ChangeThrough TimeWhat You’ll LearnChapter 14

The History of Life

Chapter 15The Theory of Evolution

Chapter 16 Primate Evolution

Chapter 17Organizing Life’s Diversity

Unit 5 ReviewBioDigest & Standardized Test Practice

Why It’s ImportantLife on Earth has a history of change that is called evolu-tion. An enormous variety of fossils, such as those of earlybirds, provide evidence of evolution. Genetic studies ofpopulations of bacteria, protists, plants, insects, and evenhumans provide further evidence of the history of changeamong organisms that live or have lived on Earth.

1593The Roman cityof Pompeii isdiscovered.

1500Leonardo da Vincirecognizes that fossilshells representancient marine life.

Understanding the PhotoThese African elephants are well-adaptedto their environment. Scientists study theseand other organisms to learn about theiradaptations and how the organisms have changed through time.

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Fossil shell

bdol.glencoe.com/webquest(t)Breck P. Kent/Earth Scenes, (crossover)Daryl Balfour/Getty Images

1773The Boston TeaParty occurs.

1974The nearly complete skeletonof Australopithecus afarensis,known as “Lucy,” is discoveredin Ethiopia.

2001A hominid fossil is discoveredin Africa that is 6 to 7 millionyears old.

1856The first humanlikefossil remains(Neandertals) are discovered in Germany.

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1778The first asser-tion that theage of Earthexceeds a fewthousand yearsis published.

1841The first university degreesare granted to women inthe United States.

1936Jesse Owens wins four goldmedals in track and field atthe Berlin Olympics.

Neandertal skull

1999Meave Leakey discovers a new fossil hominid,Kenyanthropus, in Kenya.

J. Breckett/D. Fannin/American Museum of Natural History

368

What You’ll Learn■ You will examine how rocks

and fossils provide evidenceof changes in Earth’s organisms.

■ You will correlate the geologic time scale with biological events.

■ You will sequence the steps bywhich small molecules mayhave produced living cells.

Why It’s ImportantKnowing the geological historyof Earth and understandingideas about how life began pro-vide background for an under-standing of the theory ofevolution.

Visit to• study the entire chapter

online• access Web Links for more

information on the origin oflife

• review content with theInteractive Tutor and self-check quizzes

The History of LifeThe History of Life

Erupting volcanoes and lavaflows, such as this one inHawaii, may provide a model for conditions on early Earth.

Understandingthe Photo

bdol.glencoe.com

Soames Summerhays/Photo Researchers

14.1 THE RECORD OF LIFE 369

14.1SECTION PREVIEWObjectivesIdentify the differenttypes of fossils and howthey are formed.Summarize the majorevents of the geologictime scale.

Review Vocabularyisotope: atoms of the

same element that havedifferent numbers ofneutrons (p. 144)

New Vocabularyfossilplate tectonics

Early History of EarthWhat was early Earth like? Some scientists suggest that it was proba-

bly very hot. The energy from colliding meteorites could have heated itssurface, while both the compression of minerals and the decay of radio-active materials heated its interior. Volcanoes might have frequentlyspewed lava and gases, relieving some of the pressure in Earth’s hot inte-rior. These gases helped form Earth’s early atmosphere. Although itprobably contained no free oxygen, water vapor and other gases, such ascarbon dioxide and nitrogen, most likely were present. If ancient Earth’satmosphere was like this, you would not have survived in it.

About 4.4 billion years ago, Earth might have cooled enough for thewater in its atmosphere to condense. This might have led to millions of yearsof rainstorms with lightning—enough rain to fill depressions that becameEarth’s oceans. Some scientists propose that life originated in Earth’s oceansbetween 3.9 and 3.4 billion years ago.

The Record of Life

Tape

Sequence As you read Chapter 14, arrange the divisions of the geologic scale from oldest to youngest beginning at the far left of the Foldable. Then write the major life forms and events that appeared in each era.

Geologic Time Make the following Foldable to help you organize events in Earth's history and the major life formsthat appeared during each event.

Fold two vertical sheets of paper in half from top to bottom.

Turn both papers horizontally and cut the papers in half along the folds.

Fold the four vertical pieces in half from top to bottom.

Turn the papers horizontally. Tape the shortends of the pieces together (overlapping the edges slightly) to make an accordion time line.

Label each fold.

STEP 1

STEP 3

STEP 5

STEP 2

STEP 4

PhysicalScience

Connection

Movement ofheat The move-ment of heat fromEarth’s interior outto space involvesthe processes ofconduction,convection, andradiation. Earth’sinterior can behavelike a fluid, so thatheat is transferredto the outer crustby convection andconduction. Heatthen movesthrough the solidcrust by conductionand into space byradiation.

History in RocksCan scientists be sure that Earth

formed in this way? No, they cannot.There is no direct evidence of theearliest years of Earth’s history. Thephysical processes of Earth constantlydestroy and form rocks. The oldestrocks that have been found on Earthformed about 3.9 billion years ago.Although rocks cannot provide infor-mation about Earth’s infancy, they arean important source of informationabout the diversity of life that hasexisted on the planet.

Fossils—Clues to the pastIf you’ve ever visited a zoo or toured

a botanical garden, you’ve seen evi-dence of the diversity of life. But themillions of species living today areprobably only a small fraction of all the species that ever existed. About 95 percent of the species that haveexisted are extinct—they no longer

live on Earth. Among other tech-niques, scientists study fossils to learnabout ancient species. A fossil is evidence of an organism that livedlong ago.

Because fossils can form in manydifferent ways, there are many typesof fossils, as you can see in Table 14.1.Use the MiniLab on the next page toobserve some marine fossils underyour microscope.

Paleontologists—Detectives to the past

The study of fossils is a lot like solv-ing a mystery. Paleontologists (pay leeahn TAHL uh justs), scientists whostudy ancient life, are like detectiveswho use fossils to understand eventsthat happened long ago. They use fos-sils to determine the kinds of organ-isms that lived during the past andsometimes to learn about their behav-ior. For example, fossil bones and

370(1)T.A. Wiewandt/DRK Photo, (2)Breck P. Kent/Earth Scenes, (3)Michael Collier, (4)John Gerlach/DRK Photo, (5)Jeff J. Daly/Visuals Unlimited

Table 14.1 Some Types of Fossils

Fossils Types Formation Example

Trace fossils A trace fossil is any indirect evidence left by an animal and may include a footprint, a trail, or a burrow.

Casts When minerals in rocks fill a space left by a decayed organism, they make a replica, or cast, of the organism.

Molds A mold forms when an organism is buried in sediment and then decays, leaving an empty space.

Petrified/ Petrified—minerals sometimes penetrate Permineralized and replace the hard parts of an organism.fossils Permineralized—void spaces in original

organism infilled by minerals.

Amber- At times, an entire organism was quickly preserved or trapped in ice or tree sap that hardened frozen fossils into amber.

teeth can indicate the size of animals,how they moved, and what they ate.

Paleontologists also study fossils togain knowledge about ancient cli-mate and geography. For example,when scientists find a fossil like theone in Figure 14.1, which resemblesa present-day plant that lives in a mildclimate, they may reason that theancient environment was also mild.

By studying the condition, position,and location of rocks and fossils, geolo-gists and paleontologists can makedeductions about the geography of past environments. You can use theProblem-Solving Lab on the next page totry to solve a fossil mystery.

Infer how fossilteeth could be used to determine an animal’s diet.

Fossil formationFor fossils to form, organisms usually

have to be buried in mud, sand, or claysoon after they die. These particles arecompressed over time and harden into atype of rock called sedimentary rock.Today, fossils still form at the bottomsof lakes, streams, and oceans.

Most fossils are found in sedimen-tary rocks. These rocks form at rela-tively low temperatures and pressuresthat may prevent damage to the organism. How do these fossilsbecome visible millions of years later?

14.1 THE RECORD OF LIFE 371

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Figure 14.1This fossil leaf is from rocks about 200 mil-lion years old (A). They are remarkably simi-lar to the leaves of Ginkgo biloba (B), treesthat are planted as ornamentals throughoutthe United States.

(t)Jan Hinsch/Science Photo Library/Photo Researchers, (bl)Patti Murray/Earth Scenes, (br)Gilbert S. Grant/Photo Researchers

Observe and InferMarine Fossils Certain sedimen-tary rocks are formed almost totallyfrom the fossils of once-livingmarine or ocean organisms calleddiatoms. These sedimentary rocksusually form in oceans, but can belifted above sea level during periodsof geological change.

Procedure! Prepare a wet mount of a small amount of diatomaceous

earth. CAUTION: Use care in handling microscope slidesand coverslips. Do not breathe in dry diatomaceous earth.

@ Examine the material under low-power magnification.# Draw several of the different shapes you see.$ Compare the shapes of the fossils you observe to present-

day diatoms shown in the photograph. Remember, how-ever, that the fossils you observe are probably only piecesof the whole organism.

Analysis1. Describe Describe the appearance of fossil diatoms.2. Compare and Contrast How are fossil diatoms similar to

and different from the diatoms in the photo? Can you usethese similarities and differences to predict how diatomshave changed over time? Explain your answer.

3. Infer What part of the original diatom did you observeunder the microscope? How did this part survive millionsof years? Why were the fossils you observed broken?

Present-day diatoms

Color-enhanced LM Magnification: 130�

372 THE HISTORY OF LIFE

Figure 14.2Most sedimentary rocks form in primarily horizontal layers with theyounger layers closer to the surface. Older rocks and fossils will be founddeeper in the sequence, with the oldest at the bottom. Infer What mighthave happened to a section with the oldest fossils at the top ofthe sequence?

To answer the question, look at Figure 14.3. Fossils are not usuallyfound in other types of rock because ofthe ways those rocks form. For exam-ple, metamorphic rocks form whenheat, pressure, and chemical reactionschange other rocks. The conditionsunder which metamorphic rocks formoften destroy any fossils that were inthe original sedimentary rock.

Relative dating

Scientists use a variety of methodsto determine the age of fossils. Onemethod is a technique called relativedating. To understand relative dating,imagine yourself stacking newspapersat home. As each day’s newspaper isadded to the stack, the stack becomestaller. If the stack is left undisturbed,the newspapers at the bottom areolder than ones at the top.

The relative dating of rock layersuses the same principle. In Figure 14.2,you see fossils in different layers ofrock. If the rock layers have not beendisturbed, the layers at the surface mustbe younger than the deeper layers. Thefossils in the top layer must also beyounger than those in deeper layers.Using this principle, scientists candetermine relative age and the order ofappearance of the species that are pre-served as fossils in the layers.

Radiometric datingYou cannot determine the actual

age in years of a fossil or rock by usingrelative dating techniques. To find thespecific ages of rocks, scientists useradiometric dating techniques utiliz-ing the radioactive isotopes in rocks.Most fossils and sedimentary rockscannot be directly radiometricallydated. Most dates are for volcanic orother igneous rocks, or metamorphicrocks that are closely associated withthe sedimentary rocks.

Think CriticallyCould ferns have lived in Antarctica? Scientists have dis-covered fossil remains of ferns inthe rocks of Antarctica. Thesefern fossils are related to fernsthat grow in temperate climateson Earth today.

Solve the ProblemRead each statement below and critique whether or not the statement is reasonable. Explain the reason for each of your critiques.

Thinking Critically1. Fern fossils in Antarctica are of plants that could withstand

freezing temperatures.2. The ferns in Antarctica may have been mutated forms of

ferns that grew in warm climates.3. The temperature of Earth may have been much warmer

millions of years ago than it is today.

Fern fossil from Antarctica

Louie Psihoyos/Matrix

14.1 THE RECORD OF LIFE 373

The Fossilization ProcessFigure 14.3Few organisms become fossilized because, without burial, bacteria andfungi immediately decompose their dead bodies. Occasionally, however,organisms do become fossils in a process that usually takes many years.Most fossils are found in sedimentary rocks. Critical Thinking Describehow the movements of Earth might expose a fossil.

Over time, additionallayers of sedimentcompress thesediments around thebody, forming rock.Minerals eventuallyreplace all the body’sbone material.

CC

After discovery, scientists carefullyextract the fossil from the surrounding rock.

EE

Sediments fromupstream rapidly cover thebody, slowing its decomposition.Minerals from thesediments seepinto the body.

BB

Earth movementsor erosion may exposethe fossil millions ofyears after it formed.

DD

Protoceratops skullA Protoceratops* drinking at a river falls into the water and drowns. *An adultProtoceratopswas about2.4 meterslong (8 feet).

AA

(tr)Louis Psihoyos/Matrix, (br)Associated Press/The Mesa Tribune

Recall that radioactive isotopes areatoms with unstable nuclei that breakdown, or decay, over time, giving off

radiation. A radioactive isotope formsa new isotope after it decays. The rateat which a radioactive isotope decaysis related to the half-life of the iso-tope. The half-life is the length oftime needed for half of the atoms ofthe isotope to decay.

Scientists try to determine theapproximate ages of rocks by compar-ing the amount of a radioactive iso-tope and the new isotope into whichit decays. For example, suppose thatwhen a rock forms it contains aradioactive isotope that decays to halfits original amount in one millionyears. Today, if the rock containsequal amounts of the original radioac-tive isotope and the new isotope intowhich it decays, then the rock must beabout 1 million years old.

Scientists use potassium-40, a radio-active isotope that decays to argon-40,to date rocks containing potassium-bearing minerals. Based on chemicalanalysis, chemists have determinedthat potassium-40 decays to half itsoriginal amount in 1.3 billion years.Scientists use carbon-14 to date fossils

PrecambrianCambrian Ordovician Silurian Devonian Carboniferous Permian

4000 3500 1800 543 443491 290323354417

Major LifeForm

MajorEvents

PeriodEra Paleozoic Era

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Firs

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Coni

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app

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Firs

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Million Years Ago (approximate)

Animal Keeper

Would you like to makea career out of caring

for animals? There are manyopportunities if you loveanimals.

Skills for the JobAnimal keepers or care-

takers give animals foodand water, exercise them, clean their cages, groom them,monitor their health, and sometimes administer medicines.Keepers must finish high school. Many pet shops, kennels,shelters, and stables provide on-the-job training. Humanesocieties, veterinarians, and research laboratories hire grad-uates of two-year programs in animal health. Most zoosand aquariums employ keepers with four-year degrees inzoology or biology. Taking care of animals often meansworking weekends and holidays, so keepers must careabout their work.

For more careers in related fields, visitbdol.glencoe.com/careers

374 THE HISTORY OF LIFEKathy Sloane/Photo Researchers

14.1 THE RECORD OF LIFE 375

Triassic Cretaceous Tertiary Quaternary

Cenozoic EraMesozoic Era

Jurassic

248 206 144 65 1.8

Firs

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less than 50 000 years old. Again,based on chemical analysis, they knowthat carbon-14 decays to half its origi-nal amount in 5730 years.

Use the BioLab at the end of thischapter to simulate this dating tech-nique. Scientists always analyze manysamples of a rock using as many meth-ods as possible to obtain consistent val-ues for the rock’s age. Errors can occurif the rock has been heated, causingsome of the radioactive isotopes to belost or gained. If this occurs, the ageobtained will be inaccurate.

A Trip ThroughGeologic Time

By examining sequences containingsedimentary rock and fossils and datingsome of the igneous or metamorphicrocks that are found in the sequences,scientists have put together a chronol-ogy, or calendar, of Earth’s history.This chronology, called the geologictime scale, is based on evidence fromEarth’s rocks and fossils.

The geologic time scaleRather than being based on months

or even years, the geologic time scale is divided into four large sections that you see in Figure 14.4—the Pre-cambrian (pree KAM bree un), thePaleozoic (pay lee uh ZOH ihk) Era, theMesozoic (me zuh ZOH ihk) Era, andthe Cenozoic (se nuh ZOH ihk) Era. Anera is a large division in the scale andrepresents a very long period of time.Each era is subdivided into periods.

The divisions in the geologic timescale are distinguished by the organ-isms that lived during that time inter-val. The fossil record indicates thatthere were several episodes of massextinction that fall between time divi-sions. A mass extinction is an eventthat occurs when many organismsdisappear from the fossil recordalmost at once.

The geologic time scale begins withthe formation of Earth about 4.6 bil-lion years ago. To understand the largesize of this number, try the MiniLab onthe next page, and also try scaling down

Figure 14.4The geologic time scaleis a calendar of Earth’shistory based on evi-dence found in rocks.Life probably firstappeared on Earthbetween 3.9 and 3.4 billion years ago.

376 THE HISTORY OF LIFE

Figure 14.5The filamentousfossils of theseancient organ-isms resemblesome moderncyanobacteria.

Organize DataA Time Line In this activity, you will construct a time linethat is a scale model of the geologic time scale. Use a scale in which 1 meter equals 1 billion years. Each millimeter thenrepresents 1 million years.

Procedure! Use a meterstick to draw

a continuous line downthe middle of a 5-m stripof adding-machine tape.

@ At one end of the tape,draw a vertical line andlabel it “The Present.”

# Measure off the distancethat represents 4.6 bil-lion years ago. Draw avertical line at that pointand label it “Earth’sBeginning.”

$ Using the table at right,plot the location of eachevent on your time line.Label the event, andlabel when it occurred.

Analysis1. Calculate Which era is the longest? The shortest?2. Interpret Data In which eras did dinosaurs and birds

appear on Earth?3. Interpret Data What major group first appeared after

dinosaurs became extinct?

the history of Earth into a familiar, buthypothetical, calendar year.

Life during the Precambrian In your hypothetical calendar year,

the first day of January becomes thedate on which Earth formed. The old-

est fossils are found inPrecambrian rocks that areabout 3.4 billion years old—near the end of March on thehypothetical calendar. Scien-tists found these fossils, whichare shown in Figure 14.5, inrocks found in the deserts ofwestern Australia. They havefound more examples of simi-lar types of fossils on othercontinents. The fossils resem-ble the forms of modernspecies of photosyntheticcyanobacteria (si a noh bakTIHR ee uh). You will readmore about cyanobacteria in alater chapter.

Scientists have also founddome-shaped structures called

stromatolites (stroh MAT ul ites) inAustralia and on other continents.Stromatolites still form today inAustralia from mats of cyanobacteria,Figure 14.6. Thus, the stromatolitesare evidence of the existence of photo-synthetic organisms on Earth duringthe Precambrian.

The Precambrian accounts forabout 87 percent of Earth’s history—until about the middle of October inthe hypothetical calendar year. At thebeginning of the Precambrian, unicel-lular prokaryotes—cells that do nothave a membrane-bound nucleus—appear to have been the only lifeforms on Earth. About 2.1 billionyears ago, the fossil record shows thatmore complex eukaryotic organisms,living things with membrane-boundnuclei in their cells, appeared. By the end of the Precambrian, about

Geologic Time Scale

EstimatedEvent Years Ago

Earliest evidence of life 3.4 billion

Paleozoic Era begins 543 million

First land plants 443 million

Mesozoic Era begins 248 million

Triassic Period begins 248 million

Jurassic Period begins 206 million

First dinosaurs 225 million

First birds 150 million

Cretaceous Period begins 144 million

Dinosaurs become extinct 65 million

Cenozoic Era begins 65 million

Primates appear 65 million

Humans appear 200 000

J. William Schopf/UCLA

543 million years ago, multicellulareukaryotes, such as sponges and jelly-fishes, diversified and filled theoceans.

Diversity during the PaleozoicIn the Paleozoic Era, which lasted

until 248 million years ago, manymore types of animals and plants werepresent on Earth, and some were pre-served in the fossil record. The earli-est part of the Paleozoic Era is calledthe Cambrian Period. Paleontologistsoften refer to a “Cambrian explosion”of life because the fossil record showsan enormous increase in the diversityof life forms during this time. Duringthe Cambrian Period, the oceansteemed with many types of animals,including worms, sea stars, andunusual arthropods, such as the oneshown in Figure 14.7.

During the first half of thePaleozoic, fishes, the oldest animalswith backbones, appeared in Earth’swaters. There is also fossil evidence offerns and early seed plants existing onland about 400 million years ago.Around the middle of the Paleozoic,four-legged animals such as amphib-ians appeared on Earth. During thelast half of the era, the fossil recordshows that reptiles appeared andbegan to flourish on land.

The largest mass extinction recordedin the fossil record marked the end of the Paleozoic. About 90 percent ofEarth’s marine species and 70 per-cent of the land species disappearedat this time.

Life in the MesozoicThe Mesozoic Era began about

248 million years ago, which would beabout December 10 on the hypotheti-cal one-year calendar. Many changes,in both Earth’s organisms and its geol-ogy, occurred over the span of this era.

The Mesozoic Era is divided intothree periods. Fossils from theTriassic Period, the oldest period,show that mammals appeared onEarth at this time. These fossils ofmammals indicate that early mam-mals were small and mouselike. Theyprobably scurried around in the shad-ows of huge fern forests, trying toavoid dinosaurs, reptiles that alsoappeared during this time.

The middle of the Mesozoic, calledthe Jurassic Period, began about206 million years ago, or mid-December on the hypo-thetical calendar.

Recent fossil discover-ies support the idea thatmodern birds evolvedfrom one of the groupsof dinosaurs toward theend of this period.

Figure 14.6Fossils of stromatolites,similar to the modernAustralian examplesshown here, provideevidence that photo-synthetic cyanobacterialived on Earth 3.4 bil-lion years ago.

377

Figure 14.7Arthropods, such as thisDevonian trilobite, wereamong the many groups of animals that first appeared during the Cambrian explosion.

(t)Fred Bavendam/Peter Arnold, Inc., (b0Ken Lucas/Visuals Unlimited

For example, in Figure 14.8A, you see the fossil of Archaeopteryx, a small bird discovered in Germany. Thefossil reveals that Archaeopteryx hadfeathers, a birdlike feature. You also seea present-day bird, the hoatzin, inFigure 14.8B. This bird has a reptilianfeature, claws on its wings, for its firstfew weeks of life. It also flies poorly, asthe earliest birds probably did.Scientists suggest that such evidencesupports the idea that modern birdsevolved from dinosaurs.

A mass extinctionThe last period in the Mesozoic, the

Cretaceous, began about 144 millionyears ago. During this period, manynew types of mammals appeared andflowering plants flourished on Earth.The mass extinction of the dinosaursmarked the end of the CretaceousPeriod about 65 million years ago.Scientists estimate that not onlydinosaurs, but more than two-thirds ofall living species at the time becameextinct. Some scientists propose that a

large meteorite collision caused thismass extinction. Such a collision couldhave filled the atmosphere with thick,possibly toxic dust that, in turn,changed the climate to one in whichmany species could no longer survive.Based on geological evidence of a largecrater of Cretaceous age in the watersoff eastern Mexico, scientists theorizethat this was the impact site.

Changes during the MesozoicGeological events during the

Mesozoic changed the places in whichspecies lived and affected their distri-bution on Earth. The theory of conti-nental drift, which is illustrated inFigure 14.9, suggests that Earth’scontinents have moved during Earth’shistory and are still moving today at arate of about six centimeters per year.This is about the same rate at whichyour hair grows. Early in theMesozoic, the continents weremerged into one large landmass.During the era, this supercontinentbroke up and the pieces drifted apart.

378 THE HISTORY OF LIFE

AA

tectonics from theGreek word tecton,meaning “builder”;Plate tectonics is atheory that explainsmountain building.

BB

Figure 14.8Both fossil evidence likethis Archaeopteryx (A)and some characteristicsof present-day birds likethis hoatzin (B) suggestthat dinosaurs might havebeen the ancestors oftoday’s birds.

(l)Louis Psihoyos/Matrix, (r)Michael Dick/Animals Animals

14.1 THE RECORD OF LIFE 379

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Figure 14.9The theory of continental driftdescribes the movement of the landmasses over geological time.Describe How hasAfrica moved overtime?About 245 million years ago, the

continents were joined in a landmassknown as Pangaea.

A

By 135 million years ago, Pangaeabroke apart resulting in two largelandmasses.

B

By 65 million years ago, the end of theMesozoic, most of the continents hadtaken on their modern shapes.

C

The theory that explains how thecontinents move is called plate tecton-ics (tek TAH nihks). According to thisidea, Earth’s surface consists of severalrigid plates that drift on top of a plastic(capable of flow), partially molten layerof rock. These plates are continuallymoving—spreading apart, sliding by, orpushing against each other. The move-ments affect organisms. For example,after a long time, the descendants oforganisms living on plates that are mov-ing apart may be living in areas withvery different climates.

The Cenozoic EraThe Cenozoic began about 65 mil-

lion years ago—around December 26on the hypothetical calendar of Earth’shistory. It is the era in which you nowlive. Mammals began to flourish dur-ing the early part of this era. Amongthe mammals that appeared was agroup of animals to which you belong,the primates. Primates first appearedapproximately more than 65 millionyears ago and have diversified greatly.The modern human species appearedperhaps as recently as 200 000 yearsago. On the hypothetical calendar ofEarth’s history, 200 000 years ago islate in the evening of December 31.

bdol.glencoe.com/self_check_quiz

Understanding Main Ideas1. Describe what some scientists propose Earth was

like before life arose.2. Why are most fossils found in sedimentary rocks?3. Using fossils, identify evidence showing that

species have changed over geologic time.4. Explain the difference between relative dating

and radiometric dating.

Thinking Critically5. Suppose you are examining layers of sedimentary

rock. In one layer, you discover the remains of an

extinct relative of the polar bear. In a deeperlayer, you discover the fossil of an extinct alligator.What can you hypothesize about changes overtime in this area’s environment?

6. Make and Use Tables Make a table listing thefour major divisions of the geologic time scale,their time spans, and the major life forms thatappeared during each interval. Use the informa-tion to construct a time line based on a clock face.For more help, refer to Make and Use Tables inthe Skill Handbook.

SKILL REVIEWSKILL REVIEW

14.2SECTION PREVIEWObjectivesAnalyze early experimentsthat support the conceptof biogenesis.Review, analyze, and critique modern theoriesof the origin of life.Relate hypotheses aboutthe origin of cells to theenvironmental conditionsof early Earth.

Review Vocabularyprokaryotes: unicellular

organisms that lackinternal membrane-bound structures (p. 173)

New Vocabularyspontaneous generationbiogenesisprotocellarchaebacteria

380 THE HISTORY OF LIFE

Mold and MudskippersUsing Prior Knowledge You’ve probably opened your refrigerator and found some leftovers with an unpleas-ant surprise—mold. Where did the mold comefrom? Was it in the air or in the food origi-nally? Did these mudskippers come fromthe mud or from the air?Experiment Cut a hot dog in half. Cookone half and place it in an airtight, seal-able plastic bag. Place the uncooked halfin another airtight, sealable plastic bag.Leave both bags out at room temperature untila change is observed. How did each hot dog samplechange? Which sample changed faster? Hypothesize why the changes you observed occurred.

Origins: The Early IdeasIn the past, the ideas that decaying meat produced maggots, mud pro-

duced fishes, and grain produced mice were reasonable explanations forwhat people observed occurring in their environment. After all, they sawmaggots appear on meat and young mice appear in sacks of grain. Suchobservations led people to believe in spontaneous generation—the ideathat nonliving material can produce life.

The Origin of Life

Redi placed decayingmeat in severaluncovered control jars and in coveredexperimental jars. The covers preventedflies from landing on the meat.

A

In time, maggotsand flies filled theopen jars, but notthe covered jars,showing that onlyflies produce flies.

B

Figure 14.10Francesco Redi’s controlledexperiment tested the spon-taneous generation of mag-gots from decaying meat.

Control group

Time

Time

Experimental group

Mudskippers

Fletcher & Baylis/Photo Researchers

Spontaneous generation is disproved

In 1668, an Italian physician,Francesco Redi, disproved a com-monly held belief at the time—theidea that decaying meat producedmaggots, which are immature flies.You can follow the steps of Redi’sexperiment in Figure 14.10. Redi’swell-designed, controlled experimentsuccessfully convinced many scien-tists that maggots, and probably mostlarge organisms, did not arise byspontaneous generation.

However, during Redi’s time, scien-tists began to use the latest tool in biology—the microscope. With themicroscope, they saw that microor-ganisms live everywhere. AlthoughRedi had disproved the spontaneousgeneration of large organisms, manyscientists thought that microorganismswere so numerous and widespread thatthey must arise spontaneously—prob-ably from a vital force in the air.

Pasteur’s experimentsDisproving the existence of a vital

force in air proved difficult. Finally,

in the mid-1800s, Louis Pasteurdesigned an experiment that dis-proved the spontaneous generation ofmicroorganisms. Pasteur set up anexperiment in which air, but nomicroorganisms, was allowed to con-tact a broth that contained nutrients.You can see how Pasteur carried outhis experiment in Figure 14.11.

Pasteur’s experiment showed thatmicroorganisms do not simply arisein broth, even in the presence of air.From that time on, biogenesis(bi oh JEN uh sus), the idea that livingorganisms come only from other liv-ing organisms, became a cornerstoneof biology.

Origins: The Modern Ideas

Biologists have accepted the con-cept of biogenesis for more than 100 years. However, biogenesis doesnot answer the question: How did lifebegin on Earth? No one has yetproven scientifically how life on Earthbegan. However, scientists have devel-oped theories about the origin of life

14.2 THE ORIGIN OF LIFE 381

biogenesis fromthe Greek wordbios, meaning“life,” and the Latin word genesis,meaning “birth”;Biogenesis pro-poses that livingorganisms comeonly from otherliving organisms.

Figure 14.11Some of Pasteur’sflasks, still free ofmicroorganisms, are at the Pasteur Institute in Paris.

Each of Pasteur’s broth-filledflasks was boiled to kill allmicroorganisms.

A The flask’s S-shapedneck allowed air toenter, but preventedmicroorganisms from entering the flask.

B

Pasteur tilted a flask, allow-ing the microorganisms toenter the broth.

C

Microorganisms soongrew in the broth,showing that theycome from othermicroorganisms.

D

on Earth from testing scientifichypotheses about conditions on earlyEarth. The Biology and Society at theend of this chapter summarizes someimportant viewpoints about the originof life on Earth.

Simple organic molecules formedScientists hypothesize that two

developments must have preceded theappearance of life on Earth. First,simple organic molecules, or mole-cules that contain carbon, must haveformed. Then these molecules musthave become organized into complexorganic molecules such as proteins,carbohydrates, and nucleic acids thatare essential to life.

Remember that Earth’s early atmos-phere probably contained no free oxy-gen. Instead, the atmosphere wasprobably composed of water vapor, car-bon dioxide, nitrogen, and perhapsmethane and ammonia. Many scientistshave tried to explain how these sub-stances could have joined together andformed the simple organic moleculesthat are found in all organisms today.

In the 1930s, a Russian scientist,Alexander Oparin, hypothesized that

life began in the oceans that formedon early Earth. He suggested thatenergy from the sun, lightning, andEarth’s heat triggered chemical reac-tions to produce small organic mole-cules from the substances present inthe atmosphere. Then, rain probablywashed the molecules into the oceansto form what is often called a primor-dial soup.

In 1953, two American scientists,Stanley Miller and Harold Urey,tested Oparin’s hypothesis by simu-lating the conditions of early Earth inthe laboratory. In an experiment simi-lar to the one shown in Figure 14.12,Miller and Urey mixed water vapor(steam) with ammonia, methane, andhydrogen gases. They then sent anelectric current that simulated light-ning through the mixture. Then, theycooled the mixture of gases, produceda liquid that simulated rain, and col-lected the liquid in a flask. After aweek, they analyzed the chemicals inthe flask and found several kinds of amino acids, sugars, and othersmall organic molecules, providingevidence that supported Oparin’shypothesis.

382 THE HISTORY OF LIFE

primordial fromthe Latin word pri-mordium, meaning“origin”; The ori-gin of life mayhave been in theprimordial soup.

High voltagesource

Condenserfor cooling

Solution oforganic

compounds Boilingwater

Entry forhydrogen,methane,

and ammoniagases

Electrode

Figure 14.12Miller and Urey’sexperiments showedthat under the pro-posed conditions onearly Earth, smallorganic molecules,such as amino acids,could form.

The formation of protocellsThe next step in the origin of life, as

proposed by some scientists, was theformation of complex organic com-pounds. In the 1950s, various experi-ments were performed and showedthat if the amino acids are heatedwithout oxygen, they link and formcomplex molecules called proteins. Asimilar process produces ATP andnucleic acids from small molecules.These experiments convinced manyscientists that complex organic mole-cules might have originated in poolsof water where small molecules hadconcentrated and been warmed.

How did these complex chemicalscombine to form the first cells? Thework of American biochemist SidneyFox in 1992 showed how the first cellsmay have occurred. As you can see inFigure 14.13, Fox produced proto-cells by heating solutions of aminoacids. A protocell is a large, orderedstructure, enclosed by a membrane,that carries out some life activities,such as growth and division.

Summarize the theories for how organic moleculeswere first formed on Earth.

The Evolution of CellsFossils indicate that by about

3.4 billion years ago, photosyntheticprokaryotic cells existed on Earth.But these were probably not the earli-est cells. What were the earliest cellslike, and how did they evolve?

The first true cellsThe first forms of life may have

been prokaryotic forms that evolvedfrom a protocell. Because Earth’satmosphere lacked oxygen, scientistshave proposed that these organismswere most likely anaerobic. For food,the first prokaryotes probably usedsome of the organic molecules thatwere abundant in Earth’s earlyoceans. Because they obtained foodrather than making it themselves,they would have been heterotrophs.

Over time, these heterotrophswould have used up the food supply.However, organisms that could makefood had probably evolved by thetime the food was gone. These firstautotrophs were probably similar to present-day archaebacteria.

14.2 THE ORIGIN OF LIFE 383

AA

Simple organic

molecules

Primordial soup

Aminoacid

Mixture ofamino acids

Short chainsof amino acids thatwill form protocells

Protocellsthat simulatecell division

AA

AA

AA

AA

AA

AA

AA

AAAA

AA

AAAA

AA AA

AA

AA

AA

Figure 14.13Sidney Fox showed how short chainsof amino acids could cluster to formprotocells.

Color-enhanced TEMMagnification:

7800�

Sidney Fox/Visuals Unlimited

Archaebacteria (ar kee bac TEER eeuh) are prokaryotic and live in harshenvironments, such as deep-sea ventsand hot springs like the one shown inFigure 14.14. Some early autotrophsmay have made glucose by chemosyn-thesis rather than by photosynthesis,which requires light-trapping pig-ments. These autotrophs released theenergy of inorganic compounds, suchas sulfur compounds, in their environ-ment to make their food.

Photosynthesizing prokaryotesEventually, photosynthesizing

prokaryotes capable of releasing oxy-gen from water evolve. Recall that theprocess of photosynthesis producesoxygen. As the first photosyntheticorganisms increased in number, the concentration of oxygen inEarth’s atmosphere began to increase.Organisms that could respire aerobi-cally would have evolved and thrived.In fact, the fossil record indicates that there was a large increase in thediversity of prokaryotic life about2.8 billion years ago.

The presence of oxygen in Earth’satmosphere probably affected life onEarth in another important way. Thesun’s rays would have converted muchof the oxygen into ozone moleculesthat would then have formed a layerthat contained more ozone than therest of the atmosphere. The ozonelayer, that now exists 10 to 15 miles(16–24 km) above Earth’s surface,probably shielded organisms from theharmful effects of ultraviolet radiationand enabled the evolution of morecomplex organisms, the eukaryotes.

The endosymbiont theoryComplex eukaryotic cells probably

evolved from prokaryotic cells. Use theProblem-Solving Lab on this page todetermine how long the event mighthave taken. The endosymbiont theory,

Figure 14.14Present-day archaebacteria live in places like this hot spring inYellowstone National Park. Infer What adaptations wouldan organism need to survive in this environment?

Interpret DataCan a clock model Earth’s history? As a result of studying fossils and analyzing geological events, scientists have been able to construct the geologic time scale, a timetable that shows the appearance of organisms during the history of Earth.

Solve the ProblemThe diagram shown here compresses the history of Earth intoa 12-hour clock face. On the clock, assume that the formationof Earth occurred at midnight. The oceans formed at 1:00 A.M.Use this information to help you answer the following questions.

Thinking CriticallyUse Models Based on fossil evidence, at what time on theface of the clock did prokaryotes evolve? At what time did the first eukaryotes appear?

Humansappear

Earth formsOceans form

Comstock/George Gerster

14.2 THE ORIGIN OF LIFE 385bdol.glencoe.com/self_check_quiz

Prokaryote

Aerobic bacteria Mitochondria Cyanobacteria Chloroplasts

Plantcell

Animal cell

proposed by American biologist LynnMargulis in the 1960s, explains howeukaryotic cells may have arisen.

The endosymbiont (en doh SIHM bee ont) theory as shown inFigure 14.15, proposes that eukaryotesevolved through a symbiotic relation-ship between ancient prokaryotes.Margulis based her hypothesis onobservations and experimental evi-dence of present-day unicellular organ-isms. For example, some bacteria thatare similar to cyanobacteria andchloroplasts resemble each other insize and in the ability to photosynthe-size. Likewise, mitochondria and somebacteria look similar. Experimental evi-dence revealed that both chloroplasts

and mitochondria contain DNA that issimilar to the DNA in prokaryotes andunlike the DNA in eukaryotic nuclei.

New evidence from scientificresearch supports this theory and hasshown that chloroplasts and mitochon-dria have their own ribosomes that aresimilar to the ribosomes in prokary-otes. In addition, both chloroplasts andmitochondria reproduce independ-ently of the cells that contain them.The fact that some modern prokary-otes live in close association witheukaryotes also supports the theory.

Figure 14.15The eukaryotic cells of plants and animalsprobably evolved by endosymbiosis.

A prokaryote ingested someaerobic bacteria. Theaerobes were protected andproduced energy for theprokaryote.

A Over a long time,the aerobes becomemitochondria, nolonger able to liveon their own.

B The cyanobacteriabecome chloroplasts,no longer able tolive on their own.

DSome primitiveprokaryotes also ingestedcyanobacteria, whichcontain photosyntheticpigments.

C

Understanding Main Ideas1. How did Pasteur’s experiment finally disprove

spontaneous generation?

2. Review Oparin’s hypothesis and explain how itwas tested experimentally.

3. Why do scientists think the first living cells toappear on Earth were probably anaerobic heterotrophs?

4. How would the increasing number of photosyn-thesizing organisms on Earth have affected bothEarth and its other organisms?

Thinking Critically5. Some scientists speculate that lightning was not

present on early Earth. How could you modify theMiller-Urey experiment to reflect this new idea?What energy source would you use to replacelightning?

6. Sequence Make a flowchart sequencing the evolution of life from protocells to eukaryotes. For more help, refer to Sequence in the SkillHandbook.

SKILL REVIEWSKILL REVIEW

Before You BeginTo date a rock using aradioactive isotope, thehalf-life of the radioactiveisotope and the isotopeformed when the radioac-tive isotope decays must beknown. Also, the amountof the radioactive isotopein the rock when it formedand the amount of radioac-tive isotope currently in therock must be measured. Forexample, the half-life of K-40 to decay to Ar-40 is1.3 billion years. Whenrocks form, they contain no Ar-40, so measuring theamounts of K-40 and Ar-40currently in a rock gives theinital amount of K-40 in therock. Then the age of therock can be calculated.

Determining a Rock’s Age

ProblemHow can you simulate radioactive half-life?

ObjectivesIn this BioLab you will:■ Formulate Models Simulate the radioactive decay of K-40

into Ar-40 with pennies.■ Collect Data Collect data to determine the amount of

K-40 present after several half-lives.■ Make and Use Graphs Graph your data and use its values

to determine the age of rocks.

Materialsshoe box with lid100 penniesgraph paper

Skill HandbookIf you need help with this lab, refer to the Skill Handbook.

1. Copy the data table.2. Place 100 pennies in a shoe box.3. Arrange the pennies so that their “head” sides are facing

up. Each “head” represent an atom of K-40, and each“tail” an atom of Ar-40.

4. Record the number of “heads” and “tails” present at thestart of the experiment. Use the row marked “0” in thedata table.

5. Cover the box. Then shake the box well. Let the shakerepresent one half-life of K-40, which is 1.3 billion years.

6. Remove the lid and record the number of “heads” yousee facing up. Remove all the “tail” pennies.

7. To complete the first trial, repeat steps 5 and 6 four moretimes.

8. Run two more trials and determine an average for thenumber of “heads” present at each half-life.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

(l)file photo, (r)Matt Meadows

Trial 1

0

1

2

3

4

5

Trial 2 Trial 3 Totals Average

Number of Heads (K-40 atoms left)

Data Table

Number of Shakes(half-lives)

14.2 THE ORIGIN OF LIFE 387

9. Draw a full-page graph. Plot your average values on thegraph. Plot the number of half-lives for K-40 on thex axis and the number of “heads” on the y axis. Connectthe points with a line. Remember, each half-life mark onthe graph axis for K-40 represents 1.3 billion years.

10. Return everything to its properplace for reuse. Wash hands thoroughly. CLEANUP AND DISPOSAL

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Apply Concepts What symbol represented an atom of K-40 in thisexperiment? What symbol represented an atom of Ar-40?

2. Think Critically Compare the numbers of protons and neutrons of K-40 and Ar-40. (Consult the Periodic Table on page 1112 for help.) Can Ar-40 change back to K-40? Explain your answer, pointing out what procedural part of the experiment supports your answer.

3. Define Operationally Define the term half-life. What procedural part of the simulation represented a half-life period of time in the experiment?

4. Communicate Explain how scientists use radioactive dating to approximate a rock’s age.

5. Make and Use Graphs You are attemptingto determine the age of a rock sample. Useyour graph to read the rock’s age if it has:a. 70% of its original K-40 amount.b. 35% of its original K-40 amount.c. 10% of its original K-40 amount.

6. Could the size of the boxand how vigorously the box was shaken introduce errors into the data? Explain.

ERROR ANALYSIS

Graph Suppose you had calculated the same data for an element with a half-life of 5000 years rather than 1.3 billion years.Plot a graph for the hypothetical isotope.How do the graphs compare?

Web Links To find out more about radioactive dating, visitbdol.glencoe.com/radioactive_dating

How life originated on Earth is a fascinatingand challenging question. Many have pro-

posed answers, but the mystery remains unsolved.Because it is impossible to travel in time, the ques-tion of how life originated on Earth might neverbe answered. However, a number of beliefs andhypotheses exist. Some of these are describedbelow.

Divine origins Common to human culturesthroughout history is the belief that life on Earthdid not arise spontaneously. Many of the world’smajor religions teach that life was created onEarth by a supreme being. The followers of thesereligions believe that life could only have arisenthrough the direct action of a divine force.

A variation of this belief is that organisms aretoo complex to have developed only by evolu-tion. Instead, some people believe that the com-plex structures and processes of life could nothave formed without some guiding intelligence.

Meteorites One scientific hypothesis about theorigin of life on Earth is that the molecules nec-essary for life arrived here on meteorites, rocksfrom space that collide with Earth’s surface.Many meteorites contain some organic matter.These organic molecules, which are necessaryfor the formation of cells, might have arrived onEarth and entered its oceans.

Primordial soup Another hypothesis was pro-posed by A. I. Oparin. It states that Earth’sancient atmosphere contained the gases nitro-gen, methane, and ammonia, but no free oxygen.Energy from the sun, volcanoes, and lightningcaused chemical reactions among these gases,which eventually combined into small organicmolecules such as amino acids. Rain trapped andthen carried these molecules into the oceans,making a primordial soup of organic molecules.In this soup, proteins, lipids, and the other com-plex organic molecules found in present-daycells formed. Harold Urey and Stanley Miller

provided the first experimental evidence to sup-port this idea. They produced organic moleculesin the laboratory by creating a spark in a gasmixture similar to Earth’s early atmosphere.

An RNA world Some scientists hypothesizethat the formation of self-replicating moleculespreceded the formation of cells. Today’s self-replicating molecules, DNA and RNA, provideclues about the earliest self-replicating mole-cules. Scientists hypothesize that RNA, which iscentral to the functioning of a cell, probably pre-dated DNA on Earth. However, because RNA isa more complex molecule than protein, it is noteasy to obtain data that supports the idea thatRNA was formed on early Earth.

The Origin of Life

388 THE HISTORY OF LIFE

Stanley Miller

Review, analyze, and critique the different ideasabout the origin of life presented here. Considerstrengths and weaknesses during your review.

To find out more about the origin of life, visit bdol.glencoe.com/biology_society

Roger Ressmeyer/Starlight

Section 14.1Key Concepts■ Fossils provide a record of life on Earth.

Fossils come in many forms, such as a leafimprint, a worm burrow, or a bone.

■ By studying fossils, scientists learn aboutthe diversity of life and about the behaviorof ancient organisms.

■ Fossils can provide information on ancientenvironments. For example, fossils can helpto predict whether an area had been a riverenvironment, terrestrial environment, or a marine environment. In addition, fossilsmay provide information on ancient climates.

■ Earth’s history is divided into the geologictime scale, based on evidence in rocks andfossils.

■ The four major divisions in the geologictime scale are the Precambrian, PaleozoicEra, Mesozoic Era, and Cenozoic Era. Theeras are further divided into periods.

Vocabularyfossil (p. 370)plate tectonics (p. 379)The Record

of Life

Vocabularyarchaebacteria (p. 384)biogenesis (p. 381)protocell (p. 383)spontaneous

generation (p. 380)

The Origin of Life

STUDY GUIDESTUDY GUIDE

CHAPTER 14 ASSESSMENT 389

Key Concepts■ Francesco Redi and Louis Pasteur

designed controlled experiments to dis-prove spontaneous generation. Theirexperiments and others like them con-vinced scientists to accept biogenesis.

■ Small organic molecules might haveformed from substances present in Earth’searly atmosphere and oceans. Smallorganic molecules can form complexorganic molecules.

■ The earliest organisms were probably an-aerobic, heterotrophic prokaryotes. Overtime, chemosynthetic prokaryotes evolvedand then photosynthetic prokaryotes thatproduced oxygen evolved, changing theatmosphere and triggering the evolution ofaerobic cells and eukaryotes.

Section 14.2

To help you review thegeologic time scale, use the Organiza-tional Study Fold on page 369.

bdol.glencoe.com/vocabulary_puzzlemakerJeff J. Daly/Visuals Unlimited

390 CHAPTER 14 ASSESSMENT

Review the Chapter 14 vocabulary words listed inthe Study Guide on page 389. Match the wordswith the definitions below.

1. prokaryotes that live in harsh environments 2. the idea that nonliving material can produce life 3. evidence of an organism that lived long ago 4. the idea that living organisms come only

from other living organisms

5. About how many years ago do scientists sug-gest that Earth cooled enough for watervapor to condense?A. 20 million years C. 4.4 billion yearsB. 4.6 billion years D. 5.5 billion years

6. Most fossils occur in layers of ________ rocks.A. sedimentary C. igneousB. metamorphic D. volcanic

7. Who was the scientist who showed thatmicroscopic life is not produced by spontaneous generation?A. Francesco RediB. Stanley MillerC. Louis PasteurD. Harold Urey

8. Scientists theorize that oxygen buildup inthe atmosphere resulted from ________.A. respirationB. photosynthesisC. chemosynthesisD. rock weathering

9. An entire, intact organism may be preservedin ________ and ________.A. casts—trace fossilsB. molds—castsC. trace fossils—petrified fossilsD. amber—ice

10. Open Ended Explain why there might besimilar fossils on the east coast of SouthAmerica and the west coast of Africa.

11. Open Ended Why do scientists proposethat the 3.4 billion-year-old fossils ofcyanobacteria-like prokaryotic cells found inAustralia were not the first species to haveevolved on Earth?

12. Open Ended Explain how fossils might helppaleontologists to learn about the importantbehaviors of different types of animals.Which social behaviors might they provideinformation about?

13. Interpret Scientific Illustrations Use the illustration above to explain the endo-symbiont hypothesis.

14. Recent scien-tific evidence from fossils indicates thatfeathered dinosaurs may have been a directancestor of birds. Visit to investigate these finds. How do such findsimpact our understanding of evolution?Present your findings to the class in a posteror other visual format.

15. Infer How might the way organisms obtainenergy have evolved over time?

16. Writing About Biology Why is knowledgeof geology important to paleontologists?

17. Writing About Biology Explain whyFrancesco Redi’s experiment with flies did notcompletely disprove spontaneous generation.

REAL WORLD BIOCHALLENGE

Early bacteria

Primitiveprokaryote

Aerobicbacteria

Ancestraleukaryote

bdol.glencoe.com

bdol.glencoe.com/chapter_test

CHAPTER 14 ASSESSMENT 391

Multiple ChoiceUse the graph to answer questions 18 and 19.

18. Which group of organisms had the shortesthistory?A. OrthidaB. RhynchonellidaC. TerebratulidaD. Pentamerida

19. Which group of organisms evolved first?A. OrthidaB. RhynchonellidaC. TerebratulidaD. Pentamerida

20. Which of the following rock types wouldmost likely contain fossils?A. sedimentary rock composed of limestoneB. igneous rock ejected from a volcanoC. metamorphic rockD. hardened lava

Study the graph and answer questions 21–24.

21. How long does it take for half of the element to decay?A. 1 billion yearsB. 2 billion yearsC. 3 billion yearsD. 4 billion years

22. How much of the original material is leftafter 4 billion years?A. 50%B. 25%C. 12.5%D. less than 10%

23. This element would best be used to date fossils that are ________ years old.A. a few thousandB. less than a millionC. a few millionD. a billion

Original amount ofradioactive material

After 4 billion years

After 3 billion years

After 2 billion years

Amount remainingafter 1 billion years

Decay Rate of a Radioactive Element

100%

Cenozoic

Mesozoic

Paleozoic

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

DevonianSilurian

OrdovicianCambrian

1. Orthida

2. Strophomena

3. Pentamerida

4. Rhynchonellida

5. Spiriferida

6. Terebratulida

1 2 3 4 5 6

Ranges of Brachiopod (Lamp Shell) Orders

bdol.glencoe.com/standardized_test

Constructed Response/Grid InRecord your answers on your answer document.

24. Open Ended The element in the graph above would best be used to date rocks from what era?Explain why.

25. Open Ended What kinds of clues can fossils provide about the past, including climate, whatorganisms ate, and the environment in which they lived?