Scientists think that the protobionts are the evolutionary precursors of
prokaryotic cells. Protobionts may be originated as an array of microspheres of
diverse organic and inorganic compounds enclosed by lipidic membranes.
Proteins, carbohydrates, lipids, and other organic substances were the most
important autocatalytic organic compounds. Water was a very important factor
in the assembly of the protobionts' endoplasm. After this event, several
microspheres could self-organize into organelles that were able to perform
specific functions; for example, lysosomes, peroxysomes, vacuoles, etc.
Gradually, some segments of the external membrane would invaginate for
forming membranous organelles, like endoplasmic reticulum and Golgi apparatus.
First protobionts would not have a nucleus membrane (nuclear envelope);
consequently, they could be identified like prokaryotes.
Mitochondria and chloroplasts could develop soon after as vagile and selfsufficient protobionts, which specialized in obtaining energy directly from the
environment.
Mitochondria would be heterotrophic protobionts (with their own DNA), which
obtained energy from the organic molecules that were dissolved in large
quantities in the immediate environment (chemiosmotic organisms). Some
mitochondria would be engulfed by other larger protobionts. Possibly, the
earliest mitochondria could be used as food by other protobionts, but some of
them could not be processed as food, but survived living as symbionts into more
complex protobionts. Progressively, the functional relationship would be more
vital for both mitochondria and protobionts, until they could not omit one to
another. This could be the theory about the origin of the first prokaryotic
heterotrophic protists (e.g. Archaea and Bacteria).
The same process could have happened with chloroplasts, which would be
chemoautotrophic or chemiosmotic protobionts. At present, chemoautotrophic
organisms are able to get energy from organic matter from the environment,
as well as to transform radiant energy (transported by photons) into chemical
energy (in molecular bonds) by the action of chlorophyll. Some protobionts
would incorporate chloroplasts to their endoplasm, but by some self-defense
mechanism, some chloroplasts also would survive in the endoplasm of more
complex protobionts. Comparable with mitochondria, chloroplasts would become
a vital part of those protobionts, in which they would live as symbionts. Such
protobionts could not live without chloroplasts and the chloroplasts could not
survive outside their hosts. The first autotrophic prokaryotic protists could
originate from this way (such as cyanobacteria and sulfur bacteria).
From the end of the preceding century, many biologists have been considering
that RNA was the earlier nucleic acid in protobionts instead DNA because when
the environment was too hot, the enzymes for the synthesis of DNA could not
work appropriately, and DNA is unsteady at high temperatures. Scientists
think that the Earth was extremely hot when protobionts got assembled. The
biologists that think that the RNA was the nucleic acid of the early
protobionts assume that when the environmental conditions were more
propitious, the molecules of RNA could build DNA molecules. They think that
RNA was competent to produce autocatalytic and non-autocatalytic proteins
and that some autocatalytic proteins would help to the self-synthesis of RNA
molecules. However, with the current knowledge about the physicochemical
properties of nucleic acids and considering the impossibility of an inorganic
synthesis of RNA (the processes have been always forced by an external
operator). That's why I think that both hypotheses, the DNA world and the
RNA world, are unrealistic; not only in the range of the early protobionts, but
in the whole assortment of organic compounds spontaneously synthesized on
Earth. The experimentation suggest that all depended on the synthesis of
autocatalytic proteins, which reproduced through a progression equivalent to
the reproduction of prions at present, without the involvement of nucleic acids.
EARLY EARTH AND THE ORIGIN OF LIFE
I.
Age of the Earth
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The approximate age of the Earth, based upon geologic, magnetic,
radiographic, and paleontological studies, is 4.5 billion years. Life
appears to have arisen early on the planet.
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The earliest fossils are similar to filamentous bacteria and are called
Stromatolites. These fossils have been found in western Australia
and South Africa. These fossils are approximately 3.5 billion years
old.
The Stromatolites appear to have been photosynthetic organisms.
This indicates that life arose even before they were present.
Living organisms may have appeared on Earth as early as 4.0
billion years ago.
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Early Atmosphere and Conditions on Earth
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The Miller-Urey Experiments
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In the 1920’s, A. I. Oparin and J. B. S. Haldane proposed that the
atmosphere of the early Earth was a reducing one, and contained
very little oxygen.
Because there was no protective ozone layer in the atmosphere, a
much higher percentage of ultraviolet radiation reached the surface
of the Earth.
Lightning strikes were probably frequent in the early atmosphere.
Oparin and Haldane hypothesized that these conditions were
favorable for the spontaneous, abiotic synthesis of organic
molecules.
These molecules would include some of the basic organic molecules,
and if concentrated and inclosed in some fashion, may have formed
"protocells".
The protocells are NOT living organisms, but, rather, molecules
undergoing what is known as Chemical Evolution (this came before the
process of Organic Evolution).
Stanley Miller and Harold Urey tested the Reducing Atmosphere
hypothesis of Oparin and Haldane. They simulated the conditions of
early Earth by constructing an apparatus which contained water,
hydrogen, methane, and ammonia (constituents believed to comprise
the atmosphere of the early Earth).
These constituents were subjected to heat and an electric charge for
a number of days, then the distillate was checked to determine
whether any complex organic molecules were present. They found
some amino acid precursors, and some lipids.
These results supported the Oparin-Haldane hypothesis. However, it
is now known that the early atmosphere was less reducing than than
the researcher’s model system, so it would have been more difficult
to produce complex macromolecules.
However, with slight modifications, scientists have been able to
produce all 20 amino acids, ATP, sugars, lipids, and (importantly) the
purine and pyrimidine bases of RNA and DNA.
Concentration of the Polymers and Molecules
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Simple chemical evolution of organic molecules still does not
constitute life. Some way to concentrate the molecules into polymers
would likely have been the next step in the journey toward life.
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I.
The Protobiont Stage
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Sidney Fox produced polymers from simple monomers by dripping
them onto hot clays, sands, and rocks. The water would evaporate (as
it also would in shallow pools around the margins of water bodies) and
concentrate the monomers into polymers.
This step is necessary, because enzyme like proteins would not be
present at this stage.
Polymers formed from this method were called Proteinoids and were
primarily polypeptides.
Clays are good candidates for this process because they contain iron,
zinc, sulfer, and other charged ions that would help attract and bind
some of the molecules like a workbench for synthesis.
Living cells were probably preceeded by protobionts.
Protobionts were aggregates of abiotically produced molecules which
maintained an internal environment different from that of their
surroundings.
Protobionts exhibited additional properties of life, such as
Metabolism and Excitability.
If proteinoids are mixed with cold water, they will self assemble into
Microspheres.
Microspheres have a selectively permeable protein membrane
(remember proteins have different charges on different regions;
positive and negative). These charged regions give the membrane
energy in the form of a Membrane Potential and microspheres
undergo osmotic changes.
Other Protobionts called Liposomes can spontaneously form when
phospholipids are placed in water. This membrane looks very much like
the plasma membrane around cells. It is a phospholipid bilayer as
around all living cells.
Coacervates are drops of polypeptides, nucleic acids, and
polysaccharides which self-assemble in water.
However all of these protobionts lack a mechanism of exact
inheritance. So-called reproduction would have been simply by
breaking apart, and no exact copying was usually possible.
Development of Hereditary Material
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Before our protobionts became living cells they had to develop a
mechanism or molecule of heredity. In other words, they had to
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develop a way to pass exact information for development from
parents to offspring.
Most scientists currently support the hypothesis that RNA was
utilized as the hereditary material before DNA developed.
Ribozymes help to support this view. They are RNA molecules that
act as catalysts in RNA synthesis and removal of DNA intron regions.
Short sections of RNA have been observed to self-assemble under
suitable laboratory conditions.
RNA can fold into unique shapes (unlike DNA) which might have been
useful in the process of natural selection.
As RNA molecules directed some protein synthesis, so too could
these proteins help catalyze RNA replication.
At first, RNA probably provided the template to produce DNA, but
DNA is a more stable molecule over evolutionary time (because it is
double-stranded).
Eventually, RNA assumed an intermediate role as the molecule that is
translated for a protein code within the cell.
Finally Life
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At the end of the process, prokaryotic cells would have emerged in
the organic soup of the early oceans.
Life, at first, would have been relatively easy as organic nutrients
abounded in the oceans and competition was minimal. The first
cellular organisms were likely Heterotrophic.
However, as numbers of prokaryotes increased, competition between
individuals and populations would have started the process of Natural
Selection.
Adding slightly different structures, and metabolic (chemical
pathway) specializations would have conferred selective advantages
upon certain organisms and these types would have persisted.
As the free, organic nutrients were depleted in the oceans,
photosynthesis probably developed as an alternative. With the
development of photosynthesis, our Oxidizing atmosphere would have
formed.
After the oxidizing atmosphere was in place, chemical evolution
ceased because of reduced ultraviolet radiation and raw materials for
abiotic synthesis.
I.
Alternate Hypotheses for the Origin of Life
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Some researchers believe that life may have arisen as a result of
organic compounds reaching Earth from a meteorite or comet. Some
amino acids and water have been recovered from meteorites to
support this hypothesis. This is called the Panspermia hypothesis.
Other researchers believe molecules less complex than RNA would
have been the first hereditary materials. These scientists say that
RNA strands are too complicated to be the first self-replicating
molecules.
Other researchers believe that life originated on the bottom of the
oceans instead of on the surface or in shallow waters. These
scientist state that surface conditions would have been too harsh to
support life, but conditions along the deep ocean vents would have
been perfect (constant temperatures, upwelling of minerals from the
vents for raw materials, and heat for an energy source).
Scientific Creationism or organisms being created by an omnipotent,
divine being cannot be readily tested by scientific means (cannot be
definitely proven or, more importantly, disproved) and, thus, is usually
not considered within the scope of a biology course.