Biological evidences for Snowball Earth scenario

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Biological evidences for Snowball Earth scenario?

• Snowball Earth - Journal club
• Nov. 2007
• Leehee Laronne Ben Itzhak and Eyal Rotenberg

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The geological time scale The geological history
of the earth is organized into units according to
events that took place in each period. Periods of
time are usually bordered by major geological or
paleontological events, such as mass extinction
(example the Cretaceous-Paleogene boundary is
known as the extinction event of dinosaurs and
many other marine species).
From the wikipedia
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The geological time scale
cenozoic
65.5 Ma
mesozoic
251 Ma
phanerozoic
Paleozoic
cambrian
542 Ma
Ediacaran
precambrian
4567 Ma
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Precambrian (4567-542 Ma)
The most severe glaciation known in the geologic
record occurred during the Cryogenian, when ice
sheets reached the equator and formed a possible
Snowball Earth. The earliest fossils of
multicellular life are found in the Ediacaran,
including the earliest animals.
542 Ma
Good fossils of multi-celled animals. Trace
fossils of worm-like Trichophycus, first sponges.
630 Ma
Possible snowball earth. Fossils still rare.
750 Ma
Neo-proterozoic
Trace fossils of simple multi-celled eukaryotes.
1000 Ma
Proterozoic
Meso-proterozoic
Green algae colonies in the seas. 1400 Ma
First complex single-celled life. 1800 Ma
Paleo-proterozoic
Oxygen catastrophe. 2500 Ma
First stromatolites. 3200 Ma
Archean
First known oxygen-producing bacteria. Oldest
microfossils. 3600 Ma
Simple single-celled life (bacteria and archaea)
3800 Ma
Hadean
Formation of earth began. Oldest known mineral
Zircon.
4567 Ma
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Paleozoic (542-251 Ma)
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Mesozoic (251-65.5 Ma)
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lt650 My
Protists
gt3,500 My
lt2,000 My
3,500 2,000 My
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Structure of a typical animal cell.
Eukaryotes Organisms whose cells are organized
into complex structures by internal membranes and
a cytoskeleton. The domain for the kingdoms of
animals, plants, fungi, and protists. Eukaryotes
diversity and the degree of complexity which
survived the snowball Earth (SbE) events is
considered a key indication of the severity of
the SbE episodes. Prokaryotes A group of
organisms that lack a cell nucleus (karyon).
Most are unicellular but some are multicellular.
They are divided into bacteria and archaea (or
archaebacteria).
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Does biological evidence support severe snowball
Earth events? The basic argument

• Repeated snowball Earth events, each for a period
  of 10 million years with a mean annual surface
  temperature as low as -50C and immediately
  followed by a short period of extremely hot
  weather (mean annual of 40C) and drastic
  changes in the oceans biogeochemistry cycling,
  has been proposed to impose an environmental
  filter on the course of evolution.
• Following the successive climatic events, it has
  been postulated that the biosphere is severely
  affected and that most eukaryotic lineages become
  extinct or that life survived only in refugia.
• The sharp increase in animals diversity during
  the Ediacaran-Cambrian era (following the SbE)
  can be explained as the result of evolutionary
  pressure during the icehouse-hothouse cycles. As
  such it is supportive evidence for the existence
  of snowball Earth episodes.
• But did fossils records, cell biology
  interpolations, genetic clock assessments and
  other biological insights confirm the severity of
  the Snowball Earth hypothesis?

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Complexity and diversity of life in Proterozoic
Oceans - but slow evolution development

• Evidences for moderate diversity of microfossils
  (size 1- 4 mm) and for the existence of
  eukaryotic organisms found in rocks older than
  800 M years.
• Mid-Proterozoic Eukaryotic fossils are thought to
  possess flexible membranes, cytoskeleton
  (scaffolding), cells remodeling capability,
  surface morphology...
• Phylogeny evidences pointing to major clades
  (group descending from a single ancestor)
  divergence within Eukaryas already at the Early
  Neoproterozoic (before 850 My).
• Functional abilities of early Eukaryotic cell
  morphology (like modifying cell shape), cell-cell
  communication (enables cells to grow beside
  another cells), Multicellularity in 3D (a
  prerequisite for large structure organism) and
  others.
• Given the complexity of Eukaryotes at an early
  evolutionary stage, an explaintion is required
  for their long interval untill the observed
  taxonomic explosion at the dawn of the Cambrian
  oceans (600 550 My)?

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What possibly created the Cambrian radiation?

• Generally genetics, ecology, or environmental
  conditions could possibly govern the evolution at
  the end of the Proterozoic era
• Genetic determination for that period is
  currenly poorly understood
• Ecology may be regarded as a major evolution
  driver through processes such as the appearance
  of new grazers (animals?) in Early Ediacaran
  (missing for most of the Proterozoic period),
  greater nutrient patchiness, and others.
• The dawning age of animals is associated with
  several environmental events - ice-ages (2),
  short period of shallow water anoxia and a rise
  in the oxygen level that possibly created
  evolutionary selective pressure and the emerging
  of new life forms.

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Conclusions (of the papers)

• Current available data and the level of
  understanding cannot distinguish which of the
  evolutionary driver explains the Cambrian
  radiation (also not the slow evolutionary rate in
  the Proterozoic oceans).
• California microfossils challenged the ideas of
  climate perturbation by catastrophically
  effecting the marine biosphere. This suggests
  that a completely ice-covered ocean was unlikely
  or the resiliency of life has been
  underestimated.
• These papers thus cannot resolve the question of
  the severity of the Snowball Earth events.

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Cenozoic (65.5 Ma to present)