How did life emerge on Earth? How have life and Earth co-evolved through geological time? Is life elsewhere in the universe? Take a look through the 4-billion-year history of life on Earth through the lens of the modern Tree of Life! This course will evaluate the entire history of life on Earth within the context of our cutting-edge understanding of the Tree of Life. This includes the pioneering work of Professor Carl Woese on the University of Illinois Urbana-Champaign campus which revolutionized our understanding with a new "Tree of Life." Other themes include: -Reconnaissance of ancient primordial life before the first cell evolved -The entire ~4-billion-year development of single- and multi-celled life through the lens of the Tree of Life -The influence of Earth system processes (meteor impacts, volcanoes, ice sheets) on shaping and structuring the Tree of Life This synthesis emphasizes the universality of the emergence of life as a prelude for the search for extraterrestrial life.
4.3. Cambrian Fauna: Origin of Vertebrates
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From the course by University of Illinois at Urbana-Champaign
Emergence of Life
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Week 4 - Paleozoic Life After the Advent of Skeletons
This week, you'll learn more about the Cambrian Explosion, which led to the development of external hard skeleton components at 542 million years before present. The initial successes of the invertebrates were shortly followed by the appearance of vertebrates with internal skeletons. Life then utilized these newfound evolutionary capabilities, beginning distinct cycles of radiation, diversification, and extinction, which define the three great Eukarya faunas of the Phanerozoic.
Meet the Instructors
Bruce W. Fouke, Ph.D.Director of the Roy J. Carver Biotechnology Center
Department of Geology, Department of Microbiology, and Institute for Genomic Biology
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Now, the Cambrian Fauna, it's composed of organisms that we're actually
quite familiar with in the modern day.
Things like sponges, a group called cnidarians and another group of worms.
So I'm going to explain what these are now.
But they're actually organisms that almost everybody knows about.
The first one,
the sponges are used by people around the world to cleaning up a surface.
Sponges are good to use for that.
Now the original sponges that we use for human applications,
they used to be all natural sponges, they were sponges that grew on the sea floor.
And these sponges are quite remarkable, because they originated and
radiated and developed a very simple body plan that let them
take advantage of the hydrological setting they lived in on the sea floor.
So what they became were very porous networks of holes that were in
a kind of a spongy like material that was their body mass and
then buried within that were these exoskeletons.
In this case, there were small flecks called spicules.
But what they do is that they grow off the sea floor and
they put their holes above the sea floor.
And as currents move across the sea floor, the currents actually stick and
drag as they touch the sea floor and, therefore,
as you move up from the sea floor, the lowest part of the sea floor has lower
wave velocities and the upper ones have higher wave velocities.
So basically sponges developed to make a tube that ran from the low velocities to
the high velocities, and you can run this experiment yourself.
If you have water flowing in a bathtub and you make a current at the bottom and
you just put a vertical tube into it, it will automatically passively pump.
If there's a differential gradient in the velocity the water from the bottom
to the top,
then that water will automatically pump from the bottom upward and out.
So the the sponges were remarkable engineers.
They were able to develop these body plans that took place,
that that took advantage of their place on the sea floor.
Now this next group I call them cnidarians.
And cnidarians are most commonly the ones we about the other things like jelly fish
and corals.
And cnidarians are group of metazoans that make exoskeletons
that are unified by having a modified cell.
Now this cell is remarkable, it has a very sensitive outer membrane and
it's kind of clustered together within the outer tissue of, in this case,
either jellyfish or corals.
And if that outer tissue gets brushed by something, let's say it's a fish that they
want to prey on, then those cells actually explode and they release a harpoon.
So, there's a beautifully crafted harpoon right in the middle of the cell.
They come shooting out of the cell and the harpoon has prongs on it.
So when it goes into whatever it penetrates, it sticks and
you can't pull it out.
Now on top of that, that harpoon is actually attached by a tether or
small line that goes back to the cell.
And as soon as that harpoon leaves the cell, enters whatever tissue,
in this case, let's say it's a fish, that they want to eat, enters that tissue,
hooks and stays.
As soon as the tether gets taut and gets extended,
then toxin shoots from the cell through the tether into the harpoon, and
depending on how many times it gets stung by these cells, they can either slow
down or actually kill and paralyze the organism they're trying to eat.
So, these stinging cells are called nematocysts,
and the stinging cells are what unites this group called cnidarians.
And again, for our purposes, remembering that they represent jellyfish and
corals are fine.
And in the third group, worms.
Again, worms are common in all terrestrial environments almost,
especially ones we have soils in.
These organisms are the long, segmented, and they move by a contracting and
expanding muscle sets and they slide it to the ground, they like it wet.
And there's something that, again, we're pretty familiar with in the modern day.
So things like sponges and medarians and
worms were part of this early explosion of organisms.
Now some other ones that evolved the same time then became apart of this fauna,
again this grouping of animals that lived, in this case,
on the sea floor were things like brachiopods.
Now brachiopods were first described by Aristotle, and
he called them the lamp shells.
And the brachiopod has two shells that come together.
We call them valves.
Each shell is a valve.
And the cells come together and
they have a beautiful type of symmetry where if you pass the plane of symmetry
between two valves, then they'll be equivalent on both sides.
So, they open up and what we see is that in the shell though instead of having
a large mass of tissue that might be eating organic matter in different ways,
it has a very feathery structure called a lophophore.
And the lophophore within two shells is what defines the group of brachiopods.
Brachiopods have a close cousins called Bryozoans.
They also have this feathery lophophore that's, well,
it's like a feather that comes out with small tentacles off of it, and
it allows them to capture small food particles out of the sea column.
And the Bryozoans are different, though, instead of having two valves that
are together, they're actually, they live in what looks like a coffin.
One of the valves is very large and the other one's very small
then one valve opens up and they stick out their lophophore and they feed.
Another group that came on board at that same time that's extremely important to
us are the bivalves, the mollusks.
And, again, these are some of the same ones that we still have in the modern day.
These are the classic sea shells where again, they have have two valves.
And then instead of, like the Brachiopods, which have a plane of symmetry that
cuts down through the two shells, the two valves,
this one has a plane of symmetry that runs along and between the two shells.
And so the famous that's used for all kinds of decorations,
and it's a very famous piece of art, is an example of a bivalve.
The kind of clams that people eat to make seafood and
make chowder, those are all forms of bivalves.
Another group that was present in this time period are the trilobites.
And the trilobites are quite unique.
They're actually related closely to a modern day insects.
And the trilobites were basically an insect looking organism that cruised
around on the sea floor.
Some of them lived on the bottom just directly above
the sediment water interface and other ones slightly burrowed through the mud and
they were good at extracting organic matter from those environments.
Another group that came on at this time are sea urchins.
The sea biscuits, the sea urchins.
The irregular, and regular echonoids.
The echonoids mean the spiny skin ones.
And then, the groups that are very unique, in terms of going from invertebrate,
to vertebrate, the tunicates, the sea squirts.
Those are what we call the hemichordates, the ones that have a type of early
backbone only in the embryonic stages, which gets absorbed, so as a juvenile,
they're actually more less a vertebrae and as an adult they become an invertebrate.
Another one of the stars of the Cameron fauna is an organism that still lives in
the modern day oceans and where you have an ocean that floods up
onto the shoulder of a continent and makes a large inland bay.
It's one of the places where these organisms like to still live and
they're called horseshoe crabs.
So one example in North America is
Florida Bay is a place that horseshoe crabs do extremely well.
They also live up and down the entirety of the east coast of the US, but
they occur anyplace around the world where you have those right environments.
The horseshoe crabs are remarkable.
First of all, they are very, very closely related to the trilobites.
And so the trilobites went extinct at the end of the fauna,
that was one of the organisms that did very well and
then was out competed at the environment changed.
And so we don't have any modern day trilobites, but the trilobites that we do
have living are an ancestor of the trilobites called the horseshoe crabs.
And if you look at the very earliest stages of horseshoe crab development, they
look identical to many of the types of trilobites we have in the fossil record.
What we see commonly, one of strategies within evolution,
is that as time goes on and organisms evolve and
become more successful environments, some of their earlier stages of development,
they're actually pushed further into the history of the life of one organism.
So in other words, things that evolve through time as they change and
adapt to environmental constraints and competition and predation,
some of the earlier forms of what the organism looked like are actually pushed
into the early stages of the growth of the individual organisms.
And we describe that with a phrase that is ontogeny recapitulates phylogeny.
Ontogeny is the growth history of one single organism.
Phylogeny is the history of evolution of
all lineages of that organism through time.
And so, what we see is that ontogeny,
the history of the growth of one organism going from a baby to an adult.
What we see is during ontogeny the very earliest stages of phylogeny,
which is evolution from an earlier ancestor to later organisms,
we see that those natural environmental, those natural
experiments in the face of natural selection and other processes that go on.
They're actually captured and preserved in that individual organism but
they're only shown and expressed when the organism is very young and very small.
And as they grow to be older then they have different morphologies.
So the early stages of horseshoe crabs are in that same ilk.
Another thing that's critically important about horseshoe crabs is that they're
the only Metazoan that we know of that had blood based on copper,
so their blood is actually blue.
Most organisms, like ourselves, we have an iron-based blood system.
And it has to do with, the reason that's been successful is it
is a very effective means for having iron then reacts with oxygen and it's
a good way to transport oxygen throughout the internal system of the medazone.
But the blue blooded horseshoe crabs are one that have done extremely well
in terms of being successful, but they have this unique history
of having a copper based blood, which turns the blood blue.
So the horseshoe crabs are one good example of how we have a living record of
ancient organisms that are still alive and, if you will, living fossils.
But in the history of the development of any one of these living fossils going from
juvenile into an adult, we have buried within that,
the record of the earlier stages of evolution and phylogeny,
that were part of the development of these organisms.
So the horseshoe crabs have been around since the but
they have actually evolved significantly beyond that and
their earliest ancestors were very very closely related to the trilobites.
So these organisms that live on the sea floor were, they dominated and
proliferated.
And they created these great assemblages of life on the sea
floor that we call reefs.
And the word reef, R-E-E-F is a word that, again,
most people know, but is very poorly known how to define it by most people.
And the idea there is that a reef is a structure on the sea floor, or
it could be a lake floor.
But in this case it's a sea floor.
It's a structure on the sea floor that is stuck together and
rigid, and it can resist waves.
In other words, when waves come along, if there's a bump on the sea floor, that bump
is resistant enough to push back and not be pulverized and spread out by the wave.
Now, the word reef then can be a group of organisms on the sea floor, but
it can also be a group of rocks on the sea floor.
And this word reef was actually derived from the norse word rif, R-I-F and
that means the human rib.
And so a coral reef is a group of corals along with some of the other organisms
that we talked about, the brachiopods and the bivalves and the clams and
snails and all these other groups that are living on the sea floor.
So the whole notion of having a group of animals get together and
create a community on the sea floor.
And when they live and conduct their everyday life together in these
assemblages, again, they're invertebrates.
And because they have an exoskeleton, their skeletons all accumulate in place.
And it makes an accumulation of rock.
And then that accumulation of rock, based on these exoskeletons,
is wave resistant and it sticks off a sea floor.
So the idea of communities on the sea floor, when corals were dominant,
we called them coral reefs.
When brachiopods are dominant, we call them brachiopod bryozoan bivalve reefs.
Whatever makes up the major part of the community living on the sea floor,
that's what we use as their name.
So the Cambrian fauna was the initiation and the introduction to the sea floor of
these invertebrates that would live in communities, create these
accumulations and these accumulations through time become fossilized.
And so these fossilized reef communities provided us with a dramatically detailed
record of the ecology of what was happening on the sea floor.
And again, the thinking here is that these communities of the Cambrian fauna
that were developed in these reef organizational entities on the sea floor,
the date then ended up setting the template for
the early oceans within the Paleozoic.
Now, there were a whole series of other events that took place.
Things like meteor impacts.
We had additional formation of ice.
We had other events, volcanic eruptions that took place in that early Paleozoic,
the Cambrian, the Ordovician, the Silurian, and then into the Devonian.
So, as an accumulation of all these events together with the ongoing exchange
going from some organisms originating, radiating, evolving,
and then going extinct, that Cambrian community did well, but more or
less went extinct by about the time of the Devonians.
Some of them got through, but others did not.
So that Cambrian fauna set the stage for sea floor ecology.
And some of these strategies then were used by different organisms that replaced
the through geological time.
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