The animals called vertebrates
Get their name from vertebrae, the series of bones that make up the backbone

There are approximately 52,000 species of vertebrates
Which include the largest organisms ever to live on the Earth

Chordates have a notochord and a dorsal, hollow nerve cord
Vertebrates are a subphylum of the phylum Chordata
Chordates are bilaterian animals
That belong to the clade of animals known as Deuterostomia
Two groups of invertebrate deuterostomes, the urochordates and cephalochordates
Are more closely related to vertebrates than to invertebrates
Derived Characters of Chordates
All chordates share a set of derived characters
Although some species possess some of these traits only during embryonic development
Notochord
The notochord
Is a longitudinal, flexible rod located between the digestive tube and the nerve cord
Provides skeletal support throughout most of the length of a chordate
In most vertebrates, a more complex, jointed skeleton develops
And the adult retains only remnants of the embryonic notochord
Dorsal, Hollow Nerve Cord
The nerve cord of a chordate embryo
Develops from a plate of ectoderm that rolls into a tube dorsal to the notochord
Develops into the central nervous system: the brain and the spinal cord
Pharyngeal Slits or Clefts
In most chordates, grooves in the pharynx called pharyngeal clefts
Develop into slits that open to the outside of the body
These pharyngeal slits
Function as suspension-feeding structures in many invertebrate chordates
Are modified for gas exchange in aquatic vertebrates
Develop into parts of the ear, head, and neck in terrestrial vertebrates
Muscular, Post-Anal Tail
Chordates have a tail extending posterior to the anus
Although in many species it is lost during embryonic development
The chordate tail contains skeletal elements and muscles
And it provides much of the propelling force in many aquatic species
Tunicates
Tunicates, subphylum Urochordata
Belong to the deepest-branching lineage of chordates
Are marine suspension feeders commonly called sea squirts

Tunicates most resemble chordates during their larval stage
Which may be as brief as a few minutes

As an adult
A tunicate draws in water through an incurrent siphon, filtering food particles
Early Chordate Evolution
The current life history of tunicates
Probably does not reflect that of the ancestral chordate
Lancelets
Lancelets, subphylum Cephalochordata
Are named for their bladelike shape

Lancelets are marine suspension feeders
That retain the characteristics of the chordate body plan as adults

Gene expression in lancelets
Holds clues to the evolution of the vertebrate form

Craniates are chordates that have a head
The origin of a head
Opened up a completely new way of feeding for chordates: active predation
Craniates share some common characteristics
A skull, brain, eyes, and other sensory organs
Derived Characters of Craniates
One feature unique to craniates
Is the neural crest, a collection of cells that appears near the dorsal margins of the closing neural tube in an embryo

Neural crest cells
Give rise to a variety of structures, including some of the bones and cartilage of the skull
The Origin of Craniates
Craniates evolved at least 530 million years ago
During the Cambrian explosion

The most primitive of the fossils
Are those of the 3-cm-long Haikouella

In other Cambrian rocks
Paleontologists have found fossils of even more advanced chordates, such as Haikouichthys
Hagfishes
The least derived craniate lineage that still survives
Is class Myxini, the hagfishes


Hagfishes are jawless marine craniates
That have a cartilaginous skull and axial rod of cartilage derived from the notochord
That lack vertebrae

Vertebrates are craniates that have a backbone
During the Cambrian period
A lineage of craniates evolved into vertebrates
Derived Characters of Vertebrates
Vertebrates have
Vertebrae enclosing a spinal cord
An elaborate skull
Fin rays, in aquatic forms
Lampreys
Lampreys, class Cephalaspidomorphi
Represent the oldest living lineage of vertebrates
Have cartilaginous segments surrounding the notochord and arching partly over the nerve cord

Lampreys are jawless vertebrates
Inhabiting various marine and freshwater habitats
Fossils of Early Vertebrates
Conodonts were the first vertebrates
With mineralized skeletal elements in their mouth and pharynx

Armored, jawless vertebrates called ostracoderms
Had defensive plates of bone on their skin
Origins of Bone and Teeth
Mineralization
Appears to have originated with vertebrate mouthparts
The vertebrate endoskeleton
Became fully mineralized much later

Gnathostomes are vertebrates that have jaws
Today, jawless vertebrates
Are far outnumbered by those with jaws
Derived Characters of Gnathostomes
Gnathostomes have jaws
That evolved from skeletal supports of the pharyngeal slits

Other characters common to gnathostomes include
Enhanced sensory systems, including the lateral line system
An extensively mineralized endoskeleton
Paired appendages
Fossil Gnathostomes
The earliest gnathostomes in the fossil record
Are an extinct lineage of armored vertebrates called placoderms

Another group of jawed vertebrates called acanthodians
Radiated during the Devonian period
Were closely related to the ancestors of osteichthyans
Chondrichthyans (Sharks, Rays, and Their Relatives)
Members of class Chondrichthyes
Have a skeleton that is composed primarily of cartilage
The cartilaginous skeleton
Evolved secondarily from an ancestral mineralized skeleton

The largest and most diverse subclass of Chondrichthyes
Includes the sharks and rays


A second subclass
Is composed of a few dozen species of ratfishes

Most sharks
Have a streamlined body and are swift swimmers
Have acute senses
Ray-Finned Fishes and Lobe-Fins
The vast majority of vertebrates
Belong to a clade of gnathostomes called Osteichthyes

Nearly all living osteichthyans
Have a bony endoskeleton
Aquatic osteichthyans
Are the vertebrates we informally call fishes
Control their buoyancy with an air sac known as a swim bladder

Fishes breathe by drawing water over four or five pairs of gills
Located in chambers covered by a protective bony flap called the operculum

Ray-Finned Fishes
Class Actinopterygii, the ray-finned fishes
Includes nearly all the familiar aquatic osteichthyans


The fins, supported mainly by long, flexible rays
Are modified for maneuvering, defense, and other functions
Lobe-Fins
The lobe-fins, class Sarcopterygii
Have muscular and pectoral fins
Include coelacanths, lungfishes, and tetrapods


Tetrapods are gnathostomes that have limbs and feet
One of the most significant events in vertebrate history
Was when the fins of some lobe-fins evolved into the limbs and feet of tetrapods
Derived Characters of Tetrapods
Tetrapods have some specific adaptations
Four limbs and feet with digits
Ears for detecting airborne sounds

The Origin of Tetrapods
In one lineage of lobe-fins
The fins became progressively more limb-like while the rest of the body retained adaptations for aquatic life

Extraordinary fossil discoveries over the past 20 years
Have allowed paleontologists to reconstruct the origin of tetrapods

Amphibians
Class Amphibia
Is represented by about 4,800 species of organisms
Most amphibians
Have moist skin that complements the lungs in gas exchange

Order Urodela
Includes salamanders, which have tails

Order Anura
Includes frogs and toads, which lack tails

Order Apoda
Includes caecilians, which are legless and resemble worms

Amphibian means “two lives”
A reference to the metamorphosis of an aquatic larva into a terrestrial adult

Amniotes are tetrapods that have a terrestrially adapted egg
Amniotes are a group of tetrapods
Whose living members are the reptiles, including birds, and the mammals

A phylogeny of amniotes
Derived Characters of Amniotes
Amniotes are named for the major derived character of the clade, the amniotic egg
Which contains specialized membranes that protect the embryo



The extraembryonic membranes
Have various functions


Amniotes also have other terrestrial adaptations
Such as relatively impermeable skin and the ability to use the rib cage to ventilate the lungs

Early Amniotes
Early amniotes
Appeared in the Carboniferous period
Included large herbivores and predators
Reptiles
The reptile clade includes
The tuatara, lizards, snakes, turtles, crocodilians, birds, and the extinct dinosaurs

Reptiles
Have scales that create a waterproof barrier
Lay shelled eggs on land

Most reptiles are ectothermic
Absorbing external heat as the main source of body heat
Birds are endothermic
Capable of keeping the body warm through metabolism
The Origin and Evolutionary Radiation of Reptiles
The oldest reptilian fossils
Date to about 300 million years ago
The first major group of reptiles to emerge
Were the parareptiles, which were mostly large, stocky herbivores

As parareptiles were dwindling
The diapsids were diversifying
The diapsids are composed of two main lineages
The lepidosaurs and the archosaurs

The dinosaurs
Diversified into a vast range of shapes and sizes
Included the long-necked giants called the theropods

Traditionally, dinosaurs were considered slow, sluggish creatures
But fossil discoveries and research have led to the conclusion that dinosaurs were agile and fast moving
Paleontologists have also discovered signs of parental care among dinosaurs
Lepidosaurs
One surviving lineage of lepidosaurs
Is represented by two species of lizard-like reptiles called tuatara

The other major living lineage of lepidosaurs
Are the squamates, the lizards and snakes
Lizards
Are the most numerous and diverse reptiles, apart from birds


Snakes are legless lepidosaurs
That evolved from lizards
Turtles
Turtles
Are the most distinctive group of reptiles alive today
Some turtles have adapted to deserts
And others live entirely in ponds and rivers

All turtles have a boxlike shell
Made of upper and lower shields that are fused to the vertebrae, clavicles, and ribs
Alligators and Crocodiles
Crocodilians
Belong to an archosaur lineage that dates back to the late Triassic
Birds
Birds are archosaurs
But almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight
Derived Characters of Birds
Many of the characters of birds
Are adaptations that facilitate flight

A bird’s most obvious adaptations for flight
Are its wings and feathers

The Origin of Birds
Birds probably descended from theropods
A group of small, carnivorous dinosaurs

By 150 million years ago
Feathered theropods had evolved into birds
Archaeopteryx
Remains the oldest bird known

Living Birds
The ratites, order Struthioniformes
Are all flightless

The demands of flight
Have rendered the general body form of many flying birds similar to one another


Foot structure in bird feet
Shows considerable variation

Mammals are amniotes that have hair and produce milk
Mammals, class Mammalia
Are represented by more than 5,000 species
Derived Characters of Mammals
Mammary glands, which produce milk
Are a distinctively mammalian character
Hair is another mammalian characteristic
Mammals generally have a larger brain
Than other vertebrates of equivalent size
Early Evolution of Mammals
Mammals evolved from synapsids
In the late Triassic period

The jaw was remodeled during the evolution of mammals from nonmammalian synapsids
And two of the bones that formerly made of the jaw joint were incorporated into the mammalian middle ear


Living lineages of mammals originated in the Jurassic
But did not undergo a significant adaptive radiation until after the Cretaceous
Monotremes
Monotremes
Are a small group of egg-laying mammals consisting of echidnas and the platypus

Marsupials
Marsupials
Include opossums, kangaroos, and koalas

A marsupial is born very early in its development
And completes its embryonic development while nursing within a maternal pouch called a marsupium

In some species of marsupials, such as the bandicoot
The marsupium opens to the rear of the mother’s body as opposed to the front, as in other marsupials

In Australia, convergent evolution
Has resulted in a diversity of marsupials that resemble eutherians in other parts of the world

Eutherians (Placental Mammals)
Compared to marsupials
Eutherians have a longer period of pregnancy
Young eutherians
Complete their embryonic development within a uterus, joined to the mother by the placenta

Phylogenetic relationships of mammals

The major eutherian orders
Primates
The mammalian order Primates include
Lemurs, tarsiers, monkeys, and apes
Humans are members of the ape group
Derived Characters of Primates
Most primates
Have hands and feet adapted for grasping
Primates also have
A large brain and short jaws
Forward-looking eyes close together on the face, providing depth perception
Well-developed parental care and complex social behavior
A fully opposable thumb
Living Primates
There are three main groups of living primates
The lemurs of Madagascar and the lorises and pottos of tropical Africa and southern Asia

The tarsiers of Southeast Asia
The anthropoids, which include monkeys and hominids worldwide

The oldest known anthropoid fossils, about 45 million years old
Indicate that tarsiers are more closely related to anthropoids


The fossil record indicates that monkeys
First appeared in the New World (South America) during the Oligocene
The first monkeys
Evolved in the Old World (Africa and Asia)

New World and Old World monkeys
Underwent separate adaptive radiations during their many millions of years of separation


The other group of anthropoids, the hominoids
Consists of primates informally called apes


Hominoids
Diverged from Old World monkeys about 20–25 million years ago

Humans are bipedal hominoids with a large brain
Homo sapiens is about 160,000 years old
Which is very young considering that life has existed on Earth for at least 3.5 billion years
Derived Characters of Hominids
A number of characters distinguish humans from other hominoids
Upright posture and bipedal locomotion
Larger brains
Language capabilities
Symbolic thought
The manufacture and use of complex tools
Shortened jaw
The Earliest Humans
The study of human origins
Is known as paleoanthropology

Paleoanthropologists have discovered fossils of approximately 20 species of extinct hominoids
That are more closely related to humans than to chimpanzees

These species are known as hominids


Hominids originated in Africa
Approximately 6–7 million years ago
Early hominids
Had a small brain, but probably walked upright, exhibiting mosaic evolution

Two common misconceptions of early hominids include
Thinking of them as chimpanzees
Imagining human evolution as a ladder leading directly to Homo sapiens
Australopiths
Australopiths are a paraphyletic assemblage of hominids
That lived between 4 and 2 million years ago

Some species walked fully erect
And had human-like hands and teeth

Bipedalism
Hominids began to walk long distances on two legs
About 1.9 million years ago
Tool Use
The oldest evidence of tool use—cut marks on animal bones
Is 2.5 million years old
Early Homo
The earliest fossils that paleoanthropologists place in our genus Homo
Are those of the species Homo habilis, ranging in age from about 2.4 to 1.6 million years
Stone tools have been found with H. habilis
Giving this species its name, which means “handy man”

Homo ergaster
Was the first fully bipedal, large-brained hominid
Existed between 1.9 and 1.6 million years

Homo erectus
Originated in Africa approximately 1.8 million years ago
Was the first hominid to leave Africa
Neanderthals
Neanderthals, Homo neanderthalensis
Lived in Europe and the Near East from 200,000 to 30,000 years ago
Were large, thick-browed hominids
Became extinct a few thousand years after the arrival of Homo sapiens in Europe
Homo sapiens
Homo sapiens
Appeared in Africa at least 160,000 years ago

The oldest fossils of Homo sapiens outside Africa
Date back about 50,000 years ago

The rapid expansion of our species
May have been preceded by changes to the brain that made symbolic thought and other cognitive innovations possible



Overview: Half a Billion Years of Backbones
By the end of the Cambrian period, some 540 million years ago
An astonishing variety of animals inhabited Earth’s oceans
One of these types of animals
Gave rise to vertebrates, one of the most successful groups of animals