PROGRESS IN PROTOZOOLOGY

244 D. L. NANNEY
Basing their analysis on the primary structure of the ribosomal ribonucleic
acids, these workers have stretched our understanding of evolutionary
events back almost to the beginning of life on earth, to the origin
of the ancestor of modern molecular information-processing systems.
All life forms use essentially the same nitrogenous bases in their DNA
and RNA, the same amino acids in their proteins, the same ribosomal
apparatus and the same genetic code to translate molecular information.
No stronger argument is available for the unity of origin and universality
of design of all forms of life than is found in the commonalities of the
ribosomal apparatus.
But the ribosome, although conservative, is not invariant. Subtle
differences can be found among the ribosomal molecules of different
organisms. Organisms of similar appearance, showing close relationships
by more conventional criteria and through the fossil record, also show
few differences in their RNA sequences. Organisms judged to be of
more remote common ancestry have more sequence differences. If one
assumes that sequence differences are a measure of evolutionary distance,
even beyond the limits of conventional taxonomic criteria and paleontological
records, one can reconstruct the previously hidden first
half of the history of life.
This analysis stretches back to the "progenote", this first (or most)
successful self-perpetuating information-processing machine, that managed
to become the granddaddy of us all. The time of origin of the ribosome,
and its bearer, is still uncertain, but geochemists are progressively
moving back their estimate of the time when life began to leave
chemical traces in the earth and the atmosphere. Some now estimate
that life was present in quantity some 3 1/2 or 4 billions of years ago,
within 1/2-1 billion years after the origin of the earth. The ribosome is
arguably as ancient as that. The ribosomal analysis also shows an unexpected
evolutionary cleavage among the organisms previously lumped
together as bacteria, or prokaryotes. The W o e s e group shows two
distinctive phylogenies — that of the common eubacteria and the much
rarer archaebacteria, which may have been common in earlier times,
but which are now driven into relict habitats, in salt marshes and ocean
depths. The ribosomal apparatus of eukaryotic cells apparently came
from the same ancient source, and its sequences also drifted slowly
through the long eras before the appearance of modern eukaryotes.
This molecular reconstruction of early evolutionary history, though
as yet skimpy in detail, reveals the power of the technique. It also
leads to new perspectives on the origins of the eukaryotes, and renders
obsolete many of the perennial systematic arguments corncerning the
eukaryotic protists. We cannot yet answer all the questions concern-
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