The "protozoa" [was Brain-Mind: Know Thyself!]

RoyBoy aphycho at usa.net
Mon Jun 25 22:41:02 EST 2001

Cool!  I have nothing in return to give accept my thanks...
and an open offer to assist in dissecting Blade Runner,
if that ever strikes your fancy.


"Smiles free.  Do you want fries with that?"

"If the Truth is dynamic; how will it ever be found?" - RoyBoy


Back in the "old" days (when I went to school), there were
two kingdoms, plants and animals.  The protozoa were the
single celled animals.

Around the 1950's,  advances in our understanding of cells
and biochemistry led to the development of the five Kingdom
picture of living things.  First, there are two distinct types of
cells -- eukaryotic and prokaryotic.  The prokaryotes (bacteria)
formed one Kingdom (Monera).  The animals, the plants,
and the fungi formed three Kingdoms of multicellular eukaryotes.
And the single celled eukaryotes became the Kingdom Protista.
These included both plant-like things (algae), animal-like things
(protozoa), and fungi-like things (slime molds).

Later in the 20th century, several developments occured. First,
the increasing awareness (actually, the increasing acceptance
of the fact) that the prokaryotic cell was two different kinds of
cell.  This lead to what is now finally so well accepted that it
has reached into the introductory texts -- the three "domain"
picture:  the Eubacteria (the "true" bacteria),  the Archaea
(a very different group of "bacteria-like" organisms), and the
Eukarya (including the four Kingdoms of eukarotic cells).

Second, the increasing desire to have the classification system
truly reflect what we believe to be the evolutionary relationships
between things combined with using genetic analysis (both
DNA and RNA) to measure evolutionary "distances" gave rise
to increased pressure to have "cleanly defined" (i.e, monophyletic,
to be technical) groups .  That is, a "properly" defined grouping
should include all the descendants from a single common
ancestor.  Or, if you consider the "tree of life", make a clean
cut across one major limb and pull it away from the rest
of the tree.  All of the small branches that get carried away
should have correspond to one "named" group of organisms.
(OK, the argument gets really cloudy around here, but that
is the way things are!)

In any event, lots of little pieces were thrown out of the fungi,
the plants and animals to make them relatively cleanly defined
groups with a single common ancestor.  And all the garbage
ended up in the "junk pile" called the Kingdom Protista.

To make a long story just slightly less long, what used to
be called "protozoa" because they were all single celled
and had "animal-like" characteristics (except for some like
the parasitic forms) turned out to be a hodge-podge with
very different evolutionary origins and virtually no
relationship with each other.  And the amount of diversity
within the "protozoa" as measured genetically was enormous.
A human is far more similar to a corn plant or to a mushroom
than two different protozoans are to each other.

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