17–3 evolution of multicellular life · pdf filesummary of the detailed material in ......
TRANSCRIPT
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
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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)
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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
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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)
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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.
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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.
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