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 time?

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

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.

• For a currently unknown combination of reasons, ice sheets spread from high latitudes towards the equator.  • 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 equator. 

Stage 2: Snowball Earth at It’s 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.

• 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.

Snowball Earth cont.

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 freeze.

• 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 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.

• 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.

Snowball Earth cont.

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 continue.  • Perhaps living things survived near hot springs on land. 

Interestingly, after the last of these Snowball events, multicellular animals expand dramatically. 

• 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 adult. 

• How is this accomplished?

Metazoan Development1. Cleavage

Transforming the embryo from a single, fertilized egg into a body with billions of cells

Metazoan Development cont.

2. Formation of the mouth

3. Embryonic Tissue Layers: Diploblatic vs. Triploblastic

Metazoan Development cont.

Formation of the Coelom

Metazoans with three layers can have a coelom

Metazoan Development cont.

4. Symmetry

• Asymmetry

• Radial Symmetry

• Bilateral Symmetry