Origin of life
Research into the origin of life is a limited field of
research despite its profound impact on
biology and
human understanding of our world. Progress in this field is slow and sporadic,
but it still draws the attention of many. A few facts give insight into the
conditions in which life may have emerged, but the mechanisms by which
non-life became life
are elusive.
For the observed evolution of life on earth, see the
timeline of life.
History of the concept: abiogenesis
Main article:
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.
The modern definition of abiogenesis is concerned with the
formation of the simplest forms of life from primordial chemicals. This is a
significantly different thing 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 the following
discoveries, which are listed in a rough order of postulated emergence:
- Plausible pre-biotic conditions result in the creation of the basic
small molecules of life. This was demonstrated in the
Urey-Miller experiment by
Stanley L. Miller and
Harold C. Urey in
1953.
-
Phospholipids spontaneously form
lipid bilayers, the basic structure of a
cell membrane.
- Procedures for producing random
RNA molecules can
produce "ribozymes",
which are able to produce more of themselves under very specific conditions.
The origin (see Origin of
organic molecules) of basic
biomolecules such as components of
amino
acids, while not settled, is less controversial than the significance and
order of steps 2 and 3, around which much of the current debate revolves (see
From organic
molecules to protocells).
Origin of organic molecules: Miller experiments
"Miller
experiments" (including the original Miller-Urey experiment of 1953) have
shown that under simulated conditions resembling those thought to have existed
shortly after Earth first accreted, many of the basic organic molecules that
form the building blocks of modern life are able to spontaneously form. Simple
organic molecules are of course a far cry from a fully functional
self-replicating life form, but in an environment with no pre-existing
life these molecules could accumulate and provide a rich environment for
chemical evolution. The spontaneous formation of complex
polymers
from abiotically generated
monomers
under these conditions is straightforward.
Other sources of complex molecules have been postulated including sources
of extra-terrestrial, stellar or interstellar origin. In
2004, a team
detected traces of
PAH (polycyclic aromatic hydrocarbons) in a
nebula, the
most complex molecule, to that date, found in space.
From organic molecules to protocells
There are many different hypotheses regarding the path that might have been
taken from simple organic molecules to protocells cells and metabolism. Many
of the possibilities have tended to fall into either "genes-first"
or "metabolism-first",
a recent trend is the emergence of hybrid models that combine aspects of both.
"Genes first" models: the RNA world
Main article:
RNA world hypothesis
The RNA world hypothesis, for example, suggests that short
RNA molecules could
have spontaneously formed that would then catalyze 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.
None of these various hypotheses have strong evidence behind them at this
time, however. 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 practice.
"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.
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.
The oxygen holocaust
About 2 billion years ago, during the
paleoproterozoic eon of history, there was a significant increase in
atmospheric oxygen. Before this time life was ananerobic, that is, the
metabolism of life depended on a form of
cellular respiration that did not require oxygen. The presence of large
amounts of free oxygen is poisonous to most anaerobic bacteria, and at this
time most life on Earth died out. The only life that survived was either life
that was resistant to the oxidizing and poisonous effects of oxygen, or life
that spent most or all of its life-cycle in an oxygen-free environment.
Other models
"Deep-hot biosphere" model of Gold
A 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. We now know 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.
Discovery of microbial life below the surface of another body in our
solar
system would lend significant credence to this theory.
"Primitive" extraterrestrial life
An alternative to Earthly abiogenesis is the hypothesis that primitive life
may have originally formed extraterrestrially (note this 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 and
ultraviolet light. The rain of cometary material on the early Earth could
have brought significant quantities of complex organic molecules, and it is
possible that primitive life itself may have formed in space and been 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.
Relevant fields
-
Astrobiology is a field that may shed light on the nature of life in
general, instead of just life as we know it (on Earth), and may give clues
as to how life originates.
-
Complex systems
See also
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