Download 10 - Origin of Life

Origin of Life
How old is the earth?
Radioisotope
dating
suggests a
slightly older
earth
The earth (and the other planets)
condensed out of a swirling cloud
of gas surrounding the primitive
sun.
The atmosphere of the primitive earth was rich in
hydrogen, both in the elemental state and united
with carbon in methane, with nitrogen to form
ammonia, and with oxygen as water vapor.
Atmospheric water vapor condensed into rain.
Eroded surface. Washed minerals into seas.
Chemicals from atmosphere mixed and reacted
with those in the waters to form many
hydrocarbons.
The view that life emerged
through a long process of
chemical evolution was set forth
by the Russian biochemist
Alexander Oparin in 1924.
Oparin pointed out
that the
transformation of
lifeless chemicals
into living matter
extended over a
period of almost a
billion years. He also
argued that such a
transformation would
not be possible
today, since any
particle approaching
the form of life would
be decomposed by
the oxygen of the air
or destroyed by
microorganisms.
Hydrocarbons, water and ammonia are the raw
materials for amino acids.
In the early days of
chemistry, it was believed
that organic compounds
could only be produced
by living organisms. But,
in 1828, Friedrich Woller
was able to manufacture
the organic compound
urea under laboratory
conditions. Since then,
many organic compounds
have been artificially
synthesized.
In 1953, Stanley Miller synthesized organic
compounds under conditions resembling the
primitive atmosphere of the earth. He passed
electrical sparks through a mixture of hydrogen,
water, ammonia and methane.
The electrical discharges
duplicated the effects of
violent electrical storms in
the primitive atmosphere.
In the laboratory, four
simple inorganic molecules
interacted, after just a
week, for produce several
kinds of amino acids.
A number of energy sources would have been available
to provide the “spark of life”.
Others have suggested that deep-ocean
geothermal vents provide a suitable location for the
formation of organic chemicals.
…..or under the ice covering the primitive
ocean.
However, synthesis of amino acids
is just a first step. In 1964, Sidney
Fox heated a mixture of 18 amino
acids to temperatures of 160200ºC. for varying periods of time.
He obtained stable, proteinlike
macromolecules which he termed
proteinoids.
When the proteinoid
material was cooled
and examined under
a microscope, Fox
observed small,
spherical units that
had arisen from
proteinoid
aggregations. These
microspheres showed
a general
resemblance to
simple bacteria.
It is likely that self-replicating polynucleotides
slowly became established some 3.5 billion years
ago. Natural selection likely operated on these
populations of molecules just as it does on
organisms, and the molecules became more
efficient in the storage and transfer of information.
Certain polynucleotides not only specified their
own sequences, but directed the synthesis of
polypeptides.
In present-day organisms, RNA guides protein
synthesis, which suggests that nucleotides of
RNA were the first carriers of genetic
information.
At some later stage, DNA took the place of RNA
as the repository of genetic information. Thus, in
modern cells, genetic information is stored in
DNA, transcribed into RNA, and translated into
protein.
The universality of the genetic code suggests that
this became established a very early stage or
organic evolution. Remember Kirk’s aliens.
A protein specified by a particularly efficient
variant of RNA could not speed up the
reproduction of that kind of RNA unless in were
somehow restrained in the area of that RNA. If
some form of compartment evolved to enclose
the protein, it would prevent free diffusion.
All present day cells have a plasma membrane
composed of phospholipids. It is not implausible
that the first cell was formed when polarized films
of phospholipids formed “bubbles” enclosing
aggregations of complex molecules.
The oldest known fossils are
of cyanobacteria some 3.55
billion years old.
Primitive autotrophs would require a whole array
of enzymes to direct a multistep chain of
reactions involved in the synthesis of a protein. It
is probably too much to expect that all of the
necessary enzymes evolved at the same time.
Norman Horowitz of the California Institute of
Technology suggested that the chain of steps
could have evolved backward.
What about the evolution of
eukaryotes?
The “endosymbiont hypothesis” suggests
that organelles like mitochondria originated
as endosymbiotic bacteria living within the
cell membranes of other organisms.
Many properties of
modern eukaryotic cells
and their organelles
suggest that this is a
possibility.