Changed View of the Biosphere
This view of the biosphere has dramatically changed in the last decade
with the advent of molecular taxonomy and phylogeny. The basic idea behind this approach is that there are some
molecules common to all Earthly life (16 S ribosomal RNA, for example), and that,
if one could sequence such molecules and compare the sequences, it might be
possible to use this chemical information to compare all life, even that which
can be seen only with a microscope.
While the germ of this idea is actually decades old, its demonstration
was realized only recently with new development of techniques in sequencing of
nucleic acids, and the use of this information for organismic comparisons.
The work pioneered by Dr. Karl Woese of the University of Illinois has
changed the way we look at life on Earth. From the point of view of the prokaryotes,
which lack features that can be used to compare them to each other or to the
eukaryotes, this molecular methodology allowed one, for the first time, to have
a sense of the phylogeny (a natural history which had been previously lacking)
of the various groups. Not only could the prokaryotes be compared
to each other, but also because the eukaryotes also contained these same
molecules, the comparisons could encompass all of the five kingdoms. The results of this approach were quite
dramatic: the four eukaryotic kingdoms were found to be quite homogeneous,
while the prokaryotes were found to be very diverse, and thus were expanded to
two separate kingdoms, referred to as Bacteria and Archaea (Figure 4).
A quick glance at the molecular tree reveals that the major genetic
variation among the eukaryotes is seen in the single celled protists, while the
three dominant kingdoms (plants, animals, and fungi) are actually clustered at
the end of the eukaryotic assemblage, and display only a modicum of genetic
diversity (see below). Apparently, it
is possible to achieve structural and behavioral diversity (traits that have
appeared only in the last 500 million years) while remaining genetically rather
homogeneous. Given that multicellular
eukaryotes evolved only recently, and that for nearly 3 billion years the
prokaryotes dominated the surface of the Earth, one should not be surprised
that the bulk of the apparent genetic diversity on the planet resides in this
group.
Figure
4. The three kingdoms of life. Based on sequence analyses of the 16 S ribosomal RNA gene, which
is contained in all Earthly organisms, it is possible to construct a molecular
phylogeny that can quantitatively compare all organisms, even those that can
not be cultivated in the laboratory.
Such an approach has yielded quite a different view of life, as shown
here, in which the major kingdoms of life (animals, plants, fungi, and
protists) shown in Figure 3, are grouped into one kingdom, and the prokaryotes
are expanded into two kingdoms, called the Archaea and the Bacteria.
Contributed by: Dr. Kenneth Nealson
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