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CHAPTER 17
The Evolution of Animals
Figures 17.1 – 17.3
PowerPoint® Lecture Slides for
Essential Biology, Second Edition & Essential Biology with Physiology
Neil Campbell, Jane Reece, and Eric Simon
Presentation prepared by Chris C. Romero
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• Zoologists estimate
that about a billion
billion (1018)
individual arthropods
populate the Earth
• Tapeworms can reach
lengths of 20 meters in
the human intestine
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• The blue whale, an
endangered species that
grows to lengths of nearly
30 meters, is the largest
animal that has ever
existed
• A reptile can survive on less than
10% of the calories required by a
mammal of equivalent size
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BIOLOGY AND SOCIETY:
INVASION OF THE KILLER TOADS
• The incredible diversity of animals
– Arose through hundreds of millions of years of
evolution
– Can be quickly threatened
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• The Australian quoll
– Is a catlike creature that preys on many small
animals, such as toads
Figure 17.1a
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• A non-native toad
– Was introduced from South America in 1935 to fight
beetles in sugarcane fields
– Caused considerable damage to the ecosystem
Figure 17.1b
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THE ORIGINS OF ANIMAL DIVERSITY
• Animal life began in the Precambrian seas with the
evolution of multicellular creatures that ate other
organisms
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What Is an Animal?
• Animals
– Are eukaryotic, multicellular, heterotrophic
organisms that obtain nutrients by ingestion
– Digest their food within their bodies
Figure 17.2
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• Most animals
reproduce
sexually and then
proceed through a
series of
developmental
stages
Haploid
Sperm
Egg
2
1
Meiosis
Fertilization
Zygote
(fertilized egg)
Adult
3
Diploid
Blastula
(cross
section)
7 Metamorphosis
Digestive tract
Outer cell
layer
Primitive (ectoderm)
gut
6
5
Early
gastrula
Larva
Inner cell layer
(endoderm)
Figure 17.3
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4
Later gastrula
Opening
• Most animals have muscle cells and nerve cells that
control the muscles
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Early Animals and the Cambrian Explosion
• Animals probably evolved from a colonial protist
that lived in the Precambrian seas
Digestive
cavity
Reproductive
cells
1 Early colony
of protists
(aggregate of
identical cells)
2 Hollow
sphere
(shown in
cross section)
Somatic
cells
3 Beginning of
cell
specialization
4 Infolding
5 Gastrula-like
“protoanimal”
Figure 17.4
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• At the beginning of the Cambrian period, 545 million
years ago, animals underwent a rapid diversification
Figure 17.5
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• What ignited the Cambrian explosion?
– Many hypotheses exist
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Animal Phylogeny
• To reconstruct the evolutionary history of animal
phyla, researchers must depend on clues from
comparative anatomy and embryology
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• Four key evolutionary branch points have been
hypothesized
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Sponges
Cnidarians
Flatworms Roundworms Mollusks
Annelids
Arthropods Echinoderms
Coelom from
cell masses
Chordates
Coelom from
digestive tube
4
Pseudocoelom
True coelom
No body
cavity
3 Body cavities
Radial
symmetry
Bilateral
2 symmetry
True tissues
1
Multicellularity
Figure 17.6
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• The first branch point is defined by the presence of
true tissues
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• The second major evolutionary split is based partly
on body symmetry
(a) Radial symmetry
(b) Bilateral symmetry
Figure 17.7
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• Third, the evolution of body cavities led to more
complex animals
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• A body cavity
Body covering Tissue-filled
region
(from
(from
ectoderm)
mesoderm)
– Is a fluid-filled space
separating the
digestive tract from
the outer body wall
(a) No body cavity (e.g., flatworm)
Pseudocoelom
– May be a
pseudocoelom or a
true coelom
Body
covering
(from
ectoderm)
Digestive
tract (from
endoderm)
(b) Pseudocoelom (e.g., roundworm)
Coelom
Figure 17.8
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Digestive
tract (from
endoderm)
Muscle
layer (from
mesoderm)
Body covering
(from ectoderm)
Tissue layer
lining
coelom and
suspending
Digestive
tract (from Mesentery internal
organs
endoderm)
(from
(c) True coelom (e.g., annelid)
mesoderm)
• Fourth, among animals with a true coelom, there are
two main evolutionary branches, which differ in
embryonic development
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MAJOR INVERTEBRATE PHYLA
• Invertebrates
– Are animals without backbones
– Represent 95% of the animal kingdom
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Sponges
• Phylum Porifera
– Includes sessile animals
once believed to be
plants
– Lack true tissues
Figure 17.9
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• The body of a sponge
– Resembles a sac perforated with holes
– Draws water into a central cavity, where food is
collected
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Pores
Choanocyte
in contact
with an
amoebocyte
Water flow
Skeleton fiber
Central cavity
Choanocyte
Amoebocyte
Flagella
Figure 17.10
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Cnidarians
• Phylum Cnidaria
– Is characterized by organisms with radial symmetry
and tentacles with stinging cells
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• The basic body plan of a cnidarian
– Is a sac with a gastrovascular cavity
– Has two variations: the sessile polyp and the floating
medusa
Mouth/anus
Tentacle
Gastrovascular
cavity
Tentacle
Mouth/anus
Polyp form
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Medusa form
Figure 17.11
• Examples of polyps are
– Hydras, sea anemones,
and coral animals
Figure 17.12
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• The organisms we call jellies are medusas
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• Cnidarians are carnivores that use tentacles armed
with cnidocytes, or “stinging cells,” to capture prey
Coiled
thread
Tentacle
Capsule
“Trigger”
Cnidocyte
Discharge of
thread
Prey
Figure 17.13
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Flatworms
• Phylum Platyhelminthes
– Is represented by the simplest bilateral animals
– Includes free-living forms such as planarians
Digestive tract
(gastrovascular
cavity)
Nerve cords
Mouth
Eyespots
Nervous tissue
clusters
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Figure 17.14
• Some flatworms
are parasitic
Head
– Blood flukes are
an example
– Tapeworms
parasitize many
vertebrates,
including humans
Reproductive
structures
Hooks
Sucker
Figure 17.15
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Roundworms
• Phylum Nematoda
– Includes the most
diverse and
widespread of all
animals
– Occurs in aquatic
and moist
terrestrial habitats
Figure 17.16
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• Roundworms exhibit an important evolutionary
adaptation, a digestive tube with two openings, a
mouth and an anus
• A complete digestive tract can process food and
absorb nutrients efficiently
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Mollusks
• Phylum Mollusca
– Is represented by soft-bodied animals, but most are
protected by a hard shell
– Includes snails, slugs, clams, octopuses, and squids,
to name a few
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• The body of a mollusk has three main parts: a
muscular foot, a visceral mass, and a mantle
Visceral mass
Coelom
Mantle
Kidney
Reproductive
organs
Heart
Digestive
tract
Mantle
cavity
Radula
Shell
Radula
Anus
Gill
Foot
Mouth
Nerve cords
Mouth
Figure 17.17
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• The three major classes of mollusks are
– Gastropods, which are protected by a single, spiraled
shell
Figure 17.18a
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– Bivalves, protected by shells divided into two halves
Figure 17.18b
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– Cephalopods, which may or may not have a shell
Figure 17.18c
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Annelids
• Phylum Annelida
– Includes worms with body segmentation
Anus
Brain
Main
heart
Mouth
Accessory
hearts
Coelom
Digestive
tract
Segment
walls
Nerve cord
Blood
vessels
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Excretory
organ
Figure 17.19
• There are three main classes of annelids
– Earthworms, which eat their way through soil
Figure 17.20a
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– Polychaetes, which burrow in the sea floor
Figure 17.20b
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– Leeches, some of which are parasitic
Figure 17.20c
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Arthropods
• Phylum Arthropoda
– Contains organisms named for their jointed
appendages
– Includes crustaceans, arachnids, and insects
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General Characteristics of Arthropods
• Arthropods are segmented animals with specialized
segments and appendages
Cephalothorax
Abdomen
Thorax
Antennae
(sensory
reception)
Head
Swimming
appendages
Pincer
(defense)
Walking
legs
Mouthparts (feeding)
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Figure 17.21
• The body of an arthropod is completely covered by
an exoskeleton
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Arthropod Diversity
• There are four main groups of arthropods
– Arachnids, such as spiders, scorpions, ticks, and
mites
Figure 17.22
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– Crustaceans, such as crabs, lobsters, crayfish,
shrimps, and barnacles
Figure 17.23
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– Millipedes and centipedes
Figure 17.24
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– Insects, most of which have a three-part body
Head Thorax
Abdomen
Hawk moth
Antenna
Forewing
Eye
Mosquito
Paper wasp
Mouthparts
Hindwing
Grasshopper
Damselfly
Water strider
Ground beetle
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Figure 17.25
• Many insects
undergo
metamorphosis in
their development
(a) Larva
(caterpillar)
(b) Pupa
(c) Pupa
(d) Emerging
adult
(e) Adult
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Figure 17.26
Echinoderms
• Phylum Echinodermata
– Is named for the spiny surfaces of the organisms
– Includes sea stars, sand dollars, sea urchins, and sea
cucumbers
Figure 17.27
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• Echinoderms
– Are all marine
– Lack body segments
– Usually have an endoskeleton
– Have a water vascular system that facilitates gas
exchange and waste disposal
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THE VERTEBRATE GENEALOGY
• Vertebrates
– Are represented
by mammals,
birds, reptiles,
amphibians, and
fishes
– Have unique
features,
including the
cranium and
backbone
Figure 17.28
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Characteristics of Chordates
• Phylum Chordata
– Includes the subphylum of vertebrates
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• Other subphyla include the lancelets and tunicates,
which share four key chordate characteristics
Figure 17.29
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• The four chordate hallmarks are
– A dorsal, hollow nerve cord
– A notochord
– Pharyngeal slits
– A post-anal tail
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Notochord
Dorsal,
hollow
nerve cord
Brain
Muscle segments
Mouth
Anus
Post-anal
tail
Pharyngeal
slits
Figure 17.30
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• An overview of chordate and vertebrate evolution
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Precambrian
Cambrian
Paleozoic
Triassic
Tertiary
Cenozoic
Lancelets
Tunicates
Cretaceous
Mesozoic
Jurassic
Agnatha (jawless vertebrates,
such as lampreys)
Permian
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Mammalia
(mammals)
Aves
(birds)
Reptilia (reptiles)
Amphibia (frogs and
salamanders)
Osteichthyes (bony fishes)
Chondrichthyes (sharks and rays)
Ordovician Silurian Devonian Carboniferous
Eras
Periods
Chordates
Vertebrates
Tetrapods
Amniotes
Feathers
Hair
Amniotic egg
Legs
Lungs or lung derivatives
Jaws
Vertebrae
Ancestral chordate
Figure 17.31
Fishes
• The first vertebrates probably evolved during the
early Cambrian period, about 540 million years ago
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• These early vertebrates, the agnathans, lacked jaws
• Agnathans are represented today by lampreys
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• The two major groups of living fishes are the classes
– Chondrichthyes
– Osteichthyes
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• Cartilaginous fishes have a flexible skeleton made
of cartilage
– Sharks have a lateral line system sensitive to
vibrations in the water
Figure 17.32a
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• Bony fishes
– Have a skeleton
reinforced by
hard calcium
salts
– Have a lateral
line system, a
keen sense of
smell, and
excellent
eyesight
Figure 17.32b
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• Most bony fishes are ray-finned fishes
• A second evolutionary branch includes lungfishes
and lobe-finned fishes
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Amphibians
• Members of the class
Amphibia
– Exhibit a mixture of
aquatic and terrestrial
adaptations
– Usually need water to
reproduce
Figure 17.33
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• Amphibians
– Were the first vertebrates to colonize land
– Descended from fishes that had lungs and fins with
muscles
Lobe-finned fish
Early amphibian
Figure 17.34
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• Terrestrial vertebrates are collectively called
tetrapods, which means “four legs”
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Reptiles
• Class Reptilia
– Includes snakes, lizards, turtles, crocodiles, and
alligators
– Can live totally on land
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• Adaptations for living on land include
– Scales to
prevent
dehydration
– Lungs for
breathing
– The amniotic
egg
Figure 17.35
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• Reptiles are ectotherms, which obtain their body
heat from the environment
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• Reptiles diversified extensively during the Mesozoic
Era
• Dinosaurs included the largest animals ever to live
on land
Figure 17.36
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Birds
• Class Aves
– Evolved during the great reptilian radiation of the
Mesozoic era
– Evolved the ability to fly
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• Bird anatomy and physiology are modified for flight
– Bones are honeycombed, which makes them lighter
– Some specific organs are absent, which reduces
weight
– A warm, constant body temperature is maintained
through endothermy
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• A bird’s wings
– Illustrate the same principles of aerodynamics as the
wings of an airplane
Figure 17.37
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Mammals
• Class Mammalia
– Evolved from reptiles about 225 million years ago
– Includes mostly terrestrial organisms
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• Two features are mammalian hallmarks
– Hair
– Mammary glands that produce milk and nourish the
young
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• There are three major groups of mammals
– Monotremes, the egg-laying mammals, constitute the
first group
Figure 17.38a
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• Most mammals are born rather than hatched and are
nurtured inside the mother by an organ called a
placenta
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• The second group of mammals, marsupials, are the
so-called pouched mammals
Figure 17.38b
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• Eutherians are also called
placental mammals
– Their placentas provide
more intimate and longlasting association
between the mother and
her developing young
than do marsupial
placentas
Figure 17.38c
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THE HUMAN ANCESTRY
• Humans are primates
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The Evolution of Primates
• Primate evolution
– Provides a context for understanding human origins
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• Primates
– Evolved from insect-eating mammals during the late
Cretaceous period
• Early primates
– Were small, arboreal mammals
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• The distinguishing characteristics of primates were
shaped by the demands of living in trees
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• Primate characteristics include
– Limber shoulder
joints
– Eyes in front of the
face
– Excellent eye-hand
coordination
– Extensive parental
care
Figure 17.39
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• Taxonomists divide primates into two main groups
– Prosimians
– Anthropoids
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• Prosimians include
– Lemurs, lorises,
pottos, and tarsiers
Figure 17.40a
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• Anthropoids include
– Monkeys
Figure 17.40b, c
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– Apes, the closest
relatives to humans
Figure 17.40d–g
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– Humans
Figure 17.40h
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The Emergence of Humankind
• Humans and apes have shared a common ancestry
for all but the last 5–7 million years
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Anthropoids
Prosimians
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Humans
Chimpanzees
Orangutans
Gibbons
Old World monkeys
New World monkeys
Prosimians (lemurs, lorises, pottos, and tarsiers)
Ancestral primate
Gorillas
Apes
Monkeys
Figure 17.41
Some Common Misconceptions
• Our ancestors were not chimpanzees or any other
modern apes
• Chimpanzees and humans represent two divergent
branches of the anthropoid tree
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• Human evolution
– Is not a ladder with a series of steps leading directly
to Homo sapiens
– Is more like a multibranched bush than a ladder
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Homo sapiens Homo sapiens
sapiens
neanderthalensis
Homo erectus
Australopithecus
boisei
Australopithecus
robustus
Homo habilis
Australopithecus
africanus
Ardipithecus
ramidus
Australopithecus
afarensis
Figure 17.42
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• Upright posture and an enlarged brain appeared at
separate times during human evolution
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Australopithecus and the Antiquity of Bipedalism
• Before there was the genus Homo, several hominid
species of the genus Australopithecus walked the
African savanna
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• Fossil evidence pushes bipedalism in A. afarensis
back to at least 4 million years ago
Figure 17.43
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• All Australopithecus species were extinct by about
1.4 million years
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Homo habilis and the Evolution of Inventive Minds
• Homo habilis, “handy-man”
– Had a larger brain
– Probably made stone tools
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Homo erectus and the Global Diversity of Humanity
• Homo erectus was the first species to extend
humanity’s range from Africa to other continents
• The global dispersal began about 1.8 million years
ago
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• Homo erectus
– Was taller than H. habilis
– Had a larger brain
– Gave rise to Neanderthals
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The Origin of Homo sapiens
• The oldest known post–H. erectus fossils
– Date back more than 300,000 years
– Are found in Africa
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• Many paleoanthropologists consider these fossils as
the earliest forms of our species, Homo sapiens
• The famous fossils of modern humans from the
Cro-Magnon caves of France date back about
35,000 years
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• Two hypotheses regarding the origins of modern
humans exist
– The multiregional hypothesis
– The “Out of Africa” hypothesis (also called the
replacement hypothesis)
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• The multiregional hypothesis
– States that modern humans evolved simultaneously
in different parts of the world
– States that Homo erectus spread from Africa into
other continents between 1 and 2 million years ago
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Homo sapiens
African
European
Asian
Australasian
Interbreeding
1–2 million years ago
(a) Multiregional hypothesis
Homo erectus
in Africa
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Figure 17.44a
• The “Out of Africa” hypothesis
– States that modern humans spread out from Africa
about 100,000 years ago
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Homo sapiens
African
European
Asian
Australasian
100,000 years ago
Homo sapiens
in Africa
(b) “Out of Africa” hypothesis
Homo erectus in
Africa
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Figure 17.44b
Cultural Evolution
• Culture
– Is the transmission of accumulated knowledge over
generations
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• Cultural evolution has had three major stages
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• First, nomads
who were
hunter-gatherers
– Made tools
– Created art
Figure 17.45
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• Second, the development of agriculture
• Third, the Industrial Revolution
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EVOLUTION CONNECTION:
EARTH’S NEW CRISIS
• Cultural evolution
– Made Homo sapiens a new force in the history of life
• Humans are changing the world faster than many
species can adapt
– The rate of extinction in the twentieth century was 50
times greater than the average for the past 100,000
years
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• This rapid rate of extinction is mainly a result of
habitat destruction
• The exploding human population now threatens
Earth’s ecosystems
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