EAS 105 Precambrian life I

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EAS 105 Precambrian life I
Key points The Archean world On being alive
Becoming alive hypotheses concerning lifes
origin The tree of life
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THE GEOLOGICAL CONTEXT FOR THE ORIGIN OF
LIFE Complex multicellular animals (metazoans)
are known from a range of Neoproterozoic
sediments, about 700 Ma in age The earliest
preserved sedimentary rocks are approximately 3.8
Ga (or 3800 Ma), in SW Greenland, South Africa
and/or Australia Photosynthesis can be assumed
after 2.3 Ga (2300 Ma) since this marks
oxygenation of the atmosphere, mainly witnessed
by the disappearance of reduced iron (ferrous,
Fe2) in the global ocean (banded iron
formations, of BIFs) This leaves a relatively
narrow window of 1.8 Ga (1800 Ma), during the
Archean, for the search of lifes earliest fossil
record on Earth
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The Precambrian 89 of Earth history
disappearance of BIFs
Rise of atmospheric O2
The search for life in the Archean has a long and
controversial history
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The Archean environment reduced gas atmosphere
(likely CH4, NH3) lower than present solar
luminosity intense volcanism and CO2
degassing greenhouse effect very much in
effect pinkish hue of sunlight due to spectral
absorbance
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When we refer to life - what exactly does it
mean ?

• Any entity with the capacities to
• REPLICATE (health is measured by this ability)
• MUTATE (replication with differentiation from
  parent)
• Undergo NATURAL SELECTION (preservation of
  certain mutations as adaptations to changes in
  the environment)

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The search for life begins with hypotheses
concerning the origin of organic matter (i.e.,
containing C,H, and N), and specifically
molecules that give rise to the ability of
replication, and are hence the building blocks of
life (amino acids)
THREE LEADING HYPOTHESES

1. Synthesis of organic molecules from inorganic
   precursors
2. Delivery of organic molecules from the solar
   system
3. Origins in hyperthermal vents in mid-oceanic
   spreading zones

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The Miller-Urey experiments CH4, NH3, and H2O(g)
electrified (simulating lightning) - traces of
amino acids are produced A viable yet unverified
mechanism for the first synthesis of organic
molecules in the highly reduced Archean world
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Amino acids are also present in some meteorite
and interplanetary dust samples These particles
reflect the composition of the solar system in
the early Archean shortly after Earth was
formed This opens the possibility that organic
molecules hitch-hiked to Earth from the solar
system Very frequent impacts are evident early
in Earth history, following the impact that
formed the moon - many times more frequent than
at any time during the Phanerozoic This is
referred to as the heavy bombardment period,
prior to 3.8 Ga
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A third possibility is that Earths first
life-forms originated in very hot (gt80C)
submarine environments where new sea-floor was
being formed - these are called hyperthermophilic
environments - and metabolism is based on ferrous
iron and reduced sulfur (vs. C and N)
A somewhat protected but nonetheless hostile
environment, represented today in black smokers
and volcanic hot springs, but possibly much more
abundant in early Earth history RNA analyzed
from modern thermophiles confirms their antiquity
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A tree of life, based on differences in cellular
RNA in modern organisms Distances are
proportional to the amount of natural selection
and geological age Although thermophiles are
clearly old, the chloroplast is at least as old
or older, implying the equally great antiquity of
photosynthesis
thermophiles
multicellular life
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SUMMARY Life originated on Earth during the
Archean, in a much contrasted environment to that
of the present Several possibilities exist to
account for the synthesis of the first organic
molecules. However, their presence alone does not
prove the existence of life, as defined
biologically by the ability to replicate and
evolve through natural selection through
mutation Both photosynthetic and
chemosynthetic lifestyles are primitive features
of early life The fossil record remains to be
fully explored for vestiges of such early
organisms, in order to provide definite proof of
the true antiquity of life
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EAS 105 Precambrian life II
Key points The Earths earliest fossil
record Consequences of life for the planet
Winners and losers survivors and extinct
clades the Earths earliest fossil record The
biological context for the Cambrian explosion
Recall the Archean environment
??
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Stromatolites accretions of algal and microbial
biomass that trap fine clastic material forming
typically domed and stratified morphologies. Rare
at present, limited to hypersaline shallow
embayments lacking grazers. Given relatively
frequent occurrence in the geological record,
modern stromatolites are very good modern analog
for the search of early fossils, all of which are
very small (microfossils)
Shark Bay, western Australia
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Most modern stromatolites are dominated by
prokaryotic photoautotrophs genetic material is
dispersed throughout the chloroplast where
photosynthesis occurs. Some make their own
available nitrogen.
Cyanobacteria
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Also present in stromatolites are eukaryotic cell
designs, in which the cell is more
differentiated, with genetic information
concentrated in a nucleus, and chloroplasts
dispersed elsewhere in the cell
Origin? Likely the product of endosymbiosis
between two prokaryotic cells, thus explaining
the nuclear membrane and other features
nucleus
chloroplasts
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Fossil counterparts in the Precambrian rock record
Lake Superior, north shore
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Archean stromatolites often heavily
metamorphosed as a consequence of their great
age, thus compromising their preservation
potential - here, several generations of
faulting/deformation are evident, as well as
considerable silica replacement as chert or quartz
Northern Alberta
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Apex chert, Warrawoona Fm., Australia a colonial
prokaryote or bacterium? Not unambiguous
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Bitter Springs, Australia Solitary early
eukaryotes gt3 Ga? Again, not unequivocal Some
argue that similar morphologies can be formed by
hydrothermal fluids migrating through rocks
Modern analog?
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By the late Archean, 2.3-3.0 Ga, the debate is
better settled on the grounds of more diverse
morphologies (filaments, coccoids) as well as
geochemical evidence for the first hydrocarbons
at 2.7 Ga (methyl- hopanes)
Gunflint Chert, Ontario
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However, a conundrum still exists although
photosynthesis is at least 2.7 Ga, oxygenation of
the atmosphere occurred, at a minimum, 0.4 Ga
later. Possibilities include Low initial
global primary production and hence low O2
production and release Perhaps nitrogen
limitation characterizedf the early biosphere,
if biological fixation had not yet arisen
Svanbergfjellet (Svalbard) Neoproterozoic (700
Ma) Early N2-fixation clearly in place
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By the Mesoproterozoic-Neopreoterozoic boundary,
1 Ga, many important groups that persist to the
present were clearly in place green and red
algae, in addition to diverse cyanophytes and
bacteria, as well as unicellular protistan
animals (testate amoebae), and eukaryotic fungi
(implying parasitism)
Probable dinoflagellate algae, 800 Ma, Somerset
Island, Arctic Canada
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And by 600 Ma, multicellularity in animals is
evident The Ediacaran, as of 2004, is the first
new Period added to the Geological timescale in
108 years, marking its importance at the dawn of
the Cambrian explosion
Ediacaran enigmata
Flinders Range, Australia
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The Ediacaran ecosystem populated by mysterious
and extinct fossil clades, but fuelled by primary
production from phytoplankton that have evolved
relatively little
Rangeomorphs, Spaniards Bay, NF
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SUMMARY The geological complexities of deep
time complicate the search for Earths earliest
fossils The first microfossils may be as
old as 3.5 Ga, but debates continue The
chloroplast had evolved and photosynthesis
existed by 2.7 Ga, but the atmosphere appears not
to oxygenate until somewhat later By the
mid-Proterozoic, planktonic algal life is
diverse, likely benefiting from the absence of
herbivory. Protistan animals and parasitism also
existed by this time The late Proterozoic
experienced an explosion of strange animals with
no current affinities, all supported by a
surprisingly modern-appearing collection of
primary producers
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CONCLUSIONS and IMPLICATIONS The Precambrian
contained the two most profound biological events
in Earth history (1) the origin of life on
Earth, and (2) the advent and rise of oxygenic
photosynthesis. The latter is prerequisite for
all subsequent metabolisms based on the
respiration of O2 These events
pre-conditioned the biosphere and atmosphere for
the Cambrian explosion of metazoan life, namely
the rise of shelly animals Although the
Precambrian contains marked evolutionary
successes among microbial communities including
phytoplankton, apparent failures are witnessed by
the Ediacarans Thus model, there is contrast
between the adequacy of basal design seen in
microbes and the experimental innovations in
life seen in the Ediacaran fauna. Similar
contrasts persist in Cambrian biology, whether
arising from stochastic, environmental, or
biological factors.