Fossil record of life

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Fossil record of life

• Planets formed about 4.6 Billion years ago
• 4600 Million years
• Oldest rocks on Earth 4.0 Billion years
• (most of surface lt 1 Billion years old)
• Oldest rocks on Moon 4.4 Billion years
• Oldest meteorites 4.6 Billion
  

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A time scale for life

• 4.6-3.8 Hadean Eon Earth Still Solidifying.
  Frequent Impacts. Can vaporize oceans.
  Life Impossible.
• 3.8-2.5 Archaean Eon Prokaryotes (probably)
  existed since before 3.5 Bya.
• 2.5-0.55 Proterozoic Eon Significant increase
  in atmospheric oxygen. First Eukaryotes.
• 550 Mya -present - Phanerozoic Eon period of
  visible life. First Multicellular organisms.

http//www.palaeos.com/Timescale/default.htm
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Time scale for life in the Phanerozoic

• Before 550 Mya Precambrian
• 550 Cambrian explosion. Diverse fossils appear.
• 550-250 Palaeozoic. Invertebrates. Fish.
  Amphibians.
• 250-65 Mesozoic. Reptiles.
• 65-present Caenozoic. Birds. Mammals.
• 2 Mya bipedal ancestral humans
• 50,000 genetically modern humans

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Oldest Rocks on Earth Oldest Igneous 4.0 Ga
Acasta, Northwest Territories Oldest Sedimentary
3.8 Ga Isua, Greenland These
rocks may be metamorphosed. No structural
information Some claims for life
based on ?13C However
non-biological reactions can change ?13C
????? Thought to have been formed in the ocean ?
ocean already at this stage Oldest sedimentary
rocks claimed to contain fossils 3.5 Ga
Pilbara, Western Australia (Warrawoona/ Apex
chert) 3.4 Ga Barberton Greenstone belt, South
Africa Two kinds of evidence stromatolites and
microfossils.
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Stromatolites some of the oldest fossils

• Stromatolite means layered rock.
• Stromatolites are structures formed by colonies
  of microorganisms. They are not individual
  organisms.

Section through a fossil stromatolite
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Modern stromatolites at Shark Bay, Australia

• Formed by communities of micro-organisms.
• Surface cyanobacteria. Filamentous
  photosynthetic bacteria.
• Lower layers anaerobic bacteria
• When surface gets buried by mud the cyanobacteria
  can glide up towards the light forming a new
  layer.

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J William Schopf Prof. of Palaeobiology at UCLA
Claimed to be the earliest known fossils
Apex Chert site Pilbara, Australia Age 3.5
billion years Fossilized cyanobacteria preserved
as individual filaments in what was formerly
gelatinous mucilage. But are they really fossils?
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But are they fossils?
Both the stromatolites and the microfossils might
be non-biological look-alikes. Cyanobacteria are
advanced aerobic photosynthesizers late stage
of metabolic evolution. If they existed at 3.5
Ga, life came very far very fast. Schopf (2006)
latest position Consider cases where
microfossils occur with stromatolites. Carry out
small scale chemical analysis and show that
position of carbon compounds corresponds to
observed structure position. (Only about 1 of
stromatolites have microfossils associated with
them.) Says that stromatolites are extremely
widespread in Proterozoic and that nearly all
must be biological. Acknowledges that it is more
problematic in the Archaean. Although similar to
cyanobacteria, acknowledges that the earliest
microfossils may be other kinds of filamentous
bacteria. Concludes that the evidence for life
back to 3.5 Ga is strong.
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Brasier et al (2006) Argues for non-biological
null hypothesis Co-occurrence of biology-like
morphology and carbonaceous chemistry is not
enough. Ask what they are, not what they remind
us of. Argues that the microfossils are
non-biological. Spherulite mineral crystals form
and push carbonaceous material to the boundaries.
Artefacts of symmetry breaking during
crystallization.
Concludes that cyanobacteria probably present by
2.6 Ga Early microbial mats may have been
anaerobic photosynthesizers or chemosynthetic Conc
ludes that life may still prove to go back to 3.5
Ga but this is not yet certain.
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The earliest known Eukaryotes. Han Runnegar -
1992 Empire Mine, Michigan. Found fossils of
filamentous eukaryotic algae called Grypania -
Age 2.1 Billion years
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Vendian period (end of Precambrian) around 580
Million years ago Multicellular animals found
Also called Ediacaran Fauna, because they were
discovered in the Ediacara hills in Australia in
1946
Possibly related to early metazoa (e.g.
Jellyfish) or could be entirely different branch
of evolution (see Seilacher in OI book)
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Burgess shale fossils discovered in the Rockies
in 1909. Age approx 515 Mya around the
Cambrian explosion.
Canadia spinosa (thorny Canadian) An annelid
worm
Canadapsis perfecta (completely arched
Canadian) - a crustacean
Hallucigenia nothing like it nowadays
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Eozoön canadense - the dawn animal of CanadaAn
early controversy
John William Dawson Principal of McGill.
President of Royal Society of Canada 1882.
Knighted by Queen Victoria. Discovered
Precambrian limestone age 1.1 Ga containing green
and white layered patterns. Claimed to be remains
of foraminifera. Earliest fossils then
known. Shown in 1894 that these samples were of
non-biological origin. Similar structures
produced in recent rocks from Mt Vesuvius.
Dawson continued to champion Eozoön as one of
the brightest jewels in the scientific crown of
Canada. Religiously motivated. Wanted to show
that there was a huge gap in the fossil record
and hence that gradual Darwinian evolution could
not be true.
Foraminifera - single celled marine organisms
that produce mineral shells.
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The Martian metorite a recent controversy

• Martian meteorite ALH84001 found in Antarctica in
  1984. Age approx 4.5 Billion years.
• Science (1996) possible relic biogenic
  activity. Contains polycyclic aromatic
  hydrocarbons and possible nanobacteria fossils,
  20-100nm across. Much smaller than any earth
  bacteria, therefore very controversial.

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The Red Rain of Kerala a current controversy
Fell in India in 2001 - covered a large
area. Associated with loud boom of a
meteorite(?) Contain organic molecules but no
DNA Claimed to be extraterrestrial life
???? Dr Godfrey Louis Cochin,
India http//education.vsnl.com/godfrey/ Dr N C
Wickramasinghe - Cardiff http//www.astrobiology.c
f.ac.uk/redrain.html
Electron micrograph of sections of these
particles 3 micrometres in diameter
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So were about at the end then... What do we make
of all this?
Reasons to be cheerful Genome sequencing is
allowing huge advances in our understanding of
evolution of metabolism and cellular
structure. We continue to discover new life in
unusual places on earth and to improve our
understanding of the diversity of
microorganisms. New chemistry. e.g. artificial
ribozymes. Many other planets now discovered.
Almost certainly we will find earthlike ones
soon. Spacecraft and eventually people will
explore Mars, Europa etc. There is a new
interdisciplinary field of Astrobiology. This is
a science. We are not looking for a
miracle. There must be life out there!
Reasons not to be Genome comparison cannot tell
us anything before the LUCA. LUCA was already
complicated and may be far removed from the
origin of life. Not clear that we will ever
produce life in a test tube. Exploration outside
the solar system will not be possible without a
miracle in our understanding of physics. Maybe it
is simply impossible. Maybe we will never meet
them...
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Maybe thats a good thing after all...