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Summary Microbial evolution and systematics
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HC11 Microbial Evolution andSystematics (BOOK)
Chapter 13.1-13.10
CH13 Microbial Evolution and Systematics
13.1 Formation and Early History of Earth
Earth formed about 4.5 billion years ago. And the inhospitable conditions (molten surface and
asteroids bombardments) are thought to have persisted for over 500 million years. Water originated
from volcanic outgassing of the planet’s interior and from innumerable collisions with icy comets and
asteroids. However, given the earth’s heat at the time, water would have been present only as water
vapour. Liquid water is a requirement for life, and it occurred about 4.3 billion years go.
One hypothesis holds that life may have originated at hydrothermal systems on the ocean floor. The
stability nearby these systems is higher and hydrogen and hydrogen sulfide would be more avaliable
at these hydrothermal systems.
Molecules of RNA were likely a central component of the first self-replicating systems and it is
possible that life began in an RNA world; is a component of essential cofactors and molecules, can
bind small molecules, can possess catalytic activity and can catalyse protein synthesis.
The earliest cellular forms of life likely possessed elements of this
three-part system of DNA, RNA and protein.
The last-universal common ancestor (LUCA) of al extant life likely
existed at 3.8-3.7 billion years ago —> bacteria and archaea
became distinct and diverged.
https://www.culy.nl/recepten/menugang/diner/
During most of its lifetime, the earth was anoxic. During this era
CO2 may have been the major source of carbon and H2 the major
fuel for energy metabolism.
13.2 Photosynthesis and the Oxidation of
Earth
Stromatolites are fossilised microbial formations and provide the earliest conclusive evidence of life
on earth.
Phototrophic organisms use energy from the sun to oxidise molecules and to synthesise complex
organic molecules from carbon dioxide or simple organic molecules.
Earth’s first phototrophs were anoxygenic (do not produce oxygen) but from these evolved
Cyanobacteria which were the earliest oxygenic phototrophs.
The ability to use solar radiation as an energy source allowed phototrophs to diversify extensively.
This supported the evolution of a photosystem capable of oxygenic photosynthesis which used H20
as a reluctant for CO2 and produced O2 as a waste product.
By 2.4 billion years ago, the O2 levels has risen to 1 part per million which was enough to initiate the
Great Oxidation Event.
The O2 produced by Cyanobacteria reduced minerals containing Fe2+ to iron oxides containing Fe3+
which rained down on the seafood forming banded iron formations.
, An important consequence of O2 for the evolution was the formation of ozone (O3), this creates an
ozone shield which is a barrier that protects the surface of earth from much of the UV radiation from
the sun.
13.3 Living Fossils: DNA Records the History of Life
We can reconstruct the evolutionary history (phylogeny) of a set of related DNA sequences by
analysing similarities in their nucleotide sequences.
Carl Woese was the first to construct a universal tree of life that the inferred from nucleotide
sequence similarity in the ribosomal RNA (rRNA) genes of organisms. At least 60 genes are shared by
nearly all extant cells and these genes must have been present in LUCA. Most of these genes encode
core functions in transcription, translation and DNA replication.
Eukarya show greater similarity to those of archaea than to those of bacteria.
However there are many examples of genes shared by any of two of the three domains which is
possibly due to horizontal gene transfer —> genes that provided a strong benefit may have been
transferred rapidly among early forms of life.
13.4 Endosymbiotic Origin of Eukaryotes
A well-supported explanation for the origin of organelles in the eukaryotic cell is the endosymbiotic
hypothesis:
A – shows the serial endosymbiosis hypothesis
B – shows the symbiogenesis hypothesis
The overall physiology and metabolism of mitochondria and chloroplasts and the sequence and
structures of their genomes support the endosymbiotic hypothesis. For example a 16S ribosomal
RNA (16S rRNA).
It seems clear that the modern eukaryotic cell genome is a genetic camera, made up of genes from
both bacteria and archaea.
13.5 The Evolutionary Process
Mutations are random changes in DNA sequence that accumulate over time. Mutations take several
forms including substitutions, deletions, insertions and duplications.
Recombination is a process by which segments of DNA are broken and rejoined to create new
combinations of genetic material. Homologous recombination requires short segments of highly
similar DNA sequence flanking the region of DNA being transferred. Nonhomologous recombination
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