Biomedical Engineering Reference
In-Depth Information
eubacteria. Hence, it is presumed that the eukaryotes evolved from an
archaean ancestor.
Here is another possible schema:
Class Archaea
Class Eukaryota
Class Eubacteria
This schema is much like its predecessor (see above), but the eubacteria
are not presumed to have developed from an archaean ancestor. Despite
its name, archaea, from the Latin root meaning “ancient thing,” there is no
evidence that Class Archaea is older than Class Eubacteria. Profound differ-
ences between the DNA replication mechanisms in archaeans and eubacteria
suggest that the two classes of bacteria may have arisen independently [15].
Alternately, Class Eubacteria and Class Archaea may have arisen from an
ancient ancestor that has been lost to antiquity [15], as shown here:
Class Bacteria (eventually to contain Class Eubacteria, Class Archaea)
Class Eukaryota
This schema was very popular prior to the work of Woese and colleagues,
and still has many adherents. Here, we imagine that the first living organ-
isms on earth were simple bacterial cells (cells without nuclei, also called
prokaryotes or monera). As organisms evolved, they exchanged nuclear
material somewhat promiscuously. Eventually two subclasses separated:
Class Archaea and Class Eubacteria. It has been observed that most of the
genes that code for basic cellular functions, such as enzymes, transport sys-
tems, cell wall synthesis, are quite similar in archaeans and in eubacteria [14].
This would suggest that the two classes of organisms shared a common
ancestor. After the two classes of prokaryotes were established (maybe a
billion years later), a third type of organism, characterized by a membrane-
covered nucleus, was created through a felicitous merger of two or more cells,
with contributions from Class Eubacteria and Class Archaea. This would
explain why eukaryotic organisms have features of both Class Eubacteria and
Class Archaea (discussed further in Chapter 15).
Alternately, as shown below, three classes of organisms may have arisen
by a mechanism that is unknown and beyond the pale of credible scientific
speculation.
Class Archaea
Class Eubacteria
Class Eukaryota
This agnostic schema gives Class Archaea, Class Eubacteria, and Class
Eukaryota an equivalent stature at the root level of living organisms; thus
removing precedence and ancestry for the top classes of life. There seems to
be no doubt that all three major classes share certain genetic attributes, but
