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Chapter 12 • History of Life on Earth 251

Opening ActivityTo help students visualize the enor-mity of one billion of anything, askstudents if they could physicallycarry one million dollars. Onemillion dollars would be a stack of$100 bills approximately 1 m inheight (10,000 bills). A billion dol-lars would be a stack of the samebills to a height of the WashingtonMonument (one million bills). Nowrelate this image to the first billionyears of our planet’s existence with-out any form of life.

GENERAL

• Vocabulary Worksheets• Concept Mapping

Chapter Resource File

Answers1. Proteins are polymers of amino

acids. Lipids are mostly orcompletely nonpolar substancescontaining either chains orrings of primarily carbon andhydrogen atoms. Nucleic acidsare polymers of nucleotides.

2. Enzymes interact with mole-cules to place strain on achemical bond so that it breakseasily, or to place substances inclose proximity so that a smallamount of energy input resultsin a bond being formedbetween the substances.

3. Prokaryotes are single-celledorganisms that lack membrane-bounded organelles.Eukaryotes are single or multi-cellular organisms that havemembrane-bounded organelles,including a nucleus.

4. Chloroplasts and mitochondriaare oval and have a complexsystem of internal membranes.Chloroplasts carry out all ofthe reactions of photosynthesis.Mitochondria carry out all of the reactions of cellularrespiration.

5. The DNA of an organismmakes up its genetic informa-tion. When a new organism isformed from two parents, itcontains its own genetic infor-mation, half of which camefrom each parent.

Quick Review

AnswersStudent lists will likely indicate some under-standing of the Earth’s beginnings and the origin of life. Few will likely show an under-standing of how the complexity and diversityof life on Earth today could have evolved fromthe earliest organisms that appeared on Earth.

Reading Activity

Looking AheadQuick ReviewAnswer the following without referring to earlier sections of your book. 1. Compare the structure of proteins, lipids, and

nucleic acids. (Chapter 2, Section 3)2. Describe the role of enzymes in catalyzing

chemical reactions. (Chapter 2, Section 4) 3. Contrast prokaryotes and eukaryotes.

(Chapter 3, Section 2)4. Identify the structure and function of

chloroplasts and mitochondria. (Chapter 3,Section 3)

5. Summarize the role of DNA in heredity.(Chapter 9, Section 1)

Did you have difficulty? For help, review the sections indicated.

Section 1 How Did Life Begin?

The Age of EarthFormation of the Basic Chemicals of LifePrecursors of the First Cells

Section 2 The Evolution of Cellular Life

The Evolution of ProkaryotesThe Evolution of Eukaryotes MulticellularityMass Extinctions

Section 3 Life Invaded the Land

The Ozone LayerPlants and Fungi on Land ArthropodsVertebrates

www.scilinks.orgNational Science Teachers Association sciLINKS Internet resources are located throughout this chapter.

Reading ActivityBefore you begin to read this chapter, writedown all of the key words for each section ofthe chapter. Then, write a definition next to eachword that you have heard of. As you read thechapter, write definitions next to the words thatyou did not previously know, and modify asneeded your original definitions of words familiar to you.

Billions of years ago, the combination of simplemolecules and energy from sources such as lightningmay have given rise to the complex organic moleculesnecessary for life.

History of Lifeon Earth

CHAPTER

12

251

Copyright © by Holt, Rinehart and Winston. All rights reserved.

OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. They will learn about howscientists measure the age of theEarth. They will also learn aboutseveral models scientists have devel-oped to explain how the first lifeforms originated on the early Earth.Finally, they will learn how scientiststhink the complex chemicals foundin living things could have arisenfrom the chemicals present on Earthat the time.

Ask students to draw a picture ofwhat they think Earth’s first lifeform looked like, and have themlabel the parts of the organism.

Using the FigureDirect students’ attention toFigure 1. Ask students to explainwhy the graph of the rate of decayshown in this figure is not astraight line. (The rate of radioactivedecay is based on the half-life of aradioactive material. The amount ofmaterial that decays in one half-life isone-half of the undecayed portion ofthe sample present at the beginningof the time period. With each succes-sive half-life, the total amount ofmaterial decayed increases. VisualLS

GENERAL

MotivateMotivate

Bellringer

FocusFocus

Section 1

• Directed Reading• Active Reading• Data Sheet for Quick Lab GENERAL

GENERAL


• Reading Organizers• Reading Strategies• Basic Skills Worksheet

Mass and Density

Planner CD-ROM

StrategiesStrategiesINCLUSIONINCLUSION

Ask students to draw illustrations of theirinterpretation of the Earth’s formation 4.5billion years ago. The illustrations shouldshow their idea of at least four stages of theevolution of the Earth’s surface. The stagesshould reflect information in the openingparagraphs of Section 1. Students shouldnarrate their drawings through a tape-recorded description of the events shown in their drawings.

• Attention Deficit Disorder• Learning Disability

• Developmental Delay

252 Chapter 12 • History of Life on Earth

Section 1 How Did Life Begin?

The Age of EarthWhen Earth formed, about 4.5 billion years ago, it was a fiery ballof molten rock. Eventually, the planet’s surface cooled and formeda rocky crust. Water vapor in the atmosphere condensed to formvast oceans. Most scientists think life first evolved in these oceansand that the evolution of life occurred over hundreds of millions ofyears. Evidence that Earth has existed long enough for this evolu-tion to have taken place can be found by measuring the age of rocksfound on Earth.

Measuring Earth’s AgeScientists have estimated the age of Earth using a technique calledradiometric dating. is the estimation of the ageof an object by measuring its content of certain radioactive isotopes(IE soh tohps). An isotope is a form of an element whose atomic mass(the mass of each individual atom) differs from that of other atomsof the same element. Radioactive isotopes, or , areunstable isotopes that break down and give off energy in the form ofcharged particles (radiation). This breakdown, called radioactivedecay, results in other isotopes that are smaller and more stable.

For example, certain rocks contain traces of potassium-40, an iso-tope of the element potassium. As Figure 1 shows, the decay ofpotassium-40 produces two other isotopes, argon-40 and calcium-40. The time it takes for one-half of a given amount of a radioisotopeto decay is called the radioisotope’s . By measuring theproportions of certain radioisotopes and their products of decay, sci-entists can compute how many half-lives have passed since a rockwas formed.

half-life

radioisotopes

Radiometric dating

Objectives● Summarize how radioiso-

topes can be used in determining Earth’s age.

● Compare two models thatdescribe how the chemicalsof life originated.

● Describe how cellular organization might havebegun.

● Recognize the importancethat a mechanism for heredityhas to the development of life.

Key Terms

radiometric datingradioisotopehalf-lifemicrosphere

Radioactive Decay

1/161/8

1/4

1/2

1/1

1 half-life1.3

2 half-lives2.6

3 half-lives3.9

4 half-lives5.2

Newly formed rock

Am

ount

(of r

emai

ning

po

tass

ium

-40

atom

s)

Time passed (in billions of years)

Potassium-40Argon-40 (product)Calcium-40 (product)

Figure 1 Rate of decay forpotassium-40. This graphshows the rate of decay for theradioisotope potassium-40.After one half-life has passed,half of the original amount ofthe radioisotope remains.

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Trends in Particle PhysicsTrace Analysis A new ultrasensitive traceanalysis technique—able to detect single atomsin a large sample—has been developed byArgonne National Laboratory researchers. Thetechnology, called Atom Trap Trace Analysis, orATTA for short, holds promise for advancingthe state of the art in many fields, from ground-water studies to environmental monitoring. Oneof its first applications may be dating ancientGreenland ice cores. While carbon-14 datinghas been used to date ice up to about 50,000years old, ages of older samples have to beinferred from other evidence, such as their

depth. Trace analysis of krypton-81, which lasts40 times longer than carbon-14, can be used todate samples up to one million years old.ATTA could be applied to other isotopes forapplications in a range of scientific disciplines.One possibility is using calcium-41 as a medicaltracer to monitor bone loss from osteoporosis.A patient could ingest a small, non-harmfulamount of the isotope, which would be takenup into the bones. Over the following years,ATTA could be used to detect the minisculeamounts of calcium-41 in urine samples, whichwould signal the loss of calcium from bones.

Writing Skills Have studentsresearch radioactive decay using theWeb site in the Internet Connectbox. Have students write a reportsummarizing their findings.

BUILDERSKILL

TeachTeach


ModelingRadioactive DecaySkills AcquiredForming a model, infer-ring, calculating

Teacher’s NotesHave kernels precounted andplaced in paper or plastic cups.If time allows, multiple trials(two or three) would provide abetter set of data. Make sure allkernels are accounted for; theycan be slippery if stepped on.

Analysis Answers:1. The kernels that are removed

with each step represent themolecules that decayed.

2. The half-life of the sample isthe number of trials it took toremove 50 kernels multipliedby 10 sec/trial.

3. Age of the sample = (numberof trials) ! (5,730 years/trial)

4. Answers will vary, but shouldbe supported by the data.

Transparencies

TR BellringerTR D4 Radioactive DecayTR D5 Miller-Urey ExperimentTR D6 Lerman’s Bubble Model

Formation of the Basic Chemicals of LifeMost scientists think that life on Earth developed through naturalchemical and physical processes. It is thought that the path to thedevelopment of living things began when molecules of nonlivingmatter reacted chemically during the first billion years of Earth’shistory. These chemical reactions produced many different simple,organic molecules. Energized by the sun and volcanic heat, thesesimple, organic molecules formed more-complex molecules thateventually became the building blocks of the first cells. The hypoth-esis that many of the organic molecules necessary for life can bemade from molecules of nonliving matter has been tested and sup-ported by results of laboratory experiments.

Modeling Radioactive DecayYou can use some dried corn, a box, and a watch to make a model of radioactive decay that will show you how scientists measure the age of objects.

Materials

approximately 100 dry corn kernels per group, cardboard box, clock or watch with a second hand

Procedure

1. On a separate sheet ofpaper, make a data table likethe one below.

2. Assign one member of yourteam to keep time.

3. Place 100 dry corn kernelsinto a box.

4. Shake the box gently fromside to side for 10 seconds.

5. Keep the box still and remove and count thekernels that “point” to theleft side of the box, as shownbelow. Record in your datatable the number of kernelsyou removed.

6.Repeat steps 4 and 5 until allkernels have been countedand removed.

7. Calculate the number ofkernels remaining for eachtime interval.

8. Make a graph using yourgroup’s data. Plot “Totalshake time (seconds)” on the x-axis. Plot “Numberof kernels remaining” on they-axis.

Analysis

1. Identify what the removedkernels represent in each step.

2. Calculate the half-life ofyour sample, in seconds, thatis represented in this activity.

3. Calculate the age of yoursample, in years, if each 10-second interval represents5,700 years.

4. Evaluate the ability of thismodel to demonstrateradioactive decay.

Total shake time Number of kernels Number of kernels(seconds) removed remaining

10

20

30

www.scilinks.orgTopic: Radioactive DecayKeyword: HX4154

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Teaching TipSkin Health Remind students thatultraviolet (UV) radiation in sun-light can be very damaging to theirhealth. Sunburn indicates that skinhas been damaged by UV radiation.The American Cancer Societyreminds us that people who havebeen seriously sunburned, evenonce, have an increased risk ofdeveloping skin cancer. Remindstudents that if they are going to be exposed to the sun, they shouldwear a high SPF sunscreen orprotective clothing to reduce UVdamage. Explain that the “peeling”that occurs after sunburn is thebody’s way of getting rid of cellsthat are too damaged to repair.

GENERAL

Teach, continuedTeach, continued


AnswersScientists are working on tech-nology to eventually examine (byremote telescopes) the planets inand beyond our solar systemwhose spectral “signatures” indi-cate the presence of oxygen,water, and carbon dioxide.NASA plans a sample recoverymission to Mars for 2008.

Real Life GENERAL

did you know?The Light of Life Experiments similar toMiller and Urey’s that used UV light instead of electrical sparks as an energy source alsoproduced amino acids.

Remind students that electrons occur in regionsaround atoms called orbitals which exist within eachof several discrete energy levels surrounding theatom’s nucleus. Excited electrons pushed into higherenergy levels are more reactive, since interactionsbetween the outermost electrons cause chemicalbonding. Ask students to identify the number ofvalence electrons for nitrogen, carbon, and hydrogen(the component elements found in the “primordialsoup” of the Miller-Urey experiment), and then relatethis information to their bonding pattern.

Integrating Physics and Chemistry

The “Primordial Soup” ModelIn the 1920s, the Russian scientist A. I. Oparin and the Britishscientist J.B.S. Haldane both suggested that the early Earth’s oceanscontained large amounts of organic molecules. This hypothesisbecame known as the primordial (prie MAWR dee uhl) soup model.Earth’s vast oceans were thought to be filled with many differentorganic molecules, like a soup that is filled with many different veg-etables and meats. Oparin and Haldane hypothesized that thesemolecules formed spontaneously in chemical reactions activated byenergy from solar radiation, volcanic eruptions, and lightning.

Oparin, together with the American scientist Harold Urey, andother scientists also proposed that Earth’s early atmosphere lackedoxygen. They hypothesized that the early atmosphere was insteadrich in nitrogen gas, N2; hydrogen gas, H2; and hydrogen-containinggases such as water vapor, H2O; ammonia, NH3; and methane, CH4.They reasoned that electrons in these gases would have been fre-quently pushed to higher energy levels by light particles from thesun or by electrical energy in lightning. Today, high-energy electronsare quickly soaked up by the oxygen in Earth’s atmosphere becauseoxygen atoms have a great “thirst” for such electrons. But withoutoxygen, high-energy electrons would have been free to react withhydrogen-rich molecules, forming a variety of organic compounds.

In 1953, the primordial soup model was tested by Stanley Miller,who was then working with Urey. Miller placed the gases that he andUrey proposed had existed on early Earth into a device like the oneseen in Figure 2. To simulate lightning, he provided electrical sparks.After a few days, Miller found a complex collection of organic mol-ecules in his apparatus. These chemicals included some of life’s basicbuilding blocks: amino acids, fatty acids, and other hydrocarbons(molecules made of carbon and hydrogen). These results support thehypothesis that some basic chemicals of life could have formed spon-taneously under conditions like those in the experiment.

Reevaluating the Miller-Urey ModelRecent discoveries have caused scientists to reevaluate theMiller-Urey experiment. We now know that the reductantmolecules used in Miller’s experiment could not haveexisted in abundance on the early Earth. Four billion yearsago, Earth did not have a protective layer of ozone gas, O3.Today ozone protects Earth’s surface from most of the sun’sdamaging ultraviolet radiation. Without ozone, ultravioletradiation would have destroyed any ammonia and methanepresent in the atmosphere. When these gases are absentfrom the Miller-Urey experiment, key biological moleculesare not made. This raises a very important question: If thechemicals needed to form life were not in the atmosphere,where did they come from? Some scientists argue that thechemicals were produced within ocean bubbles. Others saythat the chemicals arose in deep sea vents. The correctanswer has not been determined yet.

Spark

Organic compounds

Collectingchamber

CondenserH2Ovapor

Hotwater

N2CH4H2NH3

Figure 2 Miller-Urey experiment. Miller simulatedan atmospheric compositionthat Oparin and other scien-tists incorrectly hypothesizedexisted on early Earth. Hisexperiment produced severaldifferent organic compounds.

Real LifeIs there life on otherplanets?Many scientists think lifecould have arisen on otherplanets the same way itdid on Earth.Analyzing InformationFind out about research on extraterrestrial life, and compare scientists’predictions about possiblelife-forms on other planets.

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At issue, then as now, was whether this waterwas ever in liquid form. Even as late as 1962,respected astronomers continued to interpretdata gleaned from fifty years of telescopic obser-vation of Mars as supportive of theories of lifethere. The high-quality images returned by theMariner flybys showed Mars to be heavilycratered, more akin to the Earth’s moon than to the Earth.

Interactive Reading AssignChapter 12 of the Holt BiologyGuided Audio CD Program to helpstudents achieve greater success inreading the chapter.

Teaching TipSpace Science The origin of lifeon this planet is of interest to scien-tists in many fields. Investigators inthe fields of biology, chemistry,physics, geology, and astrophysicsare all working together to learnmore about other planets in thequest to learn more about our own.Indeed, whole new fields of study,such as planetary geology, wereunheard of a few years ago. Askstudents to discuss new fields ofstudy that might arise from explo-ration of the planets in our solarsystem. (Answers will vary, but mightinclude planetary mining and refining,and mass transit between planets.)

Verbal

Teaching TipEarth’s Changing AtmosphereStudents may think that the atmos-phere is an infinite resource thatwill always be present as it is today.Challenge this misconception byhaving students compare thechanges in the atmosphere of earlyEarth with the changes that areoccurring in our present atmos-phere. Begin by describing howoxygen levels steadily increased inthe ancient atmosphere due to theactivity of photosynthetic bacteria.Then ask students what kind ofgaseous products human activity isproducing today. (Carbon dioxide,methane, sulfur dioxide, nitrousoxide, and others) Ask which gas ofparticular importance is increasingworldwide and which is decreasingworldwide. (Carbon dioxide isincreasing, and upper atmosphericozone is decreasing.)

LS

GENERAL

GENERALSKILLBUILDER

READINGREADING


In the late 1890’s, telescopic studies of Marsspawned theories about vegetative and possi-bly intelligent life on Mars. These ideas hadgreat popular appeal and, given the poor quality of the data available, possibly even amodicum of scientific credibility. There was no question that the Martian poles were ice-locked and so Mars indeed seemed to possessone of the essential ingredients for life—water.

ASTRONOMYASTRONOMYCONNECTIONCONNECTION

The Bubble ModelIn 1986, the geophysicist Louis Lerman suggested that the keyprocesses that formed the chemicals needed for life took placewithin bubbles on the ocean’s surface. Lerman’s hypothesis,also known as the bubble model, is summarized in Figure 3.

Step Ammonia, methane, and other gases resulting fromthe numerous eruptions of undersea volcanoes weretrapped in underwater bubbles.

Step Inside the bubbles, the methane and ammonia neededto make amino acids might have been protected fromdamaging ultraviolet radiation. Chemical reactionswould take place much faster in bubbles (where reac-tants would be concentrated) than in the primordialsoup proposed by Oparin and Haldane.

Step Bubbles rose to the surface and burst, releasing simpleorganic molecules into the air.

Step Carried upward by winds, the simple organic mol-ecules were exposed to ultraviolet radiation and light-ning, which provided energy for further reactions.

Step More complex organic molecules that formed by fur-ther reactions fell into the ocean with rain, startinganother cycle.

Thus, the molecules of life could have appeared more quicklythan is accounted for by the primordial soup model alone.

Reading EffectivelyBefore reading this chapter,write the Objectives for eachsection on a sheet of paper.Rewrite each Objective as aquestion, and answer thesequestions as you read thesection.

BIOgraphic

Gases were trapped in underwater bubbles.

Lerman proposed that gases formed simple organic molecules.

Lerman’s Bubble Model

1

Gases underwent chemical reactions.

2

Gases were ejected into the atmosphere.3

Simple and complex compounds fell into the oceans.

5

Gases underwent further reactions.4

Figure 3

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Teaching TipThe First Hereditary MoleculeArmed with the knowledge of thefunction of RNA in protein synthe-sis, many students may not be ableto understand immediately howRNA, which relies on a templateprovided by DNA, could havedeveloped before DNA. Explain tostudents that today certain viruses(called retroviruses), including HIV,contain only RNA as their geneticmaterial. Their viral RNA, whenreleased into a host cell, is used as atemplate to then make DNA.

GENERAL



ModelingCoacervatesSkills AcquiredForming a model, predicting outcomes

Teacher’s Notes Solutions must be fresh andprepared accurately. Alwayswear safety goggles and labaprons when mixing chemicals.Tell students to notify you incase of an accident. Supervisethe cleanup of all spills.

Analysis Answers:1. The solution became cloudy.2. Coacervates are spherical and

membrane-bound but have nointernal structures.

3. Answers may vary but mayinclude that coacervates mightbreak up.

4. Answers will vary. Studentsmight state that coacervatesare discrete structures thatresemble cells and that arise incertain chemical environments. did you know?

Ancient Oceans Early oceans were probablysimilar in salt content to modern freshwaterlakes. The salinity of today’s oceans is theresult of millions of years of water runningover rocks, picking up mineral salts, and flow-ing to the sea.

Carbon atoms can form an almost infinitenumber of compounds by combining with eachother in long chains and in other types of large,complex molecules. They not only form a totalof four bonds, they can also join other atoms insingle, double, and triple bonds. The structureof silicon, however, is far more rigid. Their differences can be seen by comparing carbon-based plastics to silicon-based ceramics.

CHEMISTRYCHEMISTRYCONNECTIONCONNECTION

Precursors of the First CellsScientists disagree about the details of the process that led to theorigin of life. Most scientists, however, accept that under certain con-ditions, the basic molecules of life could have formed spontaneouslythrough simple chemistry. But there are enormous differencesbetween simple organic molecules and large organic moleculesfound in living cells. How did amino acids link to form proteins?How did nucleotides form the long chains of DNA that store theinstructions for making proteins? In the laboratory, scientists havenot been able to make either proteins or DNA form spontaneously inwater. However, short chains of RNA, the nucleic acid that helpscarry out DNA’s instructions, have been made to form on their ownin water.

A Possible Role As CatalystsIn the 1980s, American scientists Thomas Cech of the University ofColorado and Sidney Altman of Yale University found that certainRNA molecules can act like enzymes. RNA’s three-dimensionalstructure provides a surface on which chemical reactions can becatalyzed. Messenger RNA acts as an information-storing molecule.As a result of Cech’s and Altman’s work and other experimentsshowing that RNA molecules can form spontaneously in water, asimple hypothesis was formed: RNA was the first self-replicatinginformation-storage molecule and it catalyzed the assembly of thefirst proteins. More important, such a molecule would have beencapable of changing from one generation to the next. This hypoth-esis is illustrated in Figure 4.

Microspheres and CoacervatesObservations show that lipids, which make up cell membranes, tendto gather together in water. By shaking up a bottle of oil and vinegar,you can see something similar happen—small spheres of oil form inthe vinegar. Certain lipids, when combined with other molecules,can form a tiny droplet whose surface resembles a cell membrane.Similarly, laboratory experiments have shown that, in water, shortchains of amino acids can gather into tiny droplets called

. Another type of droplet, called a coacervate (koh ASsuhr VAYT), is composed of molecules of different types, includinglinked amino acids and sugars.

Scientists think that formation of microspheres might have beenthe first step toward cellular organization. According to this hypoth-esis, microspheres formed, persisted for a while, and then dispersed.Over millions of years, those microspheres that could persist longerby incorporating molecules and energy would have become morecommon than shorter lasting microspheres were. Microspherescould not be considered true cells, however, unless they had thecharacteristics of living things, including heredity.

microspheres

Inorganicmolecules

RNAmacromolecules

RNAnucleotides

RNA moleculescatalyze protein

synthesis

Proteins

Self-replication

Figure 4 Proposedstages leading to RNA self-replication and proteinsynthesis. Chemical reactionsbetween inorganic moleculesformed RNA nucleotides. Thenucleotides assembled intoRNA macromolecules. Thesemolecules were able to self-replicate and to catalyze theformation of proteins.

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Answers to Section Review1. The amount of the radioisotope in the rock is

compared to the amounts of isotope speciesthat resulted from decay. An older rock wouldhave less of the original radioisotope.

2. In the “primordial soup” model, the earlyatmosphere contained large amounts of nitrogenand hydrogen-containing gases. Recent discov-eries indicate that without ozone, any ammoniaand methane would have been destroyed. In thebubble model, ammonia, methane, and othergases from underwater volcanoes were trappedin bubbles in the seas. Chemical reactions tookplace faster in the bubbles, and the underwaterbubbles were protected from UV radiation. Thismodel relies on the assumption that bubbles

remained intact for a significant time.3. The formation of microspheres might have led

to cellular organization.4. Microspheres that contained replicating RNA

might have transferred both their structure andtheir RNA to offspring.

5. A. Incorrect. Nitrogen is believed to have beenpresent. B. Incorrect. Hydrogen is believed tohave been present. C. Incorrect. Water isbelieved to have been present. D. Correct. TheEarth’s early atmosphere did not have a protec-tive layer of ozone. Without ozone, the ammo-nia and methane in Miller and Urey’s modelwould have been destroyed by ultravioletradiation.

ReteachingAsk students to draw a rough dia-gram of the Miller-Urey apparatus.Be sure to have them label all com-ponents of the diagram. (Studentsshould include in their sketch an areafor holding water, an area wherewater and gases can combine and besubjected to electrical sparks, an areawhere any by-products can cool, andan area where the by-products can becollected.)

Quiz1. What is a “half-life”? (It is the

amount of time that it takes forhalf of an amount of material tobreak down.)

2.What does “spontaneous” mean,in reference to the origin of life?(It means that life originated fromnonliving things.)

3. Every theory about the origin oflife has a source of energy for thetransformation. What are someof these sources? (Possible sourcesinclude ultraviolet or other lightenergy from the sun, an electricalspark as from lightning, and vol-canic eruption.)

4. What molecule is thought to bethe first hereditary molecule?(RNA)

AlternativeAssessmentAsk students to write an answer tothis question: Were the first cellsthat evolved on Earth prokaryotesor eukaryotes? Have them justifytheir choice. (The first cells wereprokaryotes. At this point in thechapter, students should concludethat, since prokaryotes are single-celled organisms lacking a nucleus,they probably evolved earlier thanmore complex eukaryotes.)

GENERAL

GENERAL

CloseClose


Origin of HeredityAlthough scientists disagree about the details of the origin of hered-ity, many agree that double-stranded DNA evolved after RNA and thatRNA “enzymes” catalyzed the assembly of the earliest proteins. Manyscientists also tentatively accept the hypothesis that some micro-spheres or similar structures that contained RNA developed a meansof transferring their characteristics to offspring. But researchers donot yet understand how DNA, RNA and hereditary mechanisms firstdeveloped. Therefore, the subject of how life might have originatednaturally and spontaneously remains a subject of intense interest,research, and discussion.

Modeling CoacervatesBy using simple chemistry, you will see that some propertiesof coacervates resemble the properties of cells.

Materials

safety goggles, protective gloves and lab apron, graduatedcylinder, 1 percent gelatin solution, 1 percent gum arabic solution, test tube, 0.1 M HCl, pipet, microscope slide and coverslip, microscope

Procedure

1.

CAUTION: Hydrochloricacid is corrosive. Put onsafety goggles, gloves, andapron. Avoid contact withskin and eyes. Avoidbreathing vapors. If any ofthis solution should spill onyou, immediately flush thearea with water, and notifyyour teacher.

2. Mix 5 mL of a 1 percentgelatin solution with 3 mL of a 1 percent gum arabic solution in a test tube.

3. Add 0.1 M HCl to thegelatin–gum arabic

solution one drop at a timeuntil the solution turns cloudy.

4. Prepare a wet mount of thecloudy solution, and examineit under a microscope athigh power.

5. Prepare a drawing of thestructures that you see.They should resemble thestructures in the micrographabove.

Analysis

1. Describe what happened tothe solutions after the acidwas added.

2. Compare the appearance ofcoacervates with that of cells.

3. Predict what would happento the coacervates if a basewere added to the solution.

4. Critical ThinkingEvaluating HypothesesBased on the evidenceyou obtained, defend thehypothesis that coacervatescould have been the basisof life on Earth.

Section 1 Review

Explain how radioisotopes are used todetermine the age of a rock.

Critique two scientific models that explain the origin of life.

Describe the first step that may have led towardcellular organization.

Explain how heredity may have arisen.

Miller and Urey’s model isinconsistent with the finding that Earth’s earlyatmosphere lacked A nitrogen. C water.B hydrogen. D ozone.

Standardized Test PrepStandardized Test Prep

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OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. Tell students that the pur-pose of this lesson is to teach themhow the first eukaryotic cells andtheir organelles might have evolvedand how multicellular organismslikely evolved. How explosions ofnew species of organisms occurredin several different periods ofEarth’s history and how massextinctions have also played animportant role in the diversity oflife on Earth are also discussed.

Ask students to write down thenames of all the kingdoms of livingthings, writing them in the order inwhich they appeared on Earth.

DemonstrationSome students may have difficultyunderstanding the timeline ofEarth’s history. This analogy shouldprovide some perspective. Draw alarge circle on the board to repre-sent a clock. Place an arrow at12:01 to indicate when Earth wasformed. Place additional arrows at1:15 (the first bacteria appeared),8:00 (the first eukaryotes), 11:00(the first life on land), 11:50(dinosaurs became extinct), and11:59 (first humans appeared).Students will be able to relate morelife-on-Earth “milestones” to a 24-hour day when they complete theExploration Lab. VisualLS

GENERAL

MotivateMotivate

Bellringer

FocusFocus

Section 2


• Directed Reading• Active Reading• Data Sheet for Data Lab GENERAL

GENERAL

Chapter Resource File• Reading Organizers• Reading Strategies

Planner CD-ROM

Transparencies

TR BellringerTR D8 Evolution of Eukaryotes

Section 2 The Evolution of Cellular Life

The Evolution of Prokaryotes When did the first organisms form? To find out, scientists study thebest evidence of early life that we have, fossils. A is the pre-served or mineralized remains (bone, tooth, or shell) or imprint ofan organism that lived long ago. The oldest known fossils, whichare microscopic fossils of prokaryotes, come from rock that is 2.5billion years old.

Recall that prokaryotes are single-celled organisms that lackinternal membrane-bound organelles. Among the first prokaryotesto appear were marine cyanobacteria. (SIE an oh bakTIR ee ah) are photosynthetic prokaryotes. Before cyanobacteriaappeared, oxygen gas was scarce on Earth. But as ancient cyanobac-teria carried out photosynthesis, they released oxygen gas intoEarth’s oceans. After hundreds of millions of years, the oxygen pro-duced by cyanobacteria began to escape into the air, as shown inFigure 5. Over time, more oxygen was added to the air. Today oxygengas makes up 21 percent of the Earth’s atmosphere.

Two Groups of ProkaryotesEarly in the history of life, two different groups of prokaryotesevolved—eubacteria (which are commonly called bacteria) andarchaebacteria. Living examples include Escherichia coli, a species ofeubacteria, and Sulfolobus, a group of archaebacteria. are prokaryotes that contain a chemical called peptidoglycan (PEP tihdoh GLIE kan) in their cell walls. Eubacteria include many bacteriathat cause disease and decay.

are prokaryotes that lack peptidoglycan in theircell walls and have unique lipids in their cell membranes. Modernarchaebacteria are thought to closely resemble early archaebacteria.Chemical evidence indicates that archaebacteria and eubacteriadiverged very early.

Archaebacteria

Eubacteria

Cyanobacteria

fossil

Objectives● Distinguish between

the two groups ofprokaryotes.

● Describe the evolution ofeukaryotes.

● Recognize an evolutionaryadvance first seen in protists.

● Summarize how massextinctions have affected theevolution of life on Earth.

Key Terms

fossilcyanobacteriaeubacteriaarchaebacteriaendosymbiosisprotistextinctionmass extinction

Figure 5 Evolutionarytimeline. This timeline showssome of the major events thatoccurred during the evolutionof life on Earth.

PRECAMBRIAN ERA

Age (in Millions of Years Ago)

Earliest fossil bacteria Origin of O2 by photosynthesis

3,500• • • • •

2,500

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Teaching Tip Remembering What Came fromWhat Use a Graphic Organizersimilar to the one at the bottom ofthis page to help students grasp theprogression of the complexity oflife. Reproduce the diagram,without the eight terms, on theboard or overhead. Next, write analphabetical list of the eight termsbeside the graphic organizer. Askstudents to insert the terms intotheir correct rectangles.

TeachTeach


Analyzing Signs of EndosymbiosisSkills AcquiredAnalyzing data, relatingconclusions

Teacher’s NotesPoint out to students that theyare looking at a “doubled dou-ble” set of data. That is, theyare considering the nucleus withrespect to mitochondria andplants with respect to mammals.

Answers to Analysis1. The fact that in mammals the

genetic code of nuclear DNAdiffers from that of mitochon-drial DNA suggests that mitochondria have evolvedseparately from the animalcells that contain them.

2. We can infer that plant mito-chondria and plant cellstogether have been subjected to selective pressures differentfrom those that acted on mam-malian cells and on mammalianmitochondria.

3. We can infer that the prokary-otic ancestors of mammalianmitochondria probablydiverged very early from theprokaryotic ancestors of plantmitochondria because thegenetic codes are quite differ-ent in the two types.

010001011001110101000100100010011100100100010000010100100111010101001000101010010010

First eukaryotes

JURASSIC PERIOD1,500

• • • • •

Analysis

1. Defend the theory ofendosymbiosis using these data.

2. Infer what these data indicateabout the evolution of plant cells.

3. Describe how these data canbe used to support the idea thatmore than one type of cell evolvedearly in the history of life.

Analyzing Signs of EndosymbiosisBackground

You may recall that mitochondria have their own DNA and producetheir own proteins. The data below were collected by scientistsstudying the proteins produced by mitochondrial DNA. The scien-tists found that the three-nucleotide sequences (codons) in thenucleus of an organism’s cells can code for different amino acidsthan those coded for in the cell’s mitochondria. Examine the databelow, and answer the questions that follow.

010001011001110101000100100010011100100100010000010100100111010101001000101010010010

Magnification: 6930x

Amino Acids Made in the Nucleus and MitochondriaAmino acids or other Amino acids or otherinstructions coded for instructions coded for

in the nucleus in mitochondria

Codon Plants and mammals Plants Mammals

UGA Stop Stop Tryptophan

AGA Arginine Arginine Stop

AUA Isoleucine Isoleucine Methionine

AUU Isoleucine Isoleucine Methionine

CUA Leucine Leucine Leucine

The Evolution of EukaryotesAbout 1.5 billion years ago, the first eukaryotes appeared. A eukary-otic cell is much larger than a prokaryote is. Eukaryotic cells havea complex system of internal membranes. Eukaryotic DNA isenclosed within a nucleus. Almost all eukaryotes have mitochon-dria. Chloroplasts, which carry out photosynthesis, are found onlyin protists and plants. Mitochondria and chloroplasts are the size ofprokaryotes, and they contain their own DNA.

The Origins of Mitochondria and ChloroplastsMost biologists think that mitochondria and chloroplasts originatedas described by the theory of that was proposed in1966 by the American biologist Lynn Margulis. This theory proposesthat mitochondria are the descendants of symbiotic, aerobic (oxy-gen-requiring) eubacteria and cholorplasts are the descendants ofsymbiotic, photosynthetic eubacteria.

endosymbiosis

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[insert GRAPHICORGANIZER

Graphic Organizer

EubacteriaFirst

prokaryotesArchaebacteria

Mitochondria

Eukaryotes

Chloroplasts

Present-day cells

Cyanobacteria

Use this graphic organizer withTeaching Tip on this page.


Teaching TipEndosymbiosis The termendosymbiosis refers to anysymbiotic relationship in whichone organism lives inside another.Help students understandMargulis’s hypothesis for theendosymbiotic origin of mito-chondria and chloroplasts byreviewing other examples ofendosymbiosis. Examples includewood-digesting bacteria that livewithin the digestive tracts of ter-mites, bacteria that break downplant cell walls within the digestivetract of cows, and bacteria in thehuman digestive tract that breakdown otherwise undigested materi-als and synthesize vitamin K as a by-product.

Verbal

Group ActivityModels of Endosymbionts andBacteria Have students work insmall groups to create models orposters of chloroplasts, mitochon-dria, non-photosynthetic bacteriacells, and photosynthetic bacterialcells. Students should use refer-ence materials for their models orposters to ensure that they accu-rately depict these organelles andcells. Tell students that their mod-els or poster pictures should haveclearly labeled internal structuresand the colors should be realistic(i.e., green for chloroplasts orphotosynthetic bacteria). Theyshould also identify the names ofthe bacteria used for these modelsor pictures. Display the modelsand posters where all students canobserve them. Co-op Learning

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deterioration. In addition, at least some cases of diabetes and heart disease, as well as otherdisorders such as Alzheimer’s disease, havebeen linked to mitochondrial genes.When an egg is fertilized, only mitochondriafrom the egg become part of the developingfetus; all mitochondria from the sperm are discarded. Therefore, diseases caused byabnormal mitochondrial genes are transmittedby the mother. A father with abnormal mito-chondrial genes can’t transmit any such diseases to his children.


Inside nearly every cell are mitochondria, thetiny structures that provide the cell withenergy. Each mitochondrion contains a circularchromosome. Several rare diseases are causedby abnormal genes carried by the chromosomeinside a mitochondrion. Mitochondrial genescode primarily for proteins in the electrontransport chain and for ATP synthase. Defectsin one or more of these proteins thus reducethe amount of ATP the cell can make. One disease, mitochondrial myopathy, causes weakness, exercise intolerance, and muscle

MEDICINEMEDICINECONNECTIONCONNECTION

Large prokaryote

Small aerobicprokaryote

Primitive eukaryote

Mitochondrion

PRECAMBRIAN ERA

Early eukaryotes Diverse protists

1,500• • • • • •

1,000


Mitochondria are thought to have evolved from small, aerobic prokaryotes that began to live inside larger prokaryotes.

Figure 6 Endosymbiosis

According to Lynn Margulis’s theory of endosymbiosis, bacteriaentered large cells either as parasites or as undigested prey as illus-trated in Figure 6. Instead of being digested, the bacteria began tolive inside the host cell, where they performed either cellular respi-ration (mitochondria) or photosynthesis (chloroplasts). The invadingbacteria that became chloroplasts were probably closely related tocyanobacteria. Both mitochondria and chloroplasts have character-istics that are similar to those of bacteria. The following observationssupport the idea that mitochondria and chloroplasts descended frombacteria:

1. Size and structure. Mitochondria are about the same size asmost eubacteria, and chloroplasts are the same size as somecyanobacteria. Both mitochondria and chloroplasts are sur-rounded by two membranes. The smooth outer membrane ofmitochondria is thought to be derived from the endoplasmic retic-ulum of the larger host cell. The inner membrane of mitochondriais folded into many layers, so it looks like the cell membranes ofaerobic eubacteria. Inside this membrane are proteins that carryout cellular respiration. Both chloroplasts and cyanobacteria con-tain thylakoids, structures in which photosynthesis takes place.

2. Genetic material. Mitochondria and chloroplasts have circularDNA similar to the chromosomes found in bacteria. Bothchloroplasts and mitochondria contain genes that are differentfrom those found in the nucleus of the host cell.

3. Ribosomes. Mitochondrial and chloroplast ribosomes have asize and structure similar to the size and structure of bacterialribosomes.

4. Reproduction. Like bacteria, chloroplasts and mitochondriareproduce by simple fission. This replication takes place inde-pendently of the cell cycle of the host cell.

www.scilinks.orgTopic: EndosymbiosisKeyword: HX4068

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Teaching TipMulticellularity and CellularSpecialization It is importantthat students recognize the relation-ship between multicellularity andthe division of labor among differ-ent types of cells. Illustrate theseconcepts with the following anal-ogy. Modern industry usesindividual workers’ skills andabilities to perform different tasks.Use as an example a local industry(such as an automotive plant, steelmill, canning company, or meatpacking plant) to show how moreis produced, both in quantity andcomplexity, by breaking the totaltask into smaller parts that areaccomplished by workers trained todo that specific task. Point out thatcells exhibit the same type ofdepartmentalizing.

ActivityFavorite Fossils Have studentspick an extinct species that intereststhem and write a short report aboutit. Ask students to address the fol-lowing questions: When, where,and how did the species live? Whatevidence do we have that thespecies lived at all? Why might ithave become extinct? Do you see arelationship between this speciesand a modern species? Have stu-dents include with their reports anartist’s rendition of their species ora picture of a fossil of the species.

Verbal

DemonstrationBring to class a variety of fossils.Petrified wood, diatomaceous earth,trilobites, casts and molds of smallfish, mollusks, and coprolites (petri-fied feces) usually are inexpensiveand easy to obtain. Fossils are alsoavailable through biological supplyhouses or other retail dealers.Assign identification or classifica-tion activities using the specimens.

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OCEANOGRAPHYOCEANOGRAPHYCONNECTIONCONNECTION

broken from the parent sponge can generate anew sponge. Some sponges produce specializedasexual reproductive bodies called gemmules.These consist of food-filled archeocytes andamoebocytes. The amoebocytes are capable ofgiving rise to any other type of cell. Gemmulesremain viable for extended periods of time.When suitable conditions are found, a gem-mule can grow to form a new sponge that isgenetically identical to the parent. Gemmulesprovide a means of dispersal and are a way ofmaintaining local distribution and abundance.

Sponges have a variety of sexual and asexualreproductive modes. They are best known fortheir quite remarkable regenerative powers.This is illustrated by the classic experiment offorcing a piece of sponge through a sieve,which separates the cells of the organism.Within a short period of time the dissociatedcells aggregate and reform a multicellularorganism. This cannot be achieved by allsponge species.The ability to regenerate is related to asexualreproduction. A bud or small fragment

Multicellularity Many biologists group all living things into sixbroad categories called kingdoms. The two oldestkingdoms, Eubacteria and Archaebacteria, aremade up of single-celled prokaryotes. The firsteukaryotic kingdom was the kingdom Protista.

make up a large, varied group that includesboth multicellular and unicellular organisms. Theother three kingdoms (fungi, plants, and animals)evolved later and also consist of eukaryotes.

The unicellular body plan has been tremendouslysuccessful, with unicellular organisms today consti-tuting about half the biomass (the total weight of allliving things) on Earth. But a single cell must carryout all of the activities of the organism. Distincttypes of cells in one body can have specialized func-tions. For example, some organisms may have specific cells thathelp the organism protect itself from predators or disease. Othercells may help the organism resist drying out. Other examples ofspecialized cells include cells that help a multicellular organismmove about in order to find a mate or food. With all theseadvantages, it is not surprising that multicellularity has arisen inde-pendently many times.

Almost every organism large enough to see with the naked eye ismulticellular. Most protists, such as those shown in Figure 7, aresingle celled, but there are many multicellular forms. The develop-ment of multicellular organisms of the kingdom Protista marked animportant step in the evolution of life on Earth. The oldest knownfossils of multicellular organisms were found in 700 million year-old rocks.

Some of the multicellular lines thatresulted did not produce diverse groups oforganisms. Among those groups of organismsthat survive today are plantlike red, green,and brown algae, shown in Figure 8. You mayknow these algae as seaweed. Three of themulticellular groups that evolved from theprotists were very successful, producing threeseparate kingdoms—Fungi, Plantae, and Ani-malia. Each of these three kingdoms evolvedfrom a protistan ancestor.

Protists

PRECAMBRIAN ERA

Origin of all major animal phyla

CAMBRIAN PERIOD ORDOVICIAN PERIOD

Earliest multicellular organisms

500• • • • • •

Figure 8 Brown algae.Brown algae, called kelps, aremulticellular protists that formvast underwater “forests” insome coastal waters.

Figure 7 Single-celledprotists. Single-celledprotists occur in many shapesand can live in many differenttypes of environments,including water and land.

Magnification: 50x

Magnification: 230x

Paramecium bursaria

Stentor coeruleus

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Teaching TipCambrian Explosion Point out tostudents that the diversity of multi-cellular organisms expanded rapidlyduring the period known as theCambrian explosion. Changing geo-logical and atmospheric conditionsduring this time period are thoughtto have increased the number ofavailable habitats for animals.Primitive multicellular organismscould then specialize differently,become more complex, and takeadvantage of the newly availableresources. This first explosion ofbiological diversity was the founda-tion from which most complex lifeexisting today evolved.

ActivityBurgess Shale Have students uselibrary or Internet resources toresearch the Burgess Shale. Fossilevidence from the shale suggests thatanimals with a wide variety of bodyplans existed very early in evolution-ary history. Have students search fororganisms that would be consideredunusual (even bizarre) today, andmake simple sketches of them.

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Trends in AstrophysicsCause of Mass Extinction Earth’s mostsevere mass extinction, which happened 250million years ago, may have been triggered bya collision with a comet or asteroid. Over 90percent of all marine species and 70 percent ofland vertebrates perished as a result. Some sci-entists hypothesize that the collision wasn’tdirectly responsible for the extinction, butrather triggered a series of events, such as

massive volcanism and changes in ocean oxy-gen, sea level, and climate. That in turn led towholesale loss of species.Scientists don’t know the site of the impact 250 million years ago, when all Earth’s landformed a supercontinent called Pangaea.There is strong evidence suggesting the extinc-tion happened very rapidly, on the order of8,000 to 100,000 years.


Origins of Modern OrganismsMost animal phyla that exist today probably originated during a rel-atively short time (most estimates range from 10 to 100 millionyears) during the late Precambrian and early Cambrian periods. Thisrapid diversification of animals is sometimes known as the “Cam-brian explosion.” The Cambrian period was a time of great evolu-tionary expansion, as shown in Figure 9. Many unusual marineanimals also appeared at this time, animals for which there are noclose living relatives. A very rich collection of Cambrian fossils wasuncovered in 1909 in a geological formation in Canada called theBurgess Shale. The fossils in the Burgess Shale include those ofstrange animals that are not like anything alive today.

The Ordovician period, which followed the Cambrian period,lasted from about 505 million to 438 million years ago. During thistime, many different animals continued to abound in the seas.Among them were trilobites, marine arthropods that becameextinct about 250 million years ago, shown in Figure 9.

Figure 9 Appearance of a Cambrian sea. Bystudying fossils from theCambrian period, such asthe trilobite fossil below, artists re-create a scenefrom the shallow seas ofthe Cambrian period.


Origin of all major animal phyla

560• • • • • •

510PRECAMBRIAN ERA CAMBRIAN PERIOD

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Answers to Section Review1. Eubacteria contain peptidoglycan in their cell

walls. Archaebacteria lack peptidoglycan andtheir cell membrane lipids differ fromeukaryotes.

2. Eukaryotes probably evolved throughendosymbiosis. Refer to the text to check students’ answers.

3. Unlike eukaryotes, bacteria have nomembrane-bounded cellular inclusions;bacterial DNA is in a single loop, unlike thechromosomal DNA of eukaryotes; and bacteriahave cell walls, unlike eukaryotic protists.

4. The development of multicellularity in protistsallowed for cell specialization.

5. The ancestors of many living species filledniches vacated by organisms that died in massextinctions.

6. A. Incorrect. The organisms in kingdomPlantae are all multicellular eukaryotes. B. Correct. Organisms in the kingdom Protistamay be unicellular or multicellular eukaryotes.C. Incorrect. Eubacteria are single-celledprokaryotes. D. Incorrect. Archaebacteria aresingle-celled prokaryotes.

ReteachingHave students write a one-pagesummary of this section. Ask themto be sure to include a timelineindicating all of the evolutionaryadvancements they describe.

Quiz1. When did the first life form

appear on Earth? (Over 2.5 billion years ago)

2.What cellular organelles arethought to have derived frombacterial cells? (Mitochondria andchloroplasts were probably bacte-ria that became endosymbiontswith pre-eukaryotic organisms.)

3. During what geologic timeperiod did multicellular lifeforms on Earth significantlyincrease in diversity? (TheCambrian period)

AlternativeAssessmentDivide the class into two teams.Have each team write ten true orfalse questions about the materialin this section. A spokesperson forone team should pose the questionsto the other team. Give two pointsfor each correct answer. Give stu-dents one point for each of theirown questions that was answeredcorrectly. Tally the scores, anddeclare a winning team.

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Mass Extinctions The fossil record indicates that a sudden change occurred at the endof the Ordovician period. About 440 million years ago, a large per-centage of the organisms on Earth suddenly became extinct.

is the death of all members of a species. This was the firstof five major mass extinctions that have occurred on Earth. A

is an episode during which large numbers ofspecies become extinct.

Another mass extinction of about the same size happenedabout 360 million years ago. The third and most devastating of allmass extinctions occurred at the end of the Permian period,about 245 million years ago. About 96 percent of all species ofanimals living at the time became extinct. About 35 million yearslater, a fourth, less devastating mass extinction occurred.Although the specific causes of these extinctions are unknown,evidence indicates that worldwide geological and weatherchanges were likely factors. The fifth mass extinction will be dis-cussed in more detail in a later chapter. It occurred 65 millionyears ago and brought about the extinction of about two-thirds ofall land species, including most of the dinosaurs.

Some scientists think that another mass extinction is occurringtoday. These scientists reason that this new extinction is takingplace because the Earth’s ecosystems, especially tropical rain forests,are being destroyed by human activity, as shown in Figure 10. Theworld has already lost half its tropical rain forests. If the currentrate of destruction continues, from 22 percent to 47 percent ofEarth’s plant species will be lost, along with 2,000 of the world’s9,000 species of birds and countless insect species. This would bean astonishing loss of biodiversity.

mass extinction

Extinction

ORDOVICIAN PERIOD SILURIAN

Animal diversity abounds; first jawless fishes

440500• • • • • •

▲

First massextinction

Figure 10 Rain forestsare being destroyed at analarming rate. Althoughtropical rain forests cover only7 percent of the Earth’s landsurface, they contain morethan one-half of all the world’sanimal and plant species.

Section 2 Review

Contrast the two major groups of prokaryotes.

Analyze Margulis’s theory of endosymbiosis,citing its strengths and weaknesses.

Compare bacteria with eukaryotes.

Summarize how multicellularity advanced theevolution of protists.

Critical Thinking Justify the argument thattoday’s organisms would not exist if massextinctions had not occurred.

The kingdom that includesboth multicellular and unicellular eukaryotesis calledA Plantae. C Eubacteria.B Protista. D Archaebacteria.


www.scilinks.orgTopic: Extinction Keyword: HX4078

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OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. Tell students that the pur-pose of this section is to teach themabout how the conditions ofEarth’s atmosphere make it possi-ble for organisms to live on land.They will learn about the chal-lenges of living on land and aboutthe succession of organisms thatmoved from the oceans onto land.

Have students write down some ofthe differences they recognizebetween ocean-dwelling and land-dwelling organisms. (You mightsuggest that they consider howorganisms support themselves andmove around, how they obtain food,how they protect themselves fromdangers, and how they reproduce.)Ask them to consider all kinds oforganisms, not just animals.

Discussion/QuestionAsk students if ozone is a desirablepart of our atmosphere. (Most willanswer yes.) Then ask if ozone isalways a desirable part of ouratmosphere. (At lower elevations,ozone contributes to pollution; athigher elevations, however, ozoneabsorbs much of the damaging ultra-violet radiation that would otherwisereach Earth where it could harm liv-ing things.) VerbalLS

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MotivateMotivate

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Section 3


• Directed Reading• Active Reading GENERAL

Chapter Resource File• Reading Organizers• Reading Strategies• Supplemental Reading Guide

The Dinosaur Heresies

Planner CD-ROM

Transparencies

TR BellringerTR D7 Ozone Shields the Earth

Section 3 Life Invaded the Land

The Ozone LayerThe sun provides both life-giving light and dangerous ultravioletradiation. Early in Earth’s history, life formed in the seas, whereearly organisms were protected from ultraviolet radiation. Theseorganisms could not leave the water because ultraviolet radiationmade life on dry ground unsafe. What enabled life-forms to leavethe protection of the seas and live on the land?

Formation of the Ozone LayerDuring the Cambrian period and for millions of years afterward,organisms did not live on the dry, rocky surface of Earth. However, aslow change was taking place. About 2.5 billion years ago, photosyn-thesis by cyanobacteria began adding oxygen to Earth’s atmosphere.As oxygen began to reach the upper atmosphere, the sun’s rayscaused some of the molecules of oxygen, O2, to chemically react andform molecules of ozone, O3. In the upper atmosphere, ozone blocksthe ultraviolet radiation of the sun, as shown in Figure 11. After mil-lions of years, enough ozone had accumulated to make the Earth’sland a safe place to live.

Objectives● Relate the development of

ozone to the adaptation oflife to the land.

● Identify the first multicellularorganisms to live on land.

● Name the first animals to liveon land.

● List the first vertebrates toleave the oceans.

Key Terms

mycorrhizaemutualismarthropodvertebratecontinental drift


Bony fishes become abundantPlants and arthropods invade land; jawed fishes first appear

440 430 410 400 390• • • • • •

SILURIAN PERIOD DEVONIAN PERIOD

Ozoneabsent

Ozonepresent

Ultraviolet radiation Ultraviolet radiation

As ancient cyanobacteria added oxygen to the atmosphere, ozone began to form.

Figure 11 Ozone shields the Earth

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Trends in PaleobotanyClosest Living Relative of First Land PlantsSome 470 million years ago, the first land plantsemerged from prehistoric waters, put downroots in soil, and ended up ruling the plantworld. But scientists haven’t been certain aboutthe family history of those pioneer plants.By studying gene sequences of common fresh-water algae, a team of University of Marylandresearchers, funded by the National ScienceFoundation (NSF), has traced this family tree

and identified the closest living relatives of thefirst land plants. Charales, a group of greenalgae which survives today in freshwateraround the world, was confirmed to be thedescendant. Data show that land plants and the Charales both evolved from a commonancestor that was a fairly complex organism.Among its characteristics were branchingthreads and reproduction with eggs and sperm.

Teaching TipHard As a Rock Ask students:what kinds of organisms could sur-vive on bare rock? What would ittake to be successful? (Students maysuggest an association similar tolichen, in which a fungus and acyanobacterium or alga coexist in asymbiotic relationship.) Remindstudents that as life emerged fromthe oceans, living things were con-fronted with this kind of problem.

Verbal

Math Skills Tell students that anexperiment was conducted inwhich pine seedlings were grownfrom seed in soil with or withoutmycorrhizae. After several monthsof growth, the plants were har-vested and dried, and their weightsand contents of certain nutrientswere measured. The followingaveraged data were obtained:

Plant Composition

Have students calculate the percentincrease in dry weight, nitrogencontent, phosphorus content, andpotassium content for the plantsgrown in soil with mycorrhizae,compared with plants grown in soilwithout mycorrhizae. (Dry weight:26.2% higher with mycorrhizae;nitrogen content: 45.9% higher;phosphorus content: 185.7% higher;potassium: 72.1% higher) Have stu-dents prepare an appropriate graphof these results and write a para-graph of conclusions about them.

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Without WithMycorrhizae Mycorrhizae

Dry weight (g) 321 405Nitrogen (g) 0.85 1.24Phosphorus (g) 0.07 0.20Potassium (g) 0.43 0.74

Plants and Fungi on Land The first multicellular organisms to live on land may have beenfungi living together with plants or algae. Such paired organismswere able to live on land because each group possessed a qualityneeded by the other.

Plants, which likely evolved from photosynthetic protists, couldcarry out photosynthesis. In photosynthesis, plants use the energyfrom sunlight to make carbohydrates. Plants cannot, however, har-vest needed minerals from bare rock. In contrast, fungi cannotmake nutrients from sunlight but can absorb minerals—even frombare rock.

Early plants and fungi formed biological partnerships called my-corrhizae (MIE koh RIE zee), which enabled them to live on theharsh habitat of bare rock. , which exist today, are sym-biotic associations between fungi and the roots of plants, as shownin Figure 12. The fungus provides minerals to the plant, and theplant provides nutrients to the fungus. This kind of partnership iscalled mutualism. is a relationship between two speciesin which both species benefit. Plants and fungi began living togetheron the surface of the land about 430 million years ago.

Mutualism

Mycorrhizae

DEVONIAN PERIOD CARBONIFEROUS PERIOD

Early amphibians Early reptiles

330370 360 350• • • • • •

▲

Second massextinction

Figure 12 Mycorrhizaeformed on the roots of thefirst plants. This fossil is ofCooksonia, the first knownvascular plant, which lived 410million year ago. Cooksoniawas only a few centimeters tall.Cooksonia’s roots formedmycorrhizae similar to the livingmycorrhizae shown in color to the left.

Root

Example of living mycorrhizae

Fungus

Magnification: 15x

Fossilized Cooksonia

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ActivityCoevolution of Insects andPlants Have students conductlibrary or Internet research on anexample of an insect species and aplant species that have coevolved.Some examples that students couldinvestigate are yucca plants andtheir moth pollinators, and variousorchid species and their bee orwasp pollinators. Have studentswrite a brief report on their find-ings that includes a description ofthe structural and functional adap-tations of both members of thepartnership, the “costs” and “bene-fits” to both species, and character-istics of the environment in whichthe two species are found thatmight influence the partnership.



MISCONCEPTION ALERT

Evolution Some students mistakenly viewevolution as individuals changing into newforms, rather than populations changing overperiods of time longer than the lifetime of anindividual organism. An individual organismbegins life with a particular set of genes. Thisorganism may take on different characteris-tics depending on the kind of environment itgrows in, but its genes are not modified.When the organism reproduces, it passes halfof its genes (in sexual reproduction) to each

of its offspring. The organisms that repro-duce are those that have a combination ofgenes that make them well adapted to theirenvironment. The process of natural selectionoperates on every generation. Over manygenerations, the genetic composition of theorganisms living in the environment mightbecome different from what it was when theirancestors first lived in that environment. It isthis change that constitutes evolution.

StrategiesStrategiesINCLUSIONINCLUSION

Have students create their ownresource documents of animalclassification. The ArthropodGuide and the Vertebrate Guidewould each contain lists, pictures,descriptions, and approximatetime of each animal’s appearanceon Earth. Students can design acover for each guide and a pagefor each type of animal theyresearch. The completed guidesmay be used as tools for studentsto exchange with each other and study.

• Learning Disability• Developmental Delay

Arthropods By 100 million years after their first union with fungi, plants had cov-ered the surface of the Earth, forming large forests. These land plantsprovided a food source for land-dwelling animals. The first animals tosuccessfully invade land from the sea were arthropods. An is a kind of animal with a hard outer skeleton, a segmented body, andpaired, jointed limbs. Examples of arthropods include lobsters, crabs,insects, and spiders, like the one in Figure 13. Biologists think a typeof scorpion was the first arthropod to live on land.

A unique kind of terrestrial arthropod—the insect—evolved fromthe first land dwellers. Insects have since become the most plenti-ful and diverse group of animals in Earth’s history. The success ofthe insects is probably connected to their ability to fly. Insects werethe first animals to have wings. Flying allowed insects, like thedragonfly shown in Figure 14, to efficiently search for food, mates,and nesting sites. It also led to partnerships between insects andflowering plants. The oldest known fossils of flowering plants arefrom about 127 million years ago, but flowering plants may bemuch older than that.

arthropod

Figure 13 An arthropod.This marbled spider is amember of the phylumArthropoda, which includesabout 1 billion billion (1018)individuals in about 1.5 milliondescribed species.

PERMIAN PERIOD TRIASSIC PERIODCARBONIFEROUS PERIOD

▲ ▲


Third massextinction

Fourth massextinction

The first dinosaurs and mammals

220240300 280 260• • • • •

200•

Figure 14 Swamp320 million years ago.Forested swamps weredominated by tall, seedlesscanopy trees and shortertree ferns. Dragonflies hadwingspans of more than 1 m (about 3.25 ft).

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head. Its tail is powerful enough to push thewhale through the water. The pinnipeds (sealsand sea lions) are agile swimmers with taper-ing bodies and strong flippers, but they stillretain many ties to the land. Most return toshore to mate and give birth. The sea otteradapted to life in the ocean over the past fourmillion years. It still resembles the weasel, itsrelative on land. Sea otters live close to shore;they are not as well equipped for the openocean as the streamlined, deep-diving sealsand whales.

Teaching TipClamping Down AdvantagesThe lamprey, a jawless fish alivetoday, is a parasite. Without jaws,it can merely attach to a host andtry to gain nutrition by sucking.Help students appreciate theimportance of jaws by having eachname a vertebrate predator.(Answers will vary.) How does eachpredator catch and eat its prey? Forevery case, direct the discussion tothe important role of the predator’sjaws in its successful lifestyle. (Ifnot directly involved in capture, jawsare important in ingesting food.)

DemonstrationDistribute soft gelatin-based candysuch as “gummy worms” to theclass and challenge small groups ofstudents to create an internal skele-ton for some worms and an externalskeleton for others, using toothpicksand tape. Caution students not toeat the candy. Have the groupsshare their progress and results withthe class. Ask which type of skeletonrequires less material and weighsless. (The internal [endo-] skeletonshould be smaller and lighter.)

Kinesthetic

DemonstrationCrack open an egg in a Petri dish.In another Petri dish, place anintact egg. Weigh each egg-and-dishcombination and record the weight,then store the eggs in a refrigerator,if possible. At the end of 2 days,crack open the intact egg and com-pare it to the exposed egg. Recordthe similarities and differencesbetween the two, then reweighboth Petri dishes. Have studentsexplain which egg setup showedthe greater difference in weight andwhy. (The opened egg should weighless, because of evaporation, and mayshow signs of decomposition. Theother egg was protected by a shellthat prevents water loss and shouldbe in good condition.) VisualLS

Co-op LearningLS

GENERAL


did you know?Mammals of the Sea Mammals that live in the ocean range in size from the furry, five-foot-long sea otter to the hundred foot longblue whale. Like us, these creatures are warm-blooded animals that breathe air and nursetheir young. The cetaceans (whales, dolphins,porpoises) descended from a cow-like ancestorthat returned to ocean life about 65 millionyears ago. The streamlined, fishlike whaledoesn’t resemble its ancestor. With one or twonostrils on the top of its head, a whale can eas-ily breathe at the surface without lifting its

Vertebrates A is an animal with a backbone—vertebrates are the ani-mals most familiar to us. Humans are vertebrates, and almost all otherland animals bigger than our fist are vertebrates as well.

Fishes According to the fossil record, the first verte-brates were small, jawless fishes that evolvedin the oceans about 530 million years ago.Jawed fishes first appeared about 430 millionyears ago. Jaws enabled fishes to bite andchew their food instead of sucking up theirfood. As a result, jawed fishes were efficientpredators. A fossilized example of a jawedfish is shown in Figure 15. Fishes soon cameto be among the most abundant animals in the seas, and for hun-dreds of millions of years the sea is where vertebrates stayed. Fishesare the most successful living vertebrates—they make up more thanhalf of all modern vertebrate species. After nearly 200 million yearsof living in the sea, fishes have become uniquely adapted for successin water. Major changes had to occur in fish body organization,however, before some descendants of fishes became capable of liv-ing on land.

AmphibiansThe first vertebrates to inhabit the land did not come out of the seauntil 370 million years ago. Those first land vertebrates were earlyamphibians. Amphibians are smooth-skinned, four-legged animalsthat today include frogs, toads, and salamanders.

Several structural changes in the bodies of amphibians occurredas they adapted to life on land. Amphibians had moist breathingsacs—lungs—which allowed the animals to absorb oxygen from air.The limbs of amphibians are thought to have derived from the bonesof fish fins. The evolution of a strong support system of bones in theregion just behind the head made walking possible. This system ofbones provided a rigid base for the limbs to work against. Becauseof their strong, flexible internal skeleton, the bodies of vertebratescan be much larger than those of insects. While amphibians werewell adapted to their environment, a new group of animals moresuited to a drier environment evolved from them.

vertebrate

Appearance of flowering plants

CRETACEOUS PERIOD120140200 180 160

• • • • •100•

JURASSIC PERIOD CRETACEOUS PERIOD

•

Figure 15 Fossilized fishskeleton. This fish skeletonclearly shows the backbone,the structure that is character-istic of all vertebrate animals.

267


ReteachingDraw a table on the board. Acrossthe top of the table, write thesefour headings: EvolutionaryHistory, Structural Features, SpecialAdaptations, and Examples. Downthe left side of the table write theseseven classes: Jawless fishes, Sharks(cartilaginous fishes), Bony fishes,Amphibians, Reptiles, Birds, andMammals. Have students taketurns filling in the information.

Quiz1. In what ways did the evolution

of flight and an endoskeletonprovide advantages for theorganisms that possessed thesetraits? (Both flight and anendoskeleton aid an animal’smovement on land.)

2.What are amphibian legsthought to be derived from? (The fins of bony fishes arethought to be the precursors ofamphibian legs.)

AlternativeAssessmentTell students to return to the list ofthings they want to know abouthow life began. Have them placecheck marks next to the questionsthat they are now able to answer.Students should finish by making alist of what they have learned. Askstudents the following: Which ques-tions are still unanswered? Whatnew questions do you have? Youmay wish to assign projects thatrequire students to research theirunanswered questions.

GENERAL

GENERAL

CloseClose

Answers to Section Review1. Ozone was essential to protect early land

organisms from the sun’s ultraviolet rays.2. Plants and fungi (or mycorrhizae) were the first

multicellular land-dwelling organisms.3. Arthropods were the first animals to live on

land.4. The first land-dwelling vertebrates were

amphibians, which had lungs, an endoskeleton,and smooth skin.

5. Cyanobacteria contributed oxygen gas to theatmosphere. Some of this oxygen gas reacted

and formed the ozone layer, which was neces-sary to protect any multicellular organismsfrom the sun’s ultraviolet radiation.

6. A. Incorrect. Amphibians and plant roots donot form a mutualistic relationship.B. Incorrect. Insects and plant roots may forma relationship, but it is not mycorrhizal.C. Incorrect. Cyanobacteria are aquatic organ-isms that do not form relationships with plantroots. D. Correct. Mycorrhizal associationsinvolve plant roots and fungi.


Reptiles Reptiles evolved from amphibian ancestors about 340 million yearsago. Modern reptiles include snakes, lizards, turtles, and crocodiles.Reptiles are better suited to dry land than amphibians because rep-tiles’ watertight skin slows the loss of moisture. Reptiles also havea watertight egg, such as the one shown in Figure 16. Unlikeamphibians, reptiles can lay their eggs on dry land. Amphibiansmust lay their eggs in water or in very moist soil because their eggsare unable to retain enough water to remain alive.

Mammals and BirdsBirds apparently evolved from feathered dinosaurs during or afterthe Jurassic period. Therapsids, reptiles with complex teeth and legspositioned beneath their body, gave rise to mammals about the sametime dinosaurs evolved, during the Triassic period. Sixty-five millionyears ago, during the fifth mass extinction, most species disappearedforever. All of the dinosaurs except for the ancestors of birds becameextinct. The smaller reptiles, mammals, and birds survived. Althoughmany resources were available to the surviving animals, the world’sclimate was no longer largely dry. Thus the reptiles’ advantages in dryclimates were not so important. Birds and mammals then becamethe dominant vertebrates on land.

Both extinctions and continental drift played important roles inevolution. is the movement of Earth’s land massesover Earth’s surface through geologic time. Continental drift resultedin the present-day position of the continents. The movement of con-tinents helps explain why there are a large number of marsupial(pouched) mammal species in both Australia and South America,continents that were once connected.

Continental drift

Figure 16 Reptiles.Reptiles, such as thiscrocodile, were the largestgroup of land-dwellingorganisms until the end ofthe Cretaceous period.


Birds and mammals spread Mammals abundant on land

6070 50 40 30 20 10 Today• • • • • • • •

.5•

CRETACEOUS PERIOD TERTIARY PERIOD QUATERNARY PERIOD

▲

Fifth massextinction

▲

Major mammalgroups evolve

▲

Appearance ofAustralopithecus

▲

FirstHomo sapiens

Summarize why ozone was important inenabling organisms to live on land.

Name the first multicellular organisms that colonized land.

Identify the first kinds of animals to live on land.

Describe the first kinds of vertebrates thatinhabited land.

Critical Thinking Defend the argument thatinvasion of land could not have happened untilwell after the evolution of cyanobacteria.

Mycorrhizae are mutualisticrelationships between the roots of plants and A amphibians. C cyanobacteria.B insects. D fungi.


Section 3 Review

268


AlternativeAssessmentHave students prepare a blankbingo card with 25 squares, andlabel the center square FREE. Write24 terms or short phrases on theboard. Have students write in eachterm or phrase in random order,filling all of the squares on theirbingo cards. Next, have studentscross out the appropriate squareeach time a definition or clue isprovided. Students call out “bingo”whenever a horizontal, vertical, ordiagonal line is completed.

GENERAL

Answer to Concept MapThe following is one possible answer to Performance Zone item 15.


• Science Skills Worksheet• Critical Thinking Worksheet• Test Prep Pretest• Chapter Test GENERAL

GENERAL

GENERAL


Key Concepts

Study CHAPTER HIGHLIGHTS

ZONE

How Did Life Begin?● The Earth formed about 4.5 billion years ago according to

evidence obtained by radiometric dating.● The primordial soup model and the bubble model propose

explanations of the origin of the chemicals of life.● Scientists think RNA formed before DNA or proteins

formed.● Scientists think that the first cells may have developed from

microspheres.● The development of heredity made it possible for organisms

to pass traits to subsequent generations.

Complex Organisms Developed● Prokaryotes are the oldest organisms and are divided into

two groups, archaebacteria and eubacteria.● Prokaryotes likely gave rise to eukaryotes through the

process of endosymbiosis.● Mitochondria and chloroplasts are thought to have evolved

through endosymbiosis.● Multicellularity arose many times and resulted in many

different groups of multicellular organisms.● Extinctions influenced the evolution of the species alive

today.

Life Invaded the Land● Ancient cyanobacteria produced oxygen, some of which

became ozone. Ozone enabled organisms to live on land.● Plants and fungi formed mycorrhizae and were the first

multicellular organisms to live on land.● Arthropods were the first animals to leave the ocean.● The first vertebrates to invade dry land were amphibians.● The extinction of many reptile species enabled birds and

mammals to become the dominant vertebrates on land.● The movement of the continents on the surface of the

Earth has contributed to the geographic distribution ofsome species.

3

2

1

Key Terms

Section 1radiometric dating (252)radioisotope (252)half-life (252)microsphere (256)

Section 2fossil (258)cyanobacteria (258)eubacteria (258)archaebacteria (258)endosymbiosis (259)protist (261)extinction (263)mass extinction (263)

Section 3mycorrhizae (265)mutualism (265)arthropod (266)vertebrate (267)continental drift (268)

269

in

between

resemble

according to

oldest fossils

inorganic molecules organic molecules bubblemodel

primordial soup model

microspheres

radioisotopes

proteins

chemical reactions bubbles early ocean

reacted to form

may have occurred inmight have involved

canassemble

form

amino acidsRNA

form

reacted to form dated by

Spontaneous origin


ANSWERS

Understanding Key Ideas1. d2. b3. d4. d5. c6. The theory of endosymbiosis is

supported by the observationsthat mitochondria reproduce bybinary fission, and that they havetheir own genes that are separateand distinct from nuclear genes.

7. After four half-lives, the samplewould have 1.625 g of carbon-14.

8. 1 billion years9. Producers (plants) colonized the

land first, providing food for consumers ofplants and, in turn, consumers of the con-sumers, or predators. In this way,energy was gathered from the sunby land plants and then passedthrough the food web, from herbi-vores to carnivores.

10. One possible answer toPerformance Zone item 15 is onthe Study Zone page.

Critical Thinking11. Evolution would continue after a

mass extinction. In the remainingorganisms, there would be traitsthat would help ensure some sur-vival to the point of reproduc-tion. These traits would beamplified within populations.New niches would then causenew species to evolve.

12. Answers may vary. Students maypropose that changes in the seaenvironment might have

increased selection pressure and resulted inrapid evolution and diversification.

Alternative Assessment13. Answers will vary. 14. Answers will vary. 15. Students’ research may identify a variety of

scientific hypotheses for the origin of life.Possibilities include the proposed assembly ofcomplex biological molecules on clay or othermineral substrates, hypotheses of the “RNAworld” which holds that RNA acted both as acatalyst and as the first hereditary molecule,and so on.”


CHAPTER 12

Section Questions1 1, 2, 3, 7, 8, 10, 13, 152 4, 6, 11, 12, 143 5, 10

Assignment Guide

Understanding Key Ideas1. After three half-lives of a radioisotope have

passed, how much of the originalradioisotope has decayed?a. 1/8 c. 3/4b. 1/2 d. 7/8

2. Unlike the primordial soup model, Lerman’s bubble model takes ______ into account. a. ozoneb. ultraviolet radiationc. lightningd. volcanoes

3. Cells are different from microspheresbecause cellsa. contain amino acids.b. have a two-layer outer boundary.c. grow by taking in molecules from their

surroundings.d. transfer information through heredity.

4. Cell specialization came about as a result ofa. endosymbiosis. c. the Cambrian period.b. archaebacteria. d. multicellularity.

5. The first multicellular organisms to invadethe land werea. reptiles. c. fungi and plants.b. amphibians. d. mammals.

6. Describe the evidence that supports thetheory of endosymbiosis.

7. The half-life of carbon-14 is 5,700 years. If asample originally had 26 g of carbon-14, howmuch would it contain after 22,800 years?

8. What is the half-life of the radioisotoperepresented in the graph?

9. Relate the order in which different types oforganisms invaded the land to the flow ofenergy. (Hint: See Chapter 5, Section 1.)

10. Concept Mapping Construct a conceptmap that shows how life might have origi-nated by natural forces. Include the followingitems in your map: spontaneous origin,primordial soup model, bubble model, RNA,proteins, microspheres, and radioisotopes.

Critical Thinking11. Evaluating Viewpoints Several scientists

have said that if a large asteroid struck theEarth, the impact could result in a massextinction. If an asteroid impact did notkill all organisms, would evolutioncontinue or stop? Explain.

12. Recognizing Relationships Propose ahypothesis for the appearance of all animalphyla on Earth within a relatively shortperiod during the late Precambrian andearly Cambrian periods.

Alternative Assessment13. Being a Team Member Work together in

groups to design a poster to illustrate thedifferent models that describe how life’schemicals may have originated. Show howthe compounds on early Earth would haveparticipated in each of these models.

14. Finding and Communicating InformationThomas Cech and Sidney Altman shared aNobel prize in 1989 for their work on RNA.Research their work and the rewardsassociated with winning a Nobel prize.Relate your findings in an oral report.

15. Finding and Communicating InformationUse the media center or Internet resourcesto study scientific hypotheses for the originof life that are alternatives to the hypothe-ses proposed by Oparin and Lerman. Ana-lyze either Oparin’s or Lerman’s hypothesesas presented in your textbook along withone alternative scientific hypotheses thatyou discover in your research.

PerformanceZONE

CHAPTER REVIEW

Radioactive Decay

2.01.000

50

100

150

200

3.0 4.0 5.0

Gra

ms

(of r

adio

activ

em

ater

ial r

emai

ning

)

Time (in billions of years)

270



Standardized Test Prep

Understanding ConceptsDirections (1–3): For each question, write ona separate sheet of paper the letter of thecorrect answer.

1 What did the development of heredityallow organisms to store and pass on totheir offspring?A. energyB. informationC. radioisotopesD. UV radiation

2 A cell’s surface-area-to-volume ratio limitsthe size that unicellular organisms canachieve. How can multicellularity solvethis problem?F. As a cell grows in size, its surface-area-

to-volume ratio increases, whichimproves the efficiency of the cell.

G. Multicellular organisms tend to havelarger cells than unicellular organisms,which makes them stronger.

H. Communication between the nucleusand other parts of the cell is faster inlarger cells than in smaller cells.

I. An organism with many small cells hasa greater surface-area-to-volume ratiothan an organism with one large cell.

3 Photosynthesis in what organisms origi-nally formed the oxygen that becameozone in Earth’s atmosphere?A. archaebacteriaB. arthropodsC. cyanobacteriaD. mycorrhizae

Directions (4): For the following question,write a short response.

4 Identify and describe the event thatresulted in the origin of eukaryotes.

Reading SkillsDirections (5): Read the passage below.Then answer the question.

Originally, there were no organisms livingon dry land. One factor that contributed tothis occurrence was that outside of anaquatic environment, organisms could notget all of the nutrients and minerals theyneeded to survive. Then, a symbiotic rela-tionship developed between plants and fungi,which provided nutrients for the fungi andminerals for the plants. This allowed them tocolonize the bare land. Land plants served asfood for insects, which then served as foodfor amphibians and other organisms.

5 What type of relationship did the plantsand fungi have?F. aquatic H. mutualisticG. microscopic I. predatory

Interpreting GraphicsDirections (6): Base your answer to question6 on the diagram below.

Experimental Setup

6 What conclusion could be drawn from theresults of the experiment shown?A. Earth’s early atmosphere lacked N2.B. N2 and H2 can be converted into NH3

when heated.C. Water can be changed into organic

compounds if it is heated vigorously.D. Organic compounds can form under

conditions such as those in the experiment.

Spark

Organic compounds

Collectingchamber

CondenserH2Ovapor

Hotwater

N2CH4H2NH3

TestIf time permits, take short mental breaks to improveyour concentration during a test.

271

Question 2 Answer I is the correctchoice. Answer F is incorrectbecause the opposite is true. As acell grows in size, its surface-area-to-volume ratio decreases. AnswerG is incorrect because multicellularorganisms have a variety of cellsizes, and their size does not neces-sarily make them stronger. AnswerH is incorrect because the oppositetends to be true. Communicationbetween the nucleus and otherparts of the cell tends to be slowerin larger cells than in smaller cells.

Question 4 Endosymbiotic theoryproposes that mitochondria are thedescendants of symbiotic, aerobiceubacteria, and chloroplasts arethe descendants of symbiotic, pho-tosynthetic eubacteria.

Question 5 Answer H is the cor-rect choice. Both plants and fungibenefited from their relationship.Answer F is incorrect because theirrelationship allowed them to moveaway from an aquatic environ-ment. Answer G is incorrectbecause there is no clear indicationthese organisms were too small tosee without a microscope. AnswerI is incorrect because their rela-tionship did not involve predation.

Question 6 Answer D is the cor-rect choice. The experimentshowed it was possible for organiccompounds to form under certainconditions. Answer A is incorrectbecause Earth’s atmosphere wasthought to be rich in N2. AnswerB is incorrect because NH3 is notan organic compound and was notdetected in the compound in thecollecting chamber. Answer C isincorrect because other com-pounds besides water were presentin the experiment.

Answers1. B2. I3. C4. According to the theory of endosymbiosis, bac-

teria entered large cells and began to live insidehost cells. Cyanobacteria likely became chloro-plasts and carried out photosynthesis for thehost cell. Other bacteria became mitochondriaand carried out cellular respiration for the hostcell.

5. H6. D

Standardized Test Prep


MAKING A TIMELINE OFLIFE ON EARTH

Teacher’s Notes

Time Required 90 minutes

Ratings

TEACHER PREPARATION

STUDENT SETUP

CONCEPT LEVEL

CLEANUP

Skills Required• Communicating• Identifying/Recognizing Patterns• Inferring• Measuring• Organizing and Analyzing Data

Scientific MethodsIn this lab, students will:• Make Observations• Ask Questions• Draw Conclusions• Communicate the Results

Materials and EquipmentMaterials for this lab can beordered from WARD’S. See MasterMaterials List at the front of thisbook for catalog numbers.

Tips and TricksSet up stations in various parts ofthe classroom with specimens, fos-sils, photographs, or slides of dif-ferent types of organisms. Providecompound light microscopes ifneeded. During the first hour (orlab period), have students maketheir timelines and observe andrecord data for as many organismsas possible. During the secondperiod, have students complete theinvestigation.

E A S Y H A R D

Answers to Before You Begin1. Definitions: fossil—preserved remains or traces

of a past life form; fossil record—the record ofpast life-forms found in fossils; timeline—aseries of events arranged in chronologicalorder.

2. Students should each create a data table asshown.

3. Answers will vary. For example: How did lifeon Earth change after oxygen entered theatmosphere?

Answers to Procedure2. The Earth is estimated to be about 4 billion

years old. Students should label the 12 divi-sions of the geologic time scale approximatelyas follows: Precambrian time—>3.5 billion to540 million; Cambrian—540 million to 505million years ago (mya); Ordovician—505 to438 mya; Silurian—438 to 408 mya;Devonian—408 to 360 mya; Carboniferous—360 to 283 mya; Permian—283 to 247 mya;Triassic—247 to 203 mya; Jurassic—203 to144 mya; Cretaceous—144 to 67 mya;Tertiary—67 to 2 mya; and Quaternary—1 mya to the present.


Exploration Lab

Before You BeginAbout 4.5 billion years ago, Earth was a ballof molten rock. As the surface cooled, arocky crust formed and water vapor in theatmosphere condensed to form rain. By 3.9billion years ago, oceans covered much ofthe Earth’s surface. Rocks formed in theseoceans contain of bacterial cells thatlived about 3.5 billion years ago. The

shows a progression of life-forms andcontains evidence of many changes in Earth’ssurface and atmosphere.

In this lab, you will make a showing the major events in Earth’s historyand in the history of life on Earth, such asthe evolution of new groups of organisms andthe mass extinctions. This timeline can beused to study how living things have changedover time.

1. Write a definition for each boldface termin the paragraphs above.

2. Make a data table similar to the oneat right.

3. Based on the objectives for this lab, writea question you would like to explore aboutthe history of life on Earth.

ProcedurePART A: Making a Timeline1. Make a mark every 20 cm along a 5 m

length of adding-machine tape. Label oneend of the tape “5 billion years ago” andthe other end “Today.” Write “20 cm =200 million years” near the beginning ofyour timeline.

2. Locate and label a point representing theorigin of Earth on your timeline. Use yourtextbook as a reference. See the timeline atthe bottom of Section 2 and Section 3 ofthis chapter. Also locate and label the 11periods of the geologic time scale begin-ning with the Cambrian period.

3. Using your textbook as a reference, markthe following events on your timeline:the first cyanobacteria appear; oxygenenters the atmosphere; the five massextinctions; the first eukaryotes appear;the first multicellular organisms appear;the first vertebrates appear; the first plants,

timeline

recordfossil

fossils

Exploration LabMaking a Timeline of Life on Earth

SKILLS• Observing

• Inferring relationships

• Organizing data

OBJECTIVES• Compare and contrast the

distinguishing characteristicsof representative organismsof the six kingdoms.

• Organize the appearance oflife on Earth in a timeline.

MATERIALS• 5 m roll of adding-

machine tape

• meterstick

• colored pens or pencils

• photographs or drawings of organismsfrom ancient Earth topresent day

Organism Kingdom Characteristics/adaptationfor life on Earth

272


3. Mass extinctions appear to be followed closelyby the evolution of many new types oforganisms.

4. Cyanobacteria appeared before the accumula-tion of oxygen in the atmosphere, andcyanobacteria produce oxygen. Therefore, it isreasonable to conclude that they are responsi-ble for adding oxygen to Earth’s atmosphere.

5. Humans have existed for 0.014 percent of thetime that life has existed on Earth.

6. Answers will vary. Sample answer: How haveEarth’s climates changed since Precambriantimes?

3. Cyanobacteria appear—3.5 billionyears ago; oxygen enters theatmosphere—2.75 billion yearsago; five mass extinctions—440million years ago (mya), 360 mya,247 mya, 203 mya, and 65 mya;eukaryotes appear—1.5 billionyears ago; multicellular organismsappear—700 mya; vertebratesappear—500 mya; land plants andfungi appear—430 mya; land ani-mals appear—370 mya; dinosaursand mammals appear—230 mya;flowering plants appear—120mya; humans appear—500,000years ago.

4. Answers will vary. See sample datatable at the bottom of page 273.

5. Students should place the photo-graph or specimen at the point onthe timeline where that type ofspecimen appeared. For example, aphotograph of a flowering plantshould be placed on the timeline at120 million years ago.

Answers to On the JobMany types of scientists use timelinesin their work. For example, an epi-demiologist (a scientist who tracksthe emergence and spread of disease)might arrange the known cases of adisease on a timeline to track theevents of an outbreak.

Answers to Analyze andConclude1. a. Life on Earth has existed for

at least 17.5 “hours.” b. Solelyunicellular life forms existed forabout 14 “hours.” c. The firstplants appeared at about 9:30 PM.d. Mammals appeared at about 11 PM.

2. Life has changed from all prokary-otic, unicellular forms to complexmulticellular forms composed ofeukaryotic cells. A vast variety ofliving things have lived on Earthduring the last 3.5 billion years.Most life forms that evolved havebecome extinct.


ORGANISM KINGDOM CHARACTERISTICS/ADAPTATIONSFOR LIFE ON EARTH

Insect (fossil in amber) Animal Segmented body, jointed legs, and wings

Fern (fossil mold Plant Large surface area to in shale) capture sunlight;

reproductive structures on fronds

Sample Data Table

fungi, and land animals appear; the firstdinosaurs and mammals appear; thefirst flowering plants appear; the firsthumans appear.

4. Look at the photographs of organisms pro-vided by your teacher. Identify the majorcharacteristics of each organism. Recordyour observations in your data table.

5. Lay out your timeline on the floor in yourclassroom. Place photographs (or draw-ings) of the organisms you examined onyour timeline to show when they appearedon Earth.

6. Fold the timeline at the mark representing4.8 billion years ago. This leaves 24 seg-ments, each representing 200 million years,in your timeline. Now you can think ofeach segment as 1 hour in a 24-hour day.

7. When you are finished, walk slowly alongyour timeline. Note the sequence of eventsin the history of life on Earth and the rela-tive amount of time between each event.

PART B: Cleanup and Disposal8. Dispose of paper scraps in the

designated waste container.

9. Clean up your work area and all labequipment. Return lab equipment to

its proper place.

Analyze and Conclude1. Analyzing Information Think of each

segment of your timeline as 1 hour in a 24-hour day as you answer each of the following questions:

a. How long has life existed on Earth?b. For what part of the day did only unicel-

lular life-forms exist?c. At what time of day did the first plants

appear on Earth?d. At what time of day did mammals

appear on Earth?

2. Summarizing Information Identify themajor developments in life-forms that haveoccurred over the last 3.5 billion years.

3. Inferring Relationships How do massextinctions appear to be related to theappearance of new major groups oforganisms?

4. Justifying Conclusions Cyanobacteriaare thought to be responsible for addingoxygen to Earth’s atmosphere. Use yourtimeline to justify this conclusion.

5. Calculating Determine the amount oftime, as a percentage of the time that lifehas existed on Earth, that humans (Homosapiens) have existed.

6. Further Inquiry Write a new questionabout the history of life on Earth thatcould be explored in another investigation.

On the JobTimelines are used to organize events inchronological order. Do research to dis-cover how other scientists use timelinesin their work. For more about careers,visit go.hrw.com and type in the keyword HX4 Careers.

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