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Video transcript

next time someone asks you who you think you are just give them the facts you're a mammalian amniotic tetra POTUS our copter ægean hasta aqui and not the stoma vertebrate cranial core date 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 and 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 will have arrived at the most complex class within the most complex phylum the mammals but first let's start with the fundamentals we've talked before about synapomorphy traits traits that set a group of animals apart from its ancestors and from other groups that came from the same ancestors chordates share for synapomorphies that make us who we are each of them is present at some point in every chordates lifecycle how about a volunteer to demonstrate these traits ah I see that the Lancelot's are raising their mouthparts lanceolate also known as cephalic or daata 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 cord first there's the notochord a structure made of cartilage that runs between an animal's digestive tube and it's 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 discs of cartilage that we have between our vertebra 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 the lanceolate here they function as filters for feeding and fish other aquatic animals their 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 structure in the head and net and finally we can't forget our fourth synapomorphy the post anal tail which is exactly what it sounds like it helps propel aquatic animal through the water makes our dog look happy when she wags it and in humans age drinks during embryonic development and to what is known as the coccyx or tail bone straight 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 chordates subphyla even if the animals in those sub phyla look pretty much nothing like each other for instance our new friends here in cephalic or da da are the oldest living subphylum but you can't forget the other invertebrate group of chordates the aura Chordata literally tail cords there are over 2,000 species here including sea squirts and 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 core date characteristics the adults which actually have a highly developed internal structure with a heart and other organs retain the pharyngeal slits but all the other core date features disappear or reformed into other structures the third and last and most complex subphylum is the vertebra and has the most species in it because its members have a hard backbone which is 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 vertebra into its many many classes from slimy sea snaky things to us warm and fuzzy mammals and 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 for example how's this for an awesome trait a brain vertebrate with a head that contains sensory organs and a brain are called Craney aids they also always have a heart with at least two chambers so this is science you gotta 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 mixin E or hagfish the only vertebrate class that has no vertebra 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 petram eyes dadah 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 the advent of a back when we see vertebrates getting larger developing more complex skeletons and becoming more effective at catching food and avoiding predators but you notice anything missing lampreys and other early vertebrates are Agnetha's 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 now the stones or jaw mouths arrived on the scene 470 million years ago and one of the oldest and most successful groups of Malthus stones that have survived the present day are the class chondrichthyes the cartilage fish 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 contract Dean's 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 budding off delicious hunks of flesh if we're going to eventually get to the mammals we need bones and we find those with the evolution of fish meat Ostia keys which technically means the bony fish unlike cartilaginous fish members of this group have a mineralized endoskeleton nastya keys 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 the main thing to know is that the majority of all vertebrates fall under Ostia keys and that includes you it's broken up into two main groups which themselves include a bunch of classes the first is the actin after a g or ray-finned fishes and with 27 thousand species pretty much every fish you've ever heard of his in this group ray-finned fishes evolved in freshwater spread out into the oceans and some eventually then came back to freshwater and the second group things start to get really strange and interesting these are the lobe-finned fishes or thus our copter adji a name derived from bones surrounded by muscle found in their pectoral and pelvic fins and that sounds like something that could be used walking lobe fins include the coelacanths which consists of one living species lung fish which gulp air into their lungs and tetrapods which have adapted to land with four limbs this is weird right even though land animals clearly are not fish since tetrapods evolved from bony fish they are filed under this group this tax on us man I want to party with them sometime but first imagine that you're a fisherman off the coast of South Africa and the western Indian Ocean about 75 years ago let's put that in your brain 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 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 hinge joint to 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 eaten but why would you want to eat what's essentially a living fossil alright and 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 developed legs and lungs in a digestive system and often migrate to land for adulthood but amphibians lay eggs that have shells so they dehydrate quickly so they have to be laid in water so this leads us to our next evolutionary milestone for the chordates the amniotic egg amniotes are tetrapods that have eggs adapted for life on land a group that includes reptiles birds and mammals the yam meiotic 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 claws Reptilia represents the earliest amniotes like amphibians they have a three chambered heart but they're totally terrestrial and here's where we find our dinosaurs and our snakes and turtles and lizards now the here 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 on the Sun all day the oldest group of reptiles the arca soars mostly disappeared when most of the dinosaurs died out 65 million years ago but two lineages of argus Orr's survived one includes the modern reptiles crocodiles and alligators and the other is a type of dinosaur that we now call birds the class of ace there are big obvious differences between these two surviving Argus ores one is designed for like eating and fighting big animals while the other is designed for flying around it being graceful and stuff but not so obvious but equally important differences that birds are endotherms which means that they can crank up their metabolism to regulate their body temperature making all that heat requires big furnace which is provided thanks to the evolution of a four chambered heart there's only one other group of animals that develop this trait independently of birds by the way and it allowed them to spread through the planet and 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 eggshell 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 ago in this crazy core date phylum we finally made it and now you know exactly who you are you
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