1 FOCUSObjective17.3.1 Describe the key forms

of life in the Paleozoic,Mesozoic, and Cenozoiceras.

Vocabulary PreviewRead the names of the periods aloudand encourage students to repeatthem after you. Knowing how to pro-nounce the words correctly will helpstudents remember them.

Reading StrategyDepending on the amount of detailyou expect students to learn, youmight want to suggest that theymake a separate row in their table foreach period.

2 INSTRUCT

Precambrian TimeDemonstrationThe information in the text onPrecambrian Time provides a concisesummary of the detailed material inSection 17–2. Students may benefitfrom a review of the material. Workwith the class to create a simple flow-chart on the chalkboard or anoverhead transparency. Call onstudents to identify the correctsequence of events for the flowchart.

Paleozoic EraUse VisualsFigure 17–14 Call students’ atten-tion to the figure. Ask: Why are onlyaquatic organisms represented forthe Cambrian Period? (There wereno terrestrial life-forms at that time.)

The History of Life 429

Although the fossil record has missing pieces, paleontologistshave assembled good evolutionary histories for many

groups of organisms. Furthermore, the fossil record indicatesthat major changes occurred in Earth’s climate, geography, andlife-forms. In this section, you will get an overview of howmulticellular life evolved from its earliest forms to its present-day diversity.

Precambrian TimeRecall that almost 90 percent of Earth’s history occurred duringthe Precambrian. During this time, simple anaerobic forms oflife appeared and were followed by photosynthetic forms, whichadded oxygen to the atmosphere. Aerobic forms of life evolved,and eukaryotes appeared. Some of those organisms gave rise tomulticellular forms that continued to increase in complexity.Few fossils exist from this time because the animals were allsoft-bodied. Life existed only in the sea.

Paleozoic EraRich fossil evidence shows that early in the Paleozoic

Era, there was a diversity of marine life. Scientists oncethought that those different forms of life evolved rapidly at thebeginning of the Paleozoic, but increasing evidence fromPrecambrian fossils and DNA studies suggests that life began todiversify much earlier. Regardless of when these forms evolved,fossil evidence shows that life was highly diverse by the firstpart of the Paleozoic Era, the Cambrian Period. An artist’sportrayal of Cambrian life, which included many kinds ofinvertebrate animals, is shown in Figure 17–14.

Key Concept• What were the characteristic

forms of life in the Paleozoic,Mesozoic, and Cenozoic eras?

Vocabularymass extinction

Reading Strategy:Using Graphic OrganizersAs you read, make a table of thethree geologic eras described inthe section. Include informationabout the typical organisms andmain evolutionary events ofeach era.

� Figure 17–14 The fossilrecord shows evidence of manytypes of marine life early in thePaleozoic Era. These and other unfamiliar organisms dwelt in the seaduring the Cambrian Period, a timewhen animals with hard parts evolved.

17–3 Evolution of Multicellular Life Section 17–3

SECTION RESOURCES

Print:

• Teaching Resources, Lesson Plan 17–3,Adapted Section Summary 17–3, AdaptedWorksheets 17–3, Section Summary 17–3,Worksheets 17–3, Section Review 17–3

• Reading and Study Workbook A, Section 17–3• Adapted Reading and Study Workbook B,

Section 17–3

Technology:

• iText, Section 17–3• Transparencies Plus, Section 17–3

Tim

eSaver

0416_0440_bi_c07_te 3/8/06 6:53 PM Page 429

430 Chapter 17

Build Science SkillsInferring Point out that the firstorganisms with shells and outerskeletons evolved during theCambrian Period. Then, ask: Whywould having hard parts such asshells be an advantage to organ-isms? (The hard body parts would helpprotect the organisms from predators.)

Make ConnectionsEnvironmental Science Help stu-dents appreciate why the evolutionof the first land-dwelling organisms,which occurred during theOrdovician Period, was an importantevolutionary event. Point out howthe earliest terrestrial organismswould have had virtually no competi-tors for the diversity of potentialniches available on land.

Build Science SkillsDrawing Conclusions Ask: Howdid life on Earth differ before andafter the Permian extinction? (Most marine life and many land verte-brates—particularly large ones—werelost. Small reptiles and land plantswere less affected.)

Cambrian Period Paleontologists call the diversification of

life during the early Cambrian Period the “Cambrian Explosion.”

For the first time, many organisms had hard parts, including

shells and outer skeletons. During the Cambrian Period, the

first known representatives of most animal phyla evolved.

Invertebrates—such as jellyfishes, worms, and sponges—drifted

through the water, crawled along the sandy bottom, or attached

themselves to the ocean floors. Brachiopods, which were small

animals with two shells, were especially common. They resem-

bled—but were unrelated to—modern clams. Trilobites were

also common. Trilobites were arthropods, which are invertebrates

with segmented bodies, jointed limbs, and an external skeleton.

What is the “Cambrian Explosion”?

Ordovician and Silurian Periods During the

Ordovician (awr-duh-VISH-un) and Silurian (sih-LOOR-

ee-un) periods, the ancestors of the modern octopi and

squid appeared, as did aquatic arthropods like the one

in Figure 17–15. Some arthropods became the first

animals to live on land. Among the first vertebrates

(animals with backbones) to appear were jawless

fishes, which had suckerlike mouths. The first land

plants evolved from aquatic ancestors. These simple

plants grew low to the ground in damp areas.

Devonian Period By the Devonian (dih-VOH-nee-un)

Period, some plants, such as ferns, had adapted to drier areas,

allowing them to invade more habitats. Insects, which are arthro-

pods, appeared on land. In the seas, both invertebrates and

vertebrates thrived. Even though the invertebrates were far more

numerous, the Devonian is often called the Age of Fishes because

many groups of fishes were present in the oceans. Most fishes of

this time had jaws, bony skeletons, and scales on their bodies.

Sharks appeared in the late Devonian.

During the Devonian, vertebrates began to invadethe land. The first fishes to develop the ability to crawl awk-

wardly on leglike fins were still fully aquatic animals. Some of

these early four-legged vertebrates evolved into the first amphib-

ians. An amphibian (am-FIB-ee-un) is an animal that lives part

of its life on land and part of its life in water.

Carboniferous and Permian Periods Throughout the

rest of the Paleozoic Era, life expanded over Earth’s continents.

Other groups of vertebrates, such as reptiles, evolved from

certain amphibians. Reptiles are animals that have scaly skin

and lay eggs with tough, leathery shells. Winged insects evolved

into many forms, including huge dragonflies and cockroaches.

Giant ferns and other plants formed vast swampy forests,

shown in Figure 17–16. The remains of those ancient plants

formed thick deposits of sediment that changed into coal over

millions of years, giving the Carboniferous its name.

� Figure 17–15 During theOrdovician Period, aquatic arthro-pods like this eurypterid evolved.Eurypterids had segmented bodiesand lived in water. Some of themgrew to a length of almost 13 meters. Eurypterids are nowextinct. Comparing andContrasting Which of today’s animals do eurypterids resemble?

Vocabulary: WritingBeginning On the board, make a box labeledPaleozoic Era. Write several words or simplephrases describing the life-forms ofPrecambrian time in the box. To clarify thedescriptions, call students’ attention to theorganisms in Figure 17–14. Then, have thestudents draw two boxes, one labeled MesozoicEra and one labeled Cenozoic Era. Have pairs ofstudents work together to draw a picture or

write a word or phrase describing the domi-nant life-forms of that era. Intermediate Extend the beginning-levelactivity by writing a complete sentence on theboard to describe the life-forms present in thePaleozoic Era. Then, ask students to fill in theirlabeled boxes using complete sentences. If anESL student needs assistance, pair him or herwith a student who is proficient in English.

SUPPORT FOR ENGLISH LANGUAGE LEARNERS

17–3 (continued)

0416_0440_bi_c07_te 3/8/06 6:53 PM Page 430

Mesozoic EraMake ConnectionsEarth Science Show students mapsof the location of Earth’s landmassesat various times during the MesozoicEra. Include a map showing Earth ataround 225 million years ago, whenall the major landmasses were joinedtogether in the supercontinent ofPangaea, and also around 180 mil-lion years ago, when Pangaea hadsplit apart to form Laurasia andGondwanaland. Have students compare these maps with the present-day locations of the conti-nents. Ask: Why would the driftingof continents have affected climate?(Because climate is determined by thedistance north and south of the equa-tor and by the position of large bodiesof water) How might various kindsof organisms have been affectedby those climate changes? (Changesin climate might have led to the extinc-tion of some species, while favoringothers. Also, the separation of land-masses might have led to geographicisolation and the emergence of newand different species on the differentlandmasses.)

The History of Life 431

At the end of the Paleozoic, many organisms died out. Thiswas a in which many types of living thingsmass extinction,

became extinct at the same time. The mass extinction at

the end of the Paleozoic affected both plants and animals

on land and in the seas. As much as 95 percent of the

complex life in the oceans disappeared. For example,

trilobites, which had existed since early in the Paleozoic, sud-

denly became extinct. Many amphibians also became extinct.

Not all organisms disappeared, however. The mass extinction

did not affect many fishes. Numerous reptiles also survived.

Mesozoic EraThe Mesozoic Era lasted approximately 180 million years.

Events during the Mesozoic include the increasing

dominance of dinosaurs. The Mesozoic is marked by

the appearance of flowering plants.

Triassic Period Those organisms that survived the

Permian mass extinction became the main forms of life early

in the Triassic (try-AS-ik) Period. Important organisms in

this new ecosystem were fishes, insects, reptiles, and cone-

bearing plants like the one in Figure 17–17. Reptiles were so

successful during the Mesozoic Era that this time is often

called the Age of Reptiles.

About 225 million years ago, the first dinosaurs

appeared. One of the earliest dinosaurs, Coelophysis, was a

meat-eater that had light, hollow bones and ran swiftly on

its hind legs. Mammals also first appeared during the late

Triassic Period, probably evolving from mammal-like

reptiles. Mammals of the Triassic were very small, about

the size of a mouse or shrew.

� Figure 17–16 Ancient forestslike this one from the CarboniferousPeriod were characterized by a hugevariety of life-forms. At the endof the Paleozoic Era, many typesof animals and plants becameextinct.

� Figure 17–17 Among the seedplants of the Triassic Period werecone-bearing plants called cycads,which left this modern descendant.Applying Concepts What otherorganisms were important in theTriassic Period?

Answers to . . . The diversification of life

during the early Cambrian Period

Figure 17–15 Possible answersinclude a crab, a scorpion, and a lobster.

Figure 17–17 Fishes, insects, andreptiles

0416_0440_bi_c07_te 3/8/06 6:53 PM Page 431

432 Chapter 17

Address MisconceptionsSome students might think thatdinosaurs were not very successful inevolutionary terms because, with thepossible exception of the ancestors ofmodern birds, dinosaurs becameextinct at the end of the CretaceousPeriod. Point out that dinosaurs“ruled” Earth for a total period ofabout 150 million years. Put this timespan in perspective by comparing itwith the length of time that thehuman family, the hominids, hasbeen in existence, which is less than10 million years.

Use VisualsFigure 17–18 Call on students todescribe the environment that isdepicted in the figure. (They may sayit is hot and wet or tropical.) Explainthat, during the Mesozoic, manyplaces on Earth, including much ofNorth America, had this type of envi-ronment. Remind students thatdinosaurs were reptiles and probablycould not internally regulate theirbody temperature. Ask: What wouldhappen to dinosaurs if they werein North America today? (It mightbe difficult for them to survive becauseof the cold and other differences inenvironment.)

� Figure 17–18 During theMesozoic Era, dinosaurs weredominant. Dicraeosaurus (fore-ground) was a plant-eater that grew to about 20 meters in length.

Jurassic Period During the Jurassic (joo-RAS-ik) Period,

dinosaurs became the dominant animals on land. Dinosaurs

“ruled” Earth for about 150 million years, but different types

lived at different times. At 20 meters long, Dicraeosaurus,

shown in Figure 17–18, was one of the larger dinosaurs of the

Jurassic Period.

One of the first birds, called Archaeopteryx, appeared during

this time. Many paleontologists now think that birds are close

relatives of dinosaurs. Since the 1990s, scientists working in

China have found evidence for this hypothesis in other fossils

that have the skulls and teeth of dinosaurs but the body struc-

ture and feathers of birds.

Cretaceous Period Reptiles were still the dominant verte-

brates throughout the Cretaceous (krih-TAY-shus) Period.

Dinosaurs such as the meat-eating Tyrannosaurus rex dominated

land ecosystems, while flying reptiles and birds soared in the sky.

Flying reptiles, however, became extinct during the Cretaceous.

In the seas, turtles, crocodiles, and extinct reptiles such as ple-

siosaurs swam among fishes and marine invertebrates.

The Cretaceous also brought new forms of life, including

leafy trees, shrubs, and small flowering plants like those you see

today. Unlike the conifers, flowering plants produce seeds

enclosed in a fruit, which protects the seed and aids in dispers-

ing it to new locations.

At the close of the Cretaceous, another mass extinction

occurred. More than half of all plant and animal groups were

wiped out, including all of the dinosaurs.

When did flowering plants evolve?

How sweet it isFruits and seeds were a major evolutionaryadvance in the reproduction of plants. However,if animals eat unripe fruits, the immature seedsin them are not capable of sprouting and grow-ing. As a result, natural selection led to plantsthat have ways to discourage animals from eat-ing their unripe fruits. Many unripe fruits aregreen and contain bitter-tasting chemical com-pounds. The green color of the unripe fruitsmakes them more difficult to see among a

plant’s leaves, and the bitter taste helps discour-age animals from eating them. As the seedsmature, the bitter-tasting chemical compoundsbreak down, and the fruits become laden withsugars. While this process occurs, the fruits alsochange color from green to red, orange, purple,or whatever color indicates ripeness in thatspecies. These colors are more easily seen byanimals against the background of green leaves,and the fruits’ sweet taste reinforces the eatingresponse.

FACTS AND FIGURES

17–3 (continued)

0416_0440_bi_c07_te 3/8/06 6:53 PM Page 432

Cenozoic EraBuild Science SkillsDrawing Conclusions Point outthat the first mammals probablyevolved during the Triassic Period butthat mammals did not flourish untilthe Cenozoic Era. Ask: Why didmammals not become successfulfor more than 100 million yearsafter they first evolved? (During theMesozoic, early mammals had to com-pete with many kinds of reptiles,including dinosaurs, for food andplaces to live. When dinosaurs wentextinct at the end of the Mesozoic,many resources became available formammals to utilize.)

The History of Life 433

Cenozoic EraDuring the Mesozoic, early mammals competed with dinosaurs

for food and places to live. The extinction of dinosaurs at the end

of the Mesozoic, however, created a different world. Duringthe Cenozoic, mammals evolved adaptations that allowedthem to live in various environments—on land, in water,and even in the air. One land mammal from the early Ceno-

zoic is shown in Figure 17–19. Paleontologists often call the

Cenozoic the Age of Mammals.

Tertiary Period During the Tertiary Period, Earth’s climates

were generally warm and mild. In the oceans, marine mammals

such as whales and dolphins evolved. On land, flowering plants

and insects flourished. Grasses evolved, providing a food source

that encouraged the evolution of grazing mammals, the ancestors

of today’s cattle, deer, sheep, and other grass-eating mammals.

Some mammals became very large, as did some birds.

� Figure 17–19 During the Cenozoic Era,mammals evolved adaptations that allowed themto live on land, in water, and even in the air. Twoof the traits that contributed to the success ofmammals were a covering of hair that providedinsulation against the cold and the protection of theyoung before and after birth.

Fossil Preparer Job Description: work for private industries,museums, or universities to expose fossilscovered by rock or soil or to construct missingfossil parts

Education: a college degree in biology orgeology, knowledge about the fossils beingworked on

Skills: be knowledgeable about many areas ofscience, ability to use fine tools under a micro-scope, self-motivated, patient, ability to handlevery fragile specimens for long periods

Highlights: work with fossils; collaborate withmany types of people—from amateur fossilcollectors to professional paleontologists

For: Career linksVisit: PHSchool.comWeb Code: cbb-5173

Careers in Biology

Answer to . . . During the Cretaceous

Period

• Sculpting is a good skill to have forthis career because fossil preparersoccasionally must fabricate missingfossil parts.

• Some fossil preparers spendmonths working in a field camp ata paleontological dig. They mayalso spend time in the field collect-ing fossils.

Resources Students can contact auniversity paleontology department,the Paleontological Association, thePaleontological Research Institute, orthe personnel department of a natu-ral history museum.

Careers in Biology

You can have students write amore extensive job description aswell as list the educational require-ments for a career in this field.

0416_0440_bi_c07_te 3/8/06 6:53 PM Page 433

434 Chapter 17

Answer to . . . Figure 17–20 Organisms that couldnot migrate to a warmer climate oradapt to the change in climate wouldhave gone extinct.

Make ConnectionsEnvironmental Science Providestudents with background informa-tion on the ice ages. Explain thatover the past two million years, therewere four major ice ages, each last-ing at least 100,000 years or longerand between which were long peri-ods of warmer climate. During thepeak of the most recent ice age,which ended about 20,000 yearsago, ice covered much of NorthAmerica, reaching as far south as thepresent-day lower midwestern states.Scientists think the ice ages werecaused by variations in the positionof the Earth relative to the sun,changes in the sun’s energy outputdue to sunspots, and continentalmovement.

3 ASSESSEvaluate UnderstandingRead each of the Key Concepts in thesection, leaving the name of the eraor period blank. Call on students atrandom to fill in the blanks.

ReteachHave pairs of students make and quizeach other with flashcards that eachhave an important evolutionary eventon one side and the correct era andperiod on the other side.

� Figure 17–20 During theQuaternary Period, Earth’s climatecooled, producing a series of iceages. Among the characteristic ani-mals of the time were these hugemammoths. Inferring Howmight the change to a colder climatehave affected different types oforganisms?

Quaternary Period Mammals that had evolved during the

Tertiary Period eventually faced a changing environment during

the Quaternary Period. During this time, Earth’s climate cooled,

causing a series of ice ages. Repeatedly, thick continental glaciers

advanced and retreated over parts of Europe and North America.

So much of Earth’s water was frozen in continental glaciers that

the level of the oceans fell by more than 100 meters. Then, about

20,000 years ago, Earth’s climate began to warm. Over the

course of thousands of years, the continental glaciers melted.

This caused sea levels to rise again.

In the oceans, algae, coral, mollusks, fishes, and mammals

thrived. Insects and birds shared the skies. On land, mammals—

such as bats, cats, dogs, cattle, and the mammoths shown in

Figure 17–20—became common. The fossil record suggests that

the early ancestors of our species appeared about 4.5 million

years ago but that they did not look entirely human. The first

fossils assigned to our own species, Homo sapiens, may have

appeared as early as 200,000 years ago in Africa. According to

one hypothesis, members of our species began a series of migra-

tions from Africa that ultimately colonized the world.

1. Key Concept Where didlife exist during the earlyPaleozoic Era?

2. Key Concept Whatevolutionary milestone involvinganimals occurred during theDevonian Period?

3. Key Concept What aretwo key events from theMesozoic Era?

4. Critical Thinking InferringIf you were a paleontologistinvestigating fossils from theCenozoic Era, what fossils mightyou find?

Creative WritingChoose one of the periodsdescribed in this section.Then, write a story about lifeduring that time. Includeinformation about the life-forms, weather, and othercharacteristics.

17–3 Section Assessment

17–3 (continued)

If students have a hard time devel-oping ideas for their stories,suggest that they brainstorm insmall groups. Stories shouldinclude information from the textas well as additional reliablesources.

If your class subscribes to theiText, use it to review the KeyConcepts in Section 17–3.

17–3 Section Assessment1. Early life existed in the sea.2. During the Devonian Period, animals began

to invade the land.3. Events include the first appearance of

dinosaurs and the appearance of seed plants,including cone-bearing and flowering types.

4. You might find fossils of flowering plants,insects, birds, and mammals, includinghumans.

0416_0440_bi_c07_te 3/8/06 6:53 PM Page 434