Chapter 34

Vertebrates

Vertebrates

Get their name from vertebrae, the series of bones that make up the backbone

Vertebrates

• There are approximately 52,000 species of vertebrates– Which include the largest organisms ever to

live on the Earth. Plant-eating dinosaurs at 40,000 kg were the heaviest terrestrial animals. The Blue Whale at 100,000 kg is the biggest animal ever to have existed

Chordates• 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

A hypothetical phylogeny of chordates

ChordatesCraniates

VertebratesGnathostomes

OsteichthyansLobe-fins

TetrapodsAmniotes

Milk

Amniotic egg

Legs

Lobed fins

Lungs or lung derivatives

Jaws, mineralized skeleton

Vertebral column

Head

Brain

Notochord

Ancestral deuterostome

Ech

ino

de

rmat

a(s

iste

r g

rou

p to

cho

rda

tes)

Uro

ch

ord

ata

(tu

nica

tes)

Ce

ph

alo

ch

ord

ata

(lan

cele

ts)

Myx

ini

(hag

fish

es)

Ce

ph

ala

spid

om

orp

hi

(lam

pre

ys)

Ch

on

dri

ch

thy

es(s

hark

s, r

ays

, ch

ima

era

s)

Ac

tin

op

tery

gii

(ray

-fin

ned

fish

es)

Ac

tin

isti

a(c

oela

can

ths)

Dip

no

i(l

ungf

ishe

s)

Am

ph

ibia

(fro

gs,

sala

man

der

s)

Re

pti

lia

(tu

rtle

s, s

nak

es,

croc

odi

les,

bird

s)

Mam

mal

ia(m

amm

als)

Derived Characters of Chordates• All chordates share a set of derived

characters– Although some species possess some of

these traits only during embryonic development

Musclesegments

Brain

Mouth

Anus

Dorsal,hollow

nerve cord

Notochord

Muscular,post-anal tail

Pharyngealslits or clefts

1. Notochord

2. Dorsal, hollow nerve cord

3.Pharyngeal slits

4. Muscular post-anal tail

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 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 (subphylum Urochordata)• 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

Pharynx with slits

Notochord

Tail

Dorsal, hollownerve cord

AtriumStomach

Intestine

Excurrent siphon

Incurrentsiphon

Musclesegments

(c) A tunicate larva is a free-swimming butnonfeeding “tadpole” in which all fourchief characters of chordates are evident.

(a) An adult tunicate, or sea squirt, is a sessile animal (photo is approximately life-sized).

(b) In the adult, prominent pharyngeal slits function in suspension feeding, but other chordate characters are not obvious.

Tunic

Pharynxwith

numerousslits

Atrium

Excurrentsiphon

Incurrentsiphonto mouth

StomachEsophagus

IntestineAnus

Excurrent siphon

Lancelets (subphylum Cephalochordata)

• Named for their bladelike shape• Lancelets are marine suspension feeders that

retain the characteristics of the chordate body plan as adults

Tentacle

Mouth

Pharyngeal slits

Atrium

Digestive tract

Atriopore

Segmentalmuscles

Anus

Notochord

Dorsal, hollownerve cord

Tail

2 cm

Craniates• 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

Neural crest• Unique to craniates, the neural

crest, a collection of cells that appears near the dorsal margins of the closing neural tube in an embryo

Notochord

(a) The neural crest consists of bilateral bands of cells near the margins of the embryonic folds that form the neural tube.

(b) Neural crest cells migrate todistant sites in the embryo.

Migrating neuralcrest cells

Ectoderm

Ectoderm

Dorsal edgesof neural plate

Neuralcrest

Neuraltube

Neural crest cells

• Give rise to a variety of structures, including some of the bones and cartilage of the skull

(c) The cells give rise to some of the anatomical structuresunique to vertebrates, 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

(a) Haikouella. Discovered in 1999 in southern China, Haikouella had eyes and a brain but lacked a skull, a derived trait of craniates.

–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

(b) Haikouichthys. Haikouichthys had a skull and thus is considered a true craniate.

5 mm

Hagfishes• The least derived craniate lineage that still

survives is class Myxini, the hagfishes

Slime glandsHagfishes are jawless marine craniates

That have a cartilaginous skull and axial rod of cartilage derived from the notochordThey lack vertebrae

Origin of Vertebrates• Vertebrates are craniates that have a

backbone• During the Cambrian period a lineage

of craniates evolved into vertebrates• Derived Characters of Vertebrates

– Vertebrae enclosing a spinal cord– An elaborate skull– Fin rays, in aquatic forms

Lampreys (Class Cephalaspidomorphi)– Represent the oldest living lineage of

vertebrates– Have cartilaginous segments surrounding

the notochord and arching partly over the nerve cord

– Are jawless vertebrates– Inhabit various marine and freshwater

habitats

Fossils of Early Vertebrates• Conodonts were the first vertebrates

with mineralized skeletal elements in their mouth and pharynx

Dorsal viewof head

Dentalelements

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

• Gnathostome characteristics– Jaws that evolved from skeletal

supports of the pharyngeal slits.– Enhanced sensory systems,

including the lateral line system– An extensively mineralized

endoskeleton– Paired appendages

Mouth

Gill slits Cranium

Skeletal rods

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

Sharks and Rays• The largest and most diverse subclass of

Chondrichthyes includes the sharks and rays• Most sharks have a streamlined body and are

swift swimmers and have acute senses

Pectoral fins Pelvic fins

Blacktip reef shark (Carcharhinus melanopterus). Fast swimmers with acute senses, sharks have paired pectoral and pelvic fins.

Southern stingray (Dasyatis americana). Most rays are flattened bottom-dwellers thatcrush molluscs and crustaceans for food. Some rays cruise in open water and scoop food into their gaping mouth.

Ratfish

• Another subclass is composed of a few dozen species of ratfishes

Spotted ratfish (Hydrolagus colliei). Ratfishes, typically live at depths greaterthan 80 m and feed on shrimps, molluscs, and sea urchins. Some species have a poisonous spine at the front of their dorsal fin.

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

Fish anatomy

• Fishes breathe by drawing water over four or five pairs of gills – Located in chambers covered by a protective

bony flap called the operculum

Nostril Brain

Spinal cordSwim bladder

Dorsal fin Adipose fin(characteristic oftrout)

Caudal fin

Cut edge of operculum Gills

HeartLiver

KidneyStomach

Intestine

GonadAnus

Urinary bladder

Lateral line

Anal fin

Pelvic fin

Ray-Finned Fishes (Class Actinopterygii)Includes nearly all the familiar aquatic osteichthyans.

The fins, supported by long, flexible rays are modified for maneuvering, defense, and other functions.

(a) Yellowfin tuna (Thunnus albacares), a fast-swimming, schooling fish that is an important commercial fish worldwide

(b) Clownfish (Amphiprion ocellaris), a mutualistic symbiont of sea anemones

(c) Sea horse (Hippocampus ramulosus), unusual in the animal kingdom in that the male carries the young during their embryonic development

(d) Fine-spotted moray eel (Gymnothorax dovii), a predator that ambushes prey from crevices in its coral reef habitat

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

• 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

Tetrapodlimbskeleton

Bonessupportinggills

Origin of Tetrapods

Amniotes

Amphibians

GreerpetonHynerpeton

lchthyostega

Acanthostega

Metaxygnathus

Elginerpeton

Eusthenopteron

Panderichthys

Lungfishes

Coelacanths

Ray-finned fishes

Paleozoic

Silurian Devonian Carboniferous Permian To present

420 415 400 385 370 355 340 325 310 295 280 265

Millions of years ago

Amphibians• Class Amphibia is represented by about 4,800

species of organisms• Most amphibians have moist skin that

complements the lungs in gas exchange• Amphibian means “two lives” (a reference to the

metamorphosis of an aquatic larva into a terrestrial adult)

The male grasps the female, stimulating her to release eggs. The eggs are laid and fertilized in water. They have a jelly coat but lack a shell and would desiccate in air.

The tadpole is an aquatic herbivore With a fishlike tail and internal gills.

During metamorphosis, the gills and tail are resorbed, andwalking legs develop.

Origin of Amphibians

Order Urodela– Includes salamanders, which have tails

Order Urodela. Urodeles (salamanders) retain their tail as adults.

Order Anura

– Includes frogs and toads, which lack tails

Order Anura. Anurans, such as this poison arrow frog, lack a tail as adults.

Order Apoda

– Includes caecilians, which are legless and resemble worms

Order Apoda. Apodans, or caecilians, are legless, mainly burrowing amphibians.

Amniotes

• Amniotes are a group of tetrapods– Whose living members are the reptiles, including

birds, and the mammals– that have a terrestrially adapted egg– the amniotic egg contains specialized

membranes that protect the embryo– 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 appeared in the Carboniferous period

A phylogeny of amniotes

Synapsids

Ancestralamniote

Reptiles

Diapsids

Archosaurs

Saurischians

Lepidosaurs

Dinosaurs

Par

arep

tiles

Turt

les

Cro

codi

lians

Pte

rosa

urs

Orn

ithis

chia

ndi

nosa

urs

Sau

risch

ian

dino

saur

s ot

her

than

bird

s

Bir

ds

Ple

sios

aurs

Icht

hyos

aurs

Tuat

ara

Squ

amat

esM

amm

als

Traditionally, lizards, snakes, and crocs are classified together in the Class Reptilia with birds in a separate class (Aves)

But crocodiles may actually be more closely related to birds than to lizards and snakes

The extraembryonic membranes

ShellAlbumen

Yolk (nutrients)

Amniotic cavitywith amniotic fluid

Embryo

Yolk sac. The yolk sac contains the yolk, a stockpile of nutrients. Blood vessels in the yolk sac membrane transport nutrients from the yolk into the embryo. Other nutrients are stored in the albumen (“egg white”).

Allantois. The allantois is a disposalsac for certain metabolic wastes pro-duced by the embryo. The membraneof the allantois also functions withthe chorion as a respiratory organ.

Amnion. The amnion protectsthe embryo in a fluid-filled cavity that cushions againstmechanical shock.

Chorion. The chorion and the membrane of the allantois exchange gases between the embryo and the air. Oxygen and carbon dioxide diffuse freely across the shell.

Extraembryonic membranes

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)

Hatching Komodo dragon

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– Snakes and lizards are surviving forms of

lepidosaurs– Crocs and gators are an archosaur lineage

Dinosaurs

Diversified into a vast range of shapes and sizesIncluded 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

Deinonychus

Nesting Oviraptor and eggs

Lepidosaurs• One surviving lineage of lepidosaurs

is represented by two species of lizard-like reptiles called tuatara

Tuatara (Sphenodon punctatus)

Found only in New Zealand…endangered

Australian thorny devil lizard (Moloch horridus)

Squamates

• 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

• Snakes are legless lepidosaurs that evolved from lizards

Wagler’s pit viper (Tropidolaemus wagleri)

Turtles• 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

Eastern box turtle (Terrapene carolina carolina)

Alligators and Crocodiles• Crocodilians

– Belong to an archosaur lineage that dates back to the late Triassic

American alligator (Alligator mississipiensis)

Evolution of birds

Birds• Birds are archosaurs

– But almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight

• Many of the characters of birds are adaptations that facilitate flight– A bird’s most obvious adaptations for flight are its

wings and feathers

(a) wing

(b) Bone structure

Finger 1

(c) Feather structure

ShaftBarbBarbuleHook

VaneShaft

ForearmWrist

Palm

Finger 3

Finger 2

The Origin of Birds• Birds probably descended from theropods (small,

carnivorous dinosaurs)• By 150 million years ago feathered theropods

had evolved into birds• Archaeopteryx

Toothed beak

Airfoil wing with contour feathers

Long tail with many vertebrae

Wing claw

The ratites, order Struthioniformes

• Are all flightless

Emu. This ratite lives in Australia.

Flying birds• The demands of flight have rendered the

general body form of many flying birds similar to one another

Mallards. Like many bird species, the mallard exhibits pronounced color differences between the sexes.

Laysan albatrosses. Like most birds, Laysan albatrosses have specific mating behaviors, such as this courtship ritual.

Barn swallows. The barn swallow is a member of the order Passeriformes. Species in this order are called perching birds because the toes of their feet can lock around a branch or wire, enabling the bird to rest in place for long periods.

Foot Structure• Foot structure in bird feet shows considerable

variation

Grasping bird (such as a woodpecker)

Perching bird (such as a cardinal)

Raptor(such as a bald eagle)

Swimming bird(such as a duck)

Mammals, class Mammalia

• Represented by more than 5,000 species • Derived Characters of Mammals

– Mammary glands, which produce milk– 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

Sound Sound

Jaw joint Jaw joint Key

DentaryAngularSquamosal

ArticularQuadrate

Dimetrodon Morganucodon

Dimetrodon Morganucodon

Eardrum

Eardrum Middle ear Middle earStapes Inner ear

Inner ear

Stapes

Incus (evolvedfrom quadrate)

Malleus (evolvedfrom articular)

(b) During the evolutionary remodeling of the mammalian skull, the quadrate and articular bones became incorporated into the middle ear as two of the three bones that transmit sound from the eardrum to the inner ear. The steps in this evolutionary remodeling are evident in a succession of fossils.

(a) The lower jaw of Dimetrodon is composed of several fused bones; two small bones, the quadrate and articular, form part of the jaw joint. In Morganucodon, the lower jaw is reduced to a single bone, the dentary, and the location of the jaw joint has shifted.

MonotremesAre a small group of egg-laying

mammals consisting of echidnas and the platypus

Mammalian egg

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

A young brushtail possum. The young of marsupials are born very early in their development. They finish their growth while nursing from a nipple (in their mother’s pouch in most species).

Marsupials

• 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 marsupialsLong-nosed bandicoot. Most bandicoots

are diggers and burrowers that eat mainly insects but also some small vertebrates andplant material. Their rear-opening pouch helps protect the young from dirt as the mother digs. Other marsupials, such as kangaroos, have a pouch that opens to the front.

Convergent evolution

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

Marsupial mammals Eutherian mammals

Plantigale

Marsupial mole

Sugar glider

Wombat

Tasmanian devil

Kangaroo

Deer mouse

Mole

Woodchuck

Flying squirrel

Wolverine

Patagonian cavy

Eutherians (Placental Mammals)

• Have a longer period of pregnancy compared to marsupials.

• Young eutherians complete their embryonic development within a uterus, joined to the mother by the placenta.

Phylogenetic relationships of mammals

Ancestral mammal

Monotremes Marsupials Eutherians

Monotremata Marsupialia Xenarthra

Proboscidea SireniaTubulidentata Hyracoidea Afrosoricida (golden moles and tenrecs)Macroscelidea (elephant shrews)

RodentiaLagomorphaPrimatesDermoptera (flying lemurs)Scandentia (tree shrews)

CarnivoraCetartiodactylaPerissodactylaChiropteraEulipotyphlaPholidota (pangolins)

Possible phylogenetic tree of mammals. All 20 extant orders of mammals are listed at the top of the tree. Boldfaced orders are explored on the facing page.

This diverse clade includes terrestrial and marine mammals as well as bats,the only flying mammals. A growingbody of evidence, including Eocene fossils of whales with feet,supports putting whales inthe same order (Cetartiodactyla) as pigs, cows, and hippos.

This is the largest eutherian clade. It includes the rodents, which make up the largest mammalian order by far, with about 1,770 species. Humansbelong to the order Primates.

All members of this clade, which underwent an adaptive radiation in South America, belong to the order Xenarthra. One species, the nine-banded armadillo, is found in the southern United States.

This clade of eutherians evolved in Africa when the continent was isolated from other landmasses. It includesEarth’s largest living land animal (the African elephant), as well as species that weighless than 10 g.

The major eutherian ordersORDERS

AND EXAMPLESMAIN

CHARACTERISTICS

Monotremata Platypuses, echidnas

Proboscidea Elephants

SireniaManatees,dugongs

CetartiodactylaArtiodactylsSheep, pigs cattle, deer,giraffes

Lagomorpha Rabbits, hares, picas

Carnivora Dogs, wolves,bears, cats, weasels, otters,seals, walruses

Xenarthra Sloths, anteaters,armadillos

CetaceansWhales,dolphins,porpoises

Echidna

African elephant

Manatee

Tamandua

Jackrabbit

Coyote

Bighorn sheep

Pacific white-sided porpoise

Lay eggs; nonipples; young suck milk fromfur of mother

Long, musculartrunk; thick, loose skin; upper incisors elongated as tusks

Aquatic; finlikeforelimbs and no hind limbs; herbivorous

Reduced teeth orno teeth; herbivorous(sloths) or carnivorous (anteaters, armadillos)

Chisel-like incisors; hind legs longer than forelegs and adapted for running and jumping

Sharp, pointed canineteeth and molars for shearing; carnivorous

Hooves with an even number of toes on each foot; herbivorous

Aquatic; streamlinedbody; paddle-like forelimbs and no hind limbs; thicklayer of insulating blubber; carnivorous

Diet consists mainly of insects and other small invertebrates

Adapted for flight; broad skinfold that extends from elongated fingers to body and legs; carnivorous or herbivorous

Hooves with an odd number of toeson each foot; herbivorous

Opposable thumbs; forward-facing eyes; well-developed cerebral cortex; omnivorous

Chisel-like, continuously growing incisors worn down by gnawing;herbivorous

Short legs; stumpy tail; herbivorous; complex, multichamberedstomach

Teeth consisting of many thin tubes cemented together; eats ants and termites

Embryo completes development in pouch on mother

ORDERSAND EXAMPLES

MAIN CHARACTERISTICS

MarsupialiaKangaroos,opossums,koalas

TubulidentataAardvark

HyracoideaHyraxes

ChiropteraBats

PrimatesLemurs,monkeys,apes,humans

PerissodactylaHorses,zebras, tapirs,rhinoceroses

RodentiaSquirrels,beavers, rats, porcupines,mice  

Eulipotyphla“Core insecti-vores”: some moles, some shrews Star-nosed

mole

Frog-eating bat

Indian rhinoceros

Golden lion tamarin

Red squirrel

Rock hyrax

Aardvark

Koala

Primates• Order Primates include

– Lemurs, tarsiers, monkeys, and apes

• Derived Characters of Primates – 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– Most primates have hands and feet

adapted for grasping

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 Primate Family Tree

60

50

40

30

20

10

Mill

ions

of y

ear

s a

go

Ancestral primate

Lem

urs,

loris

es,

and

pot

tos

Ta

rsie

rs

Ne

w W

orld

mo

nke

ys

Old

Wo

rld

mo

nke

ys

Gib

bon

s

Ora

ngut

an

s

Go

rilla

s

Ch

im-

pan

zee

s

Hu

man

s

Anthropoids

0

Monkeys• The first monkeys evolved in the Old World (Africa

and Asia) • Monkeys first appeared in the New World (South

America) during the Oligocene• New World and Old World monkeys

– Underwent separate adaptive radiations during their many millions of years of separation

(a) New World monkeys, such as spider monkeys (shown here), squirrel monkeys, and capuchins, have a prehensile tail and nostrils that open to the sides.

(b) Old World monkeys lack a prehensile tail, and their nostrils open downward. This group includes macaques (shown here), mandrills, baboons, and rhesus monkeys.

Hominoids• Hominoids-consist of primates informally

called apes• Hominoids diverged from Old World monkeys

about 20–25 million years ago(a) Gibbons, such as this Muller's gibbon, are found only in southeastern Asia. Their very long arms and fingers are adaptations for brachiation.

(b) Orangutans are shy, solitary apes that live in the rain forests of Sumatra and Borneo. They spend most of their time in trees; note the foot adapted for grasping and the opposable thumb.

(c) Gorillas are the largest apes: some males are almost 2 m tall and weigh about 200 kg. Found only in Africa, these herbivores usually live in groups of up to about 20 individuals.

(d) Chimpanzees live in tropical Africa. They feed and sleep in trees but also spend a great deal of time on the ground. Chimpanzees are intelligent, communicative, and social.

(e) Bonobos are closely related to chimpanzees but are smaller. They survive today only in the African nation of Congo.

Humans

• 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)

• 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

HominidsHomo

sapiensHomoneanderthalensis

Homoergaster

?

Homoerectus

Homohabilis

Homorudolfensis

Paranthropusrobustus

Paranthropusboisei

Australopithecusgarhi

Australopithecusafricanus

Australopithecusafarensis

Kenyanthropusplatyops

Australopithecusanamensis

Ardipithecusramidus

Orrorin tugenensis

Sahelanthropustchadensis

7.0

6.5

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Early Hominids

• Early hominids– Had a small brain, but probably walked

upright, exhibiting mosaic evolution– Originated in Africa approximately 6–7 million

years ago

• 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 hominids that lived

between 4 and 2 million years ago• Some species walked fully erect and had

human-like hands and teeth.

(a) Lucy, a 3.24-million-year-old skeleton, represents the hominid species Australopithecus afarensis.

(b) The Laetoli footprints, more than 3.5 million years old, confirm that upright posture evolved quite early in hominid history.

(c) An artist’s reconstruction of what A. afarensis may have looked like.

Hominid evolution

• 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• 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 (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• 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