- From Second Ed. James Darnell, Harvey Lodish, and David Baltimore.
Sciam Books Wh Freeman and Co, NY.
On pages 1064 to 1073 I reread what I had reread as I made these
comments on the methanogens and their progeny. On of the evolution
of cells graphs on page 1072 is very interesting. It shows that
introns evolved precellular. And then it shows the three main
branches where single celled eukaryotes have many introns in their
genes and heterotrophism (our precursor). Heterotropic denotes a
form of meiosis (cell division) in which the chromosomes split at an
early period, the halves remaining united at the ends and opening out
into rings, each of which represents two chromosomes. Eubacteria and
Archaebacteria have autotrophism, and then it says specifically under
Archaebacteria -- "Extra DNA lost".
On page 1073 it says, and I quote: "The proposed route of evolution
multifarious eubacteria and archaebacteria species arose originally
in tandem with the eukaryotic progenitor. The present-day eubacteria
and archaebacteria now largely, though not completely, lack introns
and are designed for rapid growth . . . "
It gets better. ON page 1063-7 it reads as a block quote from the
"A Reconstrutive Analysis of Cell Lineages
Discusion about precellular chemistry may forever remain
speculative. But what about using he techniques of paleotologists,
that is fossil studies, to investigate the first cells?
Fossilized spherical objects discovered in Austrilia nad South Africa
in sedimentary rock formations taht are 3-3.5 billion years old are
widely interpreted to be remains of cells. Hoever, the basis for
indentifying thes imprints as those of microorganisms is entirly
morphologic. The sizes and shapes of of the "micro-fossils"are
similar to those of present day cyanobacteria. Some of the
structures are doublets, possibly indicating cell division. The most
convincing reason for identifying these impressions in ancient rocks
as fossils of microorganisms is that very similar structures called
stromaltolits are being laid down today in sedimentary rock
formations where ocean sediments preciptates around colonies of
cyanobacteria and other bacteria.
The current estimate of the earths age is 4.5 billion years years; if
the ancient stromatoltes were in fact living cells 3-3.5 billion
years ago, they must ahve been among the earliest cells.
Unfortunately, there is littel we can learn from the microfossil once
we have measured its size and apparent thickness of its cell wall.
It is unlikely that we will ever be able to compare the protin or
gene structures of these prsumed early cells with their possible
counterparts amoun presnet-day cells.
Comparison of the nucleic acid sequences of different present-day
organisms is by far the more promising approach to discovering
relationships that might have existed at the beginning of evolution.
Throughout the course of this book we have mentioned many protiens
whose amino acid sequences are recognizably similar in a variety of
organisms even including present-day bacteria and eukaryotes. This
is true for stretches within RNA polymerases, and translation factors
like EF-TU and or the reverse transcriptases just discussed as well
as many other proteins. So it is a cmopletely reasonabel assumption
that the earliest gene pool has been retainin mutated form to some
extent in all living cells.
The issue to which classical evolutionnist and now molecular
evolutionist are attracted is whether valid branching evolutionary
trees that project back to the first cells can be constructed from
comparative sequence data.
The first Nucleic acid sequences available for such comparisions were
the tRNAs. With the sequence of these short molecules for comparison
it was possible only to proup bacteria togehter and eukaryotes
together. Ths obvous does not settle the linage arrangements between
different types of cells.
Ribosomal RNA Comparisons Show Three Ancient Cell Lineages.
However, all cells also have two ribosomal RNAs that are much longer
in sequence and therefore afford a much better basis for comparision
of relatedness between groups of organisms. Even before complete
sequences were available for ribosomal RNAs, comparisons were made
between the oligonucleotide catalogs from the ribosomal RNAs of
different species. Thsi lead to a majore and surprising conclusion,
now fully confirmed by the complete rRNA sequences of several hundre
organisms: there are three major lineages of cells in presnt-day
organisms, each of which is distinct and euqlly different from the
other two. Therefore, no conclusion that one was earlieir than
another is acceptable.
The three linages are archaebacteria, eubacteria, and eukarotes. The
eubacteria (from the Greek eu, "good" or "normal") were so named
because they come from the unusual habitats that originally suggested
they might be more primitive then any other cells. These includ
halophilic (salt loving) bacteria that can live in environments of
0.5-3.M NaCl or KCl, methanogenic (methane-producing) bacteria, and
sulfur-metabolizing bacteria that live in hot springs and in the
vents on the sea floor under conditiosn of great heat and pressure.
The rRNSs from eubacteria, archae-bacteria, and eukarytes have the
same basci domains in their folded stem-loop structures, indicating a
common origin. But the sequence comparisons clearly show three
groups of organisms. For example, single-cell eukarotic organisms
such as baker's yeast are much closer to humans than they are to
hundreds of different kinds of bacteria. On the baiss of rRNA
sequence comparisons, Carl Woese proposed that the three present-day
linages of cells must have had an earlier common ancestor that he
labeled the "progenate". It is possible that instead of being a
single ancestral organism the progenate might signify the dividing
line before cells as we known them existed. Perhaps each of the
three present day linagesemerged from a noncellular precursor pool of
the primitive precellular genetic apparatus. Or it is possible that
an early, barely competent, gluggishly growing organism, no longer
extant, gave rise to all three present-day linages of rganisms.
Settling this isue may be difficult, but one most important message
is clear from the ribosomal RNA comparisons: the eukaryotic nuclear
ribosomal genes (and therefore presumable most of the nuclear genes)
are not a direct decendant of any know prokaryote.
A more detailed look at microbiral phylogeny arranged according to
comparisons fo rRNA sewuences shows general accord with arrangements
basded on metabolic properteis. For example, as teh primitive earth
had an atmosphere with no oxygen, the firs bacteria were presumably
anaerobes; the clostridia, a large group of anaerobes, are very
distant from teh aerobic species such s E.coli and various bacilli,
and they vary considerably among themselves as befits a very ancient
Among the most controversial but provocative interpretations of rRNA
sequences is that concerning organisms that are all grouped togeher
as archaebactria. First, there is considerable difference in rRNA
sequence between halobacteria and the methanaogens (the major group
of archaebacteria) on the one hand and a small group of very
thermophilic, sulfur-metablozing organisms. James Lake has argued hat
based on rRNA sequences a coherent group of ancient organisms
(including Thermoproteus tenax, Desulfurcoccus mobilis, and
Sulfolobus solfataricus should be seperated from archaebacria. In
addition, this small group has rRNA transcription unites seperated
from 5S transcription units and their rRNA coding regions do not
contain interspersed tRNAs as do those of many eubacteria nd som
archaebacteria. In this group there is as great a sequence
similarity to eukaryotes as to archaebactria. Furthermore, among
this group there are organisms with introns in tRNA (S. solfataricus)
and in the larg rRNA. Lake calls this group of orgnisms eocytes. Ths
group does, however, share with other archaebacteria some unusual
metabolic properties such as haveing teraether libids in their
membranes. Furthermore, in both subgroups have polymerases with
multiple subunits as eukaryotes do. Finally, some eubacteria also
have seperate 5S-rRNA transcriptin units as do the preposed eocytes.
Thus the archaebactrial classification for all these organissm is
Obviously, further sequencing not only of rRNA genes but of genes
encoding ribosomal proteins and other portiens found in all organisms
should help sort out whether archaebacteri or eocyts or some more
distant organism was the prognitor to eukaryotes or whether, as seems
more likely, a more ancient progenitor, no longer extant (the
"progenote"), was the ancestor to all types of presnt-day cells.
Again, for present purposes the most important point is that
eukaryotic nuclear linage is an ancient one, not one decended only
1.5 bilion years ago from an already established bacterium, as has
been taught in most biology courses for the last half century. "