- [Instructor] Perhaps
the most mind-blowing idea
in all of biology is the concept
that all living things we know of,
based on current evidence that we have,
all originated from a common ancestor.
So it doesn't matter
whether we're talking about
a simple bacterial cell, which actually,
in reality, isn't so simple after all,
a tree made up of trillions
and trillions of cells,
a hairy primate made up of
trillions and trillion cells,
or seemingly well-dressed
agriculture kittens,
which are also made of trillions
and trillions of cells,
that they all share a common ancestor.
You might have seen things
like these evolutionary trees.
This is an example right over here.
This is saying the same thing.
That everything that we
see in the world today,
all living things, regardless
of what domain they're in,
and we would be a subset of
animals, right over here.
There's so many animal species.
That they all share a common ancestor
several billions of years ago.
But you should be skeptical.
We are scientists here.
How do we believe this, what
is the evidence for that?
And one piece of evidence is
by looking at the cellular level,
and look at commonalities
amongst different groups,
and realize that it would
be unlikely for them
to develop independently of each other.
For example, all lifeforms
that we know of have DNA.
They all have RNA.
And it isn't just how the
encode information, it's also
processes, biochemical processes,
that occur in the cells.
They all have some form of glycolysis.
But this seems, and these
aren't the only things
that we've observed are
common to all lifeforms,
they're all based on
cells as the basic units,
which are bound by a membrane.
And so, in theory, these things, I guess,
could have developed
independently of each other,
without having a common ancestor,
but having a common ancestor
is the best explanation
of why we see these different processes.
Some of these are quite complex for these
different structures
throughout life as we know it.
And so you're saying all
right, I can maybe buy that,
that there's this common
ancestor right over here,
but how do we constuct this tree?
How do we know when things branched off?
Because some of these
branches off of these trees,
once again, these would have occurred
hundreds of millions or
billions of years ago,
and none of us were around
to observe that happening.
And once again, that goes
to more structural evidence.
So, for example, amongst what
we now classify as eukaryotes,
so everything in this brown color,
this branch of the tree right over here,
we see that all of them have
membrane-bound organelles.
Membrane-bound organelles.
These are things like a
nucleus, or mitochondria
that we study in many other videos.
They all have linear chromosomes.
So, in other groups in this tree of life,
in this evolutionary tree,
you might have circular chromosomes.
But common to all eukaryotes
are the linear chromosomes.
And they all have chromosomes
that contain introns.
Introns are sequences of DNA
that don't code for genes
that will then code into proteins.
And we're still exploring
what the point of introns are.
But the reason why all of these have been
classified together is that
they have these similarities.
And so we believe that they
would've formed their own branch.
And based on how similar
things are, that's where
we theorize when things
might have branched off.
And now that we have more sophisticated
tools of sequencing DNA and RNA,
we can look at how different
those sequences are
to construct more and more
precise trees like this.
