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The big question, for this segment,
is what happens when living systems break down?
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When thinking about the complexity of life, and the evolution of complex life,
we can see that there are quite a few points in the fossil record
of the J Chemical record,
where huge changes in the composition of the atmosphere and sediments occurred.
We can also see chemical remnants in ancient sediments, and gain insights
into changes that had occurred in the evolutionary history of life on Earth and
the evolution of biochemical events or systems.
In this lecture we will look at one evolutionary event where some very
complex-looking molecules were utilized by primitive organisms to drastically change
the face of the planet.
1:03
These bacteria evolved complex biochemical pathways,
utilizing metal ions and complex cofactors to help them divide and
reproduce, and utilize chemicals in their environment.
Environment includes the atmosphere, the hydrosphere, and the lithosphere.
They probably had limited resources.
Simple amino acids, organic acids, and
mineral salts dissolved in the hydrosphere,
methane in the atmosphere, and solid low solubility salts in the lithosphere.
There is no oxygen, and the chemical environment was highly reduced.
They made use of complex molecular cofactors to drive and
catalyze chemical reactions in order to make more complex molecules.
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So where did these cofactors come from, and what do they look like?
Vitamin B12, bacteriochlorophyll, chlorophyll, heme and
factor 430, are all examples of compounds known as tetrapyrroles.
Looking at these structures, we can say they look very complex, but
they do have some similar features that are ideal for
coordinating methylines, which are used in catalyzing chemical reactions.
These complex molecules also stabilize, and
often improve the solubility of the metals.
Vitamin B12 and factor 430 would've been used very early on in evolutionary times.
They're used extensively in methane and single carbon metabolism which would have
been some of the earliest biochemical processes.
Bacteria chlorophylls and chlorophylls capture light for photosynthesis.
Bacteria chlorophylls are more reduced in chlorophyl, and
are only able to be used in anoxygenic photosynthesis.
And it is possible that slightly different types of anoxygenic photosynthesis may
have evolved a number of times in the evolutionary history.
In contrast, it appears that oxygenic photosynthesis only evolved once, and
it's dependent on using chlorophyll lights to work.
So oxygenic photosynthesis is dependent on chlorophyll lights being synthesized.
Surprisingly, tetrapyrroles can form spontaneously by acid catalysis
from simple precursors like aminolevulinic acid.
So despite appearing to be quite complex,
the basic structure can form from simple starting precursors.
When we look at these chemicals, we see in their chemical structure, how and
why they were first utilized.
Pyrroles were formed from condensation of some type of amino acids.
And tetrapyrroles were formed from reaction of these pyrroles
with the III Isomer being the most abundant.
Organisms then involved to use and further modify these complex tetrapyrroles, and
they obviously use the ones at the highest concentration.
Thus all biological tetrapyrroles now used are derived from the III Isomer.
Because of this, they got locked in to using these isomers.
So when a biochemical pathway evolved to make them more efficiently,
they had to evolve a complex mechanism to make the most common isomer found from
spontaneous chemical synthesis.
We now see aerobic and anaerobic pathways for these compounds,
with the anaerobic pathways being found in more ancient bacteria while
the aerobic pathways are found in some bacteria and higher organisms.
4:47
Oxygen was a by-product of photosynthesis, and is an extremely toxic gas for
organisms growing anaerobically.
The gas was initially absorbed chemically.
For example, soluble metal salts became oxidized and insoluble.
But this buffering was short-lived on a geological time scale and
lasted only about 100,000 years before significant quantities of oxygen and
gas appeared in the atmosphere.
Chemical processes in cells were inhibited by oxygen.
So new systems had to evolve to cope while organisms retreated to anaerobic natures,
thick, dark and deep away from the oxygen and the oxygen produces.
So both of these things disrupted not only the atmosphere but
also has a huge impact on the materials available for primitive bacteria to use.
The composition of not only the atmosphere, but also the hydrosphere and
part of the lithosphere changed dramatically due to this oxydation.
This oxydation event was also the likely evolutionary driver for
more complex eukaryotic organisms, which based on the fossil records,
seem to appear around the same time.
5:55
These eukaryote arose due to symbiotic relationships between a primitive
anaerobic organism, and an aerobic organism, as the aerobic organism
had developed mechanisms to detoxify, and also utilize the toxic oxygen gas.
So, while one complex biological system almost vanished, as it had to retreat
to natures where the oxygen concentration was low a new complex system evolved to
take its place and harnessed the toxic material, which is oxygen gas, to thrive.
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David BakerAssociate Lecturer
David ChristianProfessor
Shawn RossAssociate Professor
