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Biodiversity flourishes in Phanerozoic eon | Cosmology & Astronomy | Khan Academy


The Earth is now
starting to get closer to being hospitable to people
like us or animals like us. In the last video, we saw
during the Proterozoic Eon, oxygen began to accumulate
in the atmosphere. This actually caused
this first snowball Earth and this mass extinction of
all the anaerobic species. But it made conditions
suitable for eukaryotic cells. And maybe even more important,
these eukaryotic cells were able to form
multicellular organisms. And we see where that starts
right here on this chart, on this time clock. Multicellular life
starts right over here. And I want to be clear. All of these things are
a bit moving targets. As we discover more things
in the geological record and we get more tools
at our disposal, these numbers get tweaked. But they do give
you a good sense, based on our current
understanding, of when these things
start to appear. And coinciding with
multicelluar life, and this is interesting
in it’s s right because it has its own
metalevel effect on evolution, you actually start also
having sexual reproduction. And what’s interesting
about this, why this has such a big
impact on evolution– and we talk about it a lot
in the biology playlist– is before evolution,
variation in DNA had to be completely dependent
really on mutations, and just random movement around within
DNA, or maybe some viruses. Now with sexual
reproduction, you had kind of a
systematic mixing of DNA so that you got more variation
in the gene pool, which allowed more
selection for– or I guess you had more
variance to select for. And so you kind of
had an acceleration in the actual pace of evolution. So we’re talking– I’ve
looked at a bunch of sources from, they say 1.2
billion, 1.5 billion, a little bit over a billion,
if you call a little bit several million
years ago, you start having these multicellular life
forms and sexual reproduction. The other thing that we talked
about in the Proterozoic Eon is the accumulation of
oxygen allowed the ozone layer to build up. Ozone is just
three oxygen atoms. It is O3. And by the end of
the Proterozoic Eon– so we’re talking, I don’t know,
maybe 550 million years ago, give or take tens of,
or hundreds, or maybe 100 million years–
these are all moving targets– the ozone
layer was dense enough to protect the
land from UV rays. We talked about that
in the last video, that the Earth is being
bombarded with UV rays. And the ozone layer
is the only thing that really keeps us from being
seriously irradiated by the Sun and allows land animals
to actually live. And so coinciding
with that time period, around 550 million
years ago, you start to have life colonizing,
especially significant life, colonizing land. So life colonizes land,
colonizes the land. And this was kind of an
interesting– when I first learned it, it was
kind of an aha moment. You always assume that
kind of trees and grasses are kind of part
of the background. They come part and
parcel with land. But it turns out that
animals colonized land before plants did. Plants didn’t come
into the picture until about 450 million
years ago, give or take a few tens of millions of years. And so we’re now entering the
end of the Proterozoic Eon. Life has started
to colonize land. We now have an ozone layer. And what happens–
and actually there’s another snowball glaciation
or a snowball Earth near the end of the Proterozoic
Era, Eon, I should say. And there’s a bunch of theories
about why it came about. And then why disappeared. Maybe there were volcanoes,
greenhouse gases, who knows. But as we enter
the end of that, we start seeing life
began to flourish. And it starts to
really flourish as we enter the Phaner– I
always have trouble saying this– the
Phanerozoic Eon. And it’s not even labeled here. The Phanerozoic Eon is this
chunk of time right over here. And let me write it out. So this right over here is the
Phanerozoic, the Phanerozoic Eon. And so this chart, these
divisions right here are eons. And then they jump into,
instead of doing eons here, they then break into eras. Eras are subsets of eons. They are hundreds of
millions of years. So this is the Paleozoic
Era, the Mesozoic Era, and the Cenozoic Era. And that’s actually
our current era. But perhaps the most
interesting– well, I don’t want to
pick favorites here. But it’s one of the
most interesting times in the geologic era–
is the first period in the Paleozoic Era,
which is the first era in the Phanerozoic Eon. And that’s the Cambrian period. You might have heard of it
before, the Cambrian period. That’s about this period of
time right over here, Cambrian. And during this period
of time, the Earth experiences what we call
the Cambrian Explosion. And that’s because there’s
just this explosion in the number of species
and genera that existed, the biodiversity, on the planet. And it might just be that we had
the ozone layer protecting us, things were colonizing land. It was an oxygen-rich
environment. We start seeing complex
multicellular organisms. It’s about that time, if
you fast forward maybe a few tens of
millions of years, you start seeing the first fish,
the first kind of preamphibians or protoamphibians. You fast forward a little bit. As we get out of
the Cambrian period, we start seeing the plants. So they actually draw
it right over here on this– land plants– or at
this point right over here. And, of course, these
are moving targets, depending on what we discover
in the fossil record. And for me, the big aha moment
here is so many of these things that you consider
fundamental to what Earth is are relatively
recent phenomena, Plants weren’t on land until
about 450 million years ago. Insects weren’t on land– or did
not even exist until about 400 million years ago. Reptiles didn’t exist until
about 300 million years ago. So we are about
right over here now. Mammals didn’t exist until
about 200 million years ago. Birds didn’t exist until
about 150 million years ago. The whole dinosaur age,
which we kind of consider in our distant past, that’s
essentially the Mesozoic Era right here. So this is the Age of the
Dinosaurs right over here. When you look at
your time clock, you can see it’s a relatively
recent time period. And it actually ends
with, we currently believe, a huge rock, a
six-mile in diameter rock, colliding with what is now the
Yucatan Peninsula in Mexico, or right off the coast
of the Yucatan Peninsula. And it destroyed all
of the large land life forms, especially the dinosaurs. And to put all of
this in perspective– and actually the
thing that really was an aha moment for
me– it’s, OK, plants are 450 million years ago. Grass, I kind of view as this
fundamental thing in nature. But grass has only been
around for about– I’ve seen multiple estimates–
40 to 70 million years. Grass is a relatively
new thing on the planet. Flowers have only been
around for 130 million years. So there was a time
where you had dinosaurs, but you did not have flowers
and you did not have grass. And so you fast
forward all the way. And so when you
look at this scale, it’s kind of funny
to look at this. This is the time period where
the dinosaurs showed up. This whole brown line is
where the mammals showed up. So the dinosaurs started to
show up along with the mammals. And then, of course, the
dinosaurs died out here. Our ancestors, when the
giant rock hit the Earth, must have been
boroughed in holes and were able to stash some
food away, or who knows what, and didn’t get fully affected. I’m sure most of the large
mammals were destroyed. But what’s almost– it’s
humbling, or almost humorous, or almost ridiculous, when
you look at this chart is they put a little dot–
you can’t even see it here, a little button. They say 2 million years
ago, the first humans– and even this is being
pretty generous when they say first humans. These are really
the first prehumans. The first humans that
are the same as us, if you took one of those
babies and your brought them up in the suburbs and gave
them haircuts and stuff, they would be the
same thing as we are, those didn’t exist until
200,000 years ago, give or take. 200,000 to 400,000 years
ago, I’ve seen estimates. So this is actually a very
generous period of time to say first humans. It’s actually 200,000 years ago. And just to give you
an idea of how new we are and how new
evolution is, it was only 5 million years
ago– and I mentioned this in a previous video– it was
only 5 million years ago– so this is just to get a sense. This is 0 years. Homo sapien sapien, only
around for 200,000 years. The Neanderthals, they
were cousin species. They weren’t our ancestors. Many people think they were. They were a cousin species. We come from the same root. Although there are
now theories that they might have remixed
in with Homo sapiens. So maybe some of us have
some Neanderthal DNA. And it shouldn’t be
viewed as an insult. They had big brains. Well, they didn’t
necessarily have big brains. They had big heads. But that seems to
imply a big brain. But who knows? We always tend to portray
them as somehow inferior. But I don’t want to get into
the political correctness of how to portray Neanderthals. But anyway, this is a very
small period of time, 200,000. If you go 2 million
years, then you get to kind of the prehuman,
the prehuman ancestors. And our family tree only
diverged from the chimpanzees 5 million years ago. If you draw that on
this clock right here, it would be like two
pixels or maybe not even two pixels as when we
diverged from the chimpanzees. So hopefully, that gives
you a sense of things. At least for me, it really
puts things in perspective.

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