Chapter 19
Microbial Taxonomy,
Evolution and Diversity
1
Microbial Evolution
• Planet earth is estimated to be 4.5 –
4.6 billion years old
• First direct evidence of cellular life
discovered in 1977 in the
Swartkoppie chert.
– microbial fossils ~3.4 billion years old
2
Figure 19.1
3
The First Self Replicating Entity:
The RNA World
• Pre-cellular life may have been an RNA
world because of the capacity of RNA to
both replicate and catalyze chemical
reactions ((ribozymes)
• RNA could have given rise to
structurally similar double stranded
DNA
4
More Evidence supporting the
RNA World
• The energy currency of the cell is
ATP, a ribonucleotide
• RNA can play a role in gene
expression
5
Evidence used against the RNA
World hypothesis
• The early hot, anoxic atmosphere on
earth would prevent the stable
formation of RNA precursors
• RNA is not a stable molecule
• Ribozyme involved in RNA replication
has not been found
6
Early Cellular Life
• FeS-based metabolism used by some archaea
may be remnant of early form of chemiosmosis
• Photosynthesis also thought to have evolved
early in Earth’s history
– fossil evidence places evolution of cyanobacteria and
oxygenic photosynthesis to ~3 billion years ago
– what appear to be fossilized remains found in
stromatolites and sedimentary rocks
• stromatolites – layered rocks formed by incorporation of
mineral sediments into microbial mat
7
Figure 19.2
8
The Three Domains of Life
• Carl Woese and George Fox using the
nucleotide sequence of the small subuit
ribosomal RNAs (rRNAs) determined
that all living organisms belong to one of
three domains
– Archaea
– Bacteria
– Eucarya
9
Table 19.1
10
The Evolutionary History of
Microbes
• The universal phylogenetic tree
– based on the rRNA sequence from three
domains of life
– evolutionary relationships based on rRNA
sequence comparisons
– the root of the tree suggests that the three
domains have a single common ancestor, but
Archaea and Eucarya evolved independently
of the Bacteria
11
Figure 19.3
12
Genome Fusion Hypothesis
• Attempts to explain evolution of the
nucleus
• Claims the combining of certain
archaeal and bacterial genes resulted in
the formation of a single eucaryotic
genome
• Origin of nucleus is still unresolved
13
The Endosymbiosis Hypothesis
• Claims that endosymbiosis was
responsible for the origin of
mitochondria and chloroplasts
– both organelles have bacteria-like
ribosomes
– most have a circular chromosome
14
Mitochondria
• Believed to be descended from an aproteobacterium
– became engulfed in a precursor cell
– provided essential function for host
• engulfed organism thought to be aerobic,
thereby eliminating oxygen toxicity to the
host cell
• host provided nutrients and a safe
environment for engulfed organism
15
More About Mitochondria and
Chloroplasts
• Engulfed organisms
– endosymbionts which evolved into
mitochrondria
• Chloroplasts are also thought to
have evolved from endosymbionts in
a similar process
16
Hydrogen Hypothesis
Another endosymbiosis Theory
• Asserts that the a-proteobacterium endosymbiont
was an anaerobic bacterium that produced H2 and
CO2 as fermentation end products
– hosts lacking external H2 source became
dependent on endosymbiont which made ATP
by substrate level phosphorylation
– symbiont ultimately evolved into a
mitochondrion or a hydrogenosome (organelle
found in protists that produce ATP by
fermentation)
17
Hydrogenosomes of Trichomonas
vaginalis
Figure 19.4
18
Serial Endosymbiotic Theory
(SET)
• Put forth by Lynn Margulis and
colleagues
• Suggests eucaryotes evolved in a
series of discrete endosymbiotic steps
starting with motility and followed
by nuclei and mitochondria
19
Evolutionary Processes
• Anagenesis (microevolution)
– small, random genetic changes over
generations which slowly drive either
speciation or extinction, both of which are
forms of macroevolution
• Punctuate equilibria
– a phenomenon caused by the slow, steady
pace of evolution being periodically
interrupted by rapid bursts of speciation due
to abrupt environmental changes
20
Figure 19.5
21