1
BUILDING THE PERFECT BEAST
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 time?
A Snowball Earth
Evidence is mounting for repeated, near total glaciations of the planet in the late
Proterozoic (~0.6 bya).
The paradoxical evidence:
1. Signs of glaciers at sea level within a few degrees latitude of the equator.
We know the rocks are from near the equator from the orientation of magnetic minerals.
Poorly sorted rocks (from clay to boulders in the same deposit) - poor sorting is common
in sediments at the bottom of glaciers.
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 that suggest it formed directly
from solution on the ocean floor.! The problem is that calcium carbonate typically
forms in warm regions, not at the cold poles.
The number of these events is debated, but there are at least two, and more likely four,
spread from 800 to 600 Mya.
Snowball Earth: Detail
1. For a currently unknown combination of reasons (lower solar intensity, position of the
continents, excessive removal of carbon dioxide [a greenhouse gas] from the atmosphere
by plants or rock weathering), ice sheets spread from high latitudes towards the equator.!
Ice is shiny, and sunlight bounces off it.!
Consequently, 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
equator.!
2
2. Computer simulations suggest that if the ice spreads to within! ~30° north or south of
the equator, this positive feedback runs away catastrophically, 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.
3. Climate modelers had assumed that, while such a catastrophic glaciation was possible
theoretically, it had never happened on Earth, because they didn't see how a planet in this
"ice house" state could ever recover.!
The recent work on the Proterozoic deposits points to the solution.!
Although the planet is iced over, with all of the water frozen out of the atmosphere,
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.!
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 until it reaches such a high concentration that
it traps enough sunlight to begin melting the ice.!
Water traps heat much better than carbon dioxide, so once the melting begins, and water
vapor gets back into the atmosphere, a second run away state takes over.!
4. This time we have a run away "greenhouse" warming.! The ice quickly melts back, and
the climate warms dramatically.!
All of the carbon dioxide previously 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.
Q. 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 continue.!
What is most intriguing is that immediately after the last of these Snowball events,
multicellular animals expand dramatically.!
3
All about Metazoans
Questions: 1) What are the costs and benefits of being multicellular? 2) How were
metazoans constructed? 3) What is the nature of the relationships among the main groups
of fossil forming metazoans?
Multicellularity: Benefits and Costs
Benefits:
Increased size: Diffusion of nutrients into the body limits how large a unicellular
organism can be.!
It is easier to get nutrients into, and waste products out of, a large body made of many
small cells.! If increased size is favored (bigger predators can eat a larger range of food;
bigger prey can be eaten by fewer organisms), multicellularity may be 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.!
This increased efficiency may sometimes provide a competitive edge.
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.
What are the 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.
!
By listing the benefits of multicellularity, I don't mean to imply that it is a "superior"
mode of life.!
Unicellular organisms have been present on the Earth for over 3.5 billion years, and they
are still going strong in nearly every conceivable (and some inconceivable)!environment.!
Still, multicellularity has arisen in eukaryotes independently over a dozen times, so it's
worth entertaining why it has happened repeatedly.
4
How are multicellular animals constructed?
We must consider two issues faced by metazoans that unicellular organisms don't
confront: Differentiation and Development
How do these processes occur in Metazoans?
Cleavage
Origin of the Mouth
How many embryonic tissue layers? Two (called diploblastic) - Endoderm and Ectoderm;
Three (called triploblastic) - Endoderm, Ectoderm, and Mesoderm
Body Symmetry: Asymmetric, Radial Symmetry, Bilateral Symmetry
!