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Lecture 4.05: History of life on Earth
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Из курса от партнера Caltech
The Science of the Solar System
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Unit 4: Life in the solar system (week 1)
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Mike BrownProfessor
Planetary Astronomy
I'm going to spin for you a very biased
history of the entire Earth in about ten minutes.
Biased, because I'm going to do it from the planetary science point
of view and from the planetary scientist who is looking to
look at the Earth and understand what you would see, what
evidence you would see that there is life, that there's habitability there.
So let's first look at the 4.5 Billion year history of
the Earth and, and see what the major events have been.
Here's a nice chart of that history made inexplicably into a circle, not sure why.
But starts here, goes this direction, 4.5 billion later, years later ends up here.
So remember how Mars was?
Mars we had a Noachian, we had something that was pre-Noachian.
And we had Hesperian, and we had Amazonian.
Pre-Noachian was, sort of, before the era of the
late heavy bombardments, and that was maybe 3.9 and greater.
And then Noachian was next, Hesperian was a couple hundred million years there.
And most of the time has been this Amazonian.
Earth is divided likewise, and the earliest is called the Hadean.
Hadean, Hadean.
I'm not a geologist, I can't pronounce these things.
And it, it basically also covers that time period from the formation of the
Earth, formation of the solar system here's the formation of the Moon in there.
But it's the end of the late heavy bombardment.
It's omewhere around 3.8, 3.9 million years.
So, this is the period where at first there's a lot of bombardment.
The Moon-forming impact occurs in through here.
We don't really know what happens in this period.
Not much survives.
We have some rocks left over from then,
and some of those show evidence for sedimentary transport.
So, there were oceans, they were rivers.
Was there life?
Some people say yes, most people say we don't know.
But the late heavy bombardment, had there been life that had
developed in through here, the late
heavy bombardment probably would've destroyed it.
It's entirely possible that the late
heavy bombardment would've sterilized the whole Earth.
And yet, as soon as this late heavy bombardment
is over, the very first life starts to show up.
That's this purple line here, this purple line is listed as
Prokaryotes, although I think it should probably really start as Archaea.
And these are the ones that are living in an atmosphere
that is rich in CO2, perhaps in methane, maybe even H2.
All these things are being outgassed from volcanoes.
This is the secondary atmosphere, Earth would have lost a
lot of its atmosphere in through all this, but then
the volcanoes would have come along, given all these materials,
and given plenty of potential chemicals for chemosynthesis of these Archaea.
Now it also lists photosynthesis starting
here, but that was actually early photosynthesis.
Early photosynthesis was anoxygenic photosynthesis.
Oxygenic meaning oxygen-creating.
An- means not.
Early photosynthesis was a less efficient form that did not create oxygen.
So early on there was chemosynthesis from materials in the water and the
atmosphere, and the beginnings of photosynthesis, but very little oxygen.
Somewhere, and it's a little controversial on
precisely when it happens, but certainly by
here the end of the Archaean, Archaean not surprisingly similar to the name archaea.
Certainly, by here, the
first oxygen-creating photosynthetic microbes appear.
This is a pretty amazing thing.
Now remember what I told you before, for these things like these methanogens.
Oxygen is toxic, and suddenly a microbe develops,
it starts to pump oxygen into the atmosphere.
What happens when it pumps oxygen into the atmosphere?
Well there are plenty of things around to react with the oxygen.
Oxygen can react with iron.
Iron was on the surface of the Earth.
Oxygen quickly reacts with iron and forms these huge bands of rust that you
can actually go and find where the sea floor sediments were banded in rust.
They're called these banded iron formations.
They're, they're incredibly cool.
They look sort of like this.
Here's a picture of a, of a sort of small one.
And you can see what happens.
There are these successive oxygenation events.
Oxygen is available in, in these periods, and all
the sediments are rusted and fall to the bottom.
Suddenly, oxygen is, is not there.
In these anoxic environments you get the iron, oxygen available again, red rust,
and you get these banded formations where you have iron, rust, iron, rust.
And they're found throughout the world.
It's a global event that's taking place.
These particular ones are from about 3 billion years ago.
Eventually though, there was nothing left for the oxygen to,
to chemically react with on the surface of the Earth.
And so, it started going into the atmosphere, and
the atmosphere started to rise in O2 based on these
photosynthetic bacteria that were very efficient at creating energy for
themselves and expelling this terrible, terrible poison into the atmosphere.
In, in a really short amount of time, the atmosphere went from this CO2, CH4, H2,
H2S atmosphere to suddenly oxygen being an important constituent in that atmosphere.
This, in general, is called the Great Oxygenation Event.
We like to call it great because we breathe oxygen.
But there's another way to think of it,
is that oxygen is a terrible, terrible poison.
So from the point of view of all of these things that presumably died out because
the atmosphere had been poisoned, this is also called the Oxy, Oxygen Catastrophe.
Once the Oxygen Catastrophe, Great Oxygenation happens, and, and
look, this was about half of the history of our
planet, half the history of our planet, we would've
been CO2, CH4, H2, H2S dominated, suddenly oxygen comes in.
Oxygen has a couple of other great effects.
Oxygen eventually leads to the creation of ozone, and an ozone layer.
And as we know, ozone protects us from
some of the ultraviolet radiation of the sun.
Ultraviolet radiation of the sun is one of the things that can cause mutations.
So, perhaps the, the, Great Oxygenation Event is one of the things that led to a
little bit more stability, that led to things
like multicellular life here in the light blue.
Sometimes, when you're talking about the history
of the Earth, you'll hear the phrase Precambrian.
I'm sure everybody's heard that.
We're talking about Hadean ,Archean Proterozoic, but Precambrian
really prefers to basically everything before about this time.
The vast history of the Earth is that ancient Precambrian.
Why from about there?
Well in about 530 million years ago, there was
what called the Cambrian explosion, and life went crazy.
First on the sea, then on the land.
Then dinosaurs, then people.
And then, you know, who knows what happens as the clock continues to go.
In this brief history of the world, there is one
dominant event that is really, if you think of it from
the planetary science point of view, the main thing that happened
to the Earth over the past four and half billion years.
Or at least, if you think of it from a planetary biologist's point of view.
And that main thing that happened to the
Earth was this change from anoxic to oxygen environment.
This Great Oxygenation Event, this Oxygen Catastrophe, is
an incredible switch that was flipped in the Earth.
And this incredible switch that was flipped led to the switch
from the dominance by these primitive Archaea, which used to dominate
everywhere and now are stuck in sort of extreme environments, to
these Photosynthetic creatures and the things
that prey on them dominating everything.
For someone interested in biology on planets, though, it's
an important thing to remember that these are both
potential things for which we might consider looking, potential
types of environment that we might consider life to need.
In both cases, the things that we've thought about before were there.
Water was clearly present.
It was necessary for, for many of these reactions and
for creating the life in the first place; energy, energy
in the form of chemical energy here, energy in the
form of sunlight was here, all available thoughout these time periods.
And each one leads to distinctive signatures, in particular
in the atmosphere of the earth at the time.
We now know enough about general principles of life on earth and really we
just skimmed the very, very, very thin
surface of general principles of life on earth.
But we know enough, I think, that we can now start to look
throughout the solar system, maybe throughout the universe, and see if we can
apply those general principles to some of the places, to some of the
environments and learn a little bit about where life could have been, could be.
Maybe we should be looking throughout the universe.
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