, attempting to discern at which
point the most minimal requirements for life
are reached.The NASA strategy on abiogenesis
states that it is necessary to identify interactions,
intermediary structures and functions, energy
sources, and environmental factors that contributed
to the diversity, selection, and replication
of evolvable macromolecular systems. Emphasis
must continue to map the chemical landscape
of potential primordial informational polymers.
The advent of polymers that could replicate,
store genetic information, and exhibit properties
subject to selection likely was a critical
step in the emergence of prebiotic chemical
evolution.In October 2018, researchers at
McMaster University announced the development
of a new technology, called a Planet Simulator,
to help study the origin of life on planet
Earth and beyond. It consists of a sophisticated
climate chamber to study how the building
blocks of life were assembled and how these
prebiotic molecules transitioned into self-replicating
RNA molecules.
== Chemical origin of organic molecules ==
The elements, except for hydrogen and helium,
ultimately derive from stellar nucleosynthesis.
On 12 October 2016, astronomers reported that
the very basic chemical ingredients of life—the
carbon-hydrogen molecule (CH, or methylidyne
radical), the carbon-hydrogen positive ion
(CH+) and the carbon ion (C+)—are largely
the result of ultraviolet light from stars,
rather than other forms of radiation from
supernovae and young stars, as thought earlier.
Complex molecules, including organic molecules,
form naturally both in space and on planets.
There are two possible sources of organic
molecules on the early Earth:
Terrestrial origins – organic molecule synthesis
driven by impact shocks or by other energy
sources (such as UV light, redox coupling,
or electrical discharges; e.g., Miller's experiments)
Extraterrestrial origins – formation of
organic molecules in interstellar dust clouds,
which rain down on planets. (See pseudo-panspermia)
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. According to the
computer studies, this same process may also
occur around other stars that acquire planets.
(Also see Extraterrestrial organic molecules).
Estimates of the production of organics from
these sources suggest that the Late Heavy
Bombardment before 3.5 Ga within the early
atmosphere made available quantities of organics
comparable to those produced by terrestrial
sources.It has been estimated that the Late
Heavy Bombardment may also have effectively
sterilized 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." Shock has found that
the available energy is maximized at around
100–150 degrees Celsius, precisely the temperatures
at which the hyperthermophilic bacteria and
thermoacidophilic archaea have been found,
at the base of the phylogenetic tree of life
closest to the Last Universal Common Ancestor
(LUCA).The accumulation and concentration
of organic molecules on a planetary surface
is also considered an essential early step
for the origin of life. Identifying and understanding
the mechanisms that led to the production
of prebiotic
molecules in various environments is critical
for establishing the inventory of ingredients
from which life originated on Earth, assuming
that the abiotic production of molecules ultimately
influenced the selection of molecules from
which life emerged.
=== 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. Stan Palasek suggested
based on a theoretical model that self-assembly
of ribonucleic acid (RNA) molecules can occur
spontaneously due to physical factors in hydrothermal
vents. 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.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. Computer simulations
also suggest that cavitation in primordial
water reservoirs such as breaking sea waves,
streams and oceans can potentially lead to
the synthesis of biogenic compounds. Unfavourable
reactions can also be driven by highly favourable
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).
Carbon fixation via iron-sulfur chemistry
is highly favourable, 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.As
early as the 1860s, experiments have demonstrated
that biologically relevant molecules can be
produced from interaction of simple carbon
sources with abundant inorganic catalysts.
In particular, experiments by Butlerov (the
formose reaction) showed that tetroses, pentoses,
and hexoses are produced when formaldehyde
is heated under basic conditions with divalent
metal ions like calcium. The reaction was
scrutinized and subsequently proposed to be
autocatalytic by Breslow in 1959. Similar
experiments (see below) demonstrate that nucleobases
like guanine and adenine could be synthesized
from simple carbon and nitrogen sources like
hydrogen cyanide and ammonia.
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 hydrogen cyanide
(HCN). It has several advantages as a biotic
precursor, including the ability to easily
become concentrated through the evaporation
of water. Although HCN is poisonous, it only
affects aerobic organisms (eukaryotes and
aerobic bacteria), which did not yet exist.
It can play roles in other chemical processes
as well, such as the synthesis of the amino
acid glycine.In 1961, it was shown that the
nucleic acid purine base adenine can be formed
by heating aqueous ammonium cyanide solutions.
Other pathways for synthesizing bases from
inorganic materials were also reported. Leslie
E. 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.
Research by Stanley L. Miller and colleagues
suggested that while adenine and guanine require
freezing conditions for synthesis, cytosine
and uracil may require boiling temperatures.
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.
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). 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. Mechanistic exploration
using quantum chemical methods provide a more
detailed understanding of some of the chemical
processes involved in chemical evolution,
and a partial answer to the fundamental question
of molecular biogenesis.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., CH4 and NH3 as opposed
to CO2 and nitrogen dioxide (NO2)). However,
current scientific consensus describes the
primitive atmosphere as either weakly reducing
or neutral (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 present
in early oceans) in the experimental conditions
have again produced a diverse array of amino
acids. Other scientific research has focused
on two other potential reducing environments:
outer space and deep-sea thermal vents.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 also formed in high concentration
during the Miller–Urey and Joan Oró experiments.
The Miller–Urey experiment, for example,
produces many substances that would react
with the amino acids or terminate their coupling
into peptide chains.A research project completed
in March 2015 by John D. Sutherland and others
found that a network of reactions beginning
with hydrogen cyanide and hydrogen sulfide,
in streams of water irradiated by UV light,
could produce the chemical components of proteins
and lipids, as well as those of RNA, while
not producing a wide range of other compounds.
The researchers used the term "cyanosulfidic"
to describe this network of reactions.
=== Autocatalysis ===
Autocatalysts are substances that catalyze
the production of themselves and therefore
are "molecular replicators." The simplest
self-replicating chemical systems are autocatalytic,
and typically contain three components: a
product molecule and two precursor molecules.
The product molecule joins together the precursor
molecules, which in turn produce more product
molecules from more precursor molecules. The
product molecule catalyzes the reaction by
providing a complementary template that binds
to the precursors, thus bringing them together.
Such systems have been demonstrated both in
biological macromolecules and in small organic
molecules. Systems that do not proceed by
template mechanisms, such as the self-reproduction
of micelles and vesicles, have also been observed.It
has been proposed that life initially arose
as autocatalytic chemical networks. British
ethologist Richard Dawkins wrote about autocatalysis
as a potential explanation for the origin
of life in his 2004 book The Ancestor's Tale.
In his book, Dawkins cites experiments performed
by Julius Rebek Jr. and his colleagues in
which they combined amino adenosine and pentafluorophenyl
esters with the autocatalyst amino adenosine
triacid ester (AATE). One product was a variant
of AATE, which catalyzed 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.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. 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 of ribozymes capable of catalyzing
their own chemical reactions. The hypercycle
theory requires the existence of complex biochemicals,
such as nucleotides, which do not form under
the conditions proposed by the Miller–Urey
experiment.
Geoffrey W. Hoffmann has shown that an early
error-prone translation machinery can be stable
against an error catastrophe of the type that
had been envisaged as problematical for the
origin of life, and was known as "Orgel's
paradox".Hoffmann has furthermore argued that
a complex nucleation event as the origin of
life involving both polypeptides and nucleic
acid is compatible with the time and space
available in the primitive oceans of Earth
Hoffmann suggests that volcanic ash may provide
the many random shapes needed in the postulated
complex nucleation event. This aspect of the
theory can be tested experimentally.
=== Homochirality ===
Homochirality refers to a geometric uniformity
of some materials 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 almost no 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
(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, statistical fluctuations
during racemic synthesis, and spontaneous
symmetry breaking.Once established, chirality
would be selected for. A small bias (enantiomeric
excess) in the population can be amplified
into a large one by asymmetric autocatalysis,
such as in the Soai reaction. In asymmetric
autocatalysis, the catalyst is a chiral molecule,
which means that a chiral molecule is catalyzing
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.Clark has suggested
that homochirality may have started in outer
space, as the studies of the amino acids on
the Murchison meteorite showed that L-alanine
is 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.
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.
== Self-enclosement, reproduction, duplication
and the RNA world ==
=== Protocells ===
A protocell is a self-organized, self-ordered,
spherical collection of lipids proposed as
a stepping-stone to the origin of life. A
central question in evolution is how simple
protocells first arose and differed in reproductive
contribution to the following generation driving
the evolution of life. Although a functional
protocell has not yet been achieved in a laboratory
setting, there are scientists who think the
goal is well within reach.Self-assembled vesicles
are essential components of primitive cells.
The second law of thermodynamics requires
that the universe move in a direction in which
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. Researchers Irene
A. Chen and Jack W. Szostak amongst others,
suggest that simple physicochemical properties
of elementary protocells can give rise to
essential cellular behaviours, including primitive
forms of differential reproduction competition
and energy storage. Such cooperative interactions
between the membrane and its encapsulated
contents could greatly simplify the transition
from simple replicating molecules to true
cells. Furthermore, competition for membrane
molecules would favour stabilized membranes,
suggesting a selective advantage for the evolution
of cross-linked fatty acids and even the phospholipids
of today. Such micro-encapsulation would allow
for metabolism within the membrane, the exchange
of small molecules but the prevention of passage
of large substances across it. The main advantages
of encapsulation include the increased solubility
of the contained cargo within the capsule
and the storage of energy in the form of an
electrochemical gradient.
A 2012 study led by Armen Y. 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. Scientists confirmed
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.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. However,
the nature and properties of the Jeewanu remains
to be clarified.
Electrostatic interactions induced by short,
positively charged, hydrophobic peptides containing
7 amino acids in length or fewer, can attach
RNA to a vesicle membrane, the basic cell
membrane.
=== RNA world ===
The RNA world hypothesis describes an early
Earth with self-replicating and catalytic
RNA but no DNA or proteins. It is widely accepted
that current life on Earth descends from an
RNA world, although RNA-based life may not
have been the first life to exist. 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.
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. The concept of the RNA world
was first proposed in 1962 by Alexander Rich,
and the term was coined by Walter Gilbert
in 1986.Possible precursors for the evolution
of protein synthesis include a mechanism to
synthesize short peptide cofactors or form
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.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 ["many worlds in one"] 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."
==== Viral origins ====
Recent evidence for a "virus first" hypothesis,
which may support theories of the RNA world,
has been suggested. One of the difficulties
for the study of the origins of viruses is
their high rate of mutation; this is particularly
the case in RNA retroviruses like HIV. A 2015
study compared protein fold structures across
different branches of the tree of life, where
researchers can reconstruct the evolutionary
histories of the folds and of the organisms
whose genomes code for those folds. They argue
that protein folds are better markers of ancient
events as their three-dimensional structures
can be maintained even as the sequences that
code for those begin to change. Thus, the
viral protein repertoire retain traces of
ancient evolutionary history that can be recovered
using advanced bioinformatics approaches.
Those researchers think that "the prolonged
pressure of genome and particle size reduction
eventually reduced virocells into modern viruses
(identified by the complete loss of cellular
makeup), meanwhile other coexisting cellular
lineages diversified into modern cells. The
data suggest that viruses originated from
ancient cells that co-existed with the ancestors
of modern cells. These ancient cells likely
contained segmented RNA genomes. Although
the virus-first hypothesis is highly controversial
today, some astrobiologists have suggested
looking for viruses on other celestial bodies
such as Mars if they do emerge before cells.
=== RNA synthesis and replication ===
A number of hypotheses of formation of RNA
have been put forward. As of 1994, there were
difficulties in the explanation of the abiotic
synthesis of the nucleotides cytosine and
uracil. 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. 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 of aqueous
solution. These are seen to form micro-spheres
which are observed to behave similarly to
membrane-enclosed compartments. Other possible
means of producing more complicated organic
molecules include chemical reactions that
take place on clay substrates or on the surface
of the mineral pyrite.
Factors supporting 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
of 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 RNA molecule under the
conditions that approximated the early Earth.
Relatively short RNA molecules have been synthesized,
capable of replication. Such replicase RNA,
which functions as both code and catalyst
provides its own template upon which copying
can occur. Jack W. Szostak has shown that
certain catalytic RNAs can join smaller RNA
sequences together, creating the potential
for self-replication. If these conditions
were present, Darwinian natural selection
would favour the proliferation of such autocatalytic
sets, to which further functionalities could
be added. Such autocatalytic systems of RNA
capable of self-sustained replication have
been identified. The RNA replication systems,
which include two ribozymes that catalyze
each other's synthesis, showed a doubling
time of the product of about one hour, and
were subject to natural selection under the
conditions that existed in the experiment.
In evolutionary competition experiments, this
led to the emergence of new systems which
replicated more efficiently. This was the
first demonstration of evolutionary adaptation
occurring in a molecular genetic system.Depending
on the definition, life started when RNA chains
began to self-replicate, initiating the three
mechanisms of Darwinian selection: heritability,
variation of type, and differential reproductive
output. The fitness of an RNA replicator (its
per capita rate of increase) would likely
be a function of its intrinsic adaptive capacities,
determined by its nucleotide sequence, and
the availability of resources. The three primary
adaptive capacities may have been: (1) replication
with moderate fidelity, giving rise to both
heritability while allowing variation of type,
(2) resistance to decay, and (3) acquisition
of process resources. These capacities would
have functioned by means of the folded configurations
of the RNA replicators resulting from their
nucleotide sequences.
Carl Zimmer has speculated that the chemical
conditions, including the presence of boron,
molybdenum and oxygen needed for the initial
production of RNA, may have been better on
early Mars than on early Earth. If so, life-suitable
molecules originating on Mars may have later
migrated to Earth via meteor ejections.
=== Pre-RNA world ===
It is possible that a different type of nucleic
acid, such as peptide nucleic acid, threose
nucleic acid or glycol nucleic acid, was the
first to emerge as a self-reproducing molecule,
only later replaced by RNA. 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.Pyrimidine ribonucleosides
and their respective nucleotides have been
prebiotically synthesized by a sequence of
reactions which by-pass the free sugars, and
are assembled in a stepwise fashion by using
nitrogenous or oxygenous chemistries. 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.
This can be viewed as a prebiotic purification
step, where the said compound spontaneously
crystallized 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 polymerize into
RNA. This paper also highlights the possibility
for the photo-sanitization of the pyrimidine-2',3'-cyclic
phosphates.A new origin-of-life theory based
on self-replicating beta-sheet structures
was recently put forward. The theory suggest
that self-replicating and self-assembling
catalytic amyloids were the first informational
polymers in a primitive pre-RNA world. The
main arguments for the amyloid hypothesis
is based on the structural stability, autocatalytic
and catalytic properties, and evolvability
of beta-sheet based informational systems.
Such systems are also error correcting. and
chiroselective
== Origin of biological metabolism ==
Metabolism-like reactions could have occurred
naturally in early oceans, before the first
organisms evolved. Metabolism may predate
the origin of life, which may have evolved
from the chemical conditions in the 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. A study at the University of Düsseldorf
created phylogenic trees based upon 6 million
genes from bacteria and archaea, and identified
355 protein families that were probably present
in the LUCA. They were based upon an anaerobic
metabolism fixing carbon dioxide and nitrogen.
It suggests that the LUCA evolved in an environment
rich in hydrogen, carbon dioxide and iron.
Following are some observed discoveries and
related hypotheses.
=== Iron–sulfur world ===
In the 1980s, Günter Wächtershäuser, encouraged
and supported by Karl R. Popper, postulated
his iron–sulfur world, a theory of the evolution
of pre-biotic chemical pathways as the starting
point in the evolution of life. It systematically
traces today's biochemistry to primordial
reactions which 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
(simulated lightning, ultraviolet irradiation),
"Wächtershäuser systems" come with a built-in
source of energy: sulfides of iron (iron pyrite)
and other minerals. The energy released from
redox reactions of these metal sulfides is
available for the synthesis of organic molecules,
and such systems may have evolved into autocatalytic
sets constituting self-replicating, metabolically
active entities predating the life forms known
today. Experiments with such sulfides in an
aqueous environment at 100 °C 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.Several
models reject the self-replication of a "naked-gene",
postulating instead the emergence of a primitive
metabolism providing a safe environment for
the later emergence of RNA replication. The
centrality of the Krebs cycle (citric acid
cycle) to energy production in aerobic organisms,
and in drawing in carbon dioxide and hydrogen
ions in biosynthesis of complex organic chemicals,
suggests that it was one of the first parts
of the metabolism to evolve. Concordantly,
geochemist Michael 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).
Physicist Jeremy England of MIT has proposed
that life was inevitable from general thermodynamic
considerations: "... 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."One of the
earliest incarnations of this idea was put
forward in 1924 with Oparin's notion of primitive
self-replicating vesicles which predated the
discovery of the structure of DNA. Variants
in the 1980s and 1990s include 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 that decade.
Orgel summarized his analysis 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-CoA pathway (another one
of the five recognized ways of carbon dioxide
fixation in nature today) would be compatible
with the idea of self-organization 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 centres and catalyzes the
formation of acetyl-CoA (similar to acetyl-thiol)
in a single step. There are increasing concerns,
however, that prebiotic thiolated and thioester
compounds are thermodynamically and kinetically
unfavourable to accumulate in presumed prebiotic
conditions (i.e. hydrothermal vents). It has
also been proposed that cysteine and homocysteine
may have reacted with nitriles resulting from
the Stecker reaction, readily forming catalytic
thiol-reach poplypeptides.
=== Zn-world hypothesis ===
The Zn-world (zinc world) theory of Armen
Y. Mulkidjanian is an extension of Wächtershäuser's
pyrite hypothesis. Wächtershäuser based
his theory of the initial chemical processes
leading to informational molecules (RNA, peptides)
on a regular mesh of electric charges at the
surface of pyrite that may have facilitated
the primeval polymerization by attracting
reactants and arranging them appropriately
relative to each other. The Zn-world theory
specifies and differentiates further. Hydrothermal
fluids rich in H2S interacting with cold primordial
ocean (or Darwin's "warm little 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 kilometres in diameter and
date back to the Archean Eon. Most abundant
are pyrite (FeS2), chalcopyrite (CuFeS2),
and sphalerite (ZnS), with additions of galena
(PbS) and alabandite (MnS). ZnS and MnS have
a unique ability to store radiation energy,
e.g. from UV light. During the relevant time
window of the origins of replicating molecules,
the primordial atmospheric pressure was high
enough (>100 bar, about 100 atmospheres) to
precipitate near the Earth's surface, and
UV irradiation was 10 to 100 times more intense
than now; hence 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 archaea, bacteria
and proto-eukaryotes evolved. Archibald Macallum
noted the resemblance of body fluids such
as blood and lymph to seawater; however, the
inorganic composition of all cells differ
from that of modern seawater, 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+, Zn2+, Mn2+, 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 vapour-dominated zones of
what we today call inland geothermal systems.
Under the oxygen depleted, CO2-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+, Zn2+,
and phosphorus compounds.
=== Deep sea vent hypothesis ===
The deep sea vent, or alkaline hydrothermal
vent, theory posits that life may have begun
at submarine hydrothermal vents, William Martin
and Michael Russell have suggested "that life
evolved in structured iron monosulphide precipitates
in a seepage site hydrothermal mound at a
redox, pH, and temperature gradient between
sulphide-rich hydrothermal fluid and iron(II)-containing
waters of the Hadean ocean floor. The naturally
arising, three-dimensional compartmentation
observed within fossilized seepage-site metal
sulphide precipitates indicates that these
inorganic compartments were the precursors
of cell walls and membranes found in free-living
prokaryotes. The known capability of FeS and
NiS to catalyze the synthesis of the acetyl-methylsulphide
from carbon monoxide and methylsulphide, constituents
of hydrothermal fluid, indicates that pre-biotic
syntheses occurred at the inner surfaces of
these metal-sulphide-walled compartments,..."
These form where hydrogen-rich fluids emerge
from below the sea floor, as a result of serpentinization
of ultra-mafic olivine with seawater and a
pH interface with carbon dioxide-rich ocean
water. The vents form a sustained chemical
energy source derived from redox reactions,
in which electron donors (molecular hydrogen)
react with electron acceptors (carbon dioxide);
see Iron–sulfur world theory. These are
highly exothermic reactions.Michael Russell
demonstrated that alkaline vents created an
abiogenic proton motive force (PMF) chemiosmotic
gradient, 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 Wächtershäuser. 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 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).
Jack W. 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, 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. This case is similar
to 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 seawater.
According to 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. 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 of Montana State University described
the formation of stromatolites in hot mineral
water at the Yellowstone National Park. Stromatolites
survive in hot mineral water and in proximity
to areas with volcanic activity. Processes
have evolved in the sea near geysers of hot
mineral water. In 2011, Tadashi Sugawara from
the University of Tokyo created a protocell
in hot water.Experimental research and computer
modelling suggest that the surfaces of mineral
particles inside hydrothermal vents have catalytic
properties similar to those of enzymes and
are able to create simple organic molecules,
such as methanol (CH3OH) and formic, acetic
and pyruvic acid out of the dissolved CO2
in the water.The research reported above by
William F. Martin in July 2016 supports the
thesis that life arose at hydrothermal vents,
that spontaneous chemistry in the Earth's
crust driven by rock–water interactions
at disequilibrium thermodynamically underpinned
life's origin and that the founding lineages
of the archaea and bacteria were H2-dependent
autotrophs that used CO2 as their terminal
acceptor in energy metabolism. Martin suggests,
based upon this evidence that LUCA "may have
depended heavily on the geothermal energy
of the vent to survive".
=== 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.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, 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, the
thermosynthesis hypothesis does not even invoke
a pathway: 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. It has been claimed that the emergence
of cyclic systems of protein catalysts is
implausible.
== Other models ==
=== Clay hypothesis ===
Montmorillonite, an abundant clay, is a catalyst
for the polymerization of RNA and for the
formation of membranes from lipids. A model
for the origin of life using clay was forwarded
by Alexander Graham Cairns-Smith in 1985 and
explored as a plausible mechanism by several
scientists. The clay hypothesis postulates
that complex organic molecules arose gradually
on pre-existing, non-organic replication surfaces
of silicate crystals in solution.
At the Rensselaer Polytechnic Institute, James
P. Ferris' studies have also confirmed that
clay minerals of montmorillonite catalyze
the formation of RNA in aqueous solution,
by joining nucleotides to form longer chains.In
2007, Bart Kahr from the University of Washington
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
behaviour 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."
=== 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 kilometres below
the surface. 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.
=== Panspermia ===
Panspermia is the hypothesis that life exists
throughout the universe, distributed by meteoroids,
asteroids, comets, planetoids, and, also,
by spacecraft in the form of unintended contamination
by microorganisms. For example, planetary
contamination by organisms like Tersicoccus
phoenicis, which has shown resistance to methods
usually used in spacecraft assembly clean
rooms.The panspermia hypothesis does not attempt
to explain how life first originated, but
merely shifts it to another planet or a comet.
The advantage of an extraterrestrial origin
of primitive life is that life is not required
to have formed on each planet it occurs on,
but rather in a single location, and then
spread about the galaxy to other star systems
via cometary 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 tests. (See
also: List of microorganisms tested in outer
space.)
=== Extraterrestrial organic molecules ===
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.
Carbon is abundant in the Sun, stars, comets,
and in the atmospheres of most planets. 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. 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. According
to the computer studies, this same process
may also occur around other stars that acquire
planets.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. 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. 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. 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. Sun Kwok of The University
of Hong Kong 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."
Glycolaldehyde, the first example of an interstellar
sugar molecule, was detected in the star-forming
region near the centre of our galaxy. It was
discovered in 2000 by Jes Jørgensen and Jan
M. Hollis. In 2012, Jørgensen's 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. Glycolaldehyde is
needed to form 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.
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.
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 the NASA 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."
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.
Amino acids which were formed extraterrestrially
may also have arrived on Earth via comets.
It is estimated that during the Late Heavy
Bombardment, meteorites may have delivered
up to five million tons of organic prebiotic
elements to Earth per year.Polycyclic aromatic
hydrocarbons (PAH) are the most common and
abundant of the known polyatomic molecules
in the observable universe, and are considered
a likely constituent of the primordial sea.
In 2010, PAHs, along with fullerenes (or "buckyballs"),
have been detected in nebulae. In April 2019,
scientists, working with the Hubble Space
Telescope, reported the confirmed detection
of the large and complex ionized molecules
of buckminsterfullerene (C60) in the interstellar
medium spaces between the stars.In March 2015,
NASA scientists reported that, for the first
time, complex DNA and RNA organic compounds
of life, including uracil, cytosine and thymine,
have been formed in the laboratory under outer
space conditions, using starting chemicals,
such as pyrimidine, found in meteorites. Pyrimidine,
like PAHs, the most carbon-rich chemical found
in the Universe, may have been formed in red
giant stars or in interstellar dust and gas
clouds. A group of Czech scientists reported
that all four RNA-bases may be synthesized
from formamide in the course of high-energy
density events like extraterrestrial impacts.
=== Lipid world ===
The lipid world theory postulates that the
first self-replicating object was lipid-like.
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 favourably. 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.
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 favoured the constitution
of a hypercycle, 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.
=== 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. Polyphosphates
are formed by polymerization of ordinary monophosphate
ions PO4−3. Several mechanisms of organic
molecule synthesis have been investigated.
Polyphosphates cause polymerization of amino
acids into peptides. They are also logical
precursors in the synthesis of such key biochemical
compounds as adenosine triphosphate (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.
=== PAH world hypothesis ===
Polycyclic aromatic hydrocarbons (PAH) are
known to be abundant in the universe, including
in the interstellar medium, in comets, and
in meteorites, and are some of the most complex
molecules so far found in space.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. In 2010,
another team also detected PAHs, along with
fullerenes, in nebulae. The use of PAHs has
also been proposed as a precursor to the RNA
world in the PAH world hypothesis. 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 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." 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."NASA
maintains a database for tracking PAHs in
the universe. 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, and are associated with new
stars and exoplanets.
=== 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. According to computer models,
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 organometallic 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.
=== Thermodynamic dissipation ===
The 19th-century Austrian physicist Ludwig
Boltzmann first recognized that the struggle
for existence of living organisms was neither
over raw material nor energy, but instead
had to do with entropy production derived
from the conversion of the solar spectrum
into heat by these systems. Boltzmann thus
realized that living systems, like all irreversible
processes, were dependent on the dissipation
of a generalized chemical potential for their
existence. In his book "What is Life", the
20th-century Austrian physicist Erwin Schrödinger
emphasized the importance of Boltzmann's deep
insight into the irreversible thermodynamic
nature of living systems, suggesting that
this was the physics and chemistry behind
the origin and evolution of life. However,
irreversible processes, and much less living
systems, could not be conveniently analyzed
under this perspective until Lars Onsager,
and later Ilya Prigogine, developed an elegant
mathematical formalism for treating the "self-organization"
of material under a generalized chemical potential.
This formalism became known as Classical Irreversible
Thermodynamics and Prigogine was awarded the
Nobel Prize in Chemistry in 1977 "for his
contributions to non-equilibrium thermodynamics,
particularly the theory of dissipative structures".
The analysis of Prigogine showed that if a
system were left to evolve under an imposed
external potential, material could spontaneously
organize (lower its entropy) forming what
he called "dissipative structures" which would
increase the dissipation of the externally
imposed potential (augment the global entropy
production). Non-equilibrium thermodynamics
has since been successfully applied to the
analysis of living systems, from the biochemical
production of ATP to optimizing bacterial
metabolic pathways to complete ecosystems.In
his "Thermodynamic Dissipation Theory of the
Origin and Evolution of Life", Karo Michaelian
has taken the insight of Boltzmann and the
work of Prigogine to its ultimate consequences
regarding the origin of life. This theory
postulates that the hallmark of the origin
and evolution of life is the microscopic dissipative
structuring of organic pigments and their
proliferation over the entire Earth surface.
Present day life augments the entropy production
of Earth in its solar environment by dissipating
ultraviolet and visible photons into heat
through organic pigments in water. This heat
then catalyzes a host of secondary dissipative
processes such as the water cycle, ocean and
wind currents, hurricanes, etc. Michaelian
argues that if the thermodynamic function
of life today is to produce entropy through
photon dissipation in organic pigments, 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 230–290
nm wavelength (UV-C) region, which is a part
of the Sun's spectrum that could have penetrated
the prebiotic atmosphere. In fact, not only
RNA and DNA, but many fundamental molecules
of life (those common to all three domains
of life) are also pigments that absorb in
the UV-C, and many of these also have a chemical
affinity to RNA and DNA. Nucleic acids may
thus have acted as acceptor molecules to the
UV-C photon excited antenna pigment donor
molecules by providing an ultrafast channel
for dissipation. Michaelian has shown using
the formalism of non-linear irreversible thermodynamics
that there would have existed during the Archean
a thermodynamic imperative to the abiogenic
UV-C photochemical synthesis and proliferation
of these pigments over the entire Earth surface
if they acted as catalysts to augment the
dissipation of the solar photons. By the end
of the Archean, with life-induced ozone dissipating
UV-C light in the Earth's upper atmosphere,
it would have become ever more improbable
for a completely new life to emerge that didn't
rely on the complex metabolic pathways already
existing since now the free energy in the
photons arriving at Earth's surface would
have been insufficient for direct breaking
and remaking of covalent bonds. It has been
suggested, however, that such changes in the
surface flux of ultraviolet radiation due
to geophysical events affecting the atmosphere
could have been what promoted the development
of complexity in life based on existing metabolic
pathways, for example during the Cambrian
explosion Many salient characteristics of
the fundamental molecules of life (those found
in all three domains) all point directly to
the involvement of UV-C light in the dissipative
structuring of incipient life. Some of the
most difficult problems concerning the origin
of life, such as enzyme-less replication of
RNA and DNA, homochirality of the fundamental
molecules, and the origin of information encoding
in RNA and DNA, also find an explanation within
the same dissipative thermodynamic framework
by considering the probable existence of a
relation between primordial replication and
UV-C photon dissipation. Michaelian suggests
that it is erroneous to expect to describe
the emergence, proliferation, or even evolution,
of life without overwhelming reference to
entropy production through the dissipation
of a generalized chemical potential, in particular,
the prevailing solar photon flux.
=== Multiple genesis ===
Different forms of life with variable origin
processes may have appeared quasi-simultaneously
in the early history of Earth. The other forms
may be extinct (having left distinctive fossils
through their different biochemistry—e.g.,
hypothetical types of biochemistry). It has
been proposed 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.
=== Fluctuating hydrothermal pools on volcanic
islands or proto-continents ===
Armid Mulkidjanian and co-authors think that
the marine environments did not provide the
ionic balance and composition universally
found in cells, as well as of ions required
by essential proteins and ribozymes found
in virtually all living organisms, especially
with respect to K+/Na+ ratio, Mn2+, Zn2+ and
phosphate concentrations. The only known environments
that mimic the needed conditions on Earth
are found in terrestrial hydrothermal pools
fed by steam vents. Additionally, mineral
deposits in these environments under an anoxic
atmosphere would have suitable pH (as opposed
to current pools in an oxygenated atmosphere),
contain precipitates of sulfide minerals that
block harmful UV radiation, have wetting/drying
cycles that concentrate substrate solutions
to concentrations amenable to spontaneous
formation of polymers of nucleic acids, polyesters
and depsipeptides, both by chemical reactions
in the hydrothermal environment, as well as
by exposure to UV light during transport from
vents to adjacent pools. Their hypothesized
pre-biotic environments are similar to the
deep-oceanic vent environments most commonly
hypothesized, but add additional components
that help explain peculiarities found in reconstructions
of the Last Universal Common Ancestor (LUCA)
of all living organisms.Bruce Damer and David
Deamer have come to the conclusion that cell
membranes cannot be formed in salty seawater,
and must therefore have originated in freshwater.
Before the continents formed, the only dry
land on Earth would be volcanic islands, where
rainwater would form ponds where lipids could
form the first stages towards cell membranes.
These predecessors of true cells are assumed
to have behaved more like a superorganism
rather than individual structures, where the
porous membranes would house molecules which
would leak out and enter other protocells.
Only when true cells had evolved would they
gradually adapt to saltier environments and
enter the ocean.Colín-García et al. (2016)
discuss the advantages and disadvantages of
hydrothermal vents as primitive environments.
They mention the exergonic reactions in such
systems could have been a source of free energy
that promoted chemical reactions, additional
to their high mineralogical diversity which
implies the induction of important chemical
gradients, thus favoring the interaction between
electron donors and acceptors. Colín-García
et al. (2016) also summarize a set of experiments
proposed to test the role of hydrothermal
vents in prebiotic synthesis.
== See also
