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Evolution Topic D: 1
The Origin of Life
http://www.youtube.com/watch?v=dYphAH2tKYE&feature=related
Early Earth Conditions
Earth formed 4.6bya
The cooling crust released, carbon dioxide, nitrogen, methane, hydrogen, water vapor
and ammonia.
There was no oxygen gas because the metals in the rocks reacted with it to form
oxides.
As the crust continued to cool, water vapor condensed to form the
oceans.
The Earth remained sterile for the next billion years before organic
molecules began to form.
Oxygen did not begin to appear in the atmosphere for another 1.3
billion years
The Spontaneous Origin of Life
Prokaryotic and eukaryotic organisms alive today all have structures
based on the cell.
Cell structure must have originated from chemicals present on early
Earth.
For this to happen four essential steps are required.
1. Living things are made of organic molecules, so simple organic
molecules such as amino acids, sugars, fatty acids, glycerol and bases
must have formed.
2.Organic molecules in living organisms (such as triglycerides,
phospholipids. Polypeptides and nucleic acids) are large, so single
molecules must have been assembled to make the more complex
molecules.
3. All living things reproduce, so molecules must have formed that
could replicate themselves and control other chemical reactions.
This is the basis of inheritance.
4. Cells have membranes, so he mixtures of these molecules
must have been enclosed within membrane-bound vesicles.
Under certain conditions, particular molecules such as small
polypeptides or phospholipids mixed in water collect together
to from small spheres, called microspheres.
If these microspheres form from larger polypeptides along
with other organic molecules, and if they become
surrounded by a skin of water, called coacervates. These
represented the first membranes.
In early oceans, microspheres and coacervates formed, many would
have broken up but some may have contained a mixture of chemicals
that increased their survival time.
They may have formed internal environments different to their
surroundings, some may have contained small molecules of RNA and
amino acids. These vesicles would’ve been able to replicate asexually,
allowing for natural selection to act on them and evolve.
Origin of Organic Compounds
The early Earth was bombarded with debris from space such as comets,
meteorites and space dust, which could have contained organic
molecules.
In September 1969, a meteorite fell in western Australia; 100kgs of
material were recovered from it and an analysis showed that is was rich in
amino acids.
Panspermia Theory- (seeds everywhere)
The water bear an small invertebrate was able to survive
outer spaces harsh conditions in an experiment done by NASA.
Providing proof that bacteria and other simple creatures could
have travelled on a meteorite from Mars.
Urey and Miller Experiment
In 1953, Harold Urey and Stanley Miller put together an
apparatus and attempted to mimic the conditions thought to
have existed on early Earth.
They wanted to test the theory that organic molecules could
have been produced when simple molecules came together.
The flask contained distilled water, which boiled and
condensed, just was it would have billions of years ago.
The gas mixture contained methane, ammonia and hydrogen,
and the electrodes produced sparks to simulate lightning.
After several days, the condensate became brown and
contained many organic compounds and amino acids.
Later other scientists used this investigation involving other
gases that were present and were able to form adenine, a base
found in RNA and DNA, and components of ATP.
When they added sand in the mixture complex amino acids were
formed.
Where could Organic molecules have formed?
Hot springs and thermal vents have been places where scientists can see
similar conditions of early Earth.
The vents and hot springs contain mineral rich water heated by volcanic
activity, containing very high temperatures and pressures forming organic
molecules and are teeming with bacteria.
The Endosymbiotic Theory
The endosymbiotic theory is an important theory explaining how
eukaryotic cells could have developed from simple prokaryotic cells.
The theory suggests that some organelles found inside eukaryotes were
once free-living prokaryotes.
There is evidence to suggest that prokaryotes were engulfed by larger cells,
and were retained inside their membranes.
Evidence includes that two organelles, mitochondria and chloroplasts share
many characteristics with prokaryotic cells.
Both mitochondria and chloroplasts:
Contain ribosomes, which are smaller than those found in other parts of the
eukaryotic cells but same size as ones found in bacteria (70s).
Contain small circular pieces of DNA similar to bacterial plasmids.
Have their own envelope surrounding them, one inner membrane where
proteins are synthesized, suggesting they may have used this ability long ago
when they were independent.
Can replicate themselves by binary fission.
These organelles (chloroplast and mitochondria) are modified bacteria
that were taken in by phagocytosis, early in evolution of eukaryotic
cells, but not digested.
Instead they became useful inclusions.
The double outer envelope of the organelles may have originated from
the bacterial membrane and the vesicle from the host.
Some of the enclosed bacteria had pigment covering their membrane and
used light energy to make organic molecules and release oxygen and
became chloroplasts.
Others may have been efficient at using oxygen molecules for aerobic
energy production and became mitochondria.
Aerobic respiration is much more efficient at releasing energy from
organic compounds than anaerobic respiration. This increase along with
sexual reproduction could of lead to a rapid evolution of life on Earth.
Critics of the theory might argue that, even if prokaryotes were engulfed
by larger cells, there is no certainty that they could be passed on to both
daughter cells when the cell divided.
However, when a cell divides by binary fission each daughter cell contains
some cytoplasm from the parent which could contain the engulfed
prokaryotes.