- [Instructor] When we look inside of eukaryotic cells,
we see membrane-bound organelles,
and some of these membrane-bound organelles
are particularly interesting.
For example, here is a diagram of a chloroplast
that are found in plant or algal cells,
and we know that this is
where the photosynthesis takes place,
but what's really interesting above and beyond that
is that it seems that chloroplasts have
a lot of the machinery necessary
for being a prokaryotic cell on its own.
We don't see it acting on its own, but it has its own DNA.
It has ribosomes, which we know are the site
where we go from messenger RNA to protein.
Similarly, another interesting membrane-bound organelle
that we see in eukaryotic cells,
and this would include even animal cells
and the cells in your and my bodies, are mitochondria,
and mitochondria are often viewed
as the energy factories of eukaryotic cells,
where we can leverage oxygen in order to produce ATP,
and like chloroplasts, mitochondria has its own DNA.
It also has mitochondrial ribosomes.
Here are some just diagrams
of how mitochondria might look
inside of a larger eukaryotic cell.
And so, evolutionary biologists
for many decades looked at this
and said, well, why do these things exist?
Why do they almost look like prokaryotic cells on their own?
And there's even examples of prokaryotic cells,
independent prokaryotic bacteria,
that live in symbiosis inside of other cells,
and they look an awful lot
like mitochondria and chloroplasts.
And so, if we fast-forward to the 1960s,
someone named Lynn Margulis comes
on the scene with endosymbiosis theory,
and her view is, is that these membrane-bound organelles
like mitochondria and chloroplasts,
if we go deep into our evolutionary past,
say, two and a half billion years ago,
their ancestors were actually
independent prokaryotic organisms
that could produce energy aerobically, or using oxygen,
and precursors to what we would consider today
to be modern eukaryotic cells
that might have already had some membrane-bound structures,
like a nucleus and maybe some other things,
that they could only metabolize things anaerobically.
They couldn't leverage oxygen,
while these other characters could leverage oxygen,
and then they could have become symbionts,
where the one that could leverage oxygen
to produce more energy
would get engulfed into the larger cell,
and that larger cell is able
to provide nutrients and protection,
while the smaller cell that's engulfed inside
of it is able to better metabolize the nutrients
and leverage oxygen to produce more energy,
and that over time, this symbiotic relationship
became even more connected,
so that the smaller organism could not operate by itself,
that it lost some of its DNA
that was necessary to act independently,
and some of it might have gotten incorporated
into the DNA of the larger cell.
And those smaller organisms are what eventually evolved
into what we consider today to be mitochondria.
This is a fascinating theory,
and it's actually been proven out.
When Lynn Margulis first published this in the late 1960s,
she wasn't taken that seriously, but in the decades since,
it's been validated as we've looked
at the DNA structures of mitochondria and chloroplasts,
that this actually is the most likely theory
of how they emerged in our cells.
And so, it's just a fascinating glimpse
of evolution in general.
We talk a lot about natural selection
and the role of variation and mutations,
but Lynn Margulis introduces another idea
that could catalyze evolution,
and that's that of symbiosis,
and we see symbiosis throughout the natural world.
And her argument is,
sometimes those symbionts can become so codependent
on each other that they merge into one organism.