Januari 30, 2017
next time someone asksyou who you think you are, just give them the facts. you're a mammalian amniotictetrapodal sarcopterygiian osteichthyan gnathostomalvertebrate cranial chordate. yeah, it's a mouthful. and in order to understandwhat it means, you're going to have to understand the mostcomplex group of animals on earth, and what it takes toget from this to this. the phylum chordata accounts forall 52,000 species of vertebrates
on earth and several thousandspecies of invertebrates. together they range fromtiny, brainless filter feeders all the way upto scarlett johansson. now, you know by now that whenwe talk about classifying animals, we're really talking abouttheir shared ancestry, each new branch on thistree marking an important new evolutionary milestone. and just like with tissuelayers and segmentation in simpler animals, thereare traits we can look for
to track theevolution of chordates. by the time all of thosetraits appear in one organism, we'll have arrived atthe most complex class within the most complexphlyum: the mammals. but first let's startwith the fundamentals. we've talked before aboutsynapomorphic traits: traits that set a group ofanimals apart from its ancestors and from other groups thatcame from the same ancestors. chordates share foursynapomorphies that
make us who we are. each of them ispresent at some point in every chordate's life cycle. how about a volunteerto demonstrate these traits? ah, i see that the lanceletsare raising their...mouthparts. the lancelets, alsoknown as cephalochordata, literally "head-cords," are one ofthe three sub-phyla of chordates. and unlike almost all otherchordates, these tiny, brainless, invertebrate filterfeeders retain all four
of these characteristicsfor their entire lives. you probably already knowwhere most of these traits are going to appear, sincethe phylum is named after it: the spinal cord, or at leastsomething that resembles a spinal chord. first there's the notochord,a structure made of cartilage that runs between an animal'sdigestive tube and its nerve cord. in most vertebrates, a skeletondevelops around the notochord and allows the muscles to attach.
in humans, the notochordis reduced to the disks of cartilage that we havebetween our vertebrae. second, we have thenerve cord itself, called the dorsal hollow nerve corda tube made of nerve fibers that develops into thecentral nervous system. this is what makes chordatesdifferent from other animal phyla, which havesolid, ventral nerve cords, meaning they run alongthe front or stomach side. third, all chordateshave pharyngeal slits.
in invertebrates like ourlancelet here, they function as filters for feeding. in fish and other aquaticanimals, they're gill slits, and in land-dwelling vertebrateslike us they disappear before we're born, but thattissue develops into areas around our jaws, ears, and otherstructures in the head and neck. and finally, we can't forgetour fourth synapomorphy, the post-anal tail, which isexactly what it sounds like. it helps propel aquaticanimals through the water,
it makes our dog look happywhen she wags it, and in humans it shrinks during embryonicdevelopment into what is known as the coccyx, or tailbone. it's right here. and trust me, when itcomes to tail placement, post-anal is the way to go. these four traitsall began to appear during the cambrian explosionmore than 500 million years ago, and today they're shared by membersof all three chordate subphyla
even if the animalsin those subphyla look pretty much nothing like each other for instance, our new friendshere in cephalochordata are the oldest living subphylum,but you can't forget the other invertebrate groupof chordates, urochordata, literally "tail-cords." there are over 2,000 species here,including sea squirts. if you're confused about whythis ended up in a phylum with us, it's because theyhave tadpole-like larvae
with all fourchordate characteristics. the adults, which actuallyhave a highly developed internal structure, witha heart and other organs, retain the pharyngeal slitsbut all other chordate features disappear or reforminto other structures. the third and last and most complexsubphylum, is the vertebrata, and has the most speciesin 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 fantasticthis diversity really is when you break down vertebratainto its many, many classes from slimy, sea-snakey thingsto us warm and fuzzy mammals. and as these classes becomemore complex, you can identify the traits they eachdeveloped that gave them an evolutionary edge overthe ones that came before. for example, how's this foran awesome trait: a brain! vertebrates with a head thatcontains sensory organs and
a brain are called craniates. they also always have a heartwith at least two chambers. since this is science,you're going to have to know that there's going to be anexception for every rule that you're goingto have to remember. and the exception in thiscase is the myxini, or hagfish the only vertebrate classthat has no vertebrae but is classified with usbecause it has a skull. this snake-like creature swimsby using segmented muscles
to exert force againstits notochord. whatever, hagfish. closely-related to itis 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 backbonewe see vertebrates getting larger, developing more complexskeletons, and becoming
more effective at catchingfood and avoiding predators. but do you noticeanything missing? lampreys and other earlyvertebrates are agnathans, literally "no-jaws." and if you want to be able tochew food, it really helps to have a jaw and teeth. most scientists think thatthe jaw evolved from structures that supported the first twopharyngeal slits near the mouth. and teeth? well, the currenttheory 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 ofgnathostomes that have survived to the present day arethe class chondrichthyes, the "cartilage fish". you know them as sharks, skatesand rays, and as their name says, their skeleton is madeup mostly of cartilage, but they show the beginningsof a calcified skeleton.
chondrichthyans haven'tchanged much over the past 300 million years or so,and their success stems from the paired fins thatallow for efficient swimming, and those jaws for bitingoff delicious hunks of flesh. if we're going toeventually get to mammals, we need bones, and we find thosewith the evolution of fish. meet osteichthyes, whichtechnically means "the bony fish" unlike cartilaginous fish,members of this group have a mineralized endoskeleton.
now, osteichthyes is sometimesconsidered a superclass, because it includes a wholeslew of diverse classes that descended from it. there's actually somecontroversy among taxonomists about what to call it,but the main thing to know is that the majorityof all vertebrates fall under osteichthyes,and that includes you. it's broken up intotwo main groups, which themselves includea bunch of classes.
the first is actinopterygii,or ray-finned fishes, and with 27,000 species,pretty much every fish you've ever heard ofis in this group. ray-finned fishesevolved in fresh water, spread out into the oceans, and some eventually cameback to fresh water. in the second group, thingsstart to get really strange and interesting. these are the lobe-finnedfishes, or sarcopterygii,
a name derived from bonessurrounded by muscle found in their pectoral and pelvic fins. and that sounds like somethingthat could be used for walking! lobe-fins include the coelacanths, which consist of one living species lungfish, which gulpair into their lungs; and tetrapods, which haveadapted to land with four limbs. so this is weird, right? even though land animalsclearly are not fish,
since tetrapods evolvedfrom bony fish, they are filed under this group. these taxonomists, man. i want to partywith them sometime. but first: imagine that you're a fishermanoff the coast of south africa in the western indianocean about 75 years ago. just put that in yourbrain and hold on to it. and you've just pulled up afish that no one has ever seen.
not only that, you've caughta fish that was thought to have become extinct75 million years ago. this is exactly whathappened in 1938, when captain hendrickgoosen hauled up a coelacanth, and it has mystifiedscientists ever since. a second population has sincebeen identified near indonesia in 1999, but the deep-seacreatures remain extremely rare. the coelacanth fascinatesscientists because of its paired lobe fins.
they extend from thebody like legs and move in an alternating pattern. in other words, they move morelike a horse than like a fish. and in fact thosepaired fins are supported by the very same bones thatwe have in our arms and legs. the coelacanth also has ahinged joint in the skull so it can widen itsmouth to eat large prey, as well as thick scales that don'texist on any living fish. it's not good eating, but whywould you want to eat what's
essentially a living fossil? alright, now we're talking abouttetrapods, which of course means "four feet," and gettingthose four feet onto land was really awesome for thoseearly creatures because that meant that they could escapethe increasingly brutal and predatory world of the ocean. tetrapods gradually replacedtheir fins with limbs, and developed entirelynew body parts that were never seenbefore, like necks,
with the help ofadditional vertebra, that separated thebody from the head. the first tetrapods are todayfound in the class amphibia, which were the first creatures todevelop a three-chambered heart. there are more than 6,000known species of amphibians like frogs and salamanders,most of which begin life as tadpoles in water, andthen later develop legs, lungs and a digestivesystem, and often migrate to land for adulthood.
but amphibians lay eggsthat don't have shells, so they dehydrate quickly, sothey have to be laid in water. so this leads us to ournext evolutionary milestone for chordates: the amniotic egg. amniotes are tetrapods that haveeggs adapted for life on land a group that includesreptiles, birds and mammals. the amniotic egg was crucialfor the success of land-dwellers, allowing embryos to developin their own "private pond" of the amniotic sac, oftensurrounded by a hard shell
in the case of reptiles and birds. the class reptilia representsthe earliest amniotes. like amphibians, theyhave a 3-chambered heart, but they're totally terrestrial. and here's where we findour dinosaurs, snakes, turtles and lizards. you often hear reptilesdescribed as "cold-blooded." this does not mean thattheir blood is cold. they're actually ectothermic,which means that they absorb
external heat as theirmain source of body heat. hence the lizard that likesto lay in the sun all day. the oldest group of reptiles,the archosaurs, mostly disappeared when most of the dinosaursdied out 65 million years ago. but two lineagesof archosaurs survived. one includes the modernreptiles crocodiles and alligators, and the other is a type ofdinosaur that we now call birds, the class aves. there are big, obvious differences
between these twosurviving archosaurs: one is designed for eatingand fighting big animals, while the other isdesigned for flying around and being graceful and stuff. the not-so-obvious butequally important difference is that birds are endotherms,which means that they can crank up their metabolism toregulate their body temperature. making all that heat requiresa big furnace, which is provided thanks to the evolutionof a four-chambered heart.
there's only one other group ofanimals that developed this trait, independently of birds, bythe way, and it allowed them to spread through the planet. i'm talking, of course,about the class mammalia, otherwise known asamniotes that have hair, three special earbones and mammary glands. and most mammals haveevolved to dispense with the hard egg shell altogether the embryo avoidingpredation and other
environmental dangers by developinginside the mother's body. in this class of chordatesyou'll find me, dame judi dench, your dog, your cat, shamu theorca, african elephants, the south american pudu, and 5,300other known species of mammals. it all began with asimple ancestor more than 500 million years in thiscrazy chordate phylum, but we've finally made it! and now you knowexactly who you are. thank you for watching thisepisode of crash course biology,
we hope that it was helpful andthat you feel like a smarter person there's review stuff nextto me that you can click on to go to those parts of the video that you maybe want towatch a little bit more of; to reinforce it in your brain. thank to everybody whohelped put this together. and if you have any questionsfor us you can get in touch with us on facebook or twitter. or, of course, downin the comments below.
goodbye.
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