CH 25 History of Life on Earth 1 4 35 min 43 s 20 ms to end

Anyway, so we think that

an aerobic, prokaryotic bacterial cell

was probably engulfed--
eaten by, if you will,

this-- this other prokaryote,

and you see that in the diagram here,
right, right here.

So here's your-- here is your
aerobic bacterium

that was-- that had, you know,

been able to make use of that oxygen

that suddenly became so--
so abundant in the atmosphere,

and it remained--
instead of being digested,

remained alive inside the host cell.

And so, this, we think now, is the,
you know,

the ancestor of...

of the modern mitochondria,
inside our eukaryotic cells.

So that's the first endosymbiont,

if that thing was beneficial to the host,

remember, it's-- you know, it's
conducting respiration,

which was much more efficient than--
than anaerobic ways

of-- of metabolism.

So, it may have provided a benefit
for this host cell,

and so it would be adaptive

to keep that thing alive inside it.

And then, I'll share with you how we think

that chloroplasts evolved.

So here we are, from our previous slide.

There's our-- you know, here's our...

our original host cell
that now has features

that look like a eukaryote, right?

It's got its nuclear envelope,

it's got some endoplasmic reticulum
from the invagination,

or the infolding, of that plasma membrane.

Now it's got its engulfed
aerobic bacterium

that may be the, you know,

the ancestor of the mitochondria.

So here we are, with our now ancestral

eukaryotic heterotrophic cell

that is able to do respiration

because of the presence of the symbiont.

Now, of course, not all eukaryotic
single-celled creatures

that we have on the planet today

are able to conduct photosynthesis,

but some of them are.

So, we think that some population

of these ancestral eukaryotes now

also engulfed another bacterium

that was able to conduct photosynthesis,

and in a similar way,

via, you know, this
endosymbiotic relationship,

remained alive,
rather than being digested.

And, you know, this would be
hugely beneficial,

presumably, for the host

because this engulfed
photosynthetic bacterium

was able to conduct photosynthesis

and make carbohydrates available
for the host cell.

So that would be adaptive as well,

and there we go, right now,
we've got our--

now we've got a ancestral photosynthetic

eukaryotic single-celled creature

that can do both respiration
and photosynthesis.

How great is that?

All right, so that's--
that's, again, that theory

of what we call serial endosymbiosis.

Let me spell that for you.

Serial...

endo...

sym...

biosis.

There we go, all right.

Before I go on, I do want to
share with you

what is the evidence to
support this theory

of serial endosymbiosis,

and so let me-- let me do that for you.

So, researchers have looked
closely at the structures

of modern mitochondria, like you see here,

and also the structures of bacteria

that we think were the ancestors
of these mitochondria,

and they see some striking similarities.

So let me share the similarities that--
that we've noticed.

I'm going to draw a little mitochondria
here for you.

And this is also true-- I was going to
share with you,

it's also true of modern chloroplasts
and other plastids.

Mitochondria, their inner membrane,

looks strikingly similar to the membranes

that are found in the prokaryotic cells

that we think were the ancestors
to these mitochondria.

If you recall from learning about
respiration,

there's a variety of enzymes embedded
in that membrane,

and those are very similar to what we see

in-- in bacterial cells.

Also, mitochondria and chloroplasts

have their own DNA, and it's circular.

If you recall from looking at
prokaryotics, also circular, right?

We looked at plasmids
a little bit last term.

So that is further evidence to--

to, you know, support this idea

that these mitochondria,
as well as chloroplasts--

I'm sorry, as well as, yeah, chloroplasts,

are descendants of prokaryotic organisms.

And then also, the--

the pattern of division,

mitochondria and chloroplasts
divide on their own,

and the way they do that is very similar

to the way that bacteria divide.

Last bit of comparison is looking
at the ribosomes.

Mitochondria and chloroplasts,
in their interior,

you're gonna find they have
their own ribosomes,

and the structure of those
ribosomes, again,

are very similar to bacterial ribosomes,

more so than they are to
eukaryotic ribosomes.

So, there's quite a bit of evidence
there that, again,

mitochondria and chloroplasts
are indeed descendants

of ancient prokaryotic cells.

All right, so we just looked at
our hypothesis

explaining the origin of
single-celled eukaryotes,

which, again, show up in the fossil record

about 1.8 million years ago.

So here we are in our timeline here,

and, you know, this allowed for the--

a greater range of-- of unicellular forms,

and-- and we see those forms represented

in the diversity of protists
that we see today, right?

And so, you know, that was
certainly a monumental event

in the-- in the evolutionary
history of life.

The other big event, of course,

was the origin of multicellularity.

So that's what we see next
in our timeline here,

based on the fossil record,
the first evidence

of multicellular eukaryotic creatures

shows up in the fossil record

about one and a half billion years ago.

The-- the organisms
that have been discovered,

we're not sure where they
fall taxonomically

into, you know, the categories of life.

So, we do think, though,

that this is the origin of--
of what gave rise

to modern fungi and plants and animals,

you know, the critters--
critters that we see today,

About 1.2 billion years ago,

so that's, you know, right here
on our timeline,

we have discovered some fossils

that appear to be red algae,

and we also see, you know,

about 635 to 541 years ago,

we find a group of organisms
that were soft-bodied,

and these are called-- what's called--

these are called the "ediacaran biota."

And they are well-represented
in the fossil record,

I'll show you some diagrams in a sec.

And then, just--

just more recently than that,

we see what's called
the Cambrian explosion,

so I want to share that with you next.

Here's another way of looking
at this timeline.

This one represents-- this a good way

of representing this phenomenon, again,

of what we call the "Cambrian explosion,"

here's the word "Cambrian" right here.

So, prior to

about 540 million years ago,

this was where we see

the representation of those
ediacaran biota,

these soft-celled animals, uh,

you know, ancient animals.

And then suddenly, in the fossil record,

we see the emergence of
all of these groups

in a fairly short time interval,

and so, we see representation of
all the modern species

that we have today showing up
at this time.

So we see sponges, cnidarians,

mollusks, and then more recently,

the groups that we call the enchinoderms,

and the chordates-- this is us, right?

Brachiopods, annelids--

these-- these that are segmented worms,

and then arthropods,

segmented, um, segmented animals that--

that don't have a backbone.

So yeah, that's-- that's the
Cambrian explosion

taking place about 540 million years ago,

although we think that explosion
lasted a long time.

So again, these organisms

all show up in the fossil record,

and we see their descendants here
on the planet today.

All right, here we are back
on our timeline,

I just want to point out another
monumental event.

Now, all of the-- all of the things

we've been talking about previously,

all those events, and all those creatures

populated a marine environment,

they were in the oceans,

and so, colonization of land
is another big-- you know,

big deal
in the evolutionary history of life.

This, of course, allowed for, you know,

diversification of organisms
as they populated

the variety of-- of environments
found on--

on dry land, on terrestrial habitat,

and so, we see fossils

of fungi and plants, as well as animals,

showing up in the fossil record

from about 500 million years ago,

so here we are right here.

And, you know, many plants
evolved adaptations

to reproduce on land and
to avoid drying out.

We see the emergence of what's called

the "vascular system" for
transporting materials

throughout the bodies of plants,

that shows up in the fossil record
in fossils

from about 420 million years ago.

And here's an interesting thing I want to
share with you, too, is that

plants and fungi

likely colonized land together

in their own symbiotic relationship.

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

some evidence of fossilized plants

that do show evidence

of mutually beneficial associations
with fungi

that are called "mycorrhize,"

and we still see these today.

So, let me share-- share that with you
before we go on.

Here is a photo...

of a cross-section

of a terrestrial plant stem,
a land plant stem, and you can see

the time here, it dates back
to 405 million years old.

And we see that there--

when we look at this closely,

we see that there are cells

that have these structures
called "arbuscules."

And this is a blow-up of just one cell
showing what looks--

always looks to me like
a cluster of grapes--

what these really are, and again, we see
these in modern plants,

these are fungal inhabitants
inside the cells of the plant.

So this is a symbiosis...

where fungal cells are living inside

and benefit-- benefiting from that--

that host plant, if you will, so
it's not a parasite,

it's actually beneficial to both.

So presumably, the fungi

were able to produce compounds

that were beneficial to the plant,

and then the plants, of course,
were probably

conducting photosynthesis and
producing carbohydrates

that then can support the existence
of these particular fungi,

and a relationship like this is called

a "mycorrhrizal association,"

and sometimes, we just
refer to these fungi

that are part of this partnership
as mycorrhizae.

So again, you know, there's evidence

that fungi might not have been able

to colonize land by themselves,

nor could plants, but these--

this association allowed them
to colonize land together.