Download Biology 399 – History of Life THE ORIGIN OF VERTEBRATES Who

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Biology 399 – History of Life
THE ORIGIN OF VERTEBRATES
Who are the living vertebrates?
Jawless fish: hagfish and lamprey
Fish with jaws & cartilage skeletons: sharks and rays
Fish with jaws & bony skeletons: all other fish (tuna, flounder, bass, etc.)
Amphibians: frogs and salamanders: cold-blooded, lay eggs in water
Reptiles: turtles, snakes and lizards: cold-blooded, lay eggs on land
Birds: warm-blooded, feathers, lay eggs
Mammals: warm-blooded, hair, eggs & live birth, nurse young.
What traits unite vertebrates?
Backbone: Structural support on our dorsal, or topside.
Spinal cord: A hollow nerve tube that runs on top or through backbone
Heads: Brains and sense organs at the front end.
Tails: Nerves, back support and muscles that extend past our butts.
Hearts: A closed circulatory system with a muscular pump to move fluid.
"Gill" Slits: Openings in pharnyx connecting throat to exterior.
Segmented muscle on body wall: V-shaped muscles masses.
Most of the vertebrates also have:
Internal skeletons of hydroxyapatite: Ca5(PO4)3OH, not CaCO3.
Jaws: Skeletal structures for capturing and processing food.
Two pairs of appendages: Paired front and hind fins or limbs.
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Where did all this stuff come from?
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Non-Chordate Next of Kin
Echinoderms: e.g., starfish, crinoids, sea cucumbers
Range: Cambrian - Recent
Habitat: Exclusively marine
Unique characters: 5-fold symmetry in adults, water-vascular system, a uniquely
constructed calcite skeleton.
What characters link vertebrates with Echinoderms?
Embryos and larvae supply the shared novelties that connect Echinoderms and
Vertebrates.
Shared novelties: Embryonic traits (Radial pattern of embryonic cleavage,
Deuterostome, Mesoderm formed by pouching); Skin-based nerve network; Bilateral,
cilia-covered larvae.
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Hemichordates: Acorn Worms
!!! Range: Cambrian - Recent
!!! Habitat: Exclusively marine
!!! Unique characters: Acorn worms are large(up to 2 m), burrowing worm!!! like filter-feeders with a long muscular proboscis and a fleshy collar.
!!! Shared novelties: Adults are bilaterally symmetric; Closed circulatory system;
!!! Paired openings in the throat.
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Chordate Next of Kin
Urochordates: Tunicates or sea squirts
!!! Range: No fossil record
!!! Habitat: Exclusively marine
!!! Unique characters: Tunicates are small, box-like filter-feeding animals that
!!! live either alone or in colonies cemented to the sea floor.
The larval tunicate is a free-living, tadpole-like animal that swims around, looking for
a good spot to settle as an adult.
Shared novelties: Notochord; Hollow nerve cord along back; Tail; Endostyle (a group of
ciliated cells with alternating mucus cells; used to entangle food) an organ used for filterfeeding.
Cephalochordates: Branchiostoma
!!! Range: Cambrian (Pikaia from the Burgess Shale) - Recent
!!! Habitat: Exclusively marine
!!! Unique characters: Branchiostoma, also known as the lancelet, is a small, free!!! living fish-like animal that lives among sand grains and filter feeds.
!!! Shared novelties: Segmented muscles on upper body wall.
The Importance of Swimming
The notochord, a stiff rod of connective tissue, provides internal support that permits
efficient side-to-side motion for swimming.
When muscles contract, the organism bends, rather than compressing like an accordian.
The tail and hollow nerve cord, coupled with a closed circulatory system, are all probably
related to this more active swimming life style.
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Tunicates and Branchiostoma are both chordates, because they both have notochords, but
neither is a vertebrate.
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Craniates: the most primitive "vertebrates"
Hagfish: the most primitive known "vertebrate"
!!! Range: Carboniferous-Recent
!!! Habitat: Exclusively marine
!!! Unique characters: Scavengers and carnivores that actively feed by rasping at
!!! prey with a bony tongue. They tie themselves into a knot to lever a chunk out of prey.
!!! They can coat themselves with mucous for defense. They contain no vertebrae
!!! and no bone.
!!! Shared novelties: A head (cartilage brain case; partial cranium); Sense organs on the
head (weak eyes); A true heart; True gills for efficient oxygen retrieval from water;
Cartilage gill supports to hold up these flimsy sheets.
Heterostracans: the first truly abundant fishes
!!! Range: Cambrian - Devonian
!!! Habitat: Originate in marine waters, later invade fresh water
!!! Unique characters: Jawless, armored body with scales on the tail. The tail was
!!! the main source of propulsion. Bottom feeding hunters and detritus feeders. Still
!!! no vertebrae.
!!! Shared novelties: Improved sense organs (better balance & vision, lateral line
!!! system for motion detection and probably electroreception (used to hunt); Bone
!!! on the outer skull but not on the braincase.
Because hagfish and heterostracans don't have vertebrae, many scientists hesitate to call
them vertebrates. They do have heads, however, so they are sometimes called Craniates.
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Why Bones of Calcium Phosphate?
Less soluble than calcite, not as subject to dissolution by metabolic acids.
Store of an important nutrient, phosphate.
May serve as insulation for electroreceptors.
Protection.
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The first true vertebrates
Lamprey: another extant jawless "fish"
!!! Range: Carboniferous - Recent
!!! Habitat: Originate in marine waters, later invade fresh water
!!! Unique characters: The adult is a parasitic bloodsucker. It is jawless, but its
!!! mouth has many hooks for latching onto prey, then they use the tongue to bore
!!! through the side of the prey. No bone on the body - an evolutionary reversal.
!!! Shared novelties: Vertebrae surrounding notochord (made of cartilage); Dorsal
!!! and anal fins; Endostyle turns into thyroid.
The larval lamprey plays a key role in our understanding of vertebrate history.
They are so different from the adults, that they were initially classified as a different
animal.
The larval lamprey has an endostyle, and it lives in the sediment and filter-feeds.
Basically it looks and acts like Branchiostoma, only it has a brain, eyes, and a heart.
Upon metamorphosis, the endostyle turns into thyroid gland, and the adult becomes
predator.
So, as with echinoderms and tunicates, larvae provide vital information.
The adults have been transformed by evolutionary pressures into very distinctive forms,
but the larvae retain similarities that provide important clues as to evolutionary ties.
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Osteostracan: another group of extinct armored jawless fish
!!! Range: Silurian - Devonian
!!! Habitat: Some marine, mostly fresh water
!!! Unique characters: Similar to heterostracans, with a bony head shield, scales on the
!!! tail, propulsion from the tail, well developed sense organs, and elaborate plumbing for
!!! gill system. They were active swimmers. Many were bottom feeders.
!!! Shared novelties: Paired pectoral fins (source of forelimbs); Braincase covered in
!!! bone.
The invertebrate-vertebrate transition seems quite complex.
Overview
At first, it's hard to imagine how the transition happened; the changes seem so complex
and unrelated.
However studies of embryos has revealed that many of the "new" structures that
vertebrates possess are derived from just a few new embryonic cell types (related to the
skin neural network) that take on a variety of functions.
Thus, the main evolutionary "advance" was the invention of these new embryonic tissues.
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The selective pressures on organisms seem to be associated with a more active life-style.
Predation or sediment feeding replaced filter feeding as the dominant style of feeding.
This requires a variety of changes in body form, from more precise and integrated sense
organs, to a more efficient respiratory and circulatory system to supply oxygenated blood
to body tissues, to more developed and efficient systems of propulsion.
Vertebrate Evolution
Recent studies in vertebrate evolution (e.g., based on studies in developmental genetics
and paleontology) suggest that evolution of the vertebrate brain may have had a
surprisingly early start in invertebrate ancestors, long before the mineralized skeleton that
makes vertebrates so distinctive
What’s more, the skeleton may have arisen as teeth
Recall that the true innovation that launched the lineage of fish and other vertebrates
seems to have been new kinds of embryonic tissue, which could form new sensory organs
That allowed protovertebrates, such as the Chinese fossil of Haikouella and
Myllokunmingia, to embark on a new way to make a living- as predators
One way to track vertebrates’ evolutionary history is to analyze their closest living
relative
Molecular and anatomical research both indicate that this is Amphioxus
It has little in the way of a skeleton, and its central nervous system consists of a nerve
cord with a barely swollen tip.
But it does possess vertebrate traits such as gill slits, rows of muscle blocks along its
flanks, and a notochord, a stiff rod of tissue that supports the nerve cord along its back.
Paleontologists have long suspected that vertebrate diverged from a lancelet-like relative
sometime in the Cambrian period
Meanwhile, molecular studies of gene similarities between lancelets and today's
vertebrates suggest that the vertebrate lineage goes all the way back to 750 million years
ago!
Brain and bone together
Until very recently, the earliest undisputed vertebrates were a mere 475 million years old.
These small, jawless fish (heterostracans?) with bodies completely covered in bony plates
of armor are thought to have dined on sea-floor invertebrates and to have used their
armor to defend against predators.
Fossils retaining the imprint of the brain reveal that these fish had already evolved many
of the major features of modern vertebrate brains, such as divisions into forebrain,
midbrain, and hindbrain.
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If these armored fishes represent the earliest vertebrates, they suggest that brains and
bone evolved together.
Yet lampreys and hagfish, the only jawless fish alive today, are squishy creatures without
a speck of armor and scant amounts of cartilage--and are far more primitive than the
fossil forms.
Then in 1983, Glenn Northcutt and Carl Gans argued that the key to vertebrate evolution
was the invention of a head, which in turn was made possible by the evolution of a new
kind of embryonic cell.
A Brief Overview of some Early Aspects of the Development of the Vertebrate Embryo
Various regions of the three germ layers develop into the rudiments of organs during the
process of organogenesis
A number of kinds of morphogenetic changes, including folding, splitting, condensation
(clustering) of cells etc. occur within the layered embryonic tissues and represent the first
evidence of organ building
The organs that first begin to take shape in the embryos of vertebrates (and chordates in
general) are the neural tube and notochord
The notochord is formed from condensation of the dorsal mesoderm just above the
archenteron, and the neural tube originates as a plate of dorsal ectoderm just above the
developing notochord
The neural plate soon undergoes folding, actually rolling itself into the neural tube, which
will eventually become the CNS
Unique to vertebrate embryos, a band of cells called the neural crest cells develop along
the border where the neural tube pinches off from the ectoderm
Cells of the neural crest later migrate to various parts of the embryo, forming pigment
cells of the skin, some of the bones and muscles of the skull, etc.
Essentially, neural crest cells breaks away and wanders around the embryo, helping to
shape many structures such as eyes, nose, nerves, head muscles, and skull bones.
It was the neural crest, Gans and Northcutt proposed, that gave vertebrates the flexibility
to build a new kind of body, one that included the complex sense organs, big brains, and
powerful pumping throats seen for the first time in lampreys and fossil jawless fish.
Along with the new body plan came an ecological shift, as vertebrates evolved from
small, passive filter feeders to large, active predators that darted about hunting their prey.
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According to Gans and Northcutt, this developmental revolution, argued, also sparked the
origin of bone.
Neural crest cells build the electroreceptors that line the bodies of fish; once these
receptors evolved, the researchers theorized, neural crest started building mineralized
bone around them to insulate them from the rest of the body.
Later, the bone spread out to form a protective coat of armor, as seen in the early bony
fish.
Supporting Research Involving Lancelets
Researchers had long been interested in whether the biology of lancelets supports the
Gans-Northcutt model
Hollands and others began studying the expression of genes that build the creatures'
bodies, using special tags to stain cells expressing the products of known developmental
genes.
After examining dozens of genes, the picture that emerges is that of a protovertebrate
brain.
The lancelet doesn't have a true neural crest, but it does have cells in the same position as
neural crest cells, and they express some of the same genes that neural crest cells express
before they begin to migrate.
These cells also migrate, but only as a sheet moving on the surface of an embryo, not as
small clusters traveling inside it.
Their work thus both confirms and refines Gans and Northcutt's notion of the importance
of the neural crest.
One innovation of vertebrates is certainly the invention of the wandering neural crest; it
opened up the potential to get awfully fancy in the vertebrate head."
What's more, Hollands found evidence that even without a true neural crest, the swollen
bud on the front end of the lancelet nerve cord bears a striking similarity to the vertebrate
brain.
The same genes that organize major regions of the forebrain, midbrain, and hindbrain of
vertebrates express themselves in a corresponding pattern in this small cluster of cells in
the lancelet's nerve cord.
It would be tempting to conclude that these patterns are an atlas of the primordial
vertebrate brain, but the lancelet genes may actually be performing quite different tasks
from the vertebrate genes, even though they're expressed in the same place
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Slicing the lancelet brain
Thurston Lacalli uncovered detailed neuroanatomy of lancelets
He photographed 2000 cross sections of the front end of the nerve cord in a lancelet
larva.
He then began painstakingly tracing out the shapes and connections of each of the
approximately 300 neurons, combining them into three-dimensional computer
reconstructions.
His research supports Hollands' claims that the lancelet nerve cord is divided like a
vertebrate brain.
In the regions of the lancelet nerve cord where the Hollands found forebrain and midbrain
genes at work, the neuronal structure matches that of the vertebrate forebrain and
midbrain.
Lacalli claims that clusters of neurons in the lancelet brain seem to perform the same
functions as their vertebrate counterparts--even though in the lancelet these clusters may
be made up of only a handful of neurons.
For example, noting a retinal-like pattern of connections near a cluster of pigment cells
near the tip of the lancelet, Lacalli claims that the cluster is a single eye, homologous to
the paired vertebrate eyes.
The lancelet eye is too crude to form images, but Lacalli suspects it can detect moving
shadows of predators.
And the hair-like projections that ring the lancelet's mouth--used to accept or reject food-are connected to nerves in much the same way as cells in vertebrate taste buds
Lacalli also claims that lancelets have a rudimentary limbic system – a cluster of nerve
cells in the lower part of the brain that interact with the cerebral cortex
The vertebrate limbic system, which includes the hypothalamus, monitors the body's
internal state, such as its temperature and hormone levels.
It then uses this information to control basic behaviors such as when to sleep, when to
eat, when to flee, and when to fight.
Lacalli has found lancelet neurons whose structure and organization resemble those of
vertebrate limbic neurons and that are located in the corresponding parts of the midbrain
and forebrain.
He suggests that the common ancestor of vertebrates and lancelets used its protolimbic
system to switch between its handful of behaviors, such as swimming and feeding.
They had [essential] decisions to make; and the way they made these decisions is part of
the limbic system
In all, the work of Lacalli, the Hollands, and others suggests that in some basic ways, the
vertebrate head is not new.
Wandering neural crest may have been a key development in the evolution of the
vertebrate nervous system, as Gans and Northcutt argued, but by the time the head arose,
some of the fundamental structure of the vertebrate brain was already in place.
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Predators on the prowl
One of the major areas of research that will go on for some time is how the vertebrates
and lancelets are different.
Because it's the difference that becomes critical for understanding vertebrate evolution
Lancelets, for example, apparently have no sense of smell. One of the parts of the
vertebrate brain that's missing from the lancelet nerve cord is the most forward portion of
the forebrain, known as the telencephalon, which among other tasks handles signals from
the nose.
Such differences add further weight to Gans and Northcutt's idea that early vertebrates
shifted from filter-feeding to predation.
But instead of a divided brain, one of the key inventions of early vertebrates might well
have been a nose.
A lancelet doesn't need to sniff out its prey, but as the early vertebrates became predators,
smell became an asset
They would also benefit from eyes to see prey and sophisticated control of their bodies to
chase prey down.
Paleontologists may have captured that crucial step from filter-feeding to active
predation.
In November 1999, Chinese researchers reported a trove of 300 specimens of a creature
called Haikouella.
In some ways these sliver-shaped impressions on ancient rocks look like lancelets, but
they also have a few key vertebrate traits unnecessary for filter feeders, such as eyes and
muscle blocks.
These clues suggest that Haikouella is poised at the transition from invertebrate to
vertebrate, closer to vertebrates than even the lancelet.
Some researchers have questioned this close kinship, noting that Haikouella has a few
anatomical peculiarities, such as in the organization of its muscle blocks.
But overall, these fossils "look like little vertebrates
That makes another feature of their anatomy significant: The fossil nerve cord has an
even larger swelling than does that of the lancelet; it appears that they do have a brain.
If so, that pushes the origin of a vertebrate-like brain back to more than 530 million years
ago.
And Haikouella is just the sort of brain-powered, sensory-enhanced predator that Gans
and Northcutt predicted 18 years ago!
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How bone was born
In a recent paper in Biological Reviews of the Cambridge Philosophical Society,
paleontologists Philip Donoghue, Peter Forey, and Richard Aldridge create a new
evolutionary tree for vertebrates that for the first time incorporates a mysterious group of
animals called conodonts.
These creatures left behind vast numbers of enigmatic little fossils in the shapes of cones
and thorns, ranging in age from 510 million to 220 million years old.
Researchers envision conodonts as eel-shaped predators with a pair of giant eyes and a
gaping mouth filled with the toothlike, bony conodont elements, which are made of
dentine and other ingredients of the vertebrate skeleton.
Conodonts may have been early chordate predators, but paleontologists have fought over
exactly what sort of chordate they might be.
Donoghue and his co-workers show that after the vertebrate lineage split from lancelets,
the first group to branch away were the hagfish; lampreys are only slightly less primitive.
Conodonts, surprisingly, turn out to be full-fledged vertebrates, even closer to living
jawed fish than to lampreys or hagfish.
Only after the rise of conodonts did the armored jawless fish, the ostracoderms, appear,
and from one of their ranks, the jawed fish eventually evolved.
According to the new tree, hagfish and lampreys offer a good representation of what the
most ancient vertebrates were like: unarmored and without mineralized skeletons.
And conodonts represent the first appearance of a mineralized skeleton.
The conodont skeleton is believed to be the primitive vertebrate skeleton
Mineralization appears not to have begun in the skin of fish, but in the mouths of
conodonts, and it presumably made them fiercer predators.
The conclusion that bone was born after the rise of vertebrates is not yet certain, as more
primitive chordates may turn out to have possessed the precursors of conodont mouth
parts.
Hagfish have "toothlets" made of keratin plus a little phosphate, which might have
originated as genuine dentine-based teeth and shifted to other materials later.
And Jun-Yuan Chen and his colleagues claim that Haikouella had mineralized
"pharyngeal teeth" in its throat.
Whether these so-called teeth have anything to do with the rise of the vertebrates will
have to wait for microscopic analyses of the fossils and for an end to the debate over
whether they are chordates at all.
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Invertebrate-vertebrate transition: When and Where
There are now fossil fish from the Cambrian that are more advanced than hagfish, so it
must have happened during the Cambrian explosion.
The transition happened in the ocean.
All invertebrate next of kin, all non-vertebrate chordates, and the most primitive living
vertebrates all come from the ocean.
Jawless, armored fish appeared in the Cambrian, were present but not diverse in the
Ordovician, and flourished in the Silurian and Devonian.
They soon began to share the waters with new kinds of fish, however.
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The first vertebrates with jaws
Placoderms: the most primitive jawed fish
!!! Range: Silurian - Carboniferous
!!! Habitat: Marine and fresh water
!!! Unique characters: Heavy armor on head and front of the trunk, scales on the tail, no
!!! teeth, just plates of bone for shearing food, heavy fish (probably slow swimmers).
!!! Shared novelties: Jaws; Paired pelvic fins (source of hindlimbs); Paired nasal
!!! openings.
These large fish were quite diverse and were the top carnivores of the Devonian seas.
Their distribution in time is odd.
While we think placoderms are the most primitive jawed fish in an evolutionary sense,
they appear in the fossil record relatively late, after several more advanced taxa are
already on the scene.
Acanthodians: earliest known jawed fishes
!!! Range: Silurian - Permian
!!! Habitat: Initially marine, later invade fresh water.
!!! Unique characters: Their fins were supported by erectable spines. Some filter fed,
!!! others had teeth. Highly manueverable swimmers propelled by their tails.
!!! Shared novelties: Teeth; Advanced jaw joint.
Chondrichthyes: Cartilaginous fish, e.g., sharks and rays
!!! Range: Silurian - Recent
!!! Habitat: Mostly marine, but some fresh water.
!!! Unique characters: No bone except in their scales (an evolutionary reversal). Fin
!!! and tail structures suggest an active, highly efficient swimming for a predatory life
!!! style. Sharks give birth to live young.! This requires internal fertilization of eggs.! They
!!! link up when breeding by using claspers.
!!! Shared novelties: Regular pattern of tooth replacement.
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! The Origin of Jaws and Teeth
Jawless fishes have gills, expanded areas of tissue for gas exchange.
These flimsy sheets of tissue are supported by gill arches.
Many fish actively pump water past the gills using muscles acting on the gill arches.
A favored hypothesis for the origin of jaws is that the pair of gill arches furthest in front
were jointed and actively involved in pumping water.
These jointed arches developed a pincher motion that was eventually used for holding
prey.
This observation is bolstered by the fact that the front gill arch in living sharks supports
the jaws and attaches them to skull.
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Vertebrates with skeletons made entirely of bone
Osteichthyes: Bony fish - fish with bony skeletons.
Range: Silurian - Recent
Habitat: Marine and fresh water.
Shared novelties: Skeleton completely composed of bone, including skull, vertebral
column, fins, and ribs; Swim bladder for buoyancy control.
Two major groups within the bony fish
Ray-finned fish
!!! Range: Silurian - Recent
!!! Habitat: Marine and fresh water
!!! Shared Novelty uniting bony fish: Fins made of bony spines connected by poorly
!!! muscled webs.
They are so diverse it is pointless to consider unique features.
They eat anything from other fish, to insects, to plankton.
They have a wide range of body forms, from torpedo shaped, to disc shaped, to flattened
for living on sea floor.
They include nearly all the common fish you can think of other than sharks.
Lobe-finned fish: lungfish, coelocanths, rhipidistians
!!! Range: Devonian - Recent
!!! Habitat: Mostly fresh water, some marine.
!!! Unique Characters: Torpedo shaped body with heavy scales, unusual bone with
!!! many pores, perhaps for electroreceptive cells.
!!! Shared novelties: Paired pectoral and pelvic fins that are fleshy and muscular;
!!! Peculiar convoluted dentin and enamel.