Next time someone asks
you who you think you are, just give them the facts. You're a mammalian amniotic
tetrapodal sarcopterygiian osteichthyan gnathostomal
vertebrate cranial chordate. Yeah, it's a mouthful. And in order to understand
what it means, you're going to have to understand the most
complex group of animals on earth, and what it takes to
get from this to this. The phylum Chordata accounts for
all 52,000 species of vertebrates on earth and several thousand
species of invertebrates. Together they range from
tiny, brainless filter feeders all the way up
to Scarlett Johansson. Now, you know by now that when
we talk about classifying animals, we're really talking about
their shared ancestry, each new branch on this
tree marking an important new evolutionary milestone. And just like with tissue
layers and segmentation in simpler animals, there
are traits we can look for to track the
evolution of chordates. By the time all of those
traits appear in one organism, we'll have arrived at
the most complex class within the most complex
phlyum: the mammals. But first let's start
with the fundamentals. We've talked before about
synapomorphic traits: traits that set a group of
animals apart from its ancestors and from other groups that
came from the same ancestors. Chordates share four
synapomorphies that make us who we are. Each of them is
present at some point in every chordate's life cycle. How about a volunteer
to demonstrate these traits? Ah, I see that the lancelets
are raising their...mouthparts. The lancelets, also
known as cephalochordata, literally "head-cords," are one of
the three sub-phyla of chordates. And unlike almost all other
chordates, these tiny, brainless, invertebrate filter
feeders retain all four of these characteristics
for their entire lives. You probably already know
where most of these traits are going to appear, since
the phylum is named after it: the spinal cord, or at least
something that resembles a spinal chord. First there's the notochord,
a structure made of cartilage that runs between an animal's
digestive tube and its nerve cord. In most vertebrates, a skeleton
develops around the notochord and allows the muscles to attach. In humans, the notochord
is reduced to the disks of cartilage that we have
between our vertebrae. Second, we have the
nerve cord itself, called the dorsal hollow nerve cord
a tube made of nerve fibers that develops into the
central nervous system. This is what makes chordates
different from other animal phyla, which have
solid, ventral nerve cords, meaning they run along
the front or stomach side. Third, all chordates
have pharyngeal slits. In invertebrates like our
lancelet here, they function as filters for feeding. In fish and other aquatic
animals, they're gill slits, and in land-dwelling vertebrates
like us they disappear before we're born, but that
tissue develops into areas around our jaws, ears, and other
structures in the head and neck. And finally, we can't forget
our fourth synapomorphy, the post-anal tail, which is
exactly what it sounds like. It helps propel aquatic
animals through the water, it makes our dog look happy
when she wags it, and in humans it shrinks during embryonic
development into what is known as the coccyx, or tailbone. It's right here. And trust me, when it
comes to tail placement, post-anal is the way to go. These four traits
all began to appear during the Cambrian explosion
more than 500 million years ago, and today they're shared by members
of all three chordate subphyla even if the animals
in those subphyla look pretty much nothing like each other For instance, our new friends
here in cephalochordata are the oldest living subphylum,
but you can't forget the other invertebrate group
of chordates, urochordata, literally "tail-cords." There are over 2,000 species here,
including sea squirts. If you're confused about why
this ended up in a phylum with us, it's because they
have tadpole-like larvae with all four
chordate characteristics. The adults, which actually
have a highly developed internal structure, with
a heart and other organs, retain the pharyngeal slits
but all other chordate features disappear or reform
into other structures. The third and last and most complex
subphylum, is the Vertebrata, and has the most species
in it because its members have a hard backbone,
which has allowed for an explosion in diversity,
from tiny minnows to the great blue whale. You can see how fantastic
this diversity really is when you break down Vertebrata
into its many, many classes from slimy, sea-snakey things
to us warm and fuzzy mammals. And as these classes become
more complex, you can identify the traits they each
developed that gave them an evolutionary edge over
the ones that came before. For example, how's this for
an awesome trait: a brain! Vertebrates with a head that
contains sensory organs and a brain are called craniates. They also always have a heart
with at least two chambers. Since this is science,
you're going to have to know that there's going to be an
exception for every rule that you're going
to have to remember. and the exception in this
case is the Myxini, or hagfish the only vertebrate class
that has no vertebrae but is classified with us
because it has a skull. This snake-like creature swims
by using segmented muscles to exert force against
its notochord. Whatever, hagfish. Closely-related to it
is the class petromyzontida, otherwise known as lampreys,
the oldest living lineage of vertebrates. Now these have a backbone,
made of cartilage, and maybe even more important,
a more complex nervous system. With the advent of the backbone
we see vertebrates getting larger, developing more complex
skeletons, and becoming more effective at catching
food and avoiding predators. But do you notice
anything missing? Lampreys and other early
vertebrates are agnathans, literally "no-jaws." And if you want to be able to
chew food, it really helps to have a jaw and teeth. Most scientists think that
the jaw evolved from structures that supported the first two
pharyngeal slits near the mouth. And teeth? Well, the current
theory is that they evolved from sharp scales on the face! Gnathostomes, or "jaw-mouths",
arrived on the scene 470 million years ago,
and one of the oldest and most successful groups of
gnathostomes that have survived to the present day are
the class chondrichthyes, the "cartilage fish". You know them as sharks, skates
and rays, and as their name says, their skeleton is made
up mostly of cartilage, but they show the beginnings
of a calcified skeleton. Chondrichthyans haven't
changed much over the past 300 million years or so,
and their success stems from the paired fins that
allow for efficient swimming, and those jaws for biting
off delicious hunks of flesh. If we're going to
eventually get to mammals, we need bones, and we find those
with the evolution of fish. Meet Osteichthyes, which
technically means "the bony fish" unlike cartilaginous fish,
members of this group have a mineralized endoskeleton. Now, Osteichthyes is sometimes
considered a superclass, because it includes a whole
slew of diverse classes that descended from it. There's actually some
controversy among taxonomists about what to call it,
but the main thing to know is that the majority
of all vertebrates fall under Osteichthyes,
and that includes you. It's broken up into
two main groups, which themselves include
a bunch of classes. The first is Actinopterygii,
or ray-finned fishes, and with 27,000 species,
pretty much every fish you've ever heard of
is in this group. Ray-finned fishes
evolved in fresh water, spread out into the oceans, and some eventually came
back to fresh water. In the second group, things
start to get really strange and interesting. These are the lobe-finned
fishes, or sarcopterygii, a name derived from bones
surrounded by muscle found in their pectoral and pelvic fins. And that sounds like something
that could be used for walking! Lobe-fins include the coelacanths, which consist of one living species lungfish, which gulp
air into their lungs; and tetrapods, which have
adapted to land with four limbs. So this is weird, right? Even though land animals
clearly are not fish, since tetrapods evolved
from bony fish, they are filed under this group. These taxonomists, man. I want to party
with them sometime. But first: Imagine that you're a fisherman
off the coast of South Africa in the western Indian
Ocean about 75 years ago. Just put that in your
brain and hold on to it. And you've just pulled up a
fish that no one has ever seen. Not only that, you've caught
a fish that was thought to have become extinct
75 million years ago. This is exactly what
happened in 1938, when Captain Hendrick
Goosen hauled up a coelacanth, and it has mystified
scientists ever since. A second population has since
been identified near Indonesia in 1999, but the deep-sea
creatures remain extremely rare. The coelacanth fascinates
scientists because of its paired lobe fins. They extend from the
body like legs and move in an alternating pattern. In other words, they move more
like a horse than like a fish. And in fact those
paired fins are supported by the very same bones that
we have in our arms and legs. The coelacanth also has a
hinged joint in the skull so it can widen its
mouth to eat large prey, as well as thick scales that don't
exist on any living fish. It's not good eating, but why
would you want to eat what's essentially a living fossil? Alright, now we're talking about
tetrapods, which of course means "four feet," and getting
those four feet onto land was really awesome for those
early creatures because that meant that they could escape
the increasingly brutal and predatory world of the ocean. Tetrapods gradually replaced
their fins with limbs, and developed entirely
new body parts that were never seen
before, like necks, with the help of
additional vertebra, that separated the
body from the head. The first tetrapods are today
found in the class Amphibia, which were the first creatures to
develop a three-chambered heart. There are more than 6,000
known species of amphibians like frogs and salamanders,
most of which begin life as tadpoles in water, and
then later develop legs, lungs and a digestive
system, and often migrate to land for adulthood. But amphibians lay eggs
that don't have shells, so they dehydrate quickly, so
they have to be laid in water. So this leads us to our
next evolutionary milestone for chordates: the amniotic egg. Amniotes are tetrapods that have
eggs adapted for life on land a group that includes
reptiles, birds and mammals. The amniotic egg was crucial
for the success of land-dwellers, allowing embryos to develop
in their own "private pond" of the amniotic sac, often
surrounded by a hard shell in the case of reptiles and birds. The class Reptilia represents
the earliest amniotes. Like amphibians, they
have a 3-chambered heart, but they're totally terrestrial. And here's where we find
our dinosaurs, snakes, turtles and lizards. You often hear reptiles
described as "cold-blooded." This does not mean that
their blood is cold. They're actually ectothermic,
which means that they absorb external heat as their
main source of body heat. Hence the lizard that likes
to lay in the sun all day. The oldest group of reptiles,
the archosaurs, mostly disappeared when most of the dinosaurs
died out 65 million years ago. But two lineages
of archosaurs survived. One includes the modern
reptiles crocodiles and alligators, and the other is a type of
dinosaur that we now call birds, the class Aves. There are big, obvious differences between these two
surviving archosaurs: one is designed for eating
and fighting big animals, while the other is
designed for flying around and being graceful and stuff. The not-so-obvious but
equally important difference is that birds are endotherms,
which means that they can crank up their metabolism to
regulate their body temperature. Making all that heat requires
a big furnace, which is provided thanks to the evolution
of a four-chambered heart. There's only one other group of
animals that developed this trait, independently of birds, by
the way, and it allowed them to spread through the planet. I'm talking, of course,
about the class Mammalia, otherwise known as
amniotes that have hair, three special ear
bones and mammary glands. And most mammals have
evolved to dispense with the hard egg shell altogether the embryo avoiding
predation and other environmental dangers by developing
inside the mother's body. In this class of chordates
you'll find me, dame Judi Dench, your dog, your cat, Shamu the
orca, African elephants, the South American pudu, and 5,300
other known species of mammals. It all began with a
simple ancestor more than 500 million years in this
crazy chordate phylum, but we've finally made it! And now you know
exactly who you are. Thank you for watching this
episode of Crash Course Biology, We hope that it was helpful and
that you feel like a smarter person There's review stuff next
to me that you can click on to go to those parts of the video that you maybe want to
watch a little bit more of; to reinforce it in your brain. Thank to everybody who
helped put this together. And if you have any questions
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