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Microfossils
I mentioned about the evidence in terms of the fossil record
for early life. What might this tell us about the origin of life? Here
are examples of microbial fossils that I found in rocks 3.5 billion
years ago from western Australia. The bar scales are ten micrometers.
As you can see, there are the thread-like structures and indeed this one
here, there's actually some partition and they're actually quite large.
They're under ten micrometers in diameter so two or three micrometers
would be this tubular structure here and this one approaches six or
seven micrometers in diameter and many tens of micrometers in length.
These are three dimensionally preserved in the rock. They are tubular
also, this one is. As you focus up and down with the microscope you can
get a three dimensional picture of this. So they're three dimensionally
preserved in the rock, they are composed of organic material and the
organic material in the rock associated with these microfossils show
that the organic material is enriched in the lighter isotope of carbon,
carbon-12. carbon-12 and carbon-13 are two stable isotopes of carbon.
So CO2 that we have out in the atmosphere has two flavors. I'm leaving
out carbon-14, the radioactive isotope of carbon.
When your green plant outside your window photosynthesize kinetically,
the carbon-12 flavor of CO2 is preferentially utilized in photosynthesis
so there's a fractionation. The organic material of the plant is
enriched with carbon-12, compared to the reservoir of carbon dioxide,
the carbon-12/carbon-13 ratio in the atmosphere. We can use that kind
of information to suggest carbon fixation, photosynthesis in the rock
record. The organic material with these fossils, is consistent with
photosynthesis. The shape, the size of these microfossils, as well as
other associated fossils, suggest that these are the remains of
Cyanobacteria, the remains of oxygen releasing phototrophs, 3.5 billion
years ago.
Now we go to the fun stuff. What does that mean with regard to the
origin of life? What does that mean with regard to early microbial
evolution? Microbiologists, molecular biologists, based on a variety of
information, have concluded that Cyanobacteria were one of the last
major groups of bacteria to have evolved. So there's sort of a more
recent kid on the block within the prokaryotic world. Therefore, that
would suggest that much of microbial evolution at the phylum or division
level, major metabolic pathways, may have already been around by 3.5
billion years and that much of microbial evolution since that time was
basically fine tuning on already well established biochemical pathways,
metabolic pathways. So in a sense, what Jere Lipps might be saying, the
large game was already over 3.5 billions years ago for prokaryotes and
probably for animals, the major game on ground plans was over by the end
of the early Cambrian. Interesting comparisons between the two and yet
they're both telling us something very interesting.
Now, what about the rates of evolution? Does this tell us something
there? Well, from the origin of life, remember we're not sure when it
originated. If life, the last common ancestor occurred 3.8 billion years
ago, then that leaves 300 million years from that first cell, presumably
some kind of fermenter utilizing organic materials of the primordial
soup as its food substance to evolve to the level of complexity, both
morphologically, genetically, metabolically of cyanobacteria 3.5 billion
years ago, 300 million years. Microbiologists seem to be very reluctant
to accept 300 million years for this time of evolution. It's not that
they think of evolution in a different way, it's that Cyanobacteria are
really very complex bacteria. A very interesting thing, that 300
million years isn't enough. Well, again, we don't know when the origin
of life occurred. Maybe it could go back 4.4 billion years, but with
the impacting, it's highly unlikely that something could have survived
from that time.
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