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Hi, everybody. Welcome back.

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Today we're covering the content

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that is in Chapter 25
of your textbook,

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and this chapter largely covers
some broad patterns

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with regard to the history of life
on the planet.

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When we were together last time,
we were looking at mechanisms

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that could promote speciation,
right?

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We talked
about allopatric speciation

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and sympatric speciation.

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Today, we're going to look at,

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again, some broad-scale events
that occurred

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over the history of life
on the planet

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that include things
like mass extinctions,

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and the phenomenon
of what we call adaptive radiation

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where we see many,
many species

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show up in-- in the fossil record.

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And yeah, a lot of what we know
with regard to what

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we're going to talk about today
comes from an exploration of

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and interpretation
of that fossil record.

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Starting
with this slide right here:

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this is a amazing photo
of a skeleton of a whale

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that was discovered
in the Sahara Desert.

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So, you might be wondering,
well, how did that happen?

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Researchers were--
were knowing where to look

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when they were--
when they were trying to figure out

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what are the origins
of marine mammals, for example.

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And so, due to conversations
with geologists about,

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you know,
where we might find fossils

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of-- of particular creatures
from a particular time

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in the history of life on Earth,

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and other patterns of--
of events that--

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that led researchers to predict

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that they
might actually find whale bones

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in the--
in the desert of the Sahara.

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And, in fact, they did.

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So, that's just one example
of some of the work

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that paleontologists do.

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Paleontology is the study
of the fossil record.

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So, yeah, let's go ahead
and get started

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building on the work that--
that we have learned

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from paleontologists today.

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So, again, when we
use the word macroevolution

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now we're really talking
about broad patterns of evolution

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that--
that are above the species level.

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So, we're looking at groups
of organisms.

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And when we look in the fossil record,
we do see some trends.

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You know, we see the emergence,
for example,

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of terrestrial vertebrates.

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We see the emergence
of other groups of species as well,

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that I'm going to share
with you today.

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We also see
that mass extinctions occurred.

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And then there's been, you know,

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there's been...

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you know, that has affected

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the evolutionary trajectory
of other species.

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So, we'll talk about that today.

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And then we'll focus
on some key adaptations

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such as flight, for example.

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So, when these new adaptations
arise,

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it allows for what we
call "adaptive radiation".

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I'll talk about that today
as well.

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The possibility
for many new species

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to emerge
over a relatively short period of time,

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and I'm talking geologic time.

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So, you know, short is relative here
in this conversation today.

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But, yeah, when you
have a novel character that arises,

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such as the ability to fly,

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that promotes the possibility

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for the--

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for a species
that has that particular character

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to diverge into many other species

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as they occupy new habitats
that are available to them

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because of that character
that emerged.

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So, yeah, we'll talk about each
of these in detail today.

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All right,
if we're going to have a conversation

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about the history of life
on Earth,

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we might as well start
at the very beginning.

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So--
So, one of our big questions is

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what is the origin of life?

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How did life ever get started
on planet Earth, right?

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And so, I'm going to share
with you some evidence

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that supports the idea

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that the origin of life

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occurred
via these sequential steps

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that you see in my slide here.

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So, it makes the most sense to us,

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and the data support this idea,

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that the first thing
that probably happened

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was number one here,
abiotic synthesis.

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Abiotic meaning non-living, right?

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Abiotic synthesis
of very small organic molecules,

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probably monomers of molecules.

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And then, finally, polymers.

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The joining of those together

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to make those polymers
probably occurred.

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And then, at some point, those--

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those molecules
were probably captured inside

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what we colloquially
call, "protocells," right.

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Proto- meaning first, these--
these precursors to modern cells.

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And then lastly,

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there is evidence
to suggest the origin

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of self-replicating molecules,

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similar to what we see today,
right?

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Self-replicating DNA and RNA.

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All right, let's take a look
at each one of these

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in detail today.

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So, one of the first things
that would have to happen,

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according to
that sequence of events

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that I shared with you previously

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is the abiotic synthesis

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of organic molecules.

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And researchers
in the 1950s were curious

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if that--
if they could get that to occur.

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and, you know, like, the conversations
at the time were

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well, what
did planet Earth's atmosphere

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look like at the time?

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Let me back up a minute.

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We think that the planet formed
about 4.6 billion years ago,

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but for
that first half a billion years,

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so from 4.6 billion years ago
to 4 billion years ago

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the planet
was probably not conducive

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to life ever forming
on the planet.

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There was--
it was constantly being bombarded

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by rocks and debris,
and it was a very hot environment.

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But about 4 point-- billion years ago,

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4 billion years ago,

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the planet cooled off,
the seas formed,

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and the environment was--

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the atmosphere, we don't know
exactly what it looked like,

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but we have some indication

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that there was methane
in the atmosphere,

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and there was ammonia
in the atmosphere,

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and there's hydrogen gas
in the atmosphere.

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And, whether or not it
was a-- a reducing environment

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or an oxidizing environment.

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Not sure.

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That refers to whether--

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you know, today we're living
in an oxidizing atmosphere, right?

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There's oxygen gas in our atmosphere
that will readily oxidize

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other compounds, right--

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steal electrons away from--
from other compounds.

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There was some indication recently

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that the environment was,
in contrast to that,

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a reducing environment full--
full of hydrogen gas,

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like you see here, H2,

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that would donate electrons
to compounds.

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So, regardless of that,

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this experiment right here
shows you an apparatus

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that was set up by a grad student

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from the University of Chicago
in 1953.

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His name was Stanley Miller.

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He and his--
his advisor worked on this project

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where they--
they attempted to simulate

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what they thought was the atmospheric
and oceanic conditions

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on the planet 4 billion years ago,

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to see if they could
get the abiotic synthesis

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of organic molecules.

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So, what you see in this apparatus

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is a container

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that might simulate the ocean.

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And, you know, we know
from volcanic activity in the ocean

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that there are areas where it's--
it's very hot.

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So, they-- they simulated that,

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you know,
providing thermal energy.

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And then, you know,
some of that water

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would, of course, evaporate.

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And this part
of the chamber, here,

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is kind of representing

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what we think
were atmospheric conditions.

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Here's methane, here's ammonia,
here's hydrogen gas.

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He simulated lightning

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with these electrodes here,

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again, providing energy.

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And then,
as that water vapor cooled--

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here's a condenser here.

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input cold--
cold water to cool it off

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to simulate
a cooler atmosphere.

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And then as that water condensed,

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he sampled it from time to time.

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And, honestly,
his graduate advisor

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actually thought
that there was no way that they

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were going to get any kind
of organic monomers anytime soon.

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You know, we thought that,
you know,

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if this happened
on the planet 4 billion years ago,

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how many million years did it take

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to get synthesis
of these organic molecules?

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But, lo and behold,
they actually got results

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in a matter of months.

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Three or four months later,

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they found in their sample,

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when they sampled it
for chemical analysis,

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they found amino acids,
amazingly enough.

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So, this did represent,

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you know, the phenomenon
that abiotic synthesis

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of organic molecules
can indeed occur.

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This was an exciting moment

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because it set the stage for--

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you know,
it set the stage chemically

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for the conditions for life.

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Another burning question,
so to speak,

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that we have is, well,
where did life originate on the planet?

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A lot of interested--
a lot of interest is being paid

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to these alkaline vents
that are in the deep sea.

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Here's a picture of one of these vents
in the slide here.

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These vents release water
with a very high pH;

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9, 10, 11.

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So, they're considered
alkaline vents.

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And also very warm water,
40 to 90 degrees C.

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And the conditions in these vents

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were probably very likely suitable

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for the formation of some
of these organic compounds.

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And, indeed,
researchers have looked at the surface

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of the structures
that formed these vents

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and found organic molecules attached
to those vents.

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So, maybe this
is where life arose.

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Other people are very interested
in looking at meteorites.

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Meteorites may have been
another source of organic molecules.

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For example,
the fragments of a meteorite

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called the Murchison meteorite,

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has been discovered
to contain more than 80 amino acids

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and other key organic molecules,

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including lipids, some sugars,
also some nitrogenous bases.

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So, maybe these are the--
maybe we can--

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maybe we can consider that that is

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where we got
these first organic molecules

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is from the meteorites
that have bombarded the planet.