Early Earth would have been very different and inhospitable compared to the
Earth today.
- Hot
- Why? - Primordial heat, collisions and compression during
accretion, decay of short-lived radioactive elements
- Consequences - Constant volcanism, surface temperature too high for
liquid water or life as we know it, molten surface or thin, unstable basaltic
crust.
- Atmosphere - early atmosphere probably completely different in
composition (H2, He)
- Cooling
- Primordial heat dissipated to space
- Condensation of water (rain), accumulation of surface water.
- Accumulation of new atmosphere due to volcanic outgassing
- Conditions appropriate for evolution of life
Atmosphere - Envelope of gases tha surrounds the Earth. Used by life as
a reservoir of chemical compounds used in living systems. Atmosphere has no
outer boundary, just fades into space. Dense part of atmosphere (97% of mass)
lies within 30 km of the Earth (so about same thickness as continental crust).
- Chemical Composition Today - Nitrogen (N2)- 78%, Oxygen (O2)- 21%, Carbon
Dioxide (CO2) - 0.03 %, plus other miscellaneous gases (H2O for one).
- Composition - Probably H2, He
- These gases are relatively rare on Earth compared to other places in the
universe and were probably lost to space early in Earth's history because
- Earth's gravity is not strong enough to hold lighter gases
- Earth still did not have a differentiated core (solid inner/liquid outer
core) which creates Earth's magnetic field (magnetosphere=Van Allen Belt) which
deflects solar winds.
- Once the core differentiated the heavier gases could be retained
Produced by volcanic outgassing.
- Gases produced were probably similar to those created by modern volcanoes
(H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane)
- No free O2 at this time (not found in volcanic gases).
- Ocean Formation - As the Earth cooled, H2O produced by outgassing
could exist as liquid in the Early Archean, allowing oceans to form.
- Evidence - pillow basalts, deep marine seds in greenstone
belts.
Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels
in the atmosphere? Revist the oxygen cycle:
- Oxygen Production
- Photochemical dissociation - breakup of water molecules by
ultraviolet
- Produced O2 levels approx. 1-2% current levels
- At these levels O3 (Ozone) can form to shield Earth surface from UV
- Photosynthesis - CO2 + H2O + sunlight = organic compounds + O2 -
produced by cyanobacteria, and eventually higher plants - supplied the rest of
O2 to atmosphere. Thus plant populations
- Oxygen Consumers
- Chemical Weathering - through oxidation of surface materials (early
consumer)
- Animal Respiration (much later)
- Burning of Fossil Fuels (much, much later)
Throughout the
Archean there was little to no free oxygen in the atmosphere (<1% of
presence levels). What little was produced by cyanobacteria, was probably
consumed by the weathering process. Once rocks at the surface were
sufficiently oxidized, more oxygen could remain free in the atmosphere.
During the Proterozoic the amount of free O2 in the atmosphere rose from 1 -
10 %. Most of this was released by cyanobacteria, which increase in abundance
in the fossil record 2.3 Ga. Present levels of O2 were probably not achieved
until ~400 Ma.
- Iron (Fe) i s extremely reactive with oxygen. If we look at the oxidation
state of Fe in the rock record, we can infer a great deal about atmospheric
evolution.
- Archean - Find occurrence of minerals that only form in non-oxidizing
environments in Archean sediments: Pyrite (Fools gold; FeS2), Uraninite (UO2)
. These minerals are easily dissolved out of rocks under present atmospheric
conditions.
- Banded Iron Formation (BIF) - Deep water deposits in which
layers of iron-rich minerals alternate with iron-poor layers, primarily chert.
Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide.
BIF's are a major source of iron ore, b/c they contain magnetite (Fe3O4) which
has a higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0
- 2.8 B.y. old, but do not form today.
- Red beds (continental siliciclastic deposits) are never
found in rocks older than 2.3 B. y., but are common during Phanerozoic time.
Red beds are red because of the highly oxidized mineral hematite (Fe2O3), that
probably forms secondarily by oxidation of other Fe minerals that have
accumulated in the sediment.
Conclusion - amount of O2 in the atmosphere
has increased with time.
- Chemical building blocks of life could not have formed in the presence of
atmospheric oxygen. Chemical reactions that yield amino acids are inhibited by
presence of very small amounts of oxygen.
- Oxygen prevents growth of the most primitive living bacteria such as
photosynthetic bacteria, methane-producing bacteria and bacteria that derive
energy from fermentation. Conclustion - Since today's most primitive life
forms are anaerobic, the first forms of cellular life probably had similar
metabolisms.
- Today these anaerobic life forms are restricted to anoxic (low
oxygen) habitats such as swamps, ponds, and lagoons.
How do you define a living thing? Minimum criteria:
- Be able to reproduce. Assures long term survival of organism as a
species.
- Be able to metabolize. Assures short term survival of organism as a
chemical system.
Conditions necessary for life as we know it:
- Protection from ultra-violet radiation/solar winds
- Free O2 in the atmosphere, creates ozone (O3), provides oxygen for
consumption
- Liquid water
As discussed above, it took some time for these
conditions to evolve on Earth.
- Fossil Evidence - for the origin and evolution of early life is
limited. But data suggest that life is at least as old as the oldest rocks now
known.
- Graphite Concentrations - graphite is a mineral consisting of pure
carbon that occurs in the oldest known sedimentary rocks. Archean graphite has
the same carbon isotope ratios as found in biological systems today.
- Stromatolites - Mats of cyanobacteria or blue-green algae. Mats
trap carbonate mud. Cyanobacteria grow up through the mud to form new mat
layer. Structures thought to be 3.4 - 3.5 by stromatolites as well as
filaments resembling cyanobacteria are found in the Warrawoona Group of Western
Australia and 2.8-3.0 B.y old rocks of southern Africa.
- Chemical Evidence - Can we find inorganic ways of synthesizing
essential organic compounds needed to get life started?
- Synthesis of Amino Acids - Stanley Miller (1953)- Organic Soup
Experiment. Synthesized amino acids (ie. built organic molecules by sending
electrical sparks through sealed vessels containing ammonia, methane, hydrogen,
and steam. Amino acids are the building blocks of proteins.
- Protobionts - In a dehydration/heating experiment, amino acids will
form chains of protenoids - protobionts. These can metabolize.
- Self Replication - not clear how this developed. Likely, starting
place is with RNA which can replicate itself without the aid of enzymes (DNA
can't replicate without enzymes, and you can't make enzymes without DNA).
- An Alternative: Submarine Hydrothermal Vents - occur on the flanks
ofMid-Oceanic Ridges and emit hot fluids rich in dissolved metals and sulphur.
Recently (1988) some of these vents were found to be supporting
chemosynthetic/sulphur metabolizing bacteria that form the food-chain base for
communities of worms/crabs/anenomes/clam. Evidence that life may have
originated in these deep sea vents includes
- Amino acids have been detected in hydrothermal fluids
- Polymerization could have occurred on clay particles
- No need for atmospheric protection from UV or for free O2