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Origin of Multicellular Animals


Evidence suggests that there were repeated, near total glaciations of the planet ... Torrents of carbonic acid rain erode the rock debris left in the wake of the ... – PowerPoint PPT presentation

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Title: Origin of Multicellular Animals

Origin of Multicellular Animals
  • The Appearance of Multicellularity
  • Multicellular algae appeared about 1.0 Bya
  • Multicellular animals appeared about 0.6 Bya
  • What were environmental conditions like at this

A Snowball Earth Evidence suggests that there
were repeated, near total glaciations of the
planet during the late Proterozoic. The
evidence 1. Signs of glaciers at sea level
within a few degrees latitude of the equator. 2.
During and immediately after the glaciations,
geochemical data suggest that basically no
photosynthesis was taking place in the oceans.
3. Thick deposits of calcium carbonate (up to
400 m) immediately overlie the glacial deposits. 
Some of these deposits show mineral forms,
suggesting they formed directly from solution on
the ocean floor. 
Calcium Carbonate Deposits
ROCKY CLIFFS along Namibia's Skeleton Coast have
provided some of the best evidence for the
snowball earth hypothesis.
More regarding the Snowball Earth Hypothesis
  • For a currently unknown combination of reasons,
    ice sheets spread from high latitudes towards the
  • Ice is shiny, and sunlight bounces off it, and
    with more of the planet covered by ice, the
    planet absorbs less solar radiation. 
  • With less solar radiation, the planet gets
    colder, and ice spreads even closer to the

Stage 1 Snowball Earth Prologue Breakup of a
single landmass 770 million years ago leaves
small continents scattered near the equator.
Formerly landlocked areas are now closer to
oceanic sources of moisture. Increased rainfall
scrubs more heat-trapping carbon dioxide out of
the air and erodes continental rocks more
quickly. Consequently, global temperatures fall,
and large ice packs form in the polar oceans. The
white ice reflects more solar energy than does
darker seawater, driving temperatures even lower.
This feedback cycle triggers an unstoppable
cooling effect that will engulf the planet in ice
within a millennium.
Snowball Earth cont.
  • Computer simulations suggest that if the ice
    spreads to within  30 north or south of the
    equator, a positive feedback develops, causing
    the planet to be totally coated with ice, right
    down to the equator. 
  • Such a run away "ice house" could explain the
    glacial deposits at low latitude, and the near
    total shutdown of the marine biosphere.

Stage 2 Snowball Earth at Its Coolest Average
global temperatures plummet to -50 degrees
Celsius shortly after the runaway freeze begins.
The oceans ice over to an average depth of more
than a kilometer, limited only by heat emanating
slowly from the earth's interior. Most
microscopic marine organisms die, but a few cling
to life around volcanic hot springs. The cold,
dry air arrests the growth of land glaciers,
creating vast deserts of windblown sand. With no
rainfall, carbon dioxide emitted from volcanoes
is not removed from the atmosphere. As carbon
dioxide accumulates, the planet warms and sea ice
slowly thins.
Snowball Earth cont.
  • Although the planet is iced over, carbon dioxide
    is still present in the atmosphere, and it is
    still being released by volcanoes on the Earth's
    surface and under the oceans.
  • But two important processes that suck carbon
    dioxide out of the atmosphere (photosynthesis and
    rock weathering) are turned off when the planet
    is in its Snowball state. 
  • Carbon dioxide builds up in the atmosphere and
    begins to trap enough sunlight to begin melting
    the ice. 
  • Water traps a lot of heat, so once the melting
    begins a second runaway process is established. 

Stage 3 Snowball Earth as it Thaws Concentrations
of carbon dioxide in the atmosphere increase
1,000-fold as a result of some 10 million years
of normal volcanic activity. The ongoing
greenhouse warming effect pushes temperatures to
the melting point at the equator. As the planet
heats up, moisture from sea ice sublimating near
the equator re-freezes at higher elevations and
feeds the growth of land glaciers. The open water
that eventually forms in the tropics absorbs more
solar energy and initiates a faster rise in
global temperatures. In a matter of centuries, a
brutally hot, wet world will supplant the deep
Snowball Earth cont.
  • A runaway "greenhouse" effect is established 
  • Carbon dioxide in the atmosphere is rapidly
    sucked up in the weathering of rocks in these
    hot, wet surface conditions, leading to rapid
    precipitation of the massive carbonate deposits
    capping the glacial deposits. 
  • Eventually, the carbon dioxide levels and
    surface temperatures stabilize, and the Earth is
    back to "normal" conditions.

Stage 4 Hothouse Aftermath As tropical oceans
thaw, seawater evaporates and works along with
carbon dioxide to produce even more intense
greenhouse conditions. Surface temperatures soar
to more than 50 degrees Celsius, driving an
intense cycle of evaporation and rainfall.
Torrents of carbonic acid rain erode the rock
debris left in the wake of the retreating
glaciers. Swollen rivers wash bicarbonate and
other ions into the oceans, where they form
carbonate sediment. New life-forms--engendered by
prolonged genetic isolation and selective
pressure--populate the world as global climate
returns to normal.
  • Q. If the Snowball Earth happened, how did the
    different types of unicellular organisms survive
    repeated encounters with ice and heat? 
  • Perhaps living things were present around
    volcanic islands that melted their way through
    the ice. 
  • Perhaps the ice was thin enough near the equator
    to allow liquid water in cracks, which would
    permit photosynthesis and heterotrophy to
  • Perhaps living things survived near hot springs
    on land. 

  • Interestingly, after the last of these Snowball
    events, multicellular animals expand
  • Were there prior attempts at multicellular life
    that were snuffed out by these Snowball events? 
  • Or did the Snowball event somehow trigger
    evolutionary innovations by providing a rigorous
    regime of natural selection unlike any that had
    come before? 

The Evolution of Multicellular Organisms
Benefits Increased size It is easier to get
nutrients into, and waste products out of, a
large body made of many small cells.  If
increased size was favored, perhaps
multicellularity was favored as well. Division
of labor When different cells and tissues within
the body are specialized for particular
functions, they can do those functions more
efficiently than a single cell that has to
simultaneously do all the bodily functions. 
Longer lives (replace cells) The life span of
a multicellular individual is not limited to the
life span of a particular cell.  Multicellular
animals can live, and produce offspring, for a
longer period of time.
The Evolution of Multicellular Organisms
cont. Risks/costs Cancer Rogue slacker cells
that decide to reproduce rather than work for the
good of the whole multicellular organism.  Their
unwillingness to do their assigned task, and
their eagerness to reproduce, can spell disaster
for the body that contains them.  
  • Construction of Multicellular Animals
  • Two issues faced by metazoans that unicellular
    organisms don't confront.
  • 1. Differentiation
  • Every cell in the body has the same DNA.  Yet
    some cells will turn into neurons, others become
    hair follicles, while others become muscle. 
  • How does a metazoan get cells in different
    tissues to turn on (or turn off) the parts the
    genetic code that cause it to specialize for a
    particular function?

  • Construction of Multicellular Animals cont.
  • 2. Development
  • A big part of building a complex, multicellular
    body is getting these differentiated
    (specialized) cells into the right place at the
    right time as the animal develops from embryo to
  • How is this accomplished?

Metazoan Development
1. Cleavage Transforming the embryo from a
single, fertilized egg into a body with billions
of cells
Metazoan Development cont.
2. Formation of the mouth
Metazoan Development cont.
3. Embryonic Tissue Layers Diploblatic vs.
Metazoans with three layers can have a coelom
Formation of the Coelom
Metazoan Development cont.
  • 4. Symmetry
  • Asymmetry
  • Radial Symmetry
  • Bilateral Symmetry
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