CH 25 History of Life on Earth 1 1 start 11min 3s 10ms

Hi, everybody. Welcome back.

Today we're covering the content

that is in Chapter 25
of your textbook,

and this chapter largely covers
some broad patterns

with regard to the history of life
on the planet.

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

that could promote speciation,
right?

We talked
about allopatric speciation

and sympatric speciation.

Today, we're going to look at,

again, some broad-scale events
that occurred

over the history of life
on the planet

that include things
like mass extinctions,

and the phenomenon
of what we call adaptive radiation

where we see many,
many species

show up in-- in the fossil record.

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

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

and interpretation
of that fossil record.

Starting
with this slide right here:

this is a amazing photo
of a skeleton of a whale

that was discovered
in the Sahara Desert.

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

Researchers were--
were knowing where to look

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

what are the origins
of marine mammals, for example.

And so, due to conversations
with geologists about,

you know,
where we might find fossils

of-- of particular creatures
from a particular time

in the history of life on Earth,

and other patterns of--
of events that--

that led researchers to predict

that they
might actually find whale bones

in the--
in the desert of the Sahara.

And, in fact, they did.

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

that paleontologists do.

Paleontology is the study
of the fossil record.

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

building on the work that--
that we have learned

from paleontologists today.

So, again, when we
use the word macroevolution

now we're really talking
about broad patterns of evolution

that--
that are above the species level.

So, we're looking at groups
of organisms.

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

You know, we see the emergence,
for example,

of terrestrial vertebrates.

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

that I'm going to share
with you today.

We also see
that mass extinctions occurred.

And then there's been, you know,

there's been...

you know, that has affected

the evolutionary trajectory
of other species.

So, we'll talk about that today.

And then we'll focus
on some key adaptations

such as flight, for example.

So, when these new adaptations
arise,

it allows for what we
call "adaptive radiation".

I'll talk about that today
as well.

The possibility
for many new species

to emerge
over a relatively short period of time,

and I'm talking geologic time.

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

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

such as the ability to fly,

that promotes the possibility

for the--

for a species
that has that particular character

to diverge into many other species

as they occupy new habitats
that are available to them

because of that character
that emerged.

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

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

about the history of life
on Earth,

we might as well start
at the very beginning.

So--
So, one of our big questions is

what is the origin of life?

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

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

that supports the idea

that the origin of life

occurred
via these sequential steps

that you see in my slide here.

So, it makes the most sense to us,

and the data support this idea,

that the first thing
that probably happened

was number one here,
abiotic synthesis.

Abiotic meaning non-living, right?

Abiotic synthesis
of very small organic molecules,

probably monomers of molecules.

And then, finally, polymers.

The joining of those together

to make those polymers
probably occurred.

And then, at some point, those--

those molecules
were probably captured inside

what we colloquially
call, "protocells," right.

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

And then lastly,

there is evidence
to suggest the origin

of self-replicating molecules,

similar to what we see today,
right?

Self-replicating DNA and RNA.

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

in detail today.

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

according to
that sequence of events

that I shared with you previously

is the abiotic synthesis

of organic molecules.

And researchers
in the 1950s were curious

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

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

well, what
did planet Earth's atmosphere

look like at the time?

Let me back up a minute.

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

but for
that first half a billion years,

so from 4.6 billion years ago
to 4 billion years ago

the planet
was probably not conducive

to life ever forming
on the planet.

There was--
it was constantly being bombarded

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

But about 4 point-- billion years ago,

4 billion years ago,

the planet cooled off,
the seas formed,

and the environment was--

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

but we have some indication

that there was methane
in the atmosphere,

and there was ammonia
in the atmosphere,

and there's hydrogen gas
in the atmosphere.

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

or an oxidizing environment.

Not sure.

That refers to whether--

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

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

other compounds, right--

steal electrons away from--
from other compounds.

There was some indication recently

that the environment was,
in contrast to that,

a reducing environment full--
full of hydrogen gas,

like you see here, H2,

that would donate electrons
to compounds.

So, regardless of that,

this experiment right here
shows you an apparatus

that was set up by a grad student

from the University of Chicago
in 1953.

His name was Stanley Miller.

He and his--
his advisor worked on this project

where they--
they attempted to simulate

what they thought was the atmospheric
and oceanic conditions

on the planet 4 billion years ago,

to see if they could
get the abiotic synthesis

of organic molecules.

So, what you see in this apparatus

is a container

that might simulate the ocean.

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

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

So, they-- they simulated that,

you know,
providing thermal energy.

And then, you know,
some of that water

would, of course, evaporate.

And this part
of the chamber, here,

is kind of representing

what we think
were atmospheric conditions.

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

He simulated lightning

with these electrodes here,

again, providing energy.

And then,
as that water vapor cooled--

here's a condenser here.

input cold--
cold water to cool it off

to simulate
a cooler atmosphere.

And then as that water condensed,

he sampled it from time to time.

And, honestly,
his graduate advisor

actually thought
that there was no way that they

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

You know, we thought that,
you know,

if this happened
on the planet 4 billion years ago,

how many million years did it take

to get synthesis
of these organic molecules?

But, lo and behold,
they actually got results

in a matter of months.

Three or four months later,

they found in their sample,

when they sampled it
for chemical analysis,

they found amino acids,
amazingly enough.

So, this did represent,

you know, the phenomenon
that abiotic synthesis

of organic molecules
can indeed occur.

This was an exciting moment

because it set the stage for--

you know,
it set the stage chemically

for the conditions for life.

Another burning question,
so to speak,

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

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

to these alkaline vents
that are in the deep sea.

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

These vents release water
with a very high pH;

9, 10, 11.

So, they're considered
alkaline vents.

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

And the conditions in these vents

were probably very likely suitable

for the formation of some
of these organic compounds.

And, indeed,
researchers have looked at the surface

of the structures
that formed these vents

and found organic molecules attached
to those vents.

So, maybe this
is where life arose.

Other people are very interested
in looking at meteorites.

Meteorites may have been
another source of organic molecules.

For example,
the fragments of a meteorite

called the Murchison meteorite,

has been discovered
to contain more than 80 amino acids

and other key organic molecules,

including lipids, some sugars,
also some nitrogenous bases.

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

maybe we can consider that that is

where we got
these first organic molecules

is from the meteorites
that have bombarded the planet.