Download The Origins of Life

The Origins of Life
Primitive Earth
The earth probably originated 4.5 - 5 x 109 years
ago. Assuming life evolved on earth and was not
seeded in from elsewhere, the conditions
necessary must have existed on earth- so how did
this occur? The earth's surface was originally
molten and as it cooled the gases escaping from
the molten magma were held by gravity as a veil
of gas above the surface - a primitive atmosphere
rich in ammonia, methane and carbon dioxide.
There was no oxygen at this stage and so no ozone
layer could form. Once the earth's surface cooled
to below 100°C, water vapour would have
condensed to form mineral rich oceans. Violent
electric storms would have been common.
How cells formed
Because of the lack of ozone, UV light could have provided the energy to combine
monomers (the basic chemical building blocks) into polymers (larger molecules which are
vital for life such as DNA) and polymers into coacervates (protein rich mixtures of polymers
which clump together). In the 1950's Urey and Miller tried to reproduce the first steps in a
lab with electrical sparks in a gas mixture similar to that thought to have existed in the
primitive earth and, using the energy from this simulated lightning, they were able to make
amino acids and other monomers needed for life in laboratory condition. On the early
earth, rain would have washed the monomers into the oceans
•Coacervates
•Urey and Miller Experiment
How the first polymers formed is
less certain, but they may have
clumped together on the surface
of marine clay particles. As water
was absorbed from the monomers
into the clay structure (perhaps at
the sea's margins as the tide went
out) the monomers joined by
condensation reactions. The
energy required for these
reactions may have come from UV
light. Several researchers,
including Oparin and Fox, have
successfully caused protein rich
mixtures of polymers to assemble
into self-sustaining clumps termed
coacervates. Given enough time
and enough polymers, some could
have formed coacervates with an
outer membrane made of lipids
(complex organic fatty acids) and
DNA on the inside - ie primitive
cells.
Evolution of Anaerobic Respiration
The difficult step is how these primitive cells
mutated to produce the enzymes required to
release and trap the energy present in the
surrounding monomers - anaerobic respiration.
Once it had occurred, however, this cell would
have divided rapidly - limited only by the
quantities of available monomers. Its ancestors
include many types of anaerobic bacteria still
alive today. Sulphur bacteria and Nitrifying
bacteria evolved the ability to take carbon
dioxide from the atmosphere and turn it into the
monomers they needed for growth and
respiration. Sulphur bacteria would have been
much more likely to evolve in the primitive
earth conditions than they would today, since
there was more volcanic activity and thus more
hydrogen sulphide, their energy source
molecule. These chemosynthetic bacteria
became free from the food shortages affecting
other organisms but were themselves limited in
distribution by the location of suitable minerals.
•Stromatolites
Cyanobacteria evolved photosynthetic pigments, capable of trapping light energy and using
it to energise the electrons in water. There was little limit to the population growth of
these organisms - water, carbon dioxide and sunlight were abundant. Some bacteria formed
into large rock-like colonies in shallow warm seas, called stromatolites. Fossils of these
have been dated at 3.2 x 109 years old and stromatolite-forming bacteria are still active
today in Shark Bay in Western Australia. The oxygen produced by cyanobacteria would have
had several effects:
• Most anaerobic bacteria became extinct, oxygen
causing breakdown of their enzymes
• Ozone build-up would shield the earth from
UV light
• Conditions favoured the evolution of aerobic
bacteria
•Cyanobacteria
This caused the atmosphere to change to that essentially similar to that found today. Life
today has evolved its many complex and diverse forms, including man, to survive in this
environment. Oxygen, which was toxic to the first bacteria, is now essential. Ultra-violet
light which was probably an essential energy source for the early steps in the evolution of
life but is now a major risk to living processes. As both photosynthesis and aerobic
respiration became established as metabolic reactions, they became the major determining
factors in the balance between CO2 and O2 in the atmosphere. This balance is necessary for
life. Subsequent evolutionary changes, such at the advent of the first true cells and the
divergence of Plants, Animals and Fungi have had only minor affects. Human influence is
the only terrestrial factor that has the potential to upset this balance
Information taken from: http://vle.camsfc.ac.uk/evs/