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Unit 10 Notes
Evolution and Classification of Microbes
Evolution of Prokaryotes
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Prokaryote = an organism whose cells do NOT have a nucleus
The microbial world was completely missing from Darwin’s fossil record – this was because they
had no bones to be preserved, and what we could see all looked alike
It wasn’t until the 1960’s that we truly began to explore the role of microbes in the tree of life.
In 1965, scientists found fossilized remnants of microscopic single-celled organisms in rock
formations that were dated at 2 billion years ago!
To date, the oldest evidence of microbial life is found in rocks that date to 3.5 billion years old.
Scientists believe that microbes were present long before this 3.5 billion year date, but have not
found proof of this yet.
Scientists also believe that a photosynthetic bacterium, Cyanobacterium, was responsible for
changing the Earth’s early atmosphere from one of methane, carbon dioxide, nitrogen, and
ammonia to one rich in oxygen.
Microbes began to be truly classified when DNA was discovered. By comparing the number of
changes in a microbe’s DNA with another microbe’s DNA, scientists can deduce how closely related
they are. This is the approach they use to construct a “tree of life” for the microbial world.
Carl Woese, a scientist from the University of Illinois, created a new tree of life in the 1980’s by
analyzing ribosomal RNA from different species. He divided it into three main branches: Archaea,
Bacteria, and Eukarya.
Mr. Woese, who also discovered the Archaea branch on the tree, noticed that the hotter the
temperature at which bacteria could grow, the closer they were to the base of the tree of life. He
hypothesized that the Archaea were probably the first living cells.
Evolutionary Timeline
Scientists estimate the world to be about 4.5 billion years old. They also estimate that the bacteria, the FIRST
living organisms, appeared about 3.5 billion years ago. They were the only inhabitants of earth for about 2
billion years. Then, approximately 1.5 billion years ago, the eukaryotes began to develop.
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Carl Woese’s Tree of Life
Eukaryotic Evolution
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Eukaryote = an organism whose cells DO have a nucleus
About 1 billion years ago, scientists believe that eukaryotes began to evolve from prokaryotes.
These new cells, in response to evolutionary pressure, invented new things to keep their DNA more
organized and their cells running smoother: a nucleus, sexual reproduction, and numerous
organelles.
Scientists, after inventing more powerful microscopes, realized that certain organelles inside of
eukaryotes resembled bacteria, including the mitochondrion and the chloroplast. Some scientists
hypothesize that eukaryotic cells arose when bacteria took up residence inside of a larger cell. This
is called the endosymbiotic theory.
Endosymbiotic Theory
Let’s examine how a mitochondria and a chloroplast may have their beginnings:
Mitochondria – the evidence scientists have that points to a mitochondrion being an entrapped bacteria is this:
mitochondria have their own circular piece of DNA inside them, like inside a bacterium; and also they have
their own cell machinery for making RNA, just like bacteria. They also reproduce independently from the cell
itself through binary fission, just like a bacterial cell. It is thought that this began as a symbiotic relationship
between a bacterium and another cell: the invaded cell gets the energy produced by the invader, and the invader
gets a steady food supply and a safe haven. Look at the picture below:
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Chloroplast – scientists believe that chloroplasts were once photosynthetic bacteria that came to live within a
host cell for the same reasons listed above about mitochondria, except they believe that these photosynthetic
bacteria were once eaten as food by the host cell, but were not digested and remained in a symbiotic relationship
with plant cells!
Nomenclature
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Bacteria, Protists, Fungi, Helminths - The system of names used for biological classification is called
nomenclature. Carolus Linnaeus was the scientist that first came up with the system. Remember the levels
of nomenclature from your Biology class:
Kingdom – Phylum – Class – Order – Family – Genus – Species
We utilize a system of binomial nomenclature to name bacteria, which consists only of the genus and species
name. When writing this, you write the genus name first, capitalized, followed by the species name. Many
times, the genus name is abbreviated to just the first letter. The whole word is italicized. Here is an example:
Escherichia coli
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or
E. coli
Viruses - the only exception to this naming rule – since they are not living and do not fall into one of the
three main domains listed in the tree of life above, they are named differently. They are called by their
common virus name, for instance, the flu virus is called Influenza virus.
Evolution on Fast Forward
As scientists tried to construct the tree of life for bacteria, it was difficult because they had conflicting data. It
was generally assumed, until this point, that genes were ONLY transferred from parent to offspring. But the
microbes had a surprise in store for us! Microbes have TWO methods of gene transfer:
1. Vertical Gene Transfer – when genes are passed from one microbe to another through the normal
division process – binary fission. See below:
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2. Horizontal Gene Transfer – when microbes pass genes to their neighboring microbes, like
handshaking! A microbe can take DNA from another microbe in a few ways (discussed next unit),
thereby gaining new genes and therefore new capabilities. See below:
Microbes’ use of both horizontal and vertical gene transfers may explain why scientists had such a hard time
constructing their tree of lineages.
Gene sharing among the various members of the microbial world gave them a tremendous advantage in rapidly
adapting to changes in their environment. It is said that the two phenomena most responsible for the
enormous biologic success of microbes on our planet are:
o The ability to reproduce so quickly
o The ability to share genes.
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