CH 25 History of Life on Earth 1 3 22 min 32 s 15 ms 35 min 43 s 20 ms

Okay, here we are.

So again, the synapsids gave rise

to a subgroup that are called the therapsids.

And so you can see examples of a synapsid here,

and this fossil dates

to about 300 million years ago.

And so you can see a couple of features, right.

Here's a-- here's a hinge

in the jawbone right here

that is a point of contact

between the articular and quadrate bones.

And there is this opening right here

called the temporal finestra.

And notice that the teeth are pointy,

and all about the same size, more or less.

But then, when we discover

fossil remains of therapsid

from about 280 million years ago--

again, these are descendants of the synapsids--

we see that represation-- representation

of all of the same four bones,

like you see represented here in different colors.

They're still present,

but they have slightly different shapes to them.

The temporal finestra is elongated.

That hinge is still present.

Notice that the teeth...

are different, right.

Now we have canine teeth,

which are distinguished from the incisors,

just like what we see in modern mammals.

And...

and here's the progression

of what we see in more recent fossils, right.

So here are the cynodonts...

that are descendants of those therapsids, right.

And so you can still see the same bones,

which are conveniently color coded

in our-- in our diagram right here.

So, so notice that that hinge, again,

is still present.

We still see a temporal finestra;

it's-- it's reduced in this example here.

And in later cynodonts, again,

still see that hinge,

but, now we have a new point of contact,

a new hinge that is formed

between the squamosal

and a dentary bones.

Here's the thing, in later cynodonts,

that original hinge is lost.

Now the only hinge that remains

is that one between the squamosal and dentary,

and that is what we see in modern mammals.

And in fact, in modern mammals, in us,

that articular bone and the quadrate bone

that served as a hinge,

in-- in the therapsids

and the early

and-- and later cynodonts,

those bones now

are part of our middle ear.

And so it makes sense to us that,

you know, over evolutionary time,

the...

opportunity that's presented by the squamosal

and dentary taking over the responsibility

of a hinge for the jaw,

which, of course, would be very important to have, right?

That frees up those original bones,

the articular and the quadrate,

that frees them up to acquire adaptations

for a new purpose, which, of course,

for us is detecting

mechanical vibrations in our environment;

in other words, we can hear with it.

So anyway, this is just an example

of the gradual changes

that we can track in--

in ancestors and their descendants.

And then, novel features that appear

and-- and-- and of course, novel uses

that are acquired...

by these-- by these features.

So, yeah. All right.

Let's use this diagram right here,

which represents the--

the geologic timeline,

if you will, that shows us the--

the timing of some major events

in the history of life.

to this diagram here,

this is a linear timeline.

So, you know, starting at the--

the point of time,

4.6 billion years ago,

when-- when Earth formed,

and then you know, moving forward in time.

This actually...

should be tied to this.

This is just one long line, right, but we can't fit it all

on the same graph, so it's in three different bars.

Anyway. So here's the thing, too,

geologists divide the geologic record

into eons.

There's the Hadean Eon,

They're listed here.

the Archaean Eon,

the Proterozoic Eon,

and the Phanerozoic Eon.

Within eons,

they further divide

those eons

into eras,

and those eras can be further divided

into periods.

We're not going to memorize the names

of the eras, eons,

and periods of the geologic record,

but they are on this diagram for reference.

And-- and sometimes their names can be helpful

to distinguish the major events

that were going on

during those-- those periods.

So anyway, you know, here we are, again,

at the origin

of the planet,

at this time that, again,

we refer to as the Hadean Eon.

This is a time when there's lots of, you know, rocks

and probably ice,

debris.

And the planet was very hot.

So you can think of "Hot as Hades," right.

And so again, this is a period of time where we don't think

there was a possibility of the origin

of organic molecules, much less the origin of cells.

Then the earth--

and also no seas.

But then, in this time span here,

between 4 and 3.5 billion years ago,

seas form.

And we see evidence

of the oldest organisms;

a group of prokaryotes show up

in the fossil record that date back

to 3.5. billion years ago,

and their fossil remains

are called stromatolites.

And these stromatolites are rocks

that are formed by the accumulation

of sedimentary layers

on these bacterial mats.

So these prokaryotes are these--

are these mats of bacteria

that leave this--

this stratification,

these-- these wavy lines in the sediment,

and you can look in your textbook,

I think there's an image

of a stromatolite fossil.

And again, you know, this is how--

this is how far back that dates.

And so, prokaryotes

were occupying the planet

all by themselves for about one and a half billion years,

until another major event happened

between about two point--

2.7 or so billion years ago,

we see a major change

in the amount of oxygen gas in the atmosphere.

And this is known as the Oxygen Revolution, O2.

At this time, you know, this major change

in the atmosphere

was not tolerable to all of those prokaryotes

that were adapted to low levels

of oxygen in the atmosphere.

So this event caused

many, many prokaryotic species

to go extinct,

although some that were able to adapt

to this new condition, of course, survived,

and they are with us today.

Here's a graph showing you the amount

of atmospheric oxygen gas.

And then this scale is in reference to today's level.

So, you know, here we are at 100%

of the oxygen gas in our atmosphere.

You know, of that 100%,

what percentage was present

over the geologic record?

And so you can see that oxygen levels

were very, very low be-fur,

you know, a billion years or so

between 4 and point 4 and 3 billion years,

and then again at about 2.4

to 2.7 billion years,

we see a major increase

in the amount of oxygen to the point

where it was between-- you know,

this is 1%,

this is 10%, of today's level.

So it hovered, you know, around that

for one and a half billion years ago.

And then, you know, half a million years ago,

it shot up again to what are our current levels of oxygen.

So you can imagine how this would impact organisms

with regard to their-- their respiration

and other physiological functions.

You know, it's hugely beneficial for--

for those of us who could respire,

but not so beneficial for organisms

who were--

who are-- are intolerant

of high levels of oxygen gas.

Okay, let's go back to our timeline here.

So we were just talking about the Oxygen Revolution

and how, you know,

many prokaryotes went extinct,

but those who could survive,

some of them survived in anaerobic environments.

So oxygen did not permeate every living--

or every single space of the planet;

there were still pockets of,

you know, pockets of environments

where oxygen gas was not abundant,

and those prokaryotes survived there.

Others adapted

to oxygen

and began the process of cellular respiration,

which we still see today, of course.

We still see both of those groups today.

So here's the thing, too, for--

for a long period of time,

we still just see...

these prokaryotes

on the planet until

about 1.8 million years ago,

we see the emergence

of single-celled eukaryotic organisms.

We talked about some protists

the other day as examples of single-celled eukaryotes.

And so...

of course, we're very curious

about how eukaryotes originated.

I'm going to share with you in my next slide

evidence for a theory called

the Theory of Endosymbiosis,

where a prokaryotic cell

engulfs a smaller cell

that would evolve into a mitochondrion.

So let's check that out

before we continue.

Let's take a look at our current hypothesis

for how we think

that eukaryotic cells descended

from-- from prokaryotic ancestors.

It makes sense to us

that that indeed is what happened

because, as we saw in our previous slide,

prokaryotic organisms

of-- of many species populated the planet

for a full 1.8 billion years

before we see the emergence

of eukaryotic-celled creatures.

So it does make sense to us

that eukaryotic cells are descendants

of a set of prokaryotic single-celled creatures.

So, based on a variety of evidence,

which I'll share with you in a sec,

this cartoon here

summarizes our hypothesis

of what we call serial endosymbiosis.

That's, uh-- that's-- that's a mouthful there.

But let me explain to you what this--

what this entails.

So we think that there was

some ancestral prokaryote,

and we actually think that it was an Archaean,

rather than a bacteria,

another prokaryotic organism, right.

So an Archaean cell

may have experienced an--

an infolding of the plasma membrane,

like you see in this diagram here.

And that probably is the origin

of that endomembrane system, right.

So now you start to see the nuclear envelope

around the nucleus,

you start to see the endoplasmic reticulum.

Those are two features of eukaryotic cells

that are absent in prokaryotic cells.

But then we also need to explain

the emergence of the various organelles, right.

And so this idea

of endosymbiosis is this:

A-- an endosymbiont

is a cell

that is living inside another cell,

a host cell,

and both the host cell

and the symbiont

benefit from this relationship.

So the-- the host cell benefits

from the presence of the symbiont

in-- in some way, and vice versa, right.

And we see evidence of this,

you know, in other systems as well.

Well, we'll talk more about symbiotic relationships...

probably in biology 213.