Hi. It's Mr. Andersen and welcome to Biology Essentials video 21. This is on
homeostatic mechanisms and how they reflect evolution. Remember homeostasis is that internal
stable environment. And what we find is when we look in different organisms and how they
maintain homeostasis, you see this continuity. In other words we see a simple strategy that
was once developed and it's kind of run through that whole lineage. We also find sometimes
that the environment changes and with that change we'll see divergence in the homeostatic
mechanisms as well. And so one of the greatest discoveries of 2004 was Neil Shubin. He discovered
the tiktaalik fossil in northern northern northern Canada. And it's an intermediary.
In other words most scientists believe that life started in the oceans. But vertebrate
life moved from fish to amphibians to reptiles to birds, dinosaurs, mammals. So there's been
this progression. But we lack some of these transitional fossils. And so tiktaalik is
one of those transitional fossils. It's pretty interesting. It had fins, so it could swim.
But it also had gills. But it also had lungs. But it also had tetrapod like legs. It also
had a mouth of really nasty looking teeth. And so this was like a fish-alligator kind of a
predator. And it also represents a transition from water to land. And so we can see all
of these throughout life. And with that we can trace evolutionary history. And so homeostatic
mechanisms show both continuity and they show change. And so continuity implies common ancestry
or homology. The example I'll talk about in this podcast is the excretory system. We can
see that the excretory system in flatworms, earthworms and finally in vertebrates serve
the same purpose. And so it's pretty much remained unchanged with some few tweaks over
the last millions of years. We also find changes in the environment and that can cause changes
in homeostatic mechanisms. Big example would be that movement from water onto land. In
other words as aquatic animals move to terrestrial animals, there were huge constraints put on
them. And as a result they had to make a switch or change in those homeostatic mechanisms.
So let's start with continuity. Continuity remains the same. Or in other words it remains
the same. The example I'll talk about, if I can start talking right, is excretory system.
Excretory system serves two purposes. First one, and in us, the big part of our excretory
system is the kidney, the first thing it does is it maintains osmolarity of the blood. In
other words it maintains the solute concentration in the blood. And in animals that's super
important. The other big thing that it does is it filters our blood. And so it serves
those purposes but it's done that through vertebrates and even invertebrates for millions
of years. And we can see some similarities through that. So again, briefly what we do
is we filter the blood. We get rid of waste. We also get rid of water or solutes if we
need to. We then eventually move that into the bladder. We can get rid of it as urine.
But this is a problem that is plagued animals from some of the first animals. And some of
the first animals on our planet were flatworms. Flatworms had to get rid of excess water or
solutes when they needed to regulate their osmolarity. And they also had to, especially
terrestrial ones, they also had to get rid of nitrogenous waste. Or urea. And so to do
that they used something called a protonephridia. And a protonephridia, there are going to be
holes throughout the back of the flatworm and on the underside of the flatworm as well.
And each of those are attached to a protonephridia. And what they have on one side is a flame
cell. And a flame cell has cilia on it. And as those cilia move back and forth, what it
does is it generates a current. And that current is going to move waste material, excess water,
excess solutes out, down a tubule and then it's going to eventually move out of one of
these holes. It's going to move into the environment. And so what I want you to get used to is this
idea of a tube where we have to generate some kind of a current so we can move material
that we don't want outside of that organism. And it worked great and continues to work
well for flatworms today. If we move forward from flatworms to something, to a worm that's
way more sophisticated than a flatworm, an example would be an earthworm. Earthworms
have a circulatory system. A number of segments within that. A worm itself is really, really
complex compared to a flatworm. What we find is that they have a tube as well. They've
got a metanephridia. And metanephridia will have a nephrostome on onside of a segment.
So this right here is one seta, or one segment within the worm. And so this would be what
it looks like inside the worm. And so what it's doing is it's generating a current. That
current, it's going to be moving down through a tube and eventually out through a hole.
And if you've ever looked very closely on a worm you'll actually see these holes on
the underside of the worm. Now another thing that's interesting that we've added here is
since we have a circulatory system you'll see that the circulatory system is going to
wrap around that tube. That's a way you can get rid of excess water or extra toxins within
the worm. So it can move that to its environment on the outside. And so it's essentially the
same with the addition of that circulatory system which wraps around it. And now let's
go towards something even more complex, like us, vertebrates. In vertebrates we have a
kidney. But if you look inside the kidney itself you see this functional unit called
the nephron. Well what does the nephron have in it? It's got a tube. That tube will move
all the way down and eventually moves to the bladder. And eventually moves outside of our
bodies. So think of that as like a hole. And they also have a circulatory system that wraps
around that tubule. And so we can get rid of wastes. And it also can generate current.
As the circulatory system moves in, excess material is actually going to move through
that. And so we can see continuity in this excretory system and this tubule of a flatworm,
earthworm, and vertebrates. It's doing the same thing. We have additional characteristics
that are added to that. But it's been added to it because of external environmental constraints.
One big one would be the move to terrestrial life. So that would show continuity. But also
in the evolutionary record we see change. And that's reflected in homeostatic mechanisms
as well. And so the example I've chose is the respiratory system. So what does the respiratory
system do? Remember it's got to get oxygen from the environment into your body and get
rid of carbon dioxide. Well there are a few characteristics on both water and land for
a good respiratory system. Number one it's got to be wet. And that allows diffusion.
Next it has to have a high surface area. And then the final thing that it has to have is
it has to have some kind of a thin membrane. So it's got to be a thin membrane. We also
require oxygen. But that's a basic respiratory system. And so in water it's generally gills.
But on land it's generally the lung. And so let's think about water. Well in water, is
it wet? Yeah, it's pretty wet in water. Do we have a lot of oxygen in water? Not so much.
We have a low amount of oxygen in water, but we have a high amount of moisture I'll say.
As we move on to land, is it wet? No. It's actually really, really dry. And so we're
going to have a low amount of moisture available. What about oxygen? Well on land we're going
to have way more oxygen available. And so since there are these different environmental
changes that organisms have to withstand, they couldn't just keep using gills on land,
they had to evolve a lung. Which I'll show you the progression of how that actually occurred.
And so if we look at the two strategies of respiratory organs, we've got the gills in
the fish and we have lungs in things like us and mammals and terrestrial organisms.
And so how do gills work? Well this right here is the operculum of a fish. And as the
operculum kind of moves like this, what it does is it draws the water over the gills.
And as it draws the water over the gills we're going to take oxygen out and we're going to
load carbon dioxide on that water. And we're going to get rid of it. Now they have to be
super efficient. They use a countercurrent exchange just to get all of the oxygen out
that water that we can. Because there's not a lot of oxygen available there. But since
it is a real moist environment, this whole thing is connected to the environment. In
other words as water moves through the mouth and out the operculum, it's not enclosed because
it's just sitting in water. So there's no big deal. What about on land? Well on land
we don't need to worry about oxygen. So we don't have a countercurrent exchange. But
we do have to worry about moisture. And so these things are tucked away in a mouse, tucked
away safely inside the mouse. Those are pretty big lugs. And so we don't have to worry about
losing or water loss as we move. And so there was a progression on to terrestrial. So once
we went terrestrial from aquatic, then we saw this divergence in organisms. So every
organism that lives terrestrial vertebrate, is going to have lungs. And those that don't
are still going to have gills. So we had this change within the homeostatic mechanisms.
Now you may be thinking, were do we see the change in organisms? A great example of that
would be in the lung fish. Lung fish, this one here is an Australian lung fish. What
does it have? Well it actually if you were to cut it open, it's going to have lungs but
it's also going to have a gill that works. And so it has both gill and lungs. And so
a lung fish is an example of a transitional kind of an organism. It can breathe air. So
it can come up and gulp air and use lungs to break that down. But it also can use a
gill. And so we can see something that has that transition. Tiktaalik has both gills
and or had both gills and lungs as well. Now how do we show that actually gills became
lungs? Well if you think about that, once we have this operculum right here, if you
close that operculum then we can have essentially the evolution of lungs coming from that. And
so an example of that would be like in a frog. How does a frog breathe? Well we breathe a
little bit different than a frog. The way we breathe is that we use our diaphragm muscle
to create negative pressure. And air pressure is actually going to push it in. But the way
a frog breathes is it'll actually take air into its mouth and then it will force that
down. So it's actually squeezing the air down into its lungs. And so this musculature, or
the genes that make that push in this what's called buccal breathing in a frog, genetically is
the same as the movement of the operculum or that two stroke movement of the operculum.
And so we can see not only transitional fossils but we can see in the genes how there was
this progression from aquatic animals to terrestrial animals. And so that's homeostatic mechanisms.
Again it shows evolutionary history and I hope that's helpful.