Chordates발음듣기
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Chordates
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You're a mammalian, amniotic, tetropodal, sarcopterygtian, osteichthyan, gnathostomal, vertebral, cranial chordate.발음듣기
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Yeah, it's a mouthful and in order to understand what it means, you're gonna have to understand the most complex group of animals on earth, and what it takes to get from this to this.발음듣기
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(lively music) The phyllum chordata accounts for all 52,000 species of vertebrates on earth, and several thousand species of invertebrates.발음듣기
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Together, they range from tiny brainless filter feeders, all the way up to Scarlet Johansson.발음듣기
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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.발음듣기
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Just like with tissue layers, and segmentation in simpler animals, there are traits we can look for to track the evolution of chordates.발음듣기
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By the time all of those traits appear in one organism, we'll have arrived at the most complex class within the most complex phylum, the mammals.발음듣기
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We've talked before about synapamorphic traits, traits that set a group of animals apart from its ancestors, and from other groups that came from the same ancestors.발음듣기
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Lancelets, also known as cephalochordata - literally head cords - are one of the three sub-phyla of chordates.발음듣기
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Unlike almost all other chordates, these tiny, brainless, invertebrate filter feeders retain all four of these characteristics for their entire lives.발음듣기
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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 the spinal cord.발음듣기
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First, there's the notochord, a structure made of cartilage, that runs between an animal's digestive tube and its nerve cord.발음듣기
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In most vertebrates, a skeleton develops around the notochord and allows the muscles to attach.발음듣기
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In humans, the notochord is reduced to the disks, the cartilage that we have beween our vertebra.발음듣기
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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.발음듣기
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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.발음듣기
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In fish and other aquatic animals, they're heel slits, and in land-dwelling vertebrates like us, they disappear before we're born.발음듣기
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But, that tissue develops into areas around our jaws, ears, and other structures in the head and neck.발음듣기
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Finally, we can't forget our fourth synapomorphy, the post-anal tail. which is exactly what it sounds like.발음듣기
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It helps propel aquatic animals through the water, 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.발음듣기
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These four traits all began to appear during the Cambrian explosion, more than 500 million years ago.발음듣기
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Today, they're shared by members of all three chordate sub-phyla, even if the animals in those sub-phyla look pretty much nothing like each other.발음듣기
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For instance, our new friends here in cephalochordata are the oldest living sub-phyla, but you can't forget the other invertebrate group of chordates, the urochordata literally, tail cords.발음듣기
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If you're confused about why this ended up in a phylum with us, it's because they have tadpole-like larva with all four chordate characteristics.발음듣기
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The adults, which actually have a highly-developed internal structure, with a heart and other organs, retain the pharyngeal slits, but all the other chordate features disappear or reform into other structures.발음듣기
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The third and last, and most complex sub-phylum, 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.발음듣기
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You can see how fantastic this diversity really is when you break down vertebrata into its many, many classes, from slimy sea snake-y things to us warm and fuzzy mammals.발음듣기
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As these classes become more complex, you can identify the traits they each develop that gave them an evolutionary edge over the ones that came before.발음듣기
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Since this is science, you're gonna have to know that there's an exception for every rule that you're gonna have to remember.발음듣기
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The exception in this case is the myxini, or hagfish, the only vertebrate class that has no vertebra, but is classified with us because it has a skull.발음듣기
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This snake-like creature swims by using segmented muscles to exert force against its notochord. Whatever, hagfish.발음듣기
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Closely related to it is the class petromyzontida, otherwise known as lampreys, the oldest-living lineage of vertebrates.발음듣기
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Now, these have a backbone made of cartilage, and, maybe even more important, a more complex nervous system.발음듣기
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With the advent of a backbone, we see vertebrates getting larger, developing more complex skeletons, and becoming more effective at catching food and avoiding predators.발음듣기
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Lampreys, and other early vertebrates, are agnafins, literally no jaws, and if you want to be able to chew food, it really helps to have a jaw and teeth.발음듣기
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Most scientists think that the jaw evolved from structures that supported the first two pharyngeal slits near the mouth.발음듣기
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Gnathostomes, or jaw mouths, arrived on the scene 470 million years ago, and one of the oldest, most successful groups of gnathostomes that have survived to present day are the class chondrichthyes - the cartilage fish.발음듣기
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You know them as the sharks, and 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.발음듣기
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Chondrichthyes haven't changed much in the past 300 million years or so, and their success stems from the paired fins that allow for fish and swimming, and those jaws for biting off delicious hunks of flesh.발음듣기
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If we're gonna eventually get to the mammals, we need bones, and we find those with the evolution of fish.발음듣기
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Now, osteoichthyes is sometimes considered a super class, because it includes a whole slew of diverse classes that descended from it.발음듣기
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The main thing to know is that the majority of all vertebrates fall under osteoichthyes, and that includes you.발음듣기
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The first is the actinopterygii, or ray-finned fishes, and with 27,000 species, pretty much every fish you've ever heard of is in this group.발음듣기
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Ray-finned fishes evolved in fresh water, spread out into the oceans, and some eventually then came back to fresh water.발음듣기
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In the second group, things started to get really strange and interesting These are the lobe-fin fishes, or the sarcopterygii, a name dervied from bones surrounded by muscle found in their pectoral and pelvic fins, and that sounds like something that could be used for walking.발음듣기
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Lobe-fins include the coelacanth, which consists of one living species; lung-fish, which gulp air into their lungs; and tetrapods, which have adapted to land with four limbs.발음듣기
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This is weird, right, even though land animals clearly are not fish, since tetrapods evolved from bony fish, they are filed under this group.발음듣기
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But first - (lively music) Young man: Imagine that you're a fisherman off the coast of South Africa, in the western Indian Ocean, about 75 years ago.발음듣기
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Not only that, you caught a fish that was thought to have become extinct 75 million years ago.발음듣기
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This is exactly what happened in 1938, when Captain Hendrick Gooson hauled up a coelacanth, and it has mystified scientists ever since.발음듣기
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A second population has since been identified near Indonesia in 1999, but the deep-sea creatures remain extremely rare.발음듣기
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In fact, those paired fins are supported by the very same bones that we have in our arms and legs.발음듣기
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The coelacanth also has a hinged joint in its skull so it can widen its mouth to eat large prey, as well as thick scales that don't exist on any living fish.발음듣기
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All right, now we're talking about tetrapods, which, of course, means four feet, and getting those four feet on to land was really awesome for those early creatures, because that meant that they could escape the increasingly brutal and predatory world of the ocean.발음듣기
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Tetrapods gradually replaced their fins with limbs, and developed entirely new body parts that were never seen before - like necks.발음듣기
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The first tetrapods are today found in the class amphibia, which were the first creatures to develop a three-chambered heart.발음듣기
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Thee 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, and lungs and a digestive system, and often migrate to land for adulthood.발음듣기
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But amphibians lay eggs that don't have shells, so they dehydrate quickly, so they have to be laid in water.발음듣기
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Amniotes are tetrapods that have eggs adapted for life on land, a group that includes reptiles, birds and mammals.발음듣기
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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.발음듣기
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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.발음듣기
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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.발음듣기
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One includes the modern reptiles crocodiles and alligators, and the other is a type of dinosaur that we now call birds, the class aves.발음듣기
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One is designed for eating and fighting big animals, while the other is designed for flying around, and being graceful and stuff.발음듣기
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A not so obvious, but equally important, difference is that birds are endotherms, which means they can crank up their metabolism to regulate their body temperature.발음듣기
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Making all that heat requires a big furnace, which is provided thanks to the evolution of a four-chambered heart.발음듣기
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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.발음듣기
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I'm talking, of course, about the class mammalia, otherwise known as amniotes that have hair, three special ear bones, and mammary glands.발음듣기
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Most mammals have evolved to dispense with the hard eggshell altogether, the embryo avoiding predation and other environmental dangers by developing inside the mother's body.발음듣기
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In this class of chordates, you'll find me, Dame Judy Dench, your dog, your cat, Shamu the Orca, African elephants, the South American pudu, and 53 hundred other known species of mammals.발음듣기
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