We left off in the last
video in the Archean Eon. And "arche" comes
from ancient Greek. It means beginning or origin. And it's the eon in which either
life first started to exist or at least it first start
to somewhat flourish. It's possible that
maybe life first started to exist at the
end of the Hadeon Eon. And, of course, this
boundary is vague. And the Archean Eon
is also the first eon where we still have
rocks from that time. So we are able to find
rocks that we can date to be roughly 3.8 billion years. Now, the other really
interesting thing that happened in
the Archean Eon, and it really has
pretty profound effects once we get into
the Proterozoic Eon, is that you started to have
cyanobacteria produce oxygen. And we said in the last video
that they were producing oxygen, but most
of that oxygen was being absorbed by
iron in the oceans. But what happens is we enter
into the Proterozoic Eon. And Proterozoic Eon,
it's right over here. I don't know if you can see it . I'll rewrite it, Proterozoic. We're now in the
Proterozoic Eon that starts up about 2.5
billion years ago. And "proterozoic" comes from
the Greek for earlier life. And I'm not a Greek scholar. So any of you Greeks
out there, forgive me if I'm not getting the
translation exactly right. But what's really interesting
about the Proterozoic Eon is that that oxygen
that was being produced by the cyanobactera,
and at some point begins to saturate the iron
and any other molecule that could have absorbed it before. And once it saturates,
it starts to get released into the atmosphere. So the oxygen starts
to get released and accumulate in
the atmosphere. And we think this
started to happen about 2.4 billion years ago. So 2.4 billion years
ago, oxygen begins to accumulate in the atmosphere. And, of course,
these dates, they might be moved around a
few hundred million years as we get more and more data. But this is the
current understanding of when things happened. And maybe we'll look at
the geological record, or the fossil record, and
we'll move these things around in the future. I can only imagine
that 50 years from now, or 100 years from now,
if someone is still watching this
video, a lot of this might say, hey,
we found out later that oxygen started to
accumulate in the atmosphere earlier or later
or that eukaryotes occurred earlier or later. But this, as far
as I can tell, is our best current understanding. But the 2.4 billion
years ago, oxygen begins to accumulate
in the atmosphere. And what's interesting about
this is once it accumulates and once it gets kind of a
critical amount of oxygen in the atmosphere--
and I touched on this in the last video--
about 2.3 billion years ago, we have something called
the Great Oxygenation Event, sometimes called the
Oxygen Catastrophe. And this is right here. They marked it this right
here, 2.3 billion years ago or 2,300 million years ago,
atmosphere becomes oxygen rich. And it's not as oxygen rich
as our current atmosphere. But it becomes
oxygen rich enough that at least the
environment becomes suitable for eurkaryotic
organisms or eukaryotic cells. Now the other interesting
thing, and we might not care so much about it
because we needed the oxygen, is that we think that
this was actually the greatest extinction event
in the history of Earth. That's why it's called
the Oxygen Catastrophe. So this right over here,
2.3 billion years ago-- I shouldn't giggle about it--
this is a serious matter. This is the greatest extinction
event in Earth's history, in History. And I'll do history
with a capital H, in the history of the Earth. And that's because
the cyanobacteria is producing all this oxygen. It eventually
saturates the iron. It accumulates in the air. Once it gets to
enough concentration, it begins to actually suffocate. It's poisonous. It's poisonous to most
of the other organisms on the planet that
were anaerobic, that did not need
oxygen, that actually found oxygen poisonous. But since we have
oxygen, there's two interesting things that
happened once that oxygen accumulated, other than
causing this mass extinction event-- actually three
interesting things. Two of them are crucial
to us eventually showing up on this planet. The first is is that it became
suitable now for eukaryotes to exist. Eukaryotic organisms,
remember, these are organisms that have nuclear
membranes around their DNA. Most eukaryotes have other
organelles, like mitochondria. They need oxygen. You can go to the
biology playlist. We actually talk
about respiration that occurs in the mitochondria. And that's obviously a
process that needs oxygen. So one, we have, now that
oxygen is in the atmosphere, we're starting to have
an environment where eukaryotes could at least exist. And based on the fossil
records, and when we look at how the DNA
has changed over time-- and we'll do multiple
videos of that-- we think that the first
eukaryotes showed up about 2.2 billion years ago. Although there's
some debate here. There's some evidence it might
have been a little earlier, some evidence it might
have a little later. I'm sure that number
will be refined. But give or take a
few hundred millions of years, one prokaryote got
engulfed by another prokaryote, and said, hey, we do pretty
well living together. I mean the current theory is
that mitochondria is actually descended from kind of an
ancient prokaryotic cell, an ancient bacteria. It actually has its own DNA. And actually, your
mitochondrial DNA is passed down from your mother,
and your mother's mother, and your mother's mother,
so on and so forth. So it's kind of like
another little animal living inside of a larger cell. And we are eukaryotes. We needed this to happen. The human body, we're not
just one eukaryotic cell. We're made up of trillions. The estimates are 50 to 100
trillion eukaryotic cells. So these are our
ancestors that had to come into being at that time. And once again, all of this is
happening inside of the oceans. Now, the other interesting
thing that happened-- remember, we're being bombarded
with UV radiation from the Sun. So if you're on the land-- let
me draw the land and the ocean. So here is the ocean and
then here is the land. Here is the land, right over
there in yellow, constantly being bombarded
with UV radiation. And "UV" stands for ultraviolet. So I'm drawing it in purple. But it's even more
violet than purple. So it's constantly
being bombarded by with ultraviolet
radiation from the Sun, which is very inhospitable
to DNA and to life. And so the only
life at this point could occur in the ocean,
where it was protected to some degree from the
ultraviolet radiation. The land was just open to it. Anything on the land would
have just gotten irradiated. It's DNA would get mutated. It just would not
be able to live. So what happened, and what
I guess has to happen, and the reason why we
are able to live on land now is that we have
an ozone layer. We have an ozone layer up
in the upper atmosphere that helps absorb, that blocks
most of the UV radiation from the Sun. And now that oxygen
began to accumulate, we have the Oxygen Catastrophe. Oxygen accumulates
in the atmosphere. Some of that oxygen goes
into the upper atmosphere. So we're now in this time
period right over here. It goes into the
upper atmosphere. It actually really
reacts with the UV light to turn into ozone, which
then can help actually block the UV light. And I'll do another video maybe
on the ozone/oxygen cycle. So this oxygen
production, it's crucial, one, to having an ozone
layer so that eventually life can exist on the land. And it's also crucial
because eukaryotic organisms need that oxygen. Now, the third
thing that happened, and this is also pretty
significant event, we believe that that
oxygen that started to accumulate in the
atmosphere, reacted with methane in the atmosphere. So it reacted with methane. And methane is an
ozone-- not an ozone. It's a greenhouse gas. It helps retain heat
in the atmosphere. And once it reacts
with the oxygen and starts dropping out of
the atmosphere as methane, we believe the
Earth cooled down. And it entered its
first, and some people believe it's longest,
snowball period. So that's what they
talk about right here on this diagram, the
first snowball Earth. It's sometimes called
the Huronic glaciation. And that happened
because we weren't able to retain our heat,
if that theory is correct. And so the whole-- as the
theory goes-- the whole Earth essentially just iced over. So as we go through
the Proterozoic Eon, I guess the big markers of it is
it's the first time that we now have an oxygen-rich atmosphere. It's the first time
that eukaryotes can now come into existence
because they now have oxygen to, I guess
we could say, breathe. And the other big thing is now
this is where the ozone forms. So this kind of sets the
stage for in the next eon, for animals or living things, to
eventually get on to the land. And we'll talk about
that in the next video.