The Microbial World:

1
The Microbial World Evidence of Earths Earliest
Life
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Oldest Known Rock on Earth
Its ours, eh !
Acasta Gneiss Northwest Territories,
Canada (3.96 Ga) (note Ga billion years, Ma
million years)
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Second Oldest Known Rocks on Earth
Isua Group, West Greenland (3.85 Ga)
4
Oldest Known Zircons on Earth
Oldest zircon crystals 4.4 Ga In coarse clastic
sedimentary rocks 3.0-3.7 y billion years
old Jack Hills, Western Australia Indicates that
some solid, granitic, crust existed prior to
oldest known rocks (but probably not much)
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Life became established relatively early
in Earths history ! Oldest crust 4.4 Ga Oldest
known life 3.5 Ga
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Oldest known evidence of life
Stromatolite (structure constructed by
cyanobacteria) Warrawoona Series, North Pole,
Western Australia 3.5 Ga
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Clues to formation of stromatolites Modern
stromatolites in Shark Bay, Western
Australia Hypersaline Intertidal Conditions
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Growth of filaments Trapping of sediment Growth
of filaments Trapping of sediment Growth of
filaments Trapping of sediment
Sediment Trapping by Bacterial Mats (forms the
fine laminated structure observed in
stromatolites)
9
Single-celled bacteria-like microfossils Fig Tree
Chert, Swaziland, South Africa recently estimated
at 3.4 Ga
10
Filamentous Prokaryote Microfossils (Probably
Cyanobacteria) Apex Chert, Marble Bar, Western
Australia 3.4-2.5 Ga
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• So there is evidence that prokaryotic life
  existed on Earth by at least 3.5 billion years
  ago.
• Carbon isotopic evidence from 3.8 Ga rocks in
  Greenland suggests that microbes may have existed
  even earlier.
• Very low d13C signatures in these rocks suggests
  some carbon fractionation by methanogens or
  photosynthesizers
• Such organisms preferentially take up 12C over
  13C, so extreme enrichment in light carbon (12C)
  might suggest primitive life
• but this evidence is tenuous at present (perhaps
  13C could be depleted by inorganic means).

12
Low-oxygen conditions on early Earth
BIF production peaks here
Note up to about 2.3-2.2 Ga, hydrosphere
contains minimal free oxygen -uranium occurs in
solid particles (uranium dissolves in presence of
oxygen) -fluvial (river) sediments contain
reduced iron (so not red) -iron oxide is
deposited in ocean in banded iron formations
(BIF) soils are iron-deficient
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2.5- 2.0 Ga
A time characterized by widespread Banded Iron
Formation (BIF) deposition -interbedded chert
(SiO2) and magnetite (Fe3O4)/hematite
(Fe2O3) Iron oxide formation may have
precipitated inorganically (via simple oxidation
of iron by free oxygen in water) or assisted
by metabolic activities of bacteria
14
2.3 - 2.0 Ga First Definite Appearance of
Redbeds (soils and river sediments containing red
iron oxides) indicates enough free oxygen in
atmosphere to oxidize iron in river water before
it reaches the sea (note BIF production shuts off
soon afterward)
15
The time of about 2.3-2.0 Ga is significant
because by this time, significant amounts of
oxygen occurred in the hydrosphere and
atmosphere, thus fundamentally changing the
dynamics of the Earths biosphere. For this
reason this time is considered to mark the
beginning of the Oxygen Revolution.
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Earths Modern Atmosphere Takes Shape
Red river sediments
Red soils
BIF production really drops off
By 2.0-1.8 Ga Redbeds well-established -Soils
and fluvial (river) sediments are enriched in
oxidized iron By about 1.8 Ga, BIF production
really drops off (indicating that transport of
dissolved iron from land has been effectively
shut off due to oxidation on land)
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• But if photosynthesizers were largely responsible
  for producing most of the free oxygen on Earth,
  why did it take them so long ?
• Possible reasons
• Oxygen was used by organisms (for metabolic
  processes) as soon as it was produced.
• Oxygen-bearing organic molecules were buried (and
  therefore was not readily released into the
  atmosphere in significant quantities)
• Oxygen was used up through oxidation of dissolved
  iron (thus forming the huge deposits of BIF)
• Nutrients such as phosphorus were in short supply
  (phosphorus tends to be adsorbed onto iron oxide
  particles), so may have limited the growth of
  cyanobacteria (and therefore oxygen production).

18
Stromatolites in Gunflint Chert, Near Schreiber
Ontario 1.9 Ga
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Even after the initial buildup of oxygen to
significant levels in the atmosphere, things were
pretty boring in terms of the variety of living
things. Stromatolites (and associated fossil
bacteria) continue to dominate the Earths
biosphere
20
Prokaryote Microfossils (probably
cyanobacteria) Belcher Islands, Arctic Canada 2 Ga
21
Cyanobacterial Filaments Bitter Springs Chert,
Northern Australia 1.5 Ga
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Bitter Springs Chert, Northern Australia, 1.5 Ga
Spherical Cyanobacteria
Filamentous Cyanobacteria
Modern Form
Modern Form
Note remarkable similarity between ancient and
modern forms
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But there are some hints of progress
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END OF LECTURE