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origin of vertebrates

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525 million years ago during the Cambrian explosion

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saw the rise in organism diversity

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humans, other mammals, reptiles, birds, amphibians, and fish are all part of a large clade of animals called the vertebrata, or vertebrates

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vertebrates are part of a larger clade called the Chordata

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animals that have a stiff chord in their back

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a sea squirt is more closely related to humans than a clam, a jellyfish or a lobster

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attach themselves to a spot on the sea floor and filter food particles out of the water around them

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yet juvenile or larval sea squirts can swim freely or float in the water

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have a structure called a notochord, a stiff but flexible rod made out of fibers of a material called collagen

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notochord is a useful thing to have because it provides an anchoring point for muscles to attach to and pull against

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the notochord is the beginning of the vertebrale indoskeleton

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this is different than the hard exoskeletons found in invertebrates like crabs, spiders and insects

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eventually the notochord was structurally replaced by the vertebrae of your backbone

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but even you had a notochord for a little while when you were a developing embryo

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above the notochord is another one of the important features found only in chordates

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the hollow dorsal nerve chord

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some animals like earthworms and insects have a nerve chord that is solid and located ventrally in the animal

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so it runs along the belly

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but chordates have a nerve that run along our back dorsally

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it is hollow and sits above the notochord

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in humans it is enclosed by the vertebrae of our backbone

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called the vertebral column, or the spinal

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a shared ancestory between two or more different features or traits in different organisms

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a charactertistic shared by some members of a larger group, a shared derived character

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humans and birds are both vertebrates

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we inherited our skeletons from a common vertebrate ancestor

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our skeleton is homologous with that of a bird

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humans and kangaroos are also part of a smaller group called mammals

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we share fur with kangaroos which birds don't

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convergent characteristics

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appear similar but arose separately

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results in analogous traits

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similar due to a shared function

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insects and birds both have wings, but they didn't inherit their wings from a winged, common ancestor

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so insect and bird wings are analogous traits

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wings of birds, bats and terrasaurs

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if we look at these animals on a simplified, philogenetic tree

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the last ancestor these animals shared was a primitive, terrestrial tetrapod with walking limbs

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these animals didn't inherit their wings from a single, common, winged ancestor

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the wings evolved independently in this distantly related lineages

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therefore these are convergent, or analogous features

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but take a look at their arm bones

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they all have the same limb bones

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a humorous, radius, carpals, metacarpals, and phalanges

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they are arranged in the same way even though the shape of the bones are different

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the bones of the similar because they inherited them from their common ancestor

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the same feature can be homologous at one level, and analogous at another

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these features are homologous as wings, and analogous as limbs

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our chordate relatives

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the notochord and the dorsal hollow nerve cord

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features found in all chordates

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therefore homologous characters within vertebrates

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in humans, the notochord is no longer present

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has been functionally replaces by our vertebral column

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if you were to cut a larval sea squirt in half

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you would see a series of narrow openings

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these are pharyngeal slits, which become the gill openings in fishes

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fishes like the shark have gills used for respiration, transferring oxygen into the bloodstream

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sea squirts use their pharyngeal slits for eating, they bring water into the pharynx and filter food out of it

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pharyngeal slits are one of the characteristic features of chordates

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terrestrial chordates lost their gills

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if you look at a human embryo, you'll see the beginnings of the pharyngeal slits for a little while during development

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a long groove on teh bottom side of the pharynx

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foudn in sea squirts and some larval vertebrates like the lampry

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the endostyle is lined witih tiny hair-like structures called cilia

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these spread a coating fo mucus that is secreted by cells which traps food particles

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the endostyle is homologous with our thyroid gland

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our thyroid gland regulates our metabolism

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in many animals, the digestive system ends in the anus which is at the end of their body

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chordates have a post-anal tail

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so we share with chordates the fact that our bodies continue beyond the end of the digestive tract

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humans don't have tails when we are born, but we do have tails when we are embryos

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sea squirts and humans look very different

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took very different evolutionary pathways

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but we still have some features in common during our early embryonic development

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those shared features show our shared evolutionary history as chordate animals