Origin of Multicellularity

A.  Cell Division
     1.  Nuclear Division
          a.  Mitosis--replication of nuclear material
          b.  Meiosis--halving of nuclear material
          c.  Both processes can result in a number of nuclei
     2.  Cytoplasm division--cytokinesis
          a.  Does not necessarily accompany nuclear division.
     3.  Cells can be come multi-nucleate if no cytokinesis takes
          place.  Some organisms, such as the vegetative parts of
          fungi are syncitial.  There are no cell membranes in the
          hyphael filaments and thus the hyphae are syncitial.
     4.  In some organism, eg., the foraminifera, nuclear division
          takes place to produce a large number of nuclei.  Later 
          cytoplasm accumulates around the nuclei and they are 
          released and depending upon their nulear state, either
          combine to produce a new organism or become a new 
          organism. 
B.  The idea of cell adhesion
     1.  Between many different materials, there is an adhesion or
          cohesion, eg., miniscus in a cylinder.
     2.  When cells divide, they might remain in contact with each
          other before they actually separate.
     3.  In addition to simple adhesion, in multicellular creatures
          there is a intercellular cement. 
C.  Advantages of larger size
     1.  Can eaten and not be eaten
     2.  Can grow in size by proliferation of cells
          a.  Most animals and plants grow this way  There are a
               few groups, rotifers and nematodes, to mention two
               that grow by increase in cell size rather than cell
               number
     3.  If all cells stay within the size limit of 1 mm diameter,
          then diffusion can still be a process to take materials
          between cells and into cells.
     4.  Affords the creature a division of labor amongst cells,
          i.e., there can be cell specialization.
     5.  Built in redundancy in case of failure. 
D.  Some examples of protists that have multiple cells
     1.  Choanoflagellates--Creatures have a collar out of which
          two flagellae extend.  Collar is used for food getting 
          purposes.
     2.  Some ciliates form chains as they divide.
     3.  Volvox is a photosynthetic protist where there is a 
          differentiation amongst the cell-types into reproductive
          and non-reproductive. 
E.  The concept of the five kingdoms
     1.  Proposed by Lynn Margulis and others.
     2.  Kingdoms include:
          a.  prokaryotes
          b.  protists
          c.  fungi
          d.  plants--metaphyta
          e.  animals--metazoa
     3.  Groups d and e are multicellular, whereas fungi are
          syncitial in vegetative stages, but may be cellular in
          reproductive structures. 
F.  Timeline for multicellularity
     1.  Perhaps about a billion years ago.
     2.  Fossil record shows dramatic change about 600 million
          years ago
     3.  Fossils found before that time are blobs.
          a.  Ediacara fauna--named after location in Australia
               fossils primarily consist of jellyfish, worms, etc.
     4.  Fossils found after that time are highly structured
          b.  Burgess shale fauna--named after location in British 
               Columbia.  Creatures of all kinds, including worms,
               aarthropods, etc.
     5.  What brought on multicellularity, what brought on increased
          structure.
          a.  One thought is the change in the atmosphere from 
               reducing to oxidizing.  Change came about by the
               evolution of photosynthesis (in the prokaryotes
               first).  Animals responded by developing oxidative
               phosphorulation as a way of avoid oxygen as a 
               cellular poison.
          b.  Changes in carbon dioxide levels may have facilitated
               the deposition of calcium carbonate in exoskeletons
               that then provided structural form and strength.