PROGRESS IN PROTOZOOLOGY

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those times PROTOZOA were regarded as unicellular eukaryotic organisms
which gave rise to METAZOA. The origin of the multicellular
animals was explained differently by various authors, starting with
the gastrea theory of E. H a e c k e 1. However, the most common of
the theories proposed through the years was that METAZOA originated
from the unicellular eukaryotes, PROTOZOA.
Origins of the protozoan taxa were considered first in the pioneering
works of Butschli (1884, 1885, 1887, 1889). The idea of a monophyletic
origin of the PROTOZOA was the basis of all theories on their
evolution. Either the naked amebae or the flagellates were considered
by different workers as the ancestors of all other eukaryotic groups,
including PROTOZOA.
During the last two decades, the originally simple scheme has undergone
numerous drastic changes. The great progress in biology, especially
in the fields of electron microscopy, molecular biology, and biochemistry,
necessitated major revisions of theories previously believed
to be unassailable. Among the questions which have arisen as a result
of recent developments is whether the PROTOZOA constitute a natural
group. This question is associated with the concept of the origin of
eukaryotes and with that of their mono- vs polyphyletic evolution.
The elucidation of these problems in a manner which would take
into account the vast body of information available in published works
would necessitate writing an entire book. Such an approach is, of course,
beyond the scope of my short presentation. This presentation is intended
merely to open a discussion. Because of time limitations, some
of the opinions I shall express cannot be illustrated by supporting factual
evidence; therefore, my assertions may sound somewhat dogmatic.
The first problem I shall address pertains to the kinds of relationships
existing between Prokaryota and Eukaryota. The question is whether
these two main forms of organisms evolved independently of each
other or if the latter found their origin in the former.
Both of the aforementioned views find adherents among scientists;
I am inclined to support the concept of prokaryote ancestry of the eukaryotes.
Indeed, although there exist obvious differences between the
two groups of organisms, their most important vital processes are essentially
the same. Among the processes shared by the pro- and eukaryotes
are: storage, replication, transcription, and translation of hereditary
information by DNA and RNA. The common origin of Pro- and
Eukaryota is supported also by their similar batteries of enzymes mediating
various metabolic processes. The results of genetic engineering can
be used as additional evidence in support of the common origin of the
two kinds or organisms, e.g., it is known that some eukaryotic genes
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