Lec 1 | MIT 7.014 Introductory Biology, Spring 2005

I am a professor in the Biology
Department at MIT. And I will be co-teaching this
course with Penny Chisholm who is a professor in the Department of Civil
and Environmental Engineering, as well as a professor in the
Biology Department. As well, Penny was recently
featured in the journal Nature. She is a very well-known
oceanographer who has become deservedly famous for discovering a
very small bacterium that's capable of carrying out photosynthesis.
For many years oceanographers used filters whose holes were big enough
that this bacterium went through. And so when they were doing their
studies of the ocean and how the biomass that was there and all the
fluxes and so on, they didn't know this organism
existed which can form up to 50% of the biomass in parts of the ocean.
So Penny is really a wonderful lecturer, a wonderful person,
and she'll be teaching the component of 7.014 which deals with ecology
and the environment.

And this is a section of 7.
14 that makes it different from the other two versions,
7.012 and 7.013. So for me personally this is an absolutely
wonderful and exciting opportunity to be able to teach this
Introductory Biology course. For some of you I know that biology
is going to either be a major part of your career or quite possibly,
even if you're in engineering or something else,
you will find yourselves working with a biological system.
You can see that happening all over campus these days that more and more
engineering departments are finding that they're working on problems
that come from biology or have a biological component.
I'm sure there are at least a few of you here who are only here
because it's a required course and you might well wish you
were somewhere else.

However, I will do my best to try to
communicate to you why you need to know some biology,
too. I think most of you know you can hardly pick up a newspaper these
days without running into something that demands a knowledge of biology,
something about stem cells, something about cloning humans,
something about a new drug, lots of things having to do with biological
affects on the environment and so on.
You're also going to be confronted with decisions about your health,
about the health of your loved ones concerning cancer,
concerning whether a child might have birth defects,
all sort of issues that will affect your personal lives that demand an
understanding of biology.

So I feel with some passion that
whether you think you're going to need biology in your professional
career or not, everyone in this institution needs
some understanding of biology to just live their ordinary lives.
I also think as MIT students you're going to be looked at,
as you go through your lives, as people who are knowledgeable
about science and engineering. And you'll be asked questions that
go far beyond your immediate area of expertise.
And again I think that's another reason for needing to know some of
biology. Anyway, it's an absolutely wonderful time to
teach biology because things have just been exploding over the last
two or three decades and things are moving faster than ever.
And another wonderful thing about teaching biology is that MIT is an
absolutely marvelous place to teach it.
Just to sort of drive this home, in the Biology Department alone
there are four Nobel laureates who got honored for critical discoveries
in biology.

Gobin Khorana who is just down the hall from me
synthesized the first gene. It was an extraordinary feat of
synthesis of organic chemistry synthesizing DNA.
When I was an undergrad I was a chemistry major but I had to take an
introductory biology course. And at that point the DNA wasn't
mentioned in the high school course. So the first time that I heard
about DNA was in my introductory biology course.
And that determined by career direction. I thought that was such
an interesting molecule that I wanted to work on it and I talked
myself into one of the labs in Ottawa, Canada where I grew up that
was trying to synthesize DNA, synthesize pieces of the gene.
And, as it turned out later, competing unsuccessfully with my now
colleague Gobin Khorana.

Susumu Tonegawa got a Nobel prize
for discovering the amazing molecular operations that underlie
the diversity of the immune system. Your immune system has the capacity
to recognize viruses and bacteria, all sorts of different pathogens,
including molecules. It can recognize molecules that
haven't even ever been synthesized in the history of life.
And we'll talk towards the end of the course about the way that
happens. And you'll see why Susumu got his Nobel prize.
Phil Sharp got his for discovering RNA splicing completely
unanticipated component of the very heart of molecular biology.
And then Bob Horvitz who was of pure mind when he started at MIT at
the same time and our labs were side by side for many years,
got his Nobel prize in 2002 for discovering a phenomenon,
the general term is ìprogrammedî cell death.
And it plays all sorts of important roles in biology from sculpting the
shapes of organs and tissues.

We initially have webs when we're
developing between our fingers, and those go away because of the
programmed cell death that the cells that were making the web disappear.
And another role of that is prevent cancer. That if cells sense that
something is very messed up they have a sort of suicide program that
would make them destroy themselves. And if that doesn't happen those
cells could go on and become more and more abnormal and eventually
turn into an invasive cancer. Anyway, there's a picture of Bob
when the institute was celebrating his Nobel prize.
He was getting a congratulatory kiss from Martha Constantine-Paton,
also a professor in the Biology Department who happens
to be Bob's wife. Most of you know that the human
genome has now been sequenced. That was one of the huge
undertakings and most important undertakings in modern biology over
the last while. It was an incredible feat.
Each cell has, as most of you know, 46 chromosomes.

And that's a total
of about two meters of DNA in every human cell.
And that two meters of DNA is composed of about 3 million DNA base
pairs, these letters A, T, G and C that we'll be talking
about as we go through the course. So to sequence the genome you had
to work out the sequence of the exact run of these A,
G, T and Cs along the backbone for 3 billion base pairs.
And somewhere that genome encodes somewhere between 20,000
and 30,000 genes. And we'll be talking about the
proteins that are encoded by most of those genes and their important
roles as we go on in the course. What some of you may not know is
the key role that MIT played in this. About a third of the genome was
sequenced at the Whitehead MIT Genome Center.
And here are some of the robots that were used to sequence
that DNA.

And that sequencing effort was led
by Eric Lander whose name some of you may recognize because he teaches
the fall version of 7. 12 along with Bob Weinberg.
So here are just a few examples of why I think it's important for you
to understand biology regardless of whether you're going to go on and
use it professionally. Most of you know one of the biggest
challenges we face on this planet is this AIDS epidemic.
It's caused by a certain kind of virus called HIV-1 that gets into
particular cells of your immune system that normally defends us
against infection and destroys those.

And then people die from infections
by other organisms that normally you can fight off.
It's a huge problem with vast societal and economic implications.
And it's one that we're still, as a mankind still trying to deal
with and grapple with. Here's another example.
Just a couple of years ago there was the scar of anthrax.
It's a bacterium that's very pathogenic and kills its host fairly
easily. It does it by making particular toxins.
The details shown here don't matter, but just reminding you that this is
something that was in the front pages of the paper just
a little while ago. A couple of years ago,
when I was teaching this, we had the scare of the SARS virus
that went all the way during the course when I was teaching it from
the initial discovery of the virus to the actual sequencing of the
genome which had happened by later on in this course.
Smallpox was a disease we thought that we eliminated,
but now it's come back as a bioterrorism treat and there is
increased study of smallpox.

It's something we have to worry
about again. Here's another example. This is a picture showing the start
of a transgenic animal. We'll talk about how this process
goes later in the course. And it's related also to this whole
issue of cloning, using the sense of the word such as
in trying to clone a human or clone an animal to make a genetically
identical copy. So you'll see there are a couple of
different uses of the word cloning as we go through the course.
There's a lot of fuss in the news and the newspapers about genetically
modified food, and people have different positions
on it. Here's a case where I think the benefit of a genetically
modified food could hardly be argued with. About two-thirds of the
world's population uses rice as their primary source of food.
One of the problems with rice is that it doesn't make beta-carotene.
And beta carotene is what our bodies take and use to make vitamin A.
And if you have a vitamin A deficiency people are prone to
infection.

They have immune deficiencies in their immune system
and blindness. And this deficiency of vitamin A
caused by eating rice afflicts about 400 million people worldwide.
Well, rice is only two chemical steps away from being able to make
beta carotene. This is a genetically modified
version of rice that has those two extra steps inserted in it.
And you can see it's golden because it's making beta carotene.
If people were to eat that form of rice then they wouldn't have this
problem with vitamin A deficiency. Penny Chisholm in her part of the
course will be considering things that are more global level.
Here's a picture of our planet, and there are issues that you know
about there. This is the carbon dioxide levels in the
atmosphere rising. And that's a real phenomenon.
This shows from 1960 to 1995.
damage our cells if we
were exposed to it.

This is associated with a global
warming that again is undeniable that it's happening.
You'll see stuff in the papers. And these gases, particularly
carbon dioxide and methane, they're known as greenhouse gases
are playing a role in that.
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And probably mankind is playing a
role in the production of those
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gases and, hence,
in global warming.
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And Penny will talk to you a little
more about that.
it would have been a real disaster.
And that seems to be, the efforts
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Right here at MIT Mario Molina in
EAPS discovered the ozone hole and
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got a Nobel prize for that.
Ozone is important because it
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absorbs UV, a critical component of
ultraviolet radiation that would And because of the emission of
fluorocarbons in the environment this large hole developed over
Antarctic. If that had spread, to cut down the release of
fluorocarbons seem to be helping with that.

But,
again, Penny will have more to say. Those of you who've lived around
here for a little bit probably know that the fishing industry just
locally has had a very hard time. There is a fishing boat just up in
Gloucester, just up the coast less than an hour from here.
And part of the problems, again, are caused by mismanagement
of the resources where the fish have been, stocks have been over-fished
so some of the fisheries have come to the point of collapse or near.
Places such as the Grand Banks off of Newfoundland there has been a
collapse, and it's not at all clear that it can be reversed and whether
they'll be cod in large quantities every again there. OK.
So there a variety of ways that we can study biology.
And I think I'm going to begin by outlining how we do that.
Biology is an experimental science and it's one of the really important
themes that we'll run through in this course.
You cannot study biology by just sitting down with a pen and paper in
a room and thinking.

You have to get out and find out
what's there. You need to make observations. You need to try
experiments. You need to formulate hypotheses and test them and either
modify your hypothesis or reject it and so on. But it's a continual
cycle of experimentation and making hypotheses and testing.
It's the Scientific Method at work. But you can carry that out at
different levels. The very highest level would be the
biosphere. Here's one example. That's earth. It's here. There
are many, many species of life that are on there.

There are many more
than a million. And the estimates of how many there
are range from, in total, 10 to the 20 million
estimated. And one of the big worries right at the moment is that
as rain forests are being depleted, parts of the world that are a rich
source of biological diversity, as those are disappearing we're
losing diversity at quite a rate. Just a couple of years ago I had a
chance to fly over a part of Brazil.
And it was just scary to see from the plane how the rain forest was
just being cut down. And you could just see how fast
some of the rain forest was disappearing, and with it many
different types of species that only can live there.
One can look instead, going down a level, at an ecosystem
which is a particular environment — — and the species of life found in
it.

And just to give you a couple of examples of that,
it could be a salt marsh as shown here. Or here's an interesting
environment that Penny Chisholm will tell you more about.
This is a black smoker. There's a vent several miles deep
in the Pacific Ocean. The water temperature that's
gushing out of here is around 360 degree centigrade.
And there's a particular community of life that's able to grow around
these deep vents.

And Penny will tell you more about
that. Then going down yet another level you can come to a population. And this is interacting — — or interbreeding organisms.
An example might be the fiddler crabs in a salt marsh.
Or here we see an interesting population. These are tube worms
that are up to a meter or more in length that you find down at these
black smokers. If we move down yet another level — — we come to organisms.
Organisms have three important sorts of characteristics that we'll
talk quite a bit about in this course. They carry out metabolism
which is the sum of all the different chemical reactions
necessary for life. They undergo regulated growth — — and they reproduce.
And the sort of fundamental unit of life that we will talk about over
and over again in this course is known as a cell.
And life comes in two kinds of species.
There is unicellular life where the organism is just a single cell and
multicellular forms of life that are made of many different types of
cells.

What's a cell? One of the secrets to life.
It's a little tiny bit of the universe that's surrounded
by a boundary. And it's given the special name of a
membrane. It's selective, not very permeable to most things.
And cells are able to put little importers and exporters and things
that control the passage of things across the membrane by isolating the
inside of a cell from all the rest of the universe.
That is one of the principles that makes life possible.
We have a couple of examples of organisms here.
Here are some clams that grow down at those black smokers,
and they can get pretty large.

And, as I say, Penny will talk a
bit more about this. And we have this really amazing
diversity of life forms that we find on this planet.
However, if we think about this division into unicellular and
multicellular organisms. Unicellular organisms include things
that you're familiar with. They're bacteria. There is a
picture of just E. coli cells. And we'll be talking
about E. coli quite a bit as a model organism as we go through the course.
By studying E. coli, scientists have learned many
important things that apply to all of life.
Another kind of important single-celled organism,
unicellular organism is yeast. Those are pictures of yeast
saccharomyces that are used in baking bread or in brewing beer or
making wine. And another one you're all familiar with are algae which
are single-celled organisms that are able to carry out photosynthesis.
And we'll be talking about that.

If we think of an example of a
multicellular organism then we see there are different levels at which
we can think about this. We could take, for example,
a picture of me, just an anatomically correct diagram here,
that I'm made up, as you are, of about ten to the fourteenth
human cells. We all started out as a fertilized
egg, which is a single cell. And by the time we're grown up,
where we have about ten to the fourteenth human cells.
Just a tremendous amount of cell growth that had to happen
and specialization.

The other thing you may not
appreciate is that we have an ecosystem inside us in our
gastrointestinal tract. This part, the intestine having the
highest concentration of microorganisms.
But there are about ten to the fourteenth bacteria also inside of
our gut. So we're actually almost the same number of human cells and
bacterial cells. And if we don't have those bacteria
then our digestive systems don't work well.
So if we go down from a whole organism, a whole multicellular
organism, a level, then we come to an organ.
An example of that might be an eye. And I think we have a diagram of an
eye which is made up of different parts. If we go down another level
we come to a tissue. Which is now you can begin to see
that tissue are made up of groups of specialized cells.
An example might be the retina of an eye. And if we continue to go
downwards we'll get to single cells. And at this point we're at the same
level of the tail as when we're talking about a unicellular
organism.

If we continue down then — — we can get to organelles.
These are involved in energy production, energy management. And mitochondrion and chloroplasts
are the two principle examples of organelles that we'll talk about.
And if we go down yet another level of organization we get to molecules.
And part of the reason that biology has flourished so well over the last
few decades at MIT is there has been a real emphasis on looking at things
at a cellular and molecular level. So you're going to be hearing a lot
about cells and a lot about molecules as we go through this
course. Here's an example of rhodopsin. That's a protein.
We'll be talking about what proteins are, but it's a very
important class of molecule in nature.

In this case,
proteins involved in sensing light and play an important
part in your vision. Here is another protein.
You cannot really tell what it's doing by just looking at it,
but in this case this is one of the lethal factors that is made by
anthrax. It's one of the proteins that anthrax makes that's capable of
killing you if you get infected with it.

Here's another molecule we'll
talk about in great detail. This is DNA. You probably all know
it's a double helix, two strands of DNA that are held
together by forces we'll be discussing.
It's an absolutely beautiful molecule. It's fascinated me
through all of my life. And we'll be talking quite a bit
about that as the course goes on. OK. So if we're thinking about
cells there are two important kinds of cells that one finds
on this planet. Prokaryotic cells. Prokaryotic organisms and
eukaryotic organisms. They each are made of cells that
are distinguishable from each other. I've indicated that a cell is a
little bit of the universe that's surrounded by a boundary or a
membrane. But inside there, inside of this is the DNA which
functions as the genetic material. It's the blueprint for everything
that that cell is going to make and be able to do.
Ultimately everything is encoded there.

And in a prokaryotic cell
the DNA is free within this membrane. The eukaryotic cell also
has a membrane. But the DNA inside is inside another
membrane compartment known as the nucleus. And this is the DNA.
These prokaryotic cells tend to be of the order of a kilometer in
length. And eukaryotic cells are usually larger,
can be ten to a hundred kilometers. There's quite a bit of variation,
but that gives you at least some sense of the range.
Now, I've for years, when I did a diagram like this,
I wanted to somehow be able to show you that these cells were impressive
than just what's on the board.

So here are a couple of pictures to
try and do that. This shows a picture of E.
coli swimming along. And the way this image is being taken lets you
see what are called flagella but which are basically the propellers
that E. coli have that let it swim through the water.
These are long structures made of proteins that are several times the
body length of the bacterium. And there's a molecular motor
imbedded in the bacterium that whirls it around at about somewhere
between 10,000 and 100, 00 RPM. And that's what drives the
bacteria forward. So that was a prokaryotic cell.
Here's a paramecium. This is a single-celled eukaryotic organism.
And, as you can see here, it's capable of movement as well.
In this case it has cilia along the outside that allow it to move.
Here's an interesting one.

I don't know if any of you can guess what
these were. These were cells from the skin of a mouse.
They're on an Auger surface. And, as you can see,
they too can move. There are a couple of things that are important
about this. I got this slide from Linda Griffith who is in the
Biological Engineering Department. At the time I got it from her I
think it was in Chemical Engineering several years ago.
And what was important about this, apart from it being a very nice
little movie showing you a mammalian cell moving around,
was that I saw Linda show this during one of her research
seminars.

So here is an engineer at MIT who
was showing this picture as part of her research talk.
And I think those of you who are going onto engineering,
you may be surprised at the extent to which you need to know about
biology as you go through your professional careers. OK. So one of the great discoveries
that has happened over the last few years that came out of our ability
to look at DNA and RNA was the discovery that the forms of life
that are prokaryotic actually split into two distinct Kingdoms that are
very, very different. The archaea and the bacteria.
And just to give you a sense of the diversity of life,
I'll just mention a couple of these. These archaea look like bacteria
but they are diverged from the bacteria as they are from the
eukaryotes.

So there were sort of three really major Kingdoms of Life.
And the archaea, many of them can live in specialized environments.
For example, sulfolobus can live at about 90 degrees centigrade and a pH
of somewhere between 1 and 2. So if you see something like a hot
springs, there are organisms such as this that are able to grow in that
environment. Or there are halophyes, salt-loving archaea that can grow,
for example, in formula sodium chloride.
And if you've, for example, ever flown into San
Francisco airport coming up from the south over San Jose,
you've seen things that look sort of like these pictures where seawater
is being evaporated down to collect the salt. And you'll see they're
colored, and the reason they're colored is that these halobacteria
are photobacteria that are able to use light as an energy source.
And they make pigments that absorb the light, and that's why these salt
areas get colored.

A third example would be methanogens.
These are organisms that produce methane. If you've walked into a
lake and stepped on the bottom and seen little bubbles come up,
those are little bubbles of methane. Or another place where you find
methanogens are inside of cows. Now, some of you may not know that
the cow is more or less a walking anaerobic fermentor here.
If we have an anatomically correct picture of a cow.
The inside of the cow, there's a large chamber known as the
rumen where there's no oxygen, and there's a culture of
microorganisms there that include methanogens.
And it's this combination of microorganisms that enables cows to
each grass that we cannot manage to get energy from.
And as a byproduct of this specialized type of metabolism
produces methane.

And a cow burps something of the
order of 400 liters a day of methane. OK. So one last thing then just to
kind of pull this all together is that these organelles that I
mentioned, which are also membrane compartments that are found in
eukaryotic cells, are the mitochondria —
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— or chloroplast.
There's pretty strong evidence at
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this point that these arose from
bacteria that were things that
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you're more familiar with.
Things like E. coli or streptococcus that causes
strep throat or the lactic acid bacteria that causes the milk to
turn into yogurt which some of you probably had for lunch today.
That these organelles, the mitochondria and the chloroplast
were derived from particular type of bacteria that probably first got
transiently associated with developing eukaryotic cells sometime
back in evolution, and eventually became captured and
became a permanent part of the eukaryotic cell.
The mitochondrion is thought to have derived from something that
looks like today's present day rizobia, which we'll talk about,
that form an intracellular infection of plants, or rickettsia which is
chronic intracellular pathogen.

The mitochondrion look as though
they came from something related to that. The chloroplasts look as
though they came from a bacterium that was able to carry on
photosynthesis which we'll also be talking about.
I want to close by giving you just a quick little snapshot of evolution
because I'm hoping this will maybe make some of the things that we talk
about in this course clearer. So what we're going to do is we're
going to look back from 4. billion years ago when the earth
was just forming — — to now. I'm just going to try and give you a few key sort of
landmarks as we go along. So about 4.5 billion years ago there
was methane, carbon dioxide, ammonium, hydrogen gas, nitrogen gas,
water, but importantly no oxygen at that point. There was a lot of
debate as to how life initially came. One of the prevalent theories at
this point is there's something called an RNA world.
This is just a hypothesis in which it's thought that perhaps the
molecule RNA, which we'll talk about, played role as both something that
was able to catalyze chemical reactions and therefore did things
actively and also stored information.

But, in any case,
the best guess is that the first life that was about 3.
billion years ago, somewhere in that vicinity.
It was something that probably resembled most closely a present-day
bacterium, a single-celled organism, something like that. Now, initially
when life got started it's thought that there were a lot of organic
chemicals that had been made as a consequence of lightening strikes
and all sorts of chemistry that had happened so there was sort of a soup
of some kind, some molecules that could be used.
So probably these first organisms where able to basically use some
preformed nutrients. And then as the soup began to get
depleted by using it they had to learn to synthesize,
at least develop systems that would synthesize these building blocks.
And they also had to begin to worry about what to use as energy.
And so somewhere in here, something that I'll call,
in a silly way, photosynthesis released number one.
But this was a system that enabled the organism to capture energy from
sunlight so that it wasn't now dependent on getting energy by
eating some preformed ingredient.

It was then able to take carbon
dioxide and make it into forms that were useful, of carbon that were
useful for life, and it produced molecules such as
sulfur as a waste product. There was a bit later in evolution,
somewhere in here, something we might think of as photosynthesis
release two. This was an improved version of photosynthesis.
It captured more energy, worked better, but it developed,
it had a waste product which was oxygen.
Well, oxygen hadn't been in our atmosphere. And the first thing
that sort of happened was that the world started to rust.
All the iron, a lot of the iron started to interact with the oxygen.
And Penny will tell you that at the base of the sea there are huge beds
of iron oxide that came from this slow rusting of the earth.
And so it took many, many years before oxygen levels started to rise.
As you know it's about 20% of our atmosphere now.
Even at this stage it was only a few percent of our,
made up a few percent of our atmosphere, even by here in
evolution.

The first eukaryotic cell is thought to have appeared
somewhere here. Again, it was likely a
single-celled organism like some of those pictures I showed you.
And evolution continued to go. Somewhere around a billion years
ago sex was evolved which enabled eukaryotic organisms to exchange
genetic material, and therefore evolve at a fast rate
than they could previously. The Cambrian Period was about a 0.
billion to 0.6 billion years ago.

And there was a veritable explosion
of life forms. And you can still see in the fossil
records how much diversity was generated at that point,
some of which went on to become life forms and other which probably were
more evolutionary dead ends. Finally we get to the dinosaurs that
were about 245 to 65 million years ago which would place them somewhere
here on this timeline. So in honor of this course,
I've commissioned a full scale model of anatomically correct
[NOISE OBSCURES]. So we'll put our dinosaur here,
if I can get him to stay put for a minute. All right.
And at this point in evolution things started to get interesting.
So somewhere about here, 4 million years ago we've got the first
evidence of hominoids. Maybe 20,000 years ago we found the
cave paintings in France. And then there was the Roman Empire
and Columbus discovered America.

And you were born and the Red Sox
won the World Series and the Patriots have just won the Super
Bowl. And we are now here at the peak of evolution which is,
as you all know, the MIT student. So we'll put our MIT student here,
who I can probably not get to stay put because you can never get MIT
students to stay anywhere. But, in any case, this is sort of a
silly demonstration. But there is a very profound reason
why I'm doing it. And I must say I don't think I'd
ever fully appreciated it until I actually thought of doing this demo
for the class. But what I think you can see is
that evolution, for the most part,
happened at the single cell level. Many people tended to think
evolution, that was about dinosaurs and all that stuff.
We can say that dinosaurs are, practically now, most of evolution
occurred at the level of single cells, and that all this amazing
diversity we see around us was very recent embellishments in evolution.
So that means when you study biology at the cellular and
molecular level you find tremendous commonalities.
If you look inside a sulfolobus growing in hot spring,
if you look inside an E.

Coli, if you look inside a yeast and
you look inside one of our cells you find that, to a huge extent,
many, many of the cellular components are common.
They arose similarly in evolution that they're shared by all forms of
life. Of course, there are some things that developed
later and are different. But that's one of the reasons that
you can learn so much by studying biology at the cellular molecular
level and why we'll emphasize it a fair bit in this course.
The other thing that I'd like to make out of this,
a theme that you'll hear along is that organisms modify their
environment.

You can see that in the case of oxygen back when the
earth formed. There was no oxygen in our atmosphere. Now
we have a lot of it. The reason it's there is because it
was generated by organisms carrying out photosynthesis and generating
oxygen as a waste product. And that was an absolutely critical
thing to enable creatures such as ourselves, which are dependent on
oxygen for us just to be alive, if we hadn't had this change in
environment things like us could have, organisms like us
couldn't have evolved. So, anyway, I hope that will give
you a little sort of snapshot of evolution and will help guide your
understanding of this course.

We'll see you at the next lecture..

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