Transcript The Hadean - Joe's Paleomag Home Page

The Hadean
The Rock Record
• The rock record is meagre.
• Oldest rocks are 4.0 billion years old from
the Slave Province in Canada
• Oldest zircons (derived from sedimentary
precursors) are 4.2 billion years old.
• Not much known, but assumed that an
early atmosphere and ocean formed via
outgassing and bombardment
The Moon
• The interval of time is
recorded on our
Early Imbrian
moon; however.
3,850
50
Nectarian
3,950
100
Basin-Groups 1-9
4,150
200
Cryptic
4560
410
Early Atmospheric Conditions
• Early crust emitted halogen gasses, ammonia, hydrogen,
carbon dioxide, methane, water vapor, and other gasses.
• This atmosphere was quite similar to the atmosphere of
Titan, one of the larger moons of Saturn.
• The primordial atmosphere is believed to have reached a
pressure of 250 atmospheres.
• Little by little our planet assumed a more familiar look,
with a dense gaseous cloud zone we could call an
atmosphere, a liquid zone with oceans, lakes and rivers,
or hydrosphere, and a solid zone, or lithosphere with the
first outlines of what would one day become continents.
Hadean Atmospheric/Hydrologic system
•
•
•
•
•
•
At the same time, another important series of events began to unfold that
led to the formation of sedimentary rocks through the processes of erosion,
drift, and accumulation.
These processes began to occur as soon as the surface cooled enough to
allow the water cycle to establish itself.
When temperatures finally cooled sufficiently, the clouds began to melt into
rain, and the primordial atmosphere produced storms of unimaginable
proportions.
Eventually, the water could accumulate in the depressed regions of the
Earth's surface, forming the first oceans.
On the primordial continents, the first river networks were created, and they
transported detritus torn from elevated regions and then deposited on the
bottom of the primordial seas.
The metamorphism and remelting of the products of the erosion ultimately
produced magmas and lava increasingly rich in silicates, and therefore of
different composition from the mantle and the primitive crust.
Hadean Continents/Oceans
• Faint young Sun Paradox
– Stars get warmer as the mature, incoming
solar radiation was reduced by 20-25% during
the late Hadean interval.
– Earth stayed ‘warm’ due to volcanic
outgassing and high levels of CO2
– High UV influx?
Orthodox View on rise of Oxygen
• Oxygen free atmosphere (anaerobic).
• The notion is that the complex
biomolecules responsible for selfreplicating organisms could not exist in an
aerobic environment.
• Evidence for anaerobic (or low oxygen)
environment?
Evidence for Anaerobic Conditions
• Primary argument is based on Banded
Iron Formations.
FeO is dissolved in acidic waters and tends to
precipitate in alkaline conditions.
Iron in BIF’s is oxidized iron (either magnetite
or hematite).
Silica tends to precipitate in acid conditions
and dissolve in acid conditions.
Suggestion is that oxygen producing
organisms led themselves to their own
demise.
Other Evidence
• Uraninite- Unoxidized uranium ore would
seem to support the generally anaerobic
conditions posited for the earliest time on
earth.
The Alternative View
• Did geologists ‘find’ what biologists
wanted?
• Biologists argued that once
photosynthesis started, oxygen levels
would rise quickly.
• It may be that atmospheric oxygen levels
reached 50% present-day levels by 2.0 Ga
The Formation of Life
One of the most interesting problems facing science is to explain how
the first replicating organisms came to be.
Darwin proposed a ‘warm little pond’ where life may have first formed.
“It is often said that all the conditions for the first production of a living
organism are now present, which could ever have been present. But if
(and oh! what a big if!) we could conceive in some warm little pond, with
all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc.
present, that a protein compound was chemically formed ready to
undergo still more complex changes, at the present day such matter
would be instantly devoured or absorbed, which would not have been
the case before living creatures were formed”.
Oparin-Miller-Urey
• In the 1920’s A.I. Oparin and Haldane each proposed
that an earth with a reducing atmosphere and with
methane (CH4) and ammonia (NH3) would have been the
ideal ‘primordial soup’
• In 1953, Stanley Miller and Harold Urey tested the
‘primordial soup’ hypothesis. They created a chamber
with methane, nitrogen, hydrogen and water vapor and
electrical discharges (lightning).
• After a week, the warm little pond turned brown and at
least 4 amino acids (building blocks of life) were formed.
• Later experiments produced 12 of the most common 20
amino acids.
Miller-Urey
• Miller and Urey did not produce life, but
only some simple amino acids necessary
for life. Still, the amino acids that formed
were far more complex than the system
input.
• Apparently, creating amino acids is not all
that difficult.
• Amino acids are found in meteorites and
cometary material as well (panspermia)
Amino Acids
Proteins
Sugars
Nucleotides- Bases for the Nucleic Acids
DNA- Double strand
deoxyribonucleic acid
RNA-Single strand ribonucleic
acid
The molecules of Life!
Problems:
(a) To produce RNA or DNA, a
base is needed to polymerize
the amino acids.
(b) Amino acids are easily
destroyed in open
environments.
(c) Even if RNA or DNA could
form, they are highly
susceptible to destruction.
(d) So we also need some way to
protect self-replicators
Some Working Hypotheses- No Answers YET!
Simple cells have an out membrane formed of fatty acids
Fatty acids are easily synthesized (found in some meteorites
Combining alcohol with fatty acids produces lipids.
Lipid molecules are polar, with an alcohol head and a tail composed of
fatty acids. The charged alcohol head is attracted to water and the tails
are repelled by water.
When fatty acids are dried and concentrated, then wetted again, they
spontaneously condense into spherical balls that can trap any biopolymers
inside (a protective membrane!).
Proteinoids & Coacervates
Deamer found that when simple proteins were trapped inside the balls with
DNA, they caused the amount of DNA to multiply 100-fold.
Sidney Fox followed up with experiments similar to Deamers which
spontaneously produced droplets of protein.
Oparin found similar behavior that produced protein droplets called
coacervates. The structures behave, in some ways, like bacteria. They
maintain their organization under changing conditions, increase in size and
bud spontaneously. They also absorb and release compounds in a manner
analagous to excretement and feeding.
RNA, DNA and Proteins-Chicken or Egg?
If life is going to form, it needs not only the protection for the complex
proteins, it also needs to self-replicate.
The ability to self-replicate is one of the hallmarks of life and thus
protenoids and coacervates are not living entities.
RNA and DNA carry the necessary proteins to communicate the
information needed for replication. In essence, these two nucleic acids
code for the proteins yet some proteins act as enzymes to catalyze the
replication of DNA.
Protein synthesis and DNA replication are both essential processes and
each depends on the other.
Which came first the chicken (proteins) or the egg (nucleic acids).
RNA WORLD OR PROTEIN WORLD?
Fox and others argue proteins came first.
Fox claims that protenoids exhibit many features of living systems.
Yet, only nucleic acids can replicate (that we know of). If protenoids
had developed a way to self-replicate, why turn over the job to nucleic
acids?
Others argue that Nucleic acids came first (RNA WORLD).
RNA can self-replicate and can produce the proteins needed to
catalyze further construction of nucleic acids.
The idea is one of a ‘naked gene’. Nucleic acids are far harder to
synthesize and only nucleic acids of a few dozen nucleotides have
been created in the lab (hundreds are needed for true genetic code).
Are these the only possibilities?
Graham Cairns-Smith claims that both scenarios are too complex and
we should look for a simpler start. The key to replication is a template
that could organize molecules in close order so that the could develop
the proper length to self-organize.
CLAY WORLD-Clays are natural catalysts with lots of space
available for complex organic molecules. Imperefections in the
clay structure would cause ‘mutations’ and life started out as a
silicate form.
Gunter Wachterhauser suggests that pyrite served as the original
template for life.
Fool’s Gold World- Pyrite has positively charged surfaces that
could attract negatively charged particles such as phosphate
backbones found in many organic molecules (including nucleic
acids). If they all lined up against the pyrite, they could
polymerize to form organic bonds and unzip themselves from
the pyrite.
Origin of Life- Final Answer
The final answer to origin of life questions is that we simply
don’t know. We only know that life did form and it formed early in the
history of life.
As organisms became self-replicating, they made copying
errors. In some cases, the copying errors led to increased survivability
and adaptibility so the organisms adopted the copying errors as the
standard.
Evolution began once life began.
Early Forms of Life
• Oldest known fossils are thought to be cyanobacteria
(blue-green algae).
• Prokaryotes- single cell with genetic material not inside a
organized nucleus.
The cyanobacteria are
photosynthesizing organisms
(autotrophs). Began to
oxygenate the earth.
CO2 + H2O + light  CH2O + O2
Oldest Fossils
• Oldest confirmed fossils are
from the Warrawoona
Complex in Australia (3.5 Ga).
• Similar fossils in the
Onverwacht and Fig Tree
Groups in Transvaal, South
Africa.
• Some of these fossils have been
questioned in the literature.
Stromatolites
• Even if the ‘cyanobacteria’ in the groups at 3.5 Ga are found
not to be organic, stromatolitic mounds have been found at
many locations in Archean strata.
Modern Stromatolites, Australia
Fossil stromatolites, India
Prokaryotes
• As far as we know, these simple forms dominated the
planet for 1.5 billion years.
• Very little or no evolution of bauplans.
• Schopf argues that these organisms (that persist to the
present day) are robust and reproduce asexually (no
mixing of genetic material).
• Significant evolution in biochemical diversity.
Heterotrophs and autotrophs.
The First Significant Stressor on Biological Systems
Between about 2.1-1.8 Ga there appears to be a major change in the
Earth’s environment. The time period is of interest for several other reasons
as well.
Oxygen Holocaust- Oxygen levels increased slowly to about this time. One
school of thought is that by 2.1 Ga oxygen levels were at about 1-2% of
modern-day values. Anaerobes were driven into the subsurface by this
poisonous environment. Some prokaryotes adapted to this new
environment.
Snowball Earth- Glacial deposits of 2.1 Ga have been found on many
continents. Some yield information suggesting they were formed at lowlatitudes and low elevation. The earth may have become totally glaciated at
this time (a tremendous stress).
First Large Supercontinent Formed- Called “Nuna” or “Columbia”, this was
the first large agglomeration of landmasses. Global circulation patterns
would change driving climatic change and opening up new environments.
Snowball Earth
Columbia according to JJW Rogers and M.
Santosh (2002)
Notable Changes at 2.1-1.8 Ga
Banded Iron Formation stops- Replaced by the formation of redbeds
(hematite-rich beds).
Uraninite Formation ends
Pyritic Rich Conglomerates end
All signal (with caveats) a rise in oxygen levels on Earth
Interestingly, a rise in oxygen levels occurs again at 1.3 Ga and again in
the Late Neoproterozoic all times of supercontinent assembly or breakup.