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Early Life on Earth

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Title: Early Life on Earth


1
Early Life on Earth
2
  • Overview
  • If you were able to travel back to visit the
    Earth during the Archaean Eon (3.8 to 2.5 bya),
    you would likely not recognize it
  • The atmosphere was very different from what we
    breathe today a reducing atmosphere of methane,
    ammonia, and other gases which would be toxic to
    most life on our planet today.
  • Also during this time, the Earth's crust cooled
    enough that rocks and continental plates began to
    form.
  • It was early in the Archaean Eon that life first
    appeared on Earth.
  • Our oldest fossils date to roughly 3.8 billion
    years ago, and consisted of bacteria
    microfossils.

3
  • Earths Oldest Rocks
  • Rocks older than 3.5 billion years are very rare
    on Earth
  • Indeed, there may have been only tiny patches of
    continental crust on the early Earth over 90 of
    the crust would have been oceanic rock
  • The oldest rocks on Earth are from northern
    Canada they are 3.9 billion years old and
  • There are also some very old rocks in the Isua
    area of west Greenland, dating at approximately
    3.85 bya
  • Rocks from both sites contain no evidence of
    life.

Major areas of exposed Precambrian rock
4
  • Carbon isotope evidence at 3.8 bya from Isua,
    Greenland
  • The Isua rocks do, however, contain quite a lot
    of carbon in the form of the mineral graphite (a
    type of elemental carbon).
  • The carbon at Isua is in the form of light
    graphite as if it had been produced by RUBSICO
    via photosynthesis
  • It is indirect evidence that there may have been
    life on earth before 3.85 bya

5
  • The Oldest Rocks with Life
  • Although there is some indirect evidence that
    there may have been life on Earth before 3.85
    bya, there is clearer isotopic evidence for
    biological carbon life at about 3.5 bya the
    first fossils are from these rocks
  • Archaen districts in both Australia and Africa
    provide evidence of stromatolites - low mounds or
    domes of finely laminated sediment composed of
    either calcium carbonate (CaCO3) or chert (SiO2).
  • Represent fossilized microbial mats formed
    mainly by photosynthetic blue-green "algae")
    called cyanobacteria

6
  • Living Stromatolites
  • We can observe them being formed today in both
    marine and freshwater systems.
  • Stromatolites contain a consortium, a complex
    associations of interacting organisms interwoven
    mats of slime-covered, filamentous cyanobacteria
    and other bacteria.
  • At the top, cyanobacteria do oxygenic
    photosynthesis.
  • Below the surface, bacteria that do
    photosynthesis without producing oxygen occur.
    Finally, deeper in the mat, heterotrophic
    bacteria feed on the decaying organic matter
    produced by photosynthesis at the top of the mat.
  • The minerals, along with grains of sediment
    precipitating from the water, are trapped within
    the sticky layer of mucilage that surrounds the
    bacterial colonies, which later continued to grow
    upwards through the sediment to form a new layer.
  • As this process occurred over and over again,
    the layers of sediment were created.

Present day columnar stromatolites in Australia
7
  • Summary
  • There appear to be several lines of morphologic
    and geochemical evidence for Archean life.
  • These data clearly document the presence of
    living things, but the record is extremely
    spotty.
  • Unfortunately, very little rock of Archean age
    is preserved at the Earths surface at present,
    and the rock that does out crop is often severely
    metamorphosed and unfossiliferous.
  • Archaen populations were probably kept in check
    by natural disasters - storms, heating, drying,
    and starvation due to lack of nutrients
  • As a consequence, there must have been very
    rapid fluctuations in bacterial populations as
    conditioned changed from day to day or season to
    season

8
  • Banded Iron Formations (BIFs)
  • The geologic record indicates a rather peculiar
    rock type between 3.5 to 2.0 bya
  • Banded iron formations or BIFs - sedimentary
    rocks formed from alternating bands of chert
    (SiO2) and iron oxide.
  • There is no evidence of these kinds of deposits
    being formed today

BIFs with red bands of hematitie and interbedded
chert
9
  • Where does the iron come from?
  • Iron is and was probably dumped into the oceans
    from erosion down rivers and from deep-sea
    volcanic vents
  • Why are there no BIFs around in present geologic
    time?
  • Iron readily precipitates out of solution in the
    presence of oxygen and organisms subsequently
    extract and use iron and silica (silica that
    could have gone into chert, SiO2) in building
    protective shells and skeletons
  • Iron dissolves readily in water that has no
    oxygen and that there was apparently little or no
    free oxygen when BIFs were being formed
  • Iron can only have precipitated from seawater in
    the amounts observed in the BIFs by an oxidizing
    chemical reaction

10
  • Evidence of no oxygen on the early Earth
  • Pyrite (iron sulfide) and Uranite (uranium
    oxide) occurring in riverbeds from 3 to 2 bya.
  • These minerals are not stable when O2 levels are
    high.
  • Their presence in rivers confirms our suspicions
    that oxygen levels were very low on the early
    Earth.

11
  • How did iron precipitate out of solution when
    there was apparently little or no free oxygen
    available?
  • The alternating iron oxide/chert beds indicate
    that there must have been periodic waves of O2
    available
  • In an oxygen-poor ocean, iron is soluble in
    water, so chert dominates the sediments on the
    ocean floor.
  • In an oxygen-rich ocean, iron is oxidized (it
    rusts), forming minerals that are insoluble in
    water, so iron oxide dominates the ocean-floor
    sediments.
  • Oxygen could have been supplied by the
    photosynthetic cyanobacteria present in
    stromatolites
  • Ultimately, this oxygen is used up by the
    "rusting" of this iron, and the ocean reverts to
    its ocean-poor state.

12
  • The Oxygen Revolution
  • Q. Why might photosynthesis have established
    itself on the early earth?
  • For bacteria, the advantages of photosynthesis
    may have occurred as soon as simple organic
    molecules began to run low, and fermenters began
    to run low of food
  • Autotrophic cells could store food and have a
    buffer against times of low food supply
  • The earliest photosynthetic cells probably used
    H from H2, H2S, or lactic acid
  • Some of the bacteria may have began to break up
    the strong H bonds of water molecules
  • H2O CO2 light ? (CH2O) 2 O

13
  • Note
  • Any bacteria that became capable of successfully
    breaking down water rather than H2S would
    immediately have multiplied their energy supply
  • However, there certainly would have been a cost
    with this switch
  • The waste product of H2O photosynthesis is
    monatomic oxygen (O), which is a poison to a cell
    because it can break down vital organic molecules
    by oxidizing them
  • Thus cells needed to evolve a natural antidote
    to this oxygen poison before they could
    consistently operate the new photosynthesis
  • We and other organisms evolved superoxide
    dismutases to serve as antidotes
  • Presumably as soon as cyanobacteria evolved an
    antidote to oxygen poisoning, they could control
    the use of it, including the use in new processes
    such as respiration

14
  • The Advantage of Respiration
  • Aerobic respiration extracts considerably more
    energy from organic molecules (C6H12O6) than does
    fermentation (anaerobic respiration)
  • Fermentation yields lactic acid which still has
    a great deal of energy
  • By using oxygen to break up a series of
    by-products all the way down t water and carbon
    dioxide, a cell can release up to 18X more energy
    from a sugar molecule via respiration than it
    can via simple fermentation

15
  • Conclusions about Stromatolites
  • Cyanobacteria, especially those in
    stromatolites, appear to be the dominant forms
    along the early ocean shorelines
  • Their success was likely due to the control over
    oxygen, which gave them an abundant and reliable
    energy supply in 2 ways
  • 1) by mastering photosynthesis based on
    water and
  • 2) by breaking down food molecules in
    respiration rather than
  • fermentation
  • Note
  • Stromatolites increase dramatically in the rock
    record with the beginning of the Proterozoic Era
    at about 2500 mya

16
  • Conclusions regarding the Oxygen Revolution
  • The increased oxygen supply by stromatolites in
    shallow water produced the first great masses of
    BIFs
  • It is probable that by oxidizing the iron, the
    BIFs served as a sort of buffer, allowing oxygen
    tolerance and utilization to evolve among some
    bacteria
  • But eventually BIF formation slackened and the
    oceans and atmosphere began to accumulate small
    amounts of oxygen
  • After about 2000 mya oxygen levels in the ocean
    reached a permanent level so high that sea water
    could no longer hold dissolved iron and BIFs
    could no longer form

17
  • Continental Red Beds
  • There is other geological evidence that confirms
    the oxygenation of the oceans around 2 bya
  • Beginning 2.3 Bya, iron minerals in soils on
    land began to be oxidized (rusted) during
    weathering soils turned red. 
  • The atmosphere must have contained O2 for this
    to occur.
  • Based on the types of oxide minerals present in
    early Proterozoic soils, it has been estimated
    that O2 levels were from 2 to 10 of modern. 
  • Today, O2 comprises 20 of the atmosphere, so in
    the late Proterozoic, it may have comprised 0.4
    to 2 of the atmosphere.

18
Red-Bed Blouberg Formation
  • The appearance of red beds, characterized by red
    iron oxide minerals, in the geological record
    marks the first appearance of significant
    quantities of oxygen in the Earth's atmosphere.

19
  • An Ozone Shield
  • One important environmental effect of higher O2
    levels is an Ozone Shield
  • With O2 levels in the 1 range, the
    stratosphere would begin to develop an effective
    ozone (O3) layer.
  • Although this ozone shield is not especially
    important to aquatic organisms, who are protected
    by water, it would be extremely important to
    living things trying to colonize land
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