Most astronomers today believe that the universe began with
a cosmic explosion, the Big Bang, that occurred throughout all
space at the beginning of time, approximately 15 billion years
In the very beginning, all matter in the universe was concentrated
in a state of infinite density. At the Big Bang, when the temperature
of the universe reached an incredible 10 billion degreee Kelvin,
the universe began to expand, marking the creation of the universe.
In the next 3 minutes, when the temperature began to drop to
less than 1 billion degree Kelvin, the nucleosynthesis began,
filling the universe completely with high-energy photons colliding
vigorously with protons and electrons. At this time, the universe
was in its "primordial fireball" state that it was
opaque. 1 million years after the Big Bang, when the energy of
the photons became too weak to keep the protons and electrons
apart, the protons and the electrons started to combine to form
hydrogen atoms as temperature dropped below 3000 K. At this time,
atoms became the most stable form, and since they are much smaller
than visible light that they do not block the photons, the universe
became transparent as we know it today.
"It is dawn, 4,600 million years ago. Earth is in the
violent red throes of its beginnings..." -- Margulis &
In its earliest stage, approximately 4.6 billion years ago,
Earth was a fire ball-- a gravitational implosion of molten rocks
and swirling metal. Its surface and atmosphere were occupied
by gases such as ammonia, hydrogen sulfide, and methane in their
superheated states, zapping everywhere and at everymoment by
lightning. Meanwhile, the sun has ignited, sending bursts of
powerful radiation onto Earth.
Meteors of different sizes, ranging from dust specks to small
planetoids, at the same time, continue their bombardments. While
these bombardments brought water and carbon compounds along onto
Earth, they also brought in incredible amounts of (kinetic) energy,
which along with the decay of radioactive isotopes, melted the
solid material collected on Earth from the earlier planetesimals(Margulis
& Sagan). Gravity then caused abundant, dense iron to sink
toward the Earth's center, forcing less dense material to the
surface. This process of chemical differentiation then produced
a layered structure within the Earth: a central core composed
of almost pure iron, surrounded by a mantle of dense, iron-rich
minerals. This mantle, in turn, is surrouned by a thin crust
of relatively light silicon-rich minerals (Kaufmann).
The Creation of life: People once believed that bacteria could
spring spontaneously from non-living things, which was later
proven "wrong" by Pasteur with his famous twitched-neck
flask experiment. Ironically, today, we've realized that the
very first life on Earth was indeed originated from abiotic surroundings.
In fact, organic molecules have been successfully generated from
abiotic elements by scientists Miller and Urey.
The abiotic chemical evolution of life follows 4 major steps:
1. the abiotic synthesis and accumulation of small organic
molecules, or monomers, such as amino acids and nucleotides;
2. the joining of these monomers into polymers, including proteins
and nucleic acids; 3. the aggregation of abiotically produced
molecules into droplets, protobionts, that had chemical characteristics
different from their surroundings; and 4. the origin of heredity.
To understand how this creation of life from abiotic material
occured, we have to consider 2 critical ideas (F. Shu): 1. The
extension of the idea of natural selection to chemical level.
2. The realization that the condition of the early Earth when
life first arose must have been vastly different from present:
a) non-oxidizing atmosphere: present level of oxygen, which began
to accumulate around 2.1 billion years ago with the presence
of cyanobacteria, would have been lethal to primitive organisms
b) abundant resources produced non-biologically c) long time
scale without competition
Stanley Miller and Harold Urey used an apparatus similar to
this one to simulate chemical dynamics on the primitive Earth.
A warmed flask of water simulated the primeval sea. The "atmosphere"
consisted of H2O, H2, CH4, and NH3. Sparks were discharged in
the synthetic atmosphere to mimic lightning. A condenser cooled
the atmosphere, raining water and any dissolved compounds back
to the miniature sea. As material circulated through the apparatus,
the solution in the flask changed from clear to murky brown.
After one week, Miller and Urey analyed the contents of the solution
and found a variety of organic compounds, including some of the
amino acids that make up the proteins of organisms. (Campbell)
As mentioned earlier under the section "The Chemical
Aspects of the Origin of Life", life did indeed originated
from its abiotic surroundings. Thus, it is important for us to
learn about the physical and chemical environments of the primitive
The atmophere of primitive Earth consisted of reactive, naturally
Nitrogen (N2), water (H2O), methane (CH4), and ammonia (NH3),
etc. These molecules are basically what was needed to creat life
-- in fact, the percentage of hydrogen, oxygen, nitrogen, and
carbon, together consists 99.5 % of all matters. Along with carbon's
versatility, constructions of an inexhaustible variety of organic
molecules using roughly the same proportions of the essential
elements thus took place. In fact, the percentage of essential
elements of life -- C, N, O, H, P, and S -- are quite similar
from individuals to individuals.
Energy wise, the primitive Earth had plenty from a variey
1. Radiation: from the cosmic and radioactive isotope decays
2. UV light: there were no protective ozone layer then 3. Electrical
discharge: from the never ending lightning 4. Heat: young crust
was volcanically acitve
Following are a few molecular clues to the origin of life
on Earth presented by Prof. Shu of UC Berkeley:
Molecules of living organisms are rich in hydrogen-containing
carbon compounds. This suggests that there were little or no
free molecular oxygen on primitive Earth.
All amino acids exist in both the right-handed state and the
left-handed state. However, only 20 amino acids of the left-handed
variety are used by living organisms in proteins. Therefore,
suggesting that there was one single origin of life.
DNA and RNA are the universal basis of all life forms on Earth.
ATP is the universal energy currency of all living organisms;
suggesting a common origin of metabolism.
In any cell, first steps of carbohydrate metabolism involve
fermentation, with the last steps in aerobic organisms the usage
of oxygen via respiration -- suggesting that aerobic organisms
evolved from anaerobic ones.
Prokaryotes (commonly known as bacteria), the simplist forms
of life, originated a few hundred million years after Earth's
crust cooled and solidified. The oldest evidence of life found
as of today are fossils resembling spherical and filamentous
prokaryotes found in stromatolites that are 3.5 billion years
old in southern Africa and Australia. These fossils appear to
be photosynthetic, however, suggesting that the earliest life
probably had evolved prior to that of 3.5 billion years ago.
(see the dating methods)
One of the essential episodes in the formation of life on
Earth involved the formation of a selectively permeable membrane
that could enclose a solution of different composition from the
surrounding solution, therefore providing a stable environment,
while still allowing exchanges of nutrients and wastes. This
may sound like a very complicated process, however, under the
goverance of the laws of chemistry, it really was not too difficult
-- all it takes was some of the abiotic elements that were spontaneously
available on the early Earth.
The formation of the biological membrane and its selective
permeability to various substances is the one most important
factor that led to the origination of life. However, this formation,
despite its importance, is quite simple. In fact, it's simultaneous
-- provided that all the essential elements are available (which
were -- please refer to the section The Conditon of the Early
When phospholipids are mixed with water, they self-assemble
to form films due to insolubility. Agitation then breaks the
films into sphere. Even these primitive membranes have some ability
to control the passage of substances between the contents of
a sphere and the aqueous environment outside. Phospholipids were
probably among the organic molecules that predated life on the
primitive Earth, and their spontaneous assembly to form membranes
was a major step toward protocells that could maintain internal
environments differing from the surroundings (Campbell).
\After the formation of the biological membrane, the protocell
then follow the 4 major steps of chemical evolution (click here
to review the 4 steps..), which then led to the formation of
the most primitive life -- the prokaryote, commonly known as
the "bacteria". The prokayrotes are the single-celled
organisms that have successfully acquired the biological membrane
system, but lack the endomembrane systems found in the more advanced
The prokaryotes thus were the earliest organisms, and they
lived and evolved all alone on Earth for 2 billion years. They
had continued to adapt and flourish on a changing Earth, and
in turn they have helped to change the Earth (Campbell).
The eukaryotes, on the other hand, are more advanced with
endomembrane systems. That is, they evolved the ability to "engulf"
other prokaryotes and acquire whatever function the prokaryotes
were capable of. For example, endocytosis of cyanobacteria (a
prokaryote) gives some eukaryotes the ability to perform photosynthesis.
Other examples include the engulfing of golgi apparatus, mitochondria,
etc. With this incredible ability, the eukaryotes were able to
evolve into more advanced organisms such as human beings.
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