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Research into the origin of life is a limited field of research despite its profound impact on
biology and human understanding of the natural world. Progress in this field is generally slow
and sporadic, though it still draws the attention of many due to the gravity of the question
being investigated. A few facts give insight into the conditions in which life may have emerged,
but the mechanisms by which non-life became life are still elusive.
History of the concept Abiogenesis
Research into the origin of life is the modern incarnation
of the ancient concept of abiogenesis. Abiogenesis, in its
most general sense, is the generation of life from
non-living matter. The term is primarily used in the
context of biology and the origin of life. Abiogenesis was
long considered to be a very common occurrence until the
Law of Biogenesis (omne vivum ex ovo or "all life from an
egg") became firmly established in modern biology as a
result of the work of Louis Pasteur.
Charles Darwin in a letter to J.D. Hooker of February 1st
1871, made the suggestion that life may have begun in a
"warm little pond, with all sorts of ammonia and
phosphoric salts, lights, heat, electricity, etc. present,
that a protein compound was chemically formed ready to
undergo still more complex changes, at the present day
such matter would be instantly devoured or absorbed, which
would not have been the case before living creatures were
formed." Thus, it is the presence of life itself,
operating in an oxygen rich atmosphere, itself a product
of life, which prevents "spontaneous generation" from
occurring on Earth today.
This modern definition of abiogenesis is concerned with
the formation of the simplest forms of life from
primordial chemicals, in an environment regarded as
similar to that at the time shortly after the formation of
the Earth. This is significantly different from the
concept of Aristotelian abiogenesis, which postulated the
formation of complex organisms. This article reviews
different hypotheses for modern abiogenetic processes that
are currently under debate.
Current models of the origin of life
There is no truly "standard" model of the origin of life,
however most currently accepted models build in one way or
another upon a number of discoveries concerning the origin
of molecular and cellular components for life, which are
listed in a rough order of postulated emergence:
Plausible pre-biotic conditions result
in the creation of certain basic small molecules
(monomers) of life, such as amino acids. This was
demonstrated in the Urey-Miller experiment by Stanley L.
Miller and Harold C. Urey in 1953. Phospholipids (of an appropriate
length) can spontaneously form lipid bilayers, one of the
two basic components of a cell membrane. The polymerization of nucleotides into
random RNA molecules might have resulted in
self-replicating ribozymes (RNA world hypothesis). Selection pressures for catalytic
efficiency and diversity result in ribozymes which
catalyse peptidyl transfer (hence formation of small
proteins), since oligopeptides complex with RNA to form
better catalysts. Thus the first ribosome is born, and
protein synthesis becomes more prevalent. Proteins outcompete ribozymes in
catalytic ability, and therefore become the dominant
biopolymer. Nucleic acids are restricted to predominantly
genomic use.
The origin of the basic bio-molecules,
while not settled, is less controversial than the
significance and order of steps 2 and 3. The basic
inorganic chemicals from which life was formed are
methane (CH4), ammonia (NH3), water (H2O), hydrogen
sulfide (H2S), carbon dioxide (CO2), and phosphate
(PO43-). As of 2004, no one has yet synthesized a
"protocell" using basic components which has the
necessary properties of life (the so-called
"bottom-up-approach"). Without such a proof-of-principle,
explanations have tended to be short on specifics.
However, some researchers are working in this field,
notably Jack Szostak at Harvard. Others have argued that
a "top-down approach" is more feasible. One such approach
attempted by Craig Venter and others at The Institute for
Genomic Research involved engineering existing
prokaryotic cells with progressively fewer genes,
attempting to discern at which point the most minimal
requirements for life were reached. The biologist John
Desmond Bernal, in coining the term Biopoesis for this
process suggested that there were a number of clearly
defined "stages" that could be recognised in explaining
the origin of life.
Stage 1: The origin of biological monomers
Stage 2: The origin of biological polymers
Stage 3: The evolution from molecules to cell
Bernal suggested that Darwinian evolution may have
commenced early, some time between Stage 1 and 2.
Origin of organic molecules: Miller, Eigen and
Wächtershäuser's theories
The "Miller experiments" (including the original Miller–Urey
experiment of 1953, by Harold Urey and his graduate
student Stanley Miller) are performed under simulated
conditions resembling those thought at the time to have
existed shortly after Earth first accreted from the
primordial solar nebula. The experiment used a highly
reduced mixture of gases (methane, ammonia and hydrogen).
However, it should be noted that the composition of the
pre-biotic atmosphere of earth is currently
controversial. Other less reducing gases produce a lower
yield and variety. It was once thought that appreciable
amounts of molecular oxygen were present in the
pre-biotic atmosphere, which would have essentially
prevented the formation of organic molecules; however,
the current scientific consensus is that such was not the
case.
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The experiment showed that some of the basic organic
monomers (such as amino acids) that form the polymeric
building blocks of modern life can be formed
spontaneously. Simple organic molecules are of course a
long way from a fully functional self-replicating life
form; however, in an environment with no pre-existing
life these molecules may have accumulated and provided a
rich environment for chemical evolution ("soup theory").
On the other hand, the spontaneous formation of complex
polymers from abiotically generated monomers under these
conditions is not at all a straightforward process.
Besides the necessary basic organic monomers, also
compounds that would have prohibited the formation of
polymers were formed in high concentration during the
experiments. Further, according to Brooks and Shaw
(1973), there is no evidence in the geological record
that any soup existed.
"If there ever was a primitive soup, then we would expect
to find at least somewhere on this planet either massive
sediments containing enormous amounts of the various
nitrogenous organic compounds, acids, purines,
pyrimidines, and the like; or in much metamorphosed
sediments we should find vast amounts of nitrogenous
cokes. In fact no such materials have been found anywhere
on earth."
Other sources of complex molecules have been postulated,
including sources of extra-terrestrial stellar or
interstellar origin. For example, from spectral analyses,
organic molecules are known to be present in comets and
meteorites. In 2004, a team detected traces of polycyclic
aromatic hydrocarbons (PAH's) in a nebula, the most
complex molecule, to that date, found in space.
It can be argued that the most crucial challenge
unanswered by this theory is how the relatively simple
organic building blocks polymerize and form more complex
structures, interacting in consistent ways to form a
protocell. For example, in an aqueous environment
hydrolysis of oligomers/polymers into their constituent
monomers would be favored over the condensation of
individual monomers into polymers. Also, the Miller
experiment produces many substances that would undergo
cross-reactions with the amino acids or terminate the
peptide chain.
In the early 1970s a major attack on the problem of the
origin of life was organized by a team of scientists
gathered around Manfred Eigen of the Max Planck
Institute. They tried to examine the transient stages
between the molecular chaos in a pre-biotic soup and the
transient stages of a self replicating hyper-cycle,
between the molecular chaos in a pre-biotic soup and
simple macromolecular self-reproducing systems.
In a hyper-cycle, the information storing system
(possibly RNA) produces an enzyme, which aids catalyze
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,
hyper-cycles could create quasi-species, which through
natural selection entered into a form of
Darwinian
evolution. A boost to hyper-cycle theory was the
discovery that RNA, in certain circumstances forms itself
into ribozymes, a form of RNA enzyme.
Another possible answer to this polymerization conundrum
was provided in 1980s by Günter Wächtershäuser, in his
iron-sulfur world theory. In this theory, he postulated
the evolution of (bio) chemical pathways as fundamentals
of the evolution of life. Moreover, he presented a
consistent system of tracing today's biochemistry back to
ancestral reactions that provide alternative pathways to
the synthesis of organic building blocks from simple
gaseous compounds. In contrast to the classical Miller
experiments, which depend on external sources of energy
(e. g. simulated lightning or UV irradiation), "Wächtershäuser
systems" come with a built-in source of energy, sulfides
of iron and other minerals (e. g. pyrite). The energy
released from redox reactions of these metal sulfides is
not only available for the synthesis of organic
molecules, but also for the formation of oligomers and
polymers. It is therefore hypothesized that such systems
may be able to evolve into autocatalytic sets of
self-replicating, metabolically active entities that
would predate the life forms known today. The experiment
as performed, produced a relatively small yield of
dipeptides (0.4–12.4%) and a smaller yield of tripeptides
(0.003%) and the authors note that: "under these same
conditions dipeptides hydrolysed rapidly." Another
criticism of the result is that the experiment did not
include any organomolecules that would most likely
cross-react or chain-terminate.
The latest modification of the iron-sulfur-hypothesis has
been provided by William Martin and Michael Russell in
2002. According to their scenario, the first cellular
life forms may have evolved inside so-called black
smokers at seafloor spreading zones in the deep sea.
These structures consist of microscale caverns that are
coated by thin membranous metal sulfide walls.
Therefore, these structures would solve several critical
points of the "pure" Wächtershäuser systems at once:
the micro-caverns provide a means of concentrating
newly synthesized molecules, thereby increasing the chance of forming oligomers; the steep temperature gradients inside a black smoker
allow for establishing "optimum zones" of partial reactions in different regions of the black
smoker (e.g. monomer synthesis in the hotter, oligomerisation in the colder parts); the flow of hydrothermal water through the structure
provides a constant source of building blocks and energy (freshly precipitated metal sulfides); the model allows for a succession of different steps of
cellular evolution (pre-biotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis,
RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between
all developmental stages; synthesis of lipids as a means of "closing" the cells
against the environment is not necessary, until basically all cellular functions are developed.
This model locates the "last universal common ancestor"
(LUCA) inside a black smoker, rather than assuming the
existence of a free-living form of LUCA. The last
evolutionary step would be the synthesis of a lipid
membrane that finally allows the organisms to leave the
microcavern system of the black smokers and start their
independent lives. This postulated late acquisition of
lipids is consistent with the presence of completely
different types of membrane lipids in archaebacteria and
eubacteria (plus eukaryotes) with highly similar cellular
physiology of all life forms in most other aspects.
Another unsolved issue in chemical evolution is the
origin of homochirality, i.e. all monomers having the
same "handedness" (amino acids being left handed, and
nucleic acid sugars being right handed). Homochirality is
essential for the formation of functional ribozymes (and
probably proteins too). The origin of homochirality might
simply be explained by an initial asymmetry by chance
followed by common descent. Work performed in 2003 by
scientists at Purdue identified the amino acid serine as
being a probable root cause of organic molecules'
homochirality. Serine forms particularly strong bonds
with amino acids of the same chirality, resulting in a
cluster of eight molecules that must be all right-handed
or left-handed. This property stands in contrast with
other amino acids which are able to form weak bonds with
amino acids of opposite chirality. Although the mystery
of why left-handed serine became dominant is still
unsolved, this result suggests an answer to the question
of chiral transmission: how organic molecules of one
chirality maintain dominance once asymmetry is
established.
From organic molecules to protocells
The question "How do simple organic molecules form a
protocell?" is largely unanswered. However, there are
many different hypotheses regarding the path that might
have been taken. Some of these postulate the early
appearance of nucleic acids ("genes-first") whereas
others postulate the evolution of biochemical reactions
and pathways first ("metabolism-first"). Recently, trends
are emerging to create hybrid models that combine aspects
of both.
"Genes first" models: the RNA world
The RNA world hypothesis, for example, suggests that
relatively short RNA molecules could have spontaneously
formed that were capable of catalyzing their own
continuing replication. Early cell membranes could have
formed spontaneously from proteinoids, protein-like
molecules that are produced when amino acid solutions are
heated. Other possibilities include systems of chemical
reactions taking place within clay substrates or on the
surface of pyrite rocks. At this time however, these
various hypotheses have incomplete evidence supporting
them. Many of them can be simulated and tested in the
lab, but a lack of undisturbed sedimentary rock from that
early in Earth's history leaves few opportunities to
determine what may have actually happened in reality. At
this time however, no prebiotically plausible experiment
has confirmed this assumption. Further, recent
experiments suggest that the original estimates of the
size of an RNA molecule capable of self-replication were
most probably vast underestimates. Worse, RNA itself does
not appear to be a prebiotically plausible molecule;
therefore, more-modern forms of the RNA World theory
propose that a simpler molecule was capable of
self-replication (that other "World" then evolved over
time to produce the RNA World).
"Metabolism first" models: iron-sulfur world and others
Several models reject the idea of the self-replication of
a "naked-gene" and postulate the emergence of a primitive
metabolism which could provide an environment for the
later emergence of RNA replication. One of the earliest
incarnations of this idea was put forward in 1924 with
Alexander Oparin's notion of primitive self-replicating
vesicles which predated the discovery of the structure of
DNA. More recent variants in the 1980s and 1990s include
Günter Wächtershäuser's iron-sulfur world theory and
models introduced by Christian de Duve based on the
chemistry of thioesters. More abstract and theoretical
arguments for the plausibility of the emergence of
metabolism without the presence of genes include a
mathematical model introduced by Freeman Dyson in the
early 1980s and Stuart Kauffman's notion of collectively
autocatalytic sets, discussed later in that decade.
However, the idea that a closed metabolic cycle, such as
the reductive citric acid cycle proposed by Günter
Wächtershäuser, could form spontaneously remains
unsupported. Further, according to Leslie Orgel, a leader
in origin-of-life studies for the past several decades,
there is reason to believe the assertion will remain so.
In an article entitled "Self-Organizing Biochemical
Cycles", Orgel summarizes his analysis of the proposal by
stating, "There is at present no reason to expect that
multi-step cycles such as the reductive citric acid cycle
will self-organize on the surface of FeS/FeS2 or some
other mineral."
The Bubble Theory
Waves breaking on the shore create a delicate foam
composed of bubbles. Winds sweeping across the ocean have
a tendency to drive things to shore, much like driftwood
collecting on the beach. It is possible that organic
molecules were concentrated on the shorelines in much the
same way. Shallow coastal waters also tend to be warmer,
further concentrating the molecules through evaporation.
While bubbles comprised of mostly water burst quickly,
oily bubbles happen to be much more stable, lending more
time to the particular bubble to perform these crucial
experiments.
The phospholipid is a good example of an oily compound
believed to have been prevalent in the prebiotic seas.
Because phospholipids contain a hydrophilic head on one
end, and a hydrophobic tail on the other, they have the
tendency to spontaneously form lipid membranes in water.
A lipid monolayer bubble can only contain oil, and is
therefore not conducive to harboring water-soluble
organic molecules. On the other hand, a lipid bilayer
bubble can contain water, and was a likely precursor to
the modern cell membrane. If a protein came along that
increased the integrity of its parent bubble, then that
bubble had an advantage, and was placed at the top of the
natural selection waiting list. Primitive reproduction
can be envisioned when the bubbles burst, releasing the
results of the experiment into the surrounding medium.
Once enough of the 'right stuff' was released into the
medium, the development of the first prokaryotes,
eukaryotes, and multi-cellular organisms could be
achieved. This theory is expanded upon in the book, "The
Cell: Evolution of the First Organism" by Joseph Panno
Ph.D.
Similarly, bubbles formed entirely out of protein-like
molecules, called micro-spheres, will form spontaneously
under the right conditions. They are not a likely
precursor to the modern cell membrane, though, as cell
membranes are composed primarily of lipid compounds
rather than amino-acid compounds.
Hybrid models
A growing realization of the inadequacy of either pure
"genes-first" or "metabolism-first" models is leading the
trend towards models that incorporate aspects of each.
Other models
Clay theory of the origin of life
A hypothesis for the origin of life based on clay was
forwarded by Dr A. Graham Cairns-Smith of Glasgow
University in 1985 and adopted as a plausible
illustration by just a handful of other scientists
(including Richard Dawkins). Clay theory postulates
complex organic molecules arising gradually on a
pre-existing, non-organic replication platform - silicate
crystals in solution. Complexity in companion molecules
developed as a function of selection pressures on types
of clay crystal is then exapted to serve the replication
of organic molecules independently of their silicate
"launch stage".
Cairns-Smith is a staunch critic of other models of
chemical evolution. However, he admits, that like many
models of the origin of life, his own also has its
shortcomings.
"Deep-hot biosphere" model of Gold
A controversial theory put forward by Thomas Gold in the
1990s has life first developing not on the surface of the
earth, but several kilometers below the surface. It is
now known that microbial life is plentiful up to five
kilometers below the earth's surface in the form of
archaea, which are generally considered to have
originated around the same time or earlier than bacteria,
most of which live on the surface including the oceans.
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. He also noted that a
trickle of food from a deep, unreachable, source promotes
survival because life arising in a puddle of organic
material is likely to consume all of its food and become
extinct.
"Primitive" extraterrestrial life
An alternative to Earthly abiogenesis is the hypothesis
that primitive life may have originally formed
extraterrestrially (note that exogenesis is related to,
but is not the same as the notion of panspermia). Organic
compounds are relatively common in space, especially in
the outer solar system where volatiles are not evaporated
by solar heating. Comets are encrusted by 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
irradiation by ultraviolet light. It is supposed that a
rain of cometary material on the early Earth could have
brought significant quantities of complex organic
molecules, and that it is possible that primitive life
itself may have formed in space was brought to the
surface along with it. A related hypothesis holds that
life may have formed first on early Mars, and been
transported to Earth when crustal material was blasted
off of Mars by asteroid and comet impacts to later fall
to Earth's surface. Both of these hypotheses are even
more difficult to find evidence for, and may have to wait
for samples to be taken from comets and Mars for study,
and neither of them actually answers the question of how
life first originated, merely shifting it to another
planet/comet. However, this hypothesis extends
tremendously the array of conditions under which life may
have have formed, from early Earth plausible conditions
to literally any conditions possible in the Universe.
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