Evidence for Early Life

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Evidence for Early Life Microfossils,
biomarkers, and preservation
Tracy Quan September 19, 2006
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The Big Questions
When did life start? What type of organisms were
first? When did photoautotrophs develop? When did
the Earth become oxygenated? How do environmental
conditions control organism development/distributi
on? How did modern organisms/distributions evolve?
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Evidence of Life
Microfossils preserved cell walls, skeletons,
cysts Biomarkers compounds specific to certain
organisms Geologic evidence isotopic shifts
correspond to enzymatic systems microbially
mediated deposits Molecular clocks backtracking
based on mutations in genes
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Microfossils
Direct evidence of the presence of organisms Can
be soft cell walls that are remineralized, hard
shells/tests, or cysts and resting
stages Particular forms are diagnostic of
specific species Presence or absence can also be
indicative of certain environmental
conditions Abundance relative to total can also
indicate environmental conditions or shifts due
to evolution
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diatom
Dinoflagellate cyst
Prasinophyte phycoma
Fossil cyanobacteria
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Fossil Preservation
Not all organisms make easily preserved
parts Even silica and carbonate shells can
dissolve Not all parts are specific to certain
taxa Environmental conditions can
influence Variations in species evolution can
make interpretation difficult Older fossils can
be hard to identify as such Can also be altered
by temperature and geologic processes
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Coccoid cyanobacteria can be thermally altered to
look like filamentous fossils due to
kerogenization of mucus envelopes rather than
individual cells
440 Mya Kazmierczak and Kramer, 2002
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What are Biomarkers?
Biomarkers are molecules that are characteristic
of certain organisms and/or processes pigments
reflect photosynthesis membrane lipids can
indicate species These compounds are often
derived from lipids and can be very
stable Diagenesis alters the compounds into more
stable forms Not all organisms leave identifiable
biomarkers some only generate during certain
conditions need to know source and diagenetic
pathways
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2-methyl bacteriohopanepolyols characteristic of
cyanobacteria
Summons et al., 1999
1.6 Gyr Barney Creek Formation Brocks et al., 2005

• Trimethylarylisoprenoids -gt carotenoid breakdown
  products
• o Carotenoid okenane -gtpurple sulfur bacteria
  (lt20m)
• i, c carotenoids -gt planktonic green and brown
  anerobic phototrophs

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The Gamble of Preservation
Organic matter passes through a series of
oxidation reactions in the water and
sediments O2 respiration CH2O O2 -gt H2O
CO2 Denitrification 4/5H CH2O 4/5NO3- -gt
CO2 2/5N2 7/5H2O Mn red 4H CH2O 2MnO2
-gt 2Mn2 3H2O CO2 Fe red 8H CH2O 4FeOOH
-gt 4Fe2 CO2 7H2O Sulfate red H CH2O
1/2SO42- -gt CO2 1/2H2S H2O 5 of OM reaches
the sediment lt1 is buried Proteins sugars
lost first lipids remain, eventually
geopolymerized to kerogen Watch for contamination!
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Geologic and Isotopic Evidence
Biological processes deplete carbon relative to
the source Modern phytoplankton between -20 and
-25 It has been shown that some high T and P
processes can deplete C to -35
Schopf 1993
Organisms can also imprint on the geologic
record precipitated stromatolites, microbial
mat-like textures, structures of trapped and
bound particles Use age of surrounding rocks to
confine age of fossils
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No Markers ? No Life
The absence of fossils, biomarkers, etc. does not
mean those organisms did not exist Preservation
requires specific circumstances Some organisms
dont have identifiable markers Tectonic and
geologic processes can eliminate or alter
signals Sampling location and biases may affect
findings Molecular clocks indicate longer times
than fossil record It can generally be assumed
that an organism existed long before it appears
in the fossil record
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Life Through the Ages
Archean (2500-3800 Mya) data is very limited
early cyanobacteria evolution possible by 3500
Mya first prokaryotes (DNA, RNA, cell
membranes)? In the Proterozoic (545-2500 Mya)
Great Oxidation Event at 2300
Mya cyanobacteria and microeukaryotes (1900 Mya)
were present limited primary photosynthetic
endosymbiosis started about 1200 Mya green
algae primary production increasing
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The Paleozoic (250-545 Mya) had minor
dinoflagelletes and heterokonts larger presence
of red and green algae and cyanobacteria largest
extinction event at the Permian-Triassic boundary
(250 Mya) 96 marine, 70 terr. extinct Modern
phytoplankton arose during the Mesozoic (65-250
Mya) dominance of diatoms, dinoflagellates and
coccoliths as primary producers on
shelves cyanobacteria primary in the open ocean
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The Oldest Fossils?
Need to know provenance and age, indigenous to
and deposited with rock surround, biologic
origin Schopf found 11 diverse species of
filamentous cyanobacterium-like organisms in
Early Archean Apex chert from Western Australia
(3465 Mya) presence of distinct
septa O2-producing photoautotropy seen 1300 My
earlier Assume similarity in form to modern
organisms equals similar function Isotopic C
signature indicates biological C
fixation Laser-Raman spectra seem to be
consistent with presence of kerogen
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Schopf 1993
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But Are They Fossils?
Braiser et al 2002 looked at same rocks and found
no evidence for fossils some shapes follow
crystal ghosts, or are part of complex branching
structures septa made by quartz interspersed
into graphite Both structures and isotopes can be
explained by non-biogenic hydrothermal
processes Laser-Raman spectra are not
characteristic of kerogen only (Dill Pasteris and
Wopenka, 2002) Could these Archean samples
contain fossils of a different type? - see
Kazmierczak and Kramer, 2002
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Fossil structures part of larger forms that
extend in 3 dimensions changing focal depth can
show additional structures
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Conclusions
Several techniques and lines of evidence must be
combined to determine how and when life
evolved All have their limitations care must be
taken to prevent contamination and
misinterpretation Absence of physical and
chemical evidence does not mean life did not
exist preservation is limiting! The earliest
life is hard to identify lack of samples, low
preservation, inconclusive evidence The debate on
the Apex chert fossils is science in action!