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

Richard Norman rsnorman at mediaone.net
Mon Jun 25 21:52:25 EST 2001


"RoyBoy" <aphycho at usa.net> wrote in message
news:wWRZ6.116689$r7.15594782 at news1.busy1.on.home.com...
>> Richard:
>> ***
>> But the point is that the whole notion of "protozoa" -- singular
>> or plural, no matter how you spell it or latinize or Englishify
>> it -- there is no such thing.
>> ***
>
>RoyBoy
> When did this happen?  5,6,7 years ago?
> I guess with analysing DNA?  Or did some other
> new examination techniques come to light?
>
> Main Entry: pro.to.zo.an
> Pronunciation: -'zO-&n
> Function: noun
> Etymology: New Latin Protozoa, from prot- + -zoa
> Date: circa 1864
> : any of a phylum or subkingdom (Protozoa) of chiefly motile
> and heterotrophic unicellular protists (as amoebas, trypanosomes,
> sporozoans, and paramecia) that are represented in almost every
> kind of habitat and include some pathogenic parasites of humans
> and domestic animals
> - protozoan adjective
>
> They have a lot of variety...but aren't they still relatively simple?
> But something at the back of my mind is telling me that we have
> a large proportion of matching DNA with bacteria and such.
>
>> Richard:
>> ***
>> There used to be a group of organisms with that name.  But
>> the more we learned about them, the more we realized that
>> the grouping was entirely artificial.  It is like talking about
>> "macrolife: all organisms larger than a bread box" which
>> includes adult humans and mature trees and some fungi
>> but not newborn babies (I have a big bread box) and
>> seedlings and most fungi.  Except that the idea of "protozoa"
>> is even less tenable than the idea of "macrolife".
> ***

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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