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C O U R S E 
Paleontology: Early Vertebrate Evolution
Alison Murray, University of Alberta
C O U R S E   L E C T U R E 
The Origin of Vertebrates
Notes taken on October 25, 2016 by Edward Tanguay
origin of vertebrates
525 million years ago during the Cambrian explosion
saw the rise in organism diversity
humans, other mammals, reptiles, birds, amphibians, and fish are all part of a large clade of animals called the vertebrata, or vertebrates
vertebrates are part of a larger clade called the Chordata
animals that have a stiff chord in their back
a sea squirt is more closely related to humans than a clam, a jellyfish or a lobster
attach themselves to a spot on the sea floor and filter food particles out of the water around them
yet juvenile or larval sea squirts can swim freely or float in the water
have a structure called a notochord, a stiff but flexible rod made out of fibers of a material called collagen
notochord is a useful thing to have because it provides an anchoring point for muscles to attach to and pull against
the notochord is the beginning of the vertebrale indoskeleton
this is different than the hard exoskeletons found in invertebrates like crabs, spiders and insects
eventually the notochord was structurally replaced by the vertebrae of your backbone
but even you had a notochord for a little while when you were a developing embryo
above the notochord is another one of the important features found only in chordates
the hollow dorsal nerve chord
some animals like earthworms and insects have a nerve chord that is solid and located ventrally in the animal
so it runs along the belly
but chordates have a nerve that run along our back dorsally
it is hollow and sits above the notochord
in humans it is enclosed by the vertebrae of our backbone
called the vertebral column, or the spinal
a shared ancestory between two or more different features or traits in different organisms
a charactertistic shared by some members of a larger group, a shared derived character
humans and birds are both vertebrates
we inherited our skeletons from a common vertebrate ancestor
our skeleton is homologous with that of a bird
humans and kangaroos are also part of a smaller group called mammals
we share fur with kangaroos which birds don't
convergent characteristics
appear similar but arose separately
results in analogous traits
similar due to a shared function
insects and birds both have wings, but they didn't inherit their wings from a winged, common ancestor
so insect and bird wings are analogous traits
wings of birds, bats and terrasaurs
if we look at these animals on a simplified, philogenetic tree
the last ancestor these animals shared was a primitive, terrestrial tetrapod with walking limbs
these animals didn't inherit their wings from a single, common, winged ancestor
the wings evolved independently in this distantly related lineages
therefore these are convergent, or analogous features
but take a look at their arm bones
they all have the same limb bones
a humorous, radius, carpals, metacarpals, and phalanges
they are arranged in the same way even though the shape of the bones are different
the bones of the similar because they inherited them from their common ancestor
the same feature can be homologous at one level, and analogous at another
these features are homologous as wings, and analogous as limbs
our chordate relatives
the notochord and the dorsal hollow nerve cord
features found in all chordates
therefore homologous characters within vertebrates
in humans, the notochord is no longer present
has been functionally replaces by our vertebral column
if you were to cut a larval sea squirt in half
you would see a series of narrow openings
these are pharyngeal slits, which become the gill openings in fishes
fishes like the shark have gills used for respiration, transferring oxygen into the bloodstream
sea squirts use their pharyngeal slits for eating, they bring water into the pharynx and filter food out of it
pharyngeal slits are one of the characteristic features of chordates
terrestrial chordates lost their gills
if you look at a human embryo, you'll see the beginnings of the pharyngeal slits for a little while during development
a long groove on teh bottom side of the pharynx
foudn in sea squirts and some larval vertebrates like the lampry
the endostyle is lined witih tiny hair-like structures called cilia
these spread a coating fo mucus that is secreted by cells which traps food particles
the endostyle is homologous with our thyroid gland
our thyroid gland regulates our metabolism
in many animals, the digestive system ends in the anus which is at the end of their body
chordates have a post-anal tail
so we share with chordates the fact that our bodies continue beyond the end of the digestive tract
humans don't have tails when we are born, but we do have tails when we are embryos
sea squirts and humans look very different
took very different evolutionary pathways
but we still have some features in common during our early embryonic development
those shared features show our shared evolutionary history as chordate animals