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Hello, my name is Andrew Knoll.
I am a Professor of
Earth Science and Biology
at Harvard University.
Today I'd like to talk about
"The Deep History of Life".
Now spoiler alert.
What most people call the
deep history of life.
You will see is actually most
of the history of
life on our planet.
Our familiar world of plants
and animals is actually
a fairly recent development
in our planet's history.
For most of its history,
the Earth has been an
alien place physically.
It has been a microbial
planet biologically.
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Let's begin with a little
bit of perspective.
Most of you know something
about the fossil record.
If nothing else, you know that
dinosaurs once ruled
landscapes on this planet.
If you know a little bit more,
you may know that long
before there were dinosaurs,
animals like trilobites,
distant relatives of
shrimp and crabs,
swam in ancient oceans.
Now, it turns out that if we
could weigh all the
organisms alive today,
you'd find that only a
very small proportion
of that weight would
be made up of animals,
much less than 1%.
It's estimated that
there's something like
30 tons of bacteria for
every ton of animals.
If we look at the diagram,
that's something called
a universal phylogeny.
It's a hypothesis of
the evolutionary relationships
of all organisms,
from humans to bacteria
based on comparisons
of molecular sequences
for DNA and proteins.
The one thing I want to
call your attention to
in this diagram is on
the right with the star and
the arrow that
represents animals.
It turns out that all the
animals that have ever lived,
from trilobites to
dinosaurs to you reside
on one short distal
branch of the tree.
The same thing is
true of plants,
but there are many branches that
precede the branch that
gave rise to animals.
The inference from
this is that life
existed long before
animals first appeared,
and that that life
was mostly microbial.
Now if we look at the
time line on the right,
we can see that the oldest
fossil evidence for animals
is about 575 million years old.
Yet our planet is more than
4.5 billion years old.
Which raises the question
of what was going
on between the origin
of the planet and the
origin of animals.
From the phylogeny, we
should estimate that
there was life through
at least part of
this interval and that
life was microbial.
That then raises a real
empirical question
if the deep history
of life is microbial.
Can such tiny and
evanescent organisms like
bacteria leave a decipherable
record in the rock record?
