Biomedical Engineering Reference
In-Depth Information
no-one can say with certainty how these genetic attributes came to be shared
or when each class arose. With this schema, there is a tacit acknowledgment
that we do not know very much about the origins of life on earth.
In this topic, we will not go into any of the arguments for or against any
of the “origin of life” hypotheses. Readers can accept the observation that
every serious evolutionary biologist is obsessed with the minutiae of these
arguments; and rightly so. Biologists believe that the cell is a living topic,
recording the history and predicting the future of life on earth [20]. It is
the responsibility of biologists to correctly interpret the contents of the topic
of life.
One of the most influential contributors to the field of molecular phylo-
genetics of bacteria is Carl Woese, who refined his earlier observations by
using ribosomal RNA sequence variations as a biologic chronometer. By
observing changes in the sequences of ribosomal RNA, he determined the
branchings, over time, that occur during the evolution of organisms. Though
he was looking at sequence similarities, his observations were focused on
those similarities that revealed the sequential phylogenetic steps of bacterial
evolution [21]. Furthermore, he built his classification on a pre-existing
nomenclature that defined organisms on a species level.
Prior to the work of Woese and others, in the 1990s, there was little hope
that the bacteria could be classified sensibly into a phylogenetic hierarchy.
It was known that bacteria exchange genetic material horizontally, from one
species of organism to another. During the several billion years of bacterial
evolution, it was likely that primitive organisms merged with one another.
There is every reason to believe that early viruses pulled and pushed frag-
ments of DNA among many different bacterial organisms. It is even possible
that native molecules of DNA, formed in the primordial soup of ancient
earth, were copied and shared by members of different branches of evolving
organisms. Biologists expected that these promiscuous exchanges of genetic
material would have created a collection of different bacterial organisms for
which a strict geneologic classification was impossible to construct. Indeed,
generations of microbiologists tried and failed to produce a credible classifi-
cation for bacteria. Because most bacteria fall into a narrow range of form
and size, taxonomists lacked a sufficient set of morphologic features from
which they could define and distinguish classes of organisms. They settled
for a systematic grouping of bacteria based on a few morphologic features:
dichotomizing staining properties (Gram positivity and Gram negativity),
and a set of growth characteristics (e.g., nutrient requirements and the
expression of traits expressed by colonies grown on various substrates).
It came as a great surprise to many when Woese and others developed
a sensible classification of bacteria based on their analyses of a single
molecule, rRNA. It remains to be seen whether Woese's one-molecule
classification will withstand further scrutiny [22].
