HISTORICAL GEOLOGY
LECTURE 7. EARLY LIFE.
The Early Atmosphere. Recap: The Earth's early atmosphere was formed by the release
of gases by volcanic activity (4.6 - 3.6 BYBP) - termed OUTGASSING. Evidence =
marine sedimentary rock 3.8 BYBP - proves existence of oceans. Composition = carbon
dioxide, water vapor, carbon monoxide, hydrogen, hydrogen chloride (similar to
modern volcanic gas). There was very little OXYGEN - when it was emitted it probably
combined very quickly with iron and precipitated as iron oxide. No oxygen meant no
OZONE (ozone is created by the breakdown of oxygen in the atmosphere) and
therefore ULTRAVIOLET RADIATION bathed the surface of the Earth (deadly now).
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The Earliest Life
Life originated sometime prior to 3.5 BYBP - the exact process is still
not known; however experiments have shown that carbon + oxygen +
hydrogen + nitrogen + phosphorus + sulfer + UV + electrical discharge
(lightning) + heat --> AMINO ACIDS, one of the major building
blocks of PROTEINS - a basic component of life. It has also been
suggested that conditions required to form organic molecules may also
have existed at mid-oceanic ridges. Today hyperthermophiles thrive
around these vents. They use chemosynthesis to get energy from
substances such as hydrogen sulfide and ammonia. Other microbes –
lithotrophs – have been found in hot rocks 3 km below the surface of
the earth. They gain energy from hydrogen, iron, magnesium and
sulfur. There is evidence, then, that life can thrive in harsh
environments – we don’t know what came first (or if life originated in
several different places).
You can read about the latest theories of how life originated here:
http://evolution.berkeley.edu/evolibrary/article/origsoflife_04
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Feeding Strategies:
1. HETEROTROPHS: at some point early in the history of life, single-celled
marine organisms developed, which consumed organic molecules for food
and, in the absence of oxygen, used fermentation (breakdown organic
molecules and rearrange the parts, releasing energy) to convert the food into
energy.
2. AUTOTROPHS: manufacture their own food (useful if organic molecules
are scarce). E.g. Sulfur bacteria make their food from carbon dioxide and
hydrogen sulfide; nitrifying bacteria make food from ammonia).
3. PHOTOAUTOTROPHS: use photosynthesis to breakdown carbon dioxide
into carbon (for growth) and oxygen (which escaped into the air). Billions of
algae-like photoautotrophs called cyanobacteria (formerly called blue/green
algae) developed in the oceans and acted like oxygen factories, slowly
transforming the Earth's atmosphere to one rich in oxygen. This was a crucial
change that allowed life as we know it to develop….
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Evidence of this is found in the form of banded iron formations. These are chert
layers with alternating rust-red and gray bands. The rusty bands are colored by
ferric iron oxide (Fe2O3), which shows that enough 02 was present to oxidize iron
at the earth’s surface. BIF’s are common in rocks 3.4 - 2 billion years old.
Michigan
Proterozoic Banded Iron Formations.
Egypt
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The cyanobacteria (Blue/green algae) responsible for adding oxygen
to the atmosphere, created mat-like colonies in intertidal areas which
collected layers of calcium carbonate mud. The algae grew through
the layer and the process was repeated to form laminations, each
layer representing a day's growth. Modern stromatolites are also
found in the intertidal zone.
Modern stromatolites in Australia
Ancient stromatolites in South Africa
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Section through 2 billion year old
stromatolite, Michigan, showing
laminated structure.
Colony of cyanobacteria from
Proterozoic rocks in Canada.
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The earliest known group of these simple (bacteria-like or prokaryotic –
cells lack a nucleus and organelles) fossils are from western Australia and
are about 3.5 billion years old. This suggests that life originated about 3.6
billion years ago.
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More advanced forms of prokaryotes evolved - a famous group are from the
Gunflint Chert of Canada (1.9 billion years old). Some resemble modern
types of algae.
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The continuing build up of oxygen in the atmosphere by
photosynthesis led to a protective ozone shield (which blocked UV
radiation) and the development of AEROBIC (“oxygen-burning”)
multicellular organisms (more than one type of cell; cells organized
into tissues, organs etc., used oxygen to convert food into energy more efficient). These are termed “Metazoans”.
These are soft-bodied
organisms e.g. soft
corals, worms,
jellyfish.
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Famous group =
Ediacaran
Fauna of
Southern
Australia. These
are about 630
million years
old (“Vendian”
time - named
after the final
period of the
Proterozoic in
Russia).
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Ediacaran Fauna
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Reconstruction of
Ediacaran sea floor about
580 - 630 million years
ago. The metazoans were
the forerunners of the
great expansion of life
that occurred in the
Phanerozoic. These
organisms were largely
soft-bodied (no hard
parts), which greatly
reduced the chances of
fossilization and
preservation. This
changed dramatically
around the beginning of
the Cambrian Period
when shelled organisms
became abundant (a lot
more fossils).
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