Biomedical Engineering Reference
In-Depth Information
not hold a monopoly on extremophilic prokaryotes; some eubacterial species
live in extreme environments (e.g. the alkaliphile Bacillus halodurans).
The third major class of organisms, the Eukaryotes, is distinguished by the
presence of a membrane-bound nucleus.
For many years, archaean species were considered just another class of
bacteria. This changed in 1977. Woese and Fox had been studying ribosomal
RNA. Because ribosomal RNA is a fundamental constituent of all living
organisms, sequence comparisons in the genes coding for ribosomal RNA
are considered a reliable way to estimate the degree of relatedness among
organisms. In 1977, Woese and Fox surprised biologists when they demon-
strated profound differences in the sequence of ribosomal RNA that distin-
guished archaean species from all other bacteria [17]. Much more shocking
was their finding that the sequence of archaean ribosomal RNA was more
closely related to eukaryotic cells than to other bacterial cells. Other sources
have since shown that the archaeans share with the eukaryotes a variety
of features that are lacking in the eubacteria. These include the presence
of histones in some archaeans, the manner in which DNA is replicated and
organized, and the finding that archaeans and eukaryotes share several tran-
scription factors (proteins that bind to DNA and control the transcription of
DNA to RNA) [18]. Woese and Fox proposed that the archaeans (then called
archaebacteria) comprised a kingdom, separate but equal to the bacteria.
That paper, and the many contributions of Woese that followed, sharpened
our understanding of terrestrial biology and sparked a controversy that shook
the foundations of taxonomic orthodoxy [14,19].
The solar system (sun, earth, moon, planets, and meteorites) all seem to
have formed about 4.5 billion years ago. The first living organisms appeared
about 3.8 billion years ago. Here are the hypothesized roots of terrestrial life,
visualized through indentation:
Class Archaea
Class Eubacteria
Class Eukaryota
The first organisms to appear on earth, about 3.8 billion years ago, were
Class Archaea, the root organism for the Tree of Life. From Class Archaea
came Class Bacteria and Class Eukaryota. The argument for this particular
schema is that many archaean species observed today are extremophiles
that thrive under environmental conditions that are similar to the conditions
on earth about 3.8 billion years ago. The members of Class Eubacteria
are better-suited for a temperate, oxygen-rich atmosphere, such as we have
today. Presumably, the eubacteria evolved after the archaeans appeared,
from archaean ancestors. The eukaryotes, like all other living organisms
on earth, replicate and transcribe DNA and translate RNA into proteins.
Key structures of the genetic machinery of eukaryotes (chromosomal
proteins and ribosomal RNA) are more similar to archaeans than to the
