BIO 1130 Lecture Notes - Lecture 6: Chitin, Mesozoic, Gram-Negative Bacteria

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7 Aug 2016
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BIO1130 - Lecture 6
Hadean eon
After the big bang some cosmology and a little astrobiology.
Introduction
The earth is approximately 4,500 million years old but multicellular life only rose
500 million years ago. So what was going on in the intervening 4,000 million
years? The first Eon in Earth’s history is the Hadean and this was a very violent
time in its history. Continuous bombardment and a molten planet gave way to the
oceans as the earth cooled and the crust solidified. The atmosphere was unlike
that of today’s with the most notable exception being the absence of oxygen.
Laboratory experiments have demonstrated that lightening and volcanic gases
reacted and created a prebiotic soup of simple organic compounds that was
supplemented by the same compounds from deep sea hydrothermal vents and
even from meteorite materials. There’s been no problem building most of the
monomers for more complex molecules like protein and lipid; the problem is
noone has been able to synthesize the required nucleotides for RNA or DNA.
There is no consensus on how the first large biopolymers appear, in the soup or
inside special lipid compartments of vesicles that form automatically in the soup
or maybe it’s a combination of both.
Geological time scale
Early palaeontologists, like Lyell and his colleagues, determined that various
layers of sedimentary rocks represented very distinct times in the history of the
earth. Using these layers they created the geological time scale that divides
Earth’s history into four eons, and within these eras and periods. Rocks that were
formed during each of these periods reflect major changes in the geology and
biology of the planet. While these early earth scientists weren’t able to accurately
date the layers more recent work using radioactive decay has, and we have fairly
precise dates for these various strata. Geological time scales are hard to
comprehend simply because of the magnitude of the numbers involved; hundreds
of millions of years, billions of years - that’s a lot of zero’s!
The earth we see today, or for which there is a form of written record to consult,
is short-lived. Most of the earth’s history has been spent in the periods when it
finally cooled to the point where organic matter makes it’s first appearance
(Hadean Eon from the formation of the earth 4,500 Ma - 3,800 Ma), followed by
the eon when single celled prokaryote bacteria predominated (Archaean Eon from
3,800 Ma - 2,500 Ma), the single celled eukaryotes (Proterozoic Eon from 2,500
Ma- 500 Ma) and finally multicellular life (Phanerozoic Eon from 550 Ma until
now). It’s probably worth noting that geologists, like biologists have controversies
about their taxonomy the geological time and you’ll see these last three eons
referred to as eras (a subdivision of eon) in a period referred to as Precambrian
times. Such a division fails to recognize the immense contribution of the Archaea
and the first unicellular protists to the changes in the planet - but that’s a
biologist’s point of view.
The shortest Eon is the one we find ourselves in now, the Phanerozoic. But even
during that time the appearance of the world has changed dramatically. A single
large super continent has broken apart and its plates have drifted on the fluid
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surface of the earth’s core. Landmasses, the continents, have drifted in a dance
where they make close contact with each other, then drifted apart. During this
dance organisms in the continental shelves and the land masses above water have
been mixed, modified and isolated from each other.
Geologists recognize three major divisions, eras, in the Phanerozoic. The first
starts with what has been described as the Cambrian explosion 550 Ma. This was
a time where there was a tremendous diversification of animal life in the oceans
and these new animals fed on the abundance of protists that populated the ancient
seas. Fossils from this period show us a bewildering array of animal architectures.
This era, the Paleozoic from 550-250m Ma, sees the worlds oceans populated
with animals and some of the first attempts by plants and animals to rise up from
the oceans and move on land. And then it all disappears in a global catastrophic
event that sees over 90% of the marine diversity disappear. During the Mesozoic
(245-65 Ma) this invasion of land is perfected and plants and animals increase in
numbers and variety, new species of marine animals fill the void in the oceans
and global diversity recovers once again. The Mesozoic comes to an end and the
disappearance of the dinosaurs marks the end of the age of reptiles as once again
another global catastrophe eliminates many of the organisms that populated the
earth. In a typically biased view we often refer to the next era, the Cenozoic, as
the age of mammals. But there’s more to this era than just the appearance and
diversification of the Mammals. Birds appear and increase in numbers and more
importantly, insects and plants continue their co-evolutionary war and the link
between flowering plants and their insect pollinators changes the appearance of
the whole planet, in the oceans a new swimming animal, chordate fish, start to
appear and have a profound impact on this environment. The Cenozoic is more
than just the age of mammals!
Hadean Eon
Please Note: This discussion of the Hadean Eon does not include information on
the Late Heavy Bombardment - refer to your notes from class for this material.
It's impossible to understand the origins of life on planet earth without first setting
the stage for its appearance, or maybe its arrival from somewhere else in the solar
system or even the galaxy! Once the Big Bang ended a massive nebula of gases
and dust started to contract and as it did, its centre reached a critical mass
resulting in a nuclear explosion igniting our sun that still burns today. Our sun is
an out of control nuclear explosion where hydrogen atoms are undergoing fusion
into helium and the resulting energy is spread through our galaxy. The closer you
are the greater the intensity, and its not just light! Almost 99% of the matter in our
solar system is tied up in the sun with the rest swirling around the new-formed
sun in a massive disk. But this wasn't a homogenous mix of material, the
explosion that ignited the sun propelled the lighter gases to the edge of the galaxy
and the heavier substances remained near the centre. The result is outer planets of
frozen gases and the inner ones have heavy iron cores. As a result of gravitational
pulls the swirling material started to stick together and the planets started to form.
One big kid on the block, Jupiter, sucked in huge amounts of material; some of it
stuck and some didn’t and was shot back across the solar system hitting other
planets in its path. These were violent times in the solar system and huge
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