Chapter 22: The Precambrian Earth - Sierra Vista Unified ... · PDF filedescribe what happened to the col- ... For about the first 4 billion years of Earth’s 4.6-billion-year existence,

To find out more aboutEarth’s earliest history, visitthe Glencoe Science Website at science.glencoe.com

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What You’ll Learn• How the age of Earth is

determined.

• How the continents,atmosphere, and oceansformed.

• When life first appearedon Earth.

• What kinds of organ-isms populated thePrecambrian Earth.

Why It’s importantMost of Earth’s historyoccurred during thePrecambrian. During thistime, the crust, atmosphereand oceans formed and lifefirst appeared. Early life-forms produced oxygenthrough photosynthesis,and, thus, changed theatmosphere and the his-tory of life on Earth.

The PrecambrianEarth

ThePrecambrianEarth

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http://science.glencoe.com

22.1 The Early Ear th 577

Earth’s core, mantle, and crusthave different average densities. Thecore is the densest, and the crust is theleast dense. Scientists hypothesize thatwhen Earth formed, temperatureswere hot enough for the materials thatmake up Earth to act, in part, like aliquid and flow. In this activity, youwill model how liquids of differentdensities react when they are mixedtogether.

1. Fill a 250-mL beaker with 50 mL oftap water.

2. Add 2–3 drops of dark food coloring to the water.

3. Poor 175 mL of vegetable oil intothe beaker and stir the contents.

CAUTION: Always wearsafety goggles and an apron in the lab.

Observe In your science journal,describe what happened to the col-ored water and vegetable oil in thebeaker. Explain how this is similar to what happened to the core andmantle when Earth formed.

Density SeparationDiscovery LabDiscovery Lab

OBJECTIVES

• Describe the evidenceused to determine the ageof Earth.

• Understand why scien-tists theorize that theearly Earth was hot.

VOCABULARY

zirconasteroidmeteorite

For most of Earth’s history, there was nothing like the plants and ani-mals that exist today. In Chapter 21, you learned about the geologictime scale and how Earth’s history is divided into time periods. Inthis chapter, you will learn about the earliest part of the geologic timescale, the Precambrian.

EARTH’S “BIRTH”For about the first 4 billion years of Earth’s 4.6-billion-year existence,most of the life-forms that inhabited Earth were unicellular organ-isms. What evidence did these organisms leave of their existence?What was Earth like when these organisms lived? How did Earthchange during the Precambrian? How did these changes set the stagefor the animal life that exists today? Answers to these questions notonly help us to understand the history of Earth, but they also serveas a model for the search for life on other planets. In 1996, theannouncement that a meteorite from Mars might contain micro-scopic fossils of bacteria rekindled scientific interest in the search forlife elsewhere in the universe. You can read more about this meteoritein this chapter’s Science in the News feature. It may be possible to

The Early Earth22.122.1

identify clues to the possible existence of life on other planets,even if rocks from those planets are the only evidence we have.After all, rocks are all that we have left of the PrecambrianEarth, and as you will learn, there is evidence of life’s humblebeginnings on Earth in Precambrian rocks. The Precambrianportion of the geologic time scale is shown in Figure 22-1.

HOW OLD IS EARTH?We know that Earth must be at least as old as the oldest rocksin the crust. Radiometric dating has determined that the age ofthe oldest rocks on Earth is between 3.96 to 3.8 billion years.But the rocks that form Earth’s crust have been eroded overtime. Evidence of 4.1- to 4.2-billion-year-old crust exists in themineral zircon that is contained in metamorphosed sedimen-tary rocks in Australia. Zircon is a very stable mineral thatcommonly occurs in small amounts in granite. Radiometric

dating has determined that the zircon grains in these sedimentaryrocks are between 4.1- and 4.2 billion years old. The zircon existedbefore it became cemented into the sedimentary rocks, and scientiststheorize that the zircon is the eroded residue left behind from 4.1 to4.2-billion-year-old granitic crustal rocks. Based on this evidence,Earth must be at least 4.2 billion years old.

Meteorites, such as the one shown in Figure 22-2, have been radio-metrically dated at between 4.5 and 4.7 billion years old. Mostastronomers agree that the solar system, including Earth, formed atthe same time, and therefore, Earth and meteorites should be aboutthe same age. In addition, the oldest rock samples from the Moon,collected during the Apollo missions, are approximately 4.6 billionyears old. Thus, taking all of the evidence into consideration, scien-tists commonly agree that the age of Earth is 4.6 billion years.

EARTH’S HEAT SOURCESEarth was most likely extremely hot shortly after it formed, andthere were three likely sources of this heat. The first source wasradioactivity. Radioactive isotopes were more abundant during thepast because, over time, radioactive decay has reduced the originalamount of Earth’s radioactive isotopes. One product of radioactivedecay is energy, which generates heat. Much of Earth’s current inter-nal heat is attributed to the energy released by radioactivity. Becausethere were more radioactive isotopes in Earth’s distant past, scien-tists infer that more heat was being generated then and that Earthwas hotter.

The second source of Earth’s heat was the impact of asteroidsand meteorites. Asteroids are metallic or silica-rich objects that are

578 CHAPTER 22 The Precambrian Ear th

Cambrian

Proterozoic

Archean

Pre

cam

bri

an

540millionyears

4.6billionyears

2.5billionyears

Indicateshuge time

span

Figure 22-1 Most of Earth’shistory is contained withinthe 4 billion years thatmake up the Precambrian.

1 km to 950 km in diameter. Today,most asteroids orbit the Sun betweenthe orbit of Mars and Jupiter.Meteoroids are small asteroids orfragments of asteroids. When mete-oroids fall to Earth, we call themmeteorites. Evidence from the sur-faces of the Moon and other planetssuggests that there were many moremeteoroids and asteroids distributedthroughout the early solar systemthan there are today, and thereforecollisions were much more common.For this reason, scientists infer thatfor the first 500 to 700 million years of Earth’s history, bombard-ment by meteorites and asteroids was common. These impacts gen-erated a tremendous amount of thermal energy.

The third source of Earth’s heat was gravitational contraction. Asa result of meteor bombardment and the subsequent accumulationof meteorite material on Earth, the size of Earth increased. Theweight of the material caused gravitational contraction of the under-lying zones. The energy of the contraction was converted to thermalenergy. The new material also caused a blanketing effect, which pre-vented the newly generated heat from escaping.

The combined effects of radioactive decay, meteorite and asteroidbombardment, and gravitational contraction made a hot and ratherinhospitable beginning for Earth. However, cooling and subsequentcrystallization laid the foundation for Earth’s crust to form and pre-pared Earth for the next phase in its development.

22.1 The Early Ear th 579

1. What is the age of Earth?

2. Describe the evidence used to determinethe age of Earth.

3. How is zircon used to date igneous rocks?

4. Which of Earth’s early sources of heat arenot major contributors to Earth’s present-day internal heat?

5. Thinking Critically If most astronomershypothesize that the solar system formed

all at once, why is it important that weuse the age of the oldest rocks on Earthto determine the age of Earth rather thanusing only the age of meteorites?

SKILL REVIEW

6. Comparing and Contrasting Compare and contrast Earth’s three early sources of heat. For more help, refer to the Skill Handbook.

Figure 22-2 This 10 000year old, 16 ton meteorite,was found in Oregon onthe tribal lands of theWillamette NativeAmericans.

22.222.2


Formation of the Crust and Continents

Were continents always present on Earth’s surface? Early in the for-mation of Earth, the planet was molten, and numerous elements andminerals were mixed throughout the magma. Over time, the miner-als became concentrated in specific zones and Earth became layered.As the magma reached the surface and cooled, landmasses began toform.

FORMATION OF THE CRUSTWhen Earth formed, iron and nickel, which are dense elements, con-centrated in its core. Minerals with low densities tend to crystallizefrom magma at cooler temperatures than denser minerals do.Therefore, near the surface of Earth, where it is cooler, the rocks aregenerally composed of a high proportion of the less-dense minerals.For example, granite is common at Earth’s surface. Granite is mainlycomposed of feldspar, quartz, and mica, which, as you learned inChapter 4, are minerals with low densities. Lava flowing from the hotand partly molten interior of Earth concentrated the less-dense min-erals near the surface of Earth over time. In contrast, the denser min-erals, which crystallize at higher temperatures, concentrated deeper

within Earth and formed the rocks that makeup Earth’s mantle. The process by which aplanet becomes internally zoned when heavymaterials sink toward its center and lightermaterials accumulate near its surface is calleddifferentiation. The differentiated zones ofEarth are illustrated in Figure 22-3.

Geologists hypothesize that Earth’s earliestcrust formed as a result of the cooling of theuppermost mantle. Thus, the crust likely con-sisted of iron and magnesium-rich mineralssimilar to those found in basalt. As these min-erals weathered, they formed sediments thatcovered the early crust. Geologists also hypoth-esize that as sediment-covered slabs of the crustwere recycled into the mantle at subductionzones, the slabs partly melted and generatedmagmas with different mineral compositions.These magmas crystallized to form the firstgranitic continental crust, which was rich infeldspar, quartz, and mica. The formation of

Crust

Uppermantle

Lowermantle

Outercore

Innercore

Figure 22-3 Earth’s layers formed as a resultof differentiation. The density of the miner-als found within each layer decreases towardthe crust.

OBJECTIVES

• Explain the origin ofEarth’s crust.

• Describe the formationof the Archean andProterozoic continents.

VOCABULARY

differentiationPrecambrian shieldCanadian ShieldmicrocontinentLaurentia

22.2 Formation of the Crust and Continents 581

Figure 22-4 The differencein density between theheavier ocean crust and thelighter continental crustallows the continental crustto float higher on the man-tle, even when its thicknessis greater.

Lithosphere

Continental crust

Asthenosphere

Upper mantle

Mantle

Oceanic crust

granitic crust was a slow process. Most geologists hypothesize that the formation of the majority of crustal rocks that make up the low-density, granitic cores of continents was completed by about 2.5 billionyears ago. The rocks of the earliest crust no longer exist because theywere recycled in subduction zones long ago.

Less-dense material has a tendency to float on more-dense mater-ial. In the Discovery Lab at the beginning of this chapter, you observedthat oil floats on top of water. This happens because oil is less densethan water. For this same reason, continental crust “floats” on top ofthe mantle below it. In addition, basaltic crust is more dense thangranitic crust, and therefore, it does not float as high on the mantle asgranitic, continental crust does. This difference in density is whatcauses the basaltic ocean floor to be lower in elevation than the less-dense granitic continental crust, as illustrated in Figure 22-4.

THE CORES OF THE CONTINENTSToday’s continents each contain a core of Archean and Proterozoicrock called a Precambrian shield. In some areas, the Precambrianshields are exposed at the surface, whereas in other areas, younger sed-imentary rocks bury them. The buried and exposed parts of a shieldtogether compose the craton, which is the stable part of a continent. InNorth America, the Precambrian shield is called the Canadian Shieldbecause much of it is exposed in Canada. As shown in Figure 22-5 onpage 582, the Canadian Shield is also exposed in the northern parts ofMinnesota, Wisconsin, and Michigan; in the Adirondack Mountainsof New York; and over a large part of Greenland.

GROWTH OF CONTINENTSEarly during the Proterozoic, small pieces ofcontinental crust, called microcontinents,that formed during the Archean began tocollide as a result of plate tectonics. Theimpact of the collisions jammed the micro-continents together, and they became largercontinents. At each of these collision sites,the Archean microcontinents were suturedor fused together at orogens. These orogensare belts of rocks that were deformed by theimmense energy of the colliding continents.The resulting mountain ranges have beendeeply eroded since that time. By about 1.8billion years ago, the core of modern-dayNorth America had been assembled; itformed the ancient continent known asLaurentia, as shown in Figure 22-6. Youwill explore the technique of interpretingcontinental growth in the Mapping GeoLabat the end of this chapter.


Canadian shield-exposedand covered rocks

Belts of folded strata

Stable platforms

The Precambrian Shield in North America

1.0 – 1.3 billion1.8 – 2.0 billion1.6 – 1.8 billion>2.5 billionMidcontinent rift

Present-day Greenland

Outline of present-dayNorth America

Age

Laurentia

Figure 22-5 The oldestrocks in North America arefound in the Precambrianshield rocks in Canada. Theywere the first-formed rocksof the North American continent.

Figure 22-6 From the beginning of its forma-tion, the continent of Laurentia resembled thefamiliar shape of present-day North America.

Near the end of the Early Proterozoic, between1.8 and 1.6 billion years ago, volcanic island arcs collided with the southern margin of Laurentia.This added more than 1000 km of continental crust to southern Laurentia. The final phase ofProterozoic growth of Laurentia is called theGrenville Orogeny. Recall that an orogeny is amountain-building event. The Grenville Orogenyoccurred between 1.2 billion and 900 million yearsago and added a considerable amount of continen-tal crust to the southern and eastern margins ofLaurentia. Also by the end of the Proterozoic, nearly75 percent of present-day North America hadformed. The remaining 25 percent, as you will learnin Chapter 24, was added to the eastern and westernmargins of the North American craton during thePhanerozoic.

By the end of the Proterozoic, all of the majormasses of continental lithosphere had formed onEarth. The lithospheric plates were moving around,periodically colliding with each other and suturingtogether. By the end of the Proterozoic, so many ofthese collisions had occurred that Rodinia, the firstsupercontinent, had formed. It was positioned sothat the equator ran through Laurentia, as shown inFigure 22-7. Rodinia began to break apart at the endof the Proterozoic and continued to do so during theEarly Phanerozoic. During this time, Earth alsoacquired an atmosphere and oceans. You will learnhow they formed in the next section.

22.2 Formation of the Crust and Continents 583

1. Describe the origin of Earth’s crust.

2. How did the Archean and Proterozoiccontinents form?

3. How does a planet become internallyzoned?

4. Thinking Critically The oceans are under-lain by basaltic crust, and the continentsare underlain by granitic crust. What

would Earth be like if all of its crust weremade of the same material?

SKILL REVIEW

5. Sequencing Suppose that you are areporter about to witness the formationof a large continent. In your science jour-nal, write your step-by-step eyewitnessaccount of the event. For more help,refer to the Skill Handbook.

Rodinia

NorthAmerica

Equator

750 million years ago

Figure 22-7 Rodinia was the first super-continent to form on Earth’s surface. Similarrock types in eastern North America andwestern Africa are evidence of its existence.

22.322.3 Formation of the Atmosphereand Oceans

If you could travel back in time to the Early Precambrian, whatwould you take with you? Probably the most important thing thatyou could take for your survival would be a supply of oxygen! This isbecause Earth’s early atmosphere was nothing like what it is today.The oxygen that early forms of algae produced through the processof photosynthesis affected the development of life on Earth in twovery important ways. First, it changed the composition of the atmo-sphere and thus made life possible for oxygen-breathing animals.Second, it produced the ozone layer that filters ultraviolet (UV) radi-ation. Scientists refer to these types of processes, which modify a sys-tem, as feedback.

THE PRECAMBRIAN ATMOSPHEREHydrogen and helium probably dominated Earth’s earliest atmo-sphere. However, because of their small masses, these gases could notremain near Earth for long. Earth’s gravity is not strong enough tokeep hydrogen and helium from escaping to space. However, gasesthat have greater masses, such as carbon dioxide and nitrogen, can-not escape Earth’s gravity. This is why Earth’s atmosphere is rich incarbon dioxide and nitrogen today.

There was considerable volcanic activity during the EarlyPrecambrian. In Chapter 18, you learned that lava is not the onlysubstance that erupts from volcanoes. Tremendous amounts ofgases are also vented during volcanic eruptions in a process calledoutgassing, shown in Figure 22-8. The most abundant gases ventedfrom volcanoes are water vapor (H2O), carbon dioxide (CO2),nitrogen (N2), and carbon monoxide (CO). Many geologistshypothesize that outgassing formed Earth’s early atmosphere. Thus,

the early atmosphere must have contained largeconcentrations of water vapor, carbon dioxide, andnitrogen. In addition, the early atmosphere mostlikely contained methane (CH4) and ammonia(NH3), both of which may have formed as a result ofchemical reactions among the volcanic gases. Theargon (Ar) that is present in today’s atmospherebegan to accumulate during the Early Precambrian.It forms when the radioactive isotope potassium-40(K-40) decays to argon-40 (Ar-40). Through thisreaction, the amount of argon has increased in theatmosphere at about the same rate that K-40 decays.

OBJECTIVES

• Describe the formationof Earth’s atmosphereand oceans.

• Identify the origin of oxygen in the atmosphere.

• Explain the evidencethat oxygen existed in the atmosphere duringthe Proterozoic.

VOCABULARY

cyanobacteriastromatolitebanded iron formationred bed

584

Figure 22-8 Steam and gasfrom Poas Volcano in PoasNational Park, Costa Rica,rise high above the vol-cano’s summit.

OXYGEN IN THE ATMOSPHERENo wonder life as we know it today didnot exist during the Precambrian—therewas no oxygen in the atmosphere tobreathe! Volcanoes do not commonly giveoff oxygen; so, scientists do not think thatthe oxygen in Earth’s atmosphere camefrom volcanoes. Where did the oxygencome from? The oldest known fossilswhich can help answer this question, arepreserved in rocks that are about 3.5 bil-lion years old. These fossils are the remains of tiny, threadlikechlorophyl bearing filaments of cyanobacteria. Such fossils arecontained in 3.46-billion-year-old rocks called the WarrawoonaGroup, from western Australia. Figure 22-9 compares fossilizedcyanobacteria and modern cyanobacteria. Like their present-daycounterparts, ancient cyanobacteria used photosynthesis to producethe nutrients they needed to survive. In the process of photosynthe-sis, solar energy is used to convert carbon dioxide and water intosugar. Oxygen is given off as a waste product.

Oxygen Producers Could microscopic cyanobacteria have pro-duced enough oxygen to change the composition of Earth’s earlyatmosphere? The abundance of cyanobacteria increased throughoutthe Archean until they became truly abundant during theProterozoic. Large mats and mounds of billions of cyanobacteria,called stromatolites, dominated the shallow oceans of the Protero-zoic. Modern stromatolites, such as those found in Australia todayand shown in Figure 22-10, do not differ much from their ancientcounterparts. The oldest stromatolites are preserved in 3.4-billion-year-old rocks from the Swaziland Supergroup of South Africa.

Evidence in the Rocks One way to test the hypothesis thatthere was oxygen in Earth’s early atmosphere is to look for oxidizediron, or iron oxides, in Archean and Proterozoic rocks. The iron in rocks

22.3 Formation of the Atmosphere and Oceans 585

B

Figure 22-9 The cells ofmodern cyanobacteria areoften identical in shape andsize to some fossil cyano-bacteria that are billions ofyears old. This micrograph isof the filamentouscyanobacterium, Oscillatoria(A). This fossil cyanobac-terium is from the GunflintChert, Ontario, Canada (B).

A

Figure 22-10 Colonies ofcyanobacteria form modernstromatolites in HamelinPool, Australia. Millions ofindividual cyanobacteriacells make up each colony(A). Fossil stromatolites arenearly identical to modernstromatolites. They provideevidence for the existenceof a shallow sea in theareas where they are found (B).

A B

EnvironmentalConnection

Magnification: 140�

Magnification: 780�

reacts with free oxygen in the atmosphere to form iron oxides. Ironoxides are identified by their red color and provide undeniable evidenceof free oxygen in the atmosphere. Some metamorphosed Archean sed-imentary rocks contain mineral grains that would have been oxidized ifthere had been oxygen in the atmosphere. However, these mineralswere not oxidized, which indicates that there was little or no free oxy-gen in the atmosphere throughout most of the Archean. However, thereis evidence that near the end of the Archean and by the beginning of theProterozoic, photosynthesizing stromatolites in shallow marine waterincreased oxygen levels in localized areas. These locally high concentra-tions of oxygen in otherwise oxygen-poor, shallow, ocean water allowedunique deposits to form. These deposits, which consist of alternatingbands of chert and iron oxides are called banded iron formations.Today, these formations are mined as a source of iron, as shown in Figure 22-11. The Problem-Solving Lab should give you an example


A

Figure 22-11 The beauti-ful rocks of the Banded IronFormation give testimonyto the oxygenated atmo-sphere of the Precambrian.This sample is from the 2.1billion year old NegauneeFormation, Michigan (A).The Empire Iron Mine inIshpeming, Michigan is wellknown for its iron produc-tion (B).

B

Calculate mining costs Precambrianrocks contain many important mineraldeposits, such as uranium, which is used innuclear reactors. In a uranium oxide (U3O8)ore deposit in southern Ontario, the ore-containing rocks are an average of 3 mthick over an area that is 750 m long and1500 m wide. Geochemical analysis of thedeposit indicates that there are, on aver-age, 0.9 kg of uranium oxide per metricton of rock. Additionally, 0.3 cubic metersof the uranium-bearing rock weighs oneton (2000 lbs).

Analysis1. How many pounds of ore does this

new deposit contain?2. It will cost $45 per cubic yard and 10

years to mine and extract the ore fromthe deposit. How much will this cost?

Thinking Critically3. The current market price is $9.25 per

pound of uranium oxide. Based onyour answer to question 2, can the ore be mined for a profit?

Profits from the Precambrian

of mining for a profit. Many sedimentaryrocks that are younger than 1.8 billion yearsare rusty red in color and are called red beds.The presence of red beds in rocks that areProterozoic and younger is strong evidencethat the atmosphere by this time containedfree oxygen. You will observe oxidation whenyou complete the MiniLab on this page.

IMPORTANCE OF OXYGENOxygen is important not only because mostanimals require it for respiration, but alsobecause it provides protection against UVradiation from the Sun. If you have everread a label on sunscreen, you know thatUV radiation can be harmful. Today, only asmall fraction of the UV radiation that theSun radiates toward Earth reaches its sur-face. This is because Earth is naturally pro-tected from this radiation by ozone that ispresent in the lower part of Earth’s upperatmosphere. An ozone molecule (O3) con-sists of three oxygen atoms bondedtogether. Ozone forms when high-energyUV radiation splits oxygen gas molecules(O2) and the single oxygen atoms combinewith other oxygen molecules. This ozonelayer filters out much of the Sun’s UV radi-ation. Oxygen in Earth’s atmosphere thatwas produced mainly through photosyn-thesis also contributes to the ozone layer.Early life, mainly the cyanobacteria thatmade up stromatolites, modified the atmo-sphere by generating large amounts of oxy-gen. Some of this oxygen also formedozone, which, in turn, filtered out UV radi-ation so that other forms of life could sur-vive on Earth’s surface. It appears thatnearly all the oxygen that we breathe today,and the oxygen that all animals havebreathed in the geologic past, was releasedinto the atmosphere by photosynthesis.

22.3 Formation of the Atmosphere and Oceans 587

Why are red beds red?

Model the formationof red beds with iron,oxygen, and water.

Procedure CAUTION: Steelwool can be sharp. Wear gloves in the lab.

1. Place 40 mL of white sand in a 150-mLbeaker.

2. Add water so that the total volume is 120 mL.

3. Add 15 mL of bleach.4. Place a piece of steel wool about the size

of your thumbnail, in the beaker. Coverthe beaker with a petri dish and allow itto stand in a quiet place for one day.

5. Remove the steel wool and stir the con-tents of the beaker. Allow the mixture tosettle for five minutes after stirring.

6. Slowly pour off the water so that the iron-oxide sediment is left behind.

7. Stir the mixture again, then spoon someof the sand onto a watch glass and allowit to dry.

Analyze and Conclude1. In your science journal, describe how the

color of the sediment changed.2. Where does the iron in the experiment

come from?3. Where in nature does the red in rocks

come from?4. What do you think is the function of

the bleach?

FORMATION OF THE OCEANSOceans are thought to have originated largely from the same processof outgassing that formed the atmosphere. A major component of thegas was water vapor. As the early atmosphere and the surface of Earthcooled, the water vapor condensed to form liquid water. You haveprobably observed the result of condensation on the sides of a coldglass of water. During the Archean, the entire atmosphere was richwith water vapor. When it began to cool, the result was a tremendousamount of rain, which slowly filled the low-lying, basalt-flooredbasins, thus forming the oceans. Rainwater dissolved the soluble min-erals exposed at Earth’s surface and just as they do today, rivers, runoffand groundwater transported these minerals to the oceans. These dis-solved minerals made the oceans of the Precambrian salty just as theymake the oceans salty today.

Another source of water may have played an important role inadding water vapor to Earth’s atmosphere. A recent but controversialhypothesis suggests that some of Earth’s water may have come fromouter space! Earth is constantly bombarded with very small cometsmade of frozen gas and water. Based on the current rate of micro-comet bombardment, some scientists calculate that a significant por-tion of Earth’s surface waters might be extraterrestrial in origin.

Oxygen Causes Change The Precambrian began with an oxygen-free atmosphere and simple life-forms. Cyanobacteria thenevolved, and their oxygen contribution caused the atmosphere tobecome filled with oxygen. This oxygen not only enabled new life-forms to evolve, but it also protected Earth’s surface from the Sun’sUV rays. Oceans formed from abundant water vapor in the atmo-sphere and possibly from outer space. Earth was then a hospitableplace for new life-forms to inhabit.


1. What are banded iron formations?

2. Describe the origin of the oxygen in theatmosphere.

3. Explain the relationship between red bedsand oxygen in the atmosphere.

4. Thinking Critically If cyanobacteria hadnot produced as much oxygen as they

did, how might life on Earth be differentfrom how it is today?

SKILL REVIEW

5. Comparing and Contrasting Compare andcontrast the formation of the atmosphereand the formation of the oceans. Formore help, refer to the Skill Handbook.

To find out more aboutozone in Earth’s atmo-sphere, visit the GlencoeScience Web site atscience.glencoe.com


OBJECTIVES

• Describe the experimen-tal evidence of how lifedeveloped on Earth.

• Distinguish betweenprokaryotes and eukaryotes.

• Identify when the first multicellular animals appeared in geologic time.

VOCABULARY

amino acidshydrothermal ventprokaryoteeukaryoteVarangian GlaciationEdiacara fauna

22.422.4 Early Life on Earth

Of all the questions that humans have ever asked, none fascinates usmore than those about the origin of life. “Where did life come from?”is a question that has been explored from many different perspec-tives. Today, we know that life comes from other life through repro-duction. But where did the first life come from? What does science tell us about the origin of life?

ORIGIN OF LIFE ON EARTHYou have learned that Earth is about 4.6 billion years old and thatfossil evidence indicates that life existed on Earth about 3.5 billionyears ago. Thus, life must have begun during Earth’s first 1.1 billionyears. Earth probably could not have supported life until about 3.9billion years ago because meteorites were constantly striking its sur-face. If life did begin during this time of meteorite bombardment, itis unlikely that it could have survived for long. This places the originof life somewhere between 3.9 and 3.5 billion years ago.

Experimental Evidence During the first half of the twentiethcentury, scientists hypothesized that the early atmosphere containedcarbon dioxide, nitrogen, water vapor, methane, and ammonia, butno free oxygen. They also theorized that numerous storms producedlightning and that the surface of Earth was relatively warm. Molecularbiologists in the 1920s also suggested that an atmospherecontaining abundant ammonia and methane but lackingfree oxygen would be an ideal setting for the “primordialsoup” in which life may have begun. A young graduatestudent named Stanley Miller, who was working with hisgraduate advisor, Nobel prize-winning chemist HaroldUrey in 1953, was aware of these hypotheses.

Miller and Urey decided to create their own primor-dial soup. They set up an apparatus, like that shown inFigure 22-12, that contained a chamber filled withhydrogen, methane, and ammonia to simulate the earlyatmosphere. This atmospheric chamber was connectedto a lower chamber that was designed to catch any par-ticles that condensed in the atmospheric chamber.

Figure 22-12 Dr. Stanley Miller isshown with a replica of the apparatusused to model Earth’s early atmospherein the Miller-Urey experiment.

22.4 Early Life on Ear th 589


Miller and Urey added sparks from tungsten electrodes to simulatelightning in the atmosphere. Only one week after the start of theexperiment, the lower chamber contained a murky, brown liquid—the primordial soup! The “soup” that formed in this experimentcontained organic molecules such as cyanide (CN), formaldehyde(H2CO), and four different amino acids. Amino acids are the build-ing blocks of proteins, and proteins are the basic substances fromwhich life is built.

Continued experiments showed that 13 of the 20 amino acidsknown to occur in living things could be formed using the Miller-Urey method. Further experiments demonstrated that heat, cyanide,and certain clay minerals could cause amino acids to join together in chains like proteins. Proteins provide structure for tissues andorgans, and are important agents in cell metabolism. Thus, the dis-covery that amino acids could be formed in this way was amazing.What Miller and Urey demonstrated, is that however life firstformed, the basic building blocks of life were most likely present onEarth during the Archean.

The Role of RNA Not much is known about the next step requiredfor the development of life. It is one thing for the proteins that arerequired for life to exist on the Archean Earth, but quite another fororganic life to actually exist. One essential characteristic of life is theability to reproduce. The nucleic acids RNA and DNA, shown inFigure 22-13, are the basic requirements for reproduction.

In modern organisms, DNA carries the instructions necessary forcells in all living things to function. Both RNA and DNA needenzymes to replicate and at least one of them is necessary for the syn-

thesis of enzymes. Recent experiments haveshown that types of RNA molecules,

called ribozymes, can act as anenzyme. These RNA ribozymes

can, therefore, replicate without

Using NumbersSome scientists arguethat the influx ofsmall comets into theatmosphere couldadd about one inchof water to Earth’ssurface every 20 000years. The averagedepth of Earth’soceans is 3795meters. If this influxhas been constant forthe past 4.6 billionyears, what should bethe minimum averagedepth of the oceans?How can you accountfor the difference inwater volume?

Figure 22-13 These are ribonucleicacid (RNA) polymerase molecules(A). This is a single strand ofdeoxyribonucleic acid (DNA) (B).

A

B

Magnification: 27 000�

Magnification: 10 800�

the aid of enzymes. This suggests that RNA molecules mayhave been the first replicating molecules on Earth. AnRNA-based world may have been intermediate between aninorganic world and the DNA-based organic world thatfollowed. The remaining mystery is figuring out how thefirst RNA molecule formed because RNA cannot be easilysynthesized under the conditions that likely existed at thesurface of the Archean Earth.

Hydrothermal Vents and the Beginnings of LifeThe Urey-Miller model places the origin of life in shallowsurface waters. But, life on Earth may have originated deepin the ocean, near active volcanic seafloor rifts. Ocean waterseeps through the cracks in the ocean floor and is heated bythe magma at the rifts. This heated water rises and isexpelled from the ocean floor at hot-water vents calledhydrothermal vents, as shown in Figure 22-14. All of the energy and nutrients necessary for the origin of life are present at thesedeep-sea hydrothermal vents. In fact, amino acids have been foundthere. This has led some scientists to hypothesize that during theArchean, near hydrothermal vents, amino acids joined together onthe surfaces of clay minerals to form proteins. Other scientists con-tend that this is not possible in such an environment. It is importantto note that life is not being synthesized at these vents today, onlybecause amino acids are quickly devoured by organisms that livenear the vents. But in the Archaean, no organisms existed to eat theamino acids being produced.

PROTEROZOIC LIFEAt the beginning of the Proterozoic, life-forms were still quite simple.The only evidence of life-forms that existed before the Proterozoic isthe fossilized remains of unicellular organisms called prokaryotes. Aprokaryote is an organism that is composed of a single cell, whichdoes not contain a nucleus and is the simplest kind of cell. Allprokaryotes, including the cyanobacteria that make up stromatolitesbelong to Kingdom Monera.

A eukaryote is an organism that is composed of cells that containa nucleus. One way to determine whether an organism is a prokary-ote or a eukaryote is by its size. As a general rule, eukaryotes are largerthan prokaryotes. This general observation is useful in determiningwhether a fossil was a prokaryote or a eukaryote because it is rare for a fossil to be preserved in enough detail to determine whether itscells had nuclei. The oldest known fossil eukaryotes occur in a 2.1-billion-year-old banded iron formation in northern Michigan.


Update For an on-lineupdate on recent discov-eries of life-forms athydrothermal vents, visitthe Glencoe Science WebSite at science.glencoe.comand select the appropriatechapter.

Figure 22-14 These sulfurencrusted, underwatergeothermal vents bubble asvolcanic gasses escape. Theyare located off Dobu Island,Papua, New Guinea.


There is growing evidence that a widespread glaciation, whichoccurred between 800 and 700 million years ago, played a criticalrole in the extinction of many members of a group of possibleeukaryotes, the acritarchs. This glaciation event, called theVarangian Glaciation, was so widespread that some geologistsliken Earth at that time to a giant snowball. Evidence from ancientglacial deposits suggests that glacial ice advanced nearly to the equa-tor. Shortly after the ice retreated toward the poles, 700 million yearsago, multicellular organisms first appeared in the fossil record.

EDIACARA FOSSILSIn 1947, the impressions of soft-bodied organisms were discoveredin Late Proterozoic rocks in the Ediacara Hills of southern Australia.These fossils are collectively referred to as the Ediacara fauna.Figure 22-15 shows an interpretation of the Ediacaran world. Therehas been much debate in the scientific community about the precisenature of these remarkable fossils. It is generally agreed that thesefossils represent animals that were composed of different types ofeukaryotic cells. Scientists are unsure, however, whether the Ediacarafauna are relatives of modern animal groups or whether they werecompletely different types of organisms.

The discovery of the Ediacara fauna at first seemed to solve oneof the great mysteries in geology—why were there no fossils of theancestors of the complex and diverse organisms that existed duringthe Cambrian Period. The Ediacara fauna seem to provide fossil


Figure 22-15 The Ediacarafauna contained a widevariety of organisms. Itincluded floating organismsas well as those which wereattached to the sea floorand possibly some organ-isms that were activelymobile.

evidence of an ancestral stock of complex Proterozoic animals.Indeed, many of the Ediacara fossils look quite similar in overallbody shape to jellyfish, sea pens, segmented worms, arthropods,and echinoderms—just the kind of ancestral stock that geologistshad been hoping to find.

Some scientists, however, hypothesize that the Ediacara faunadoes not represent an ancestral stock of any modern group. Thesescientists consider the similarity in shape to animals in other phylacoincidental and that the Ediacara fauna represents a virtual deadend. None of the Ediacara fossils shows any evidence of a mouth,anus, or gut, and there is little evidence these animals could move.Arthropods, for example, leave tracks and trails when they moveacross the seafloor, but there is no evidence of such trace fossils asso-ciated with the Ediacara fossils. This has led some geologists tohypothesize that the Ediacara organisms were relatively immobileand that they fed by passively absorbing nutrients from seawater.These geologists point out that in the absence of any animal preda-tors, there would have been no disadvantage to being a defenselesscreature basking in the warm seawater and absorbing nutrients.

In recent years, geologists have found Ediacara fossils in all partsof the world. This suggests that these organisms were widely distrib-uted throughout the shallow oceans of the Late Proterozoic. Theyseemed to have flourished between 670 and 570 million years ago.Then, in an apparent mass extinction, they disappeared before organ-isms that are undisputedly related to modern phyla took over theoceans of the world.


1. Explain why the Miller-Urey experimentwas important.

2. What kind of organisms do the earliestfossils represent?

3. What is the significance of the VarangianGlaciation?

4. Discuss the differences between prokary-otes and eukaryotes.

5. Thinking Critically Describe how early life might have changed if some of theEdiacaran fauna had been able to moveand if predators had been present in their environment.

SKILL REVIEW

6. Concept Mapping Rearrange the followingevents into an events chain that describesthe results of the Miller-Urey experiment.For more help, refer to the Skill Handbook.

chamber with cyanide,formaldehyde, and

amino acids

chains of amino acids

heat and clay minerals added

simulated atmospheric lightning

chamber with hydrogen,methane, and ammonia

To find out more about theEdiacara fauna, visit theGlencoe Science Web Siteat science.glencoe.com


ProblemHow can the distribution of the ages ofrocks plotted on a map be used to inter-pret the growth of a continent?

Materialspaper colored pencilspencil metric ruler


Mapping ContinentalGrowth

P lotting the distribution of the ages of rocks onto a maphelps geologists to reconstruct the history of continental

accretion. During the Precambrian, microcontinents andisland arcs collided to form what would become the moderncontinents.

1. Use the ruler to measure the east-to-west width of Province A. Convertthe map scale to ground distance byusing the scale 1 cm = 1 km.

2. Why do some of your classmates havedifferent answers? Who is right?

3. Where is the oldest province locatedrelative to all the other provinces?

1. Your teacher will set up locationswith a rock sample at each location.

2. Make an outline map of your class-room similar to the map on the nextpage, using the scale 1 cm = 100 km.

3. Visit each location where a rock sample has been set out. Plot eachlocation and record the age of eachrock on the map.

4. After you have recorded all the loca-tions, use a pencil to draw lines on the map that separate rocks of

different ages. Be careful not to sim-ply connect the dots.

5. Use a different colored pencil toshade in the areas on the map thatcontain rocks of the same age. Theseare your geologic age provinces.

6. Make a key for your map by drawinga small rectangle for each differentgeologic age province. Name the oldest province “Province A,” the next oldest “Province B,” and so onfor all provinces.

Preparation

Procedure

Analyze

Conclude & Apply

1 23

45

6

7

10

8

11

9

18

23 22

1920

21

1514

13

1716

100 km

N

Mapping GeoLab 595

1. Based on the distribution of the geo-logic age provinces, describe the sequence of collisional events thatformed the craton represented byyour map.

2. According to your map, where wouldyou find metamorphic rocks? Whattype of metamorphism would haveoccurred?

3. If your map represents an area com-posed of Precambrian-aged rocks,

would the mountains that formedfrom collisions still be high andrugged? Explain.

4. Compare the distribution of ageprovinces on your map with Figure22-6. What are the similarities?

5. Based on what you learned in thisactivity, describe the formation ofthe North American Craton.

Locality Data

What other kinds of geologic features canbe mistaken for fossils? Visit the GlencoeScience Web Site at science.glencoe.com ora library to find more examples of pseudo-fossils. Choose one and, in your sciencejournal, describe how it forms.

Activity

actually lived on the bottom of an ancient sea.Second, mapping showed that Eozoon canadenseoccurred around igneous intrusions. The heatfrom the cooling magma, it seemed, had createdthe appearance of fossils. Finally, nearly identicalfeatures were found in limestone blocks ejectedby a volcano, again showing that heat, not life,had formed these features.

What are they? What about the so-called fossils in the Martian

meteorite? Many scientists now do not think thatthey are fossils. One reason is their size. Their vol-ume is 2000 times smaller than the smallestknown living things—parasitic organisms that livein other cells. Many scientists think that somethingso tiny probably could not contain enough geneticinformation to direct life processes. The searchgoes on for extraterrestrial life.


ALH 84001 is a potato-shaped rock thatweighs 1.9 kg. It was found in the Allan Hills IceField, Antarctica, in 1984. The meteorite formedabout 4.5 billion years ago, when Mars did. The“fossils,” shown above, are found in cracks inthe rock and are more than one million timessmaller than a typical bacterial cell and are1/100th the diameter of human hair.

Mystery on EarthA similar situation took place in the 1860s,

when features that were assumed to be fossilswere found in limestone in Canada. The featureswere millimeter-thick bands of dark minerals sepa-rated into blobs and layers by light minerals. This“fossil” was named Eozoon canadense, the “dawnanimal of Canada.” This discovery created quite a stir, because Eozoon canadense came from thePrecambrian, in some of the oldest rocks in NorthAmerica. Scientists hailed it as “the greatest dis-covery in geology for half a century.”

Not everyone was so taken with Eozooncanadense, however. Within months, the firstdoubts that it was truly a fossil were published.

Gradually, the weight of evidence turnedagainst the possibility that Eozoon canadense hadan organic origin. There were three main lines ofevidence. First, it was found that the original layer-ing of the rock cut across the “fossil,” rather thanbeing parallel to it as it should if the animal had

Martians or Meteorites?Some of the rarest meteorites found on Earth come from Mars.Recently, one of these, ALH 84001, caused quite a stir in the scientificcommunity when some scientists claimed to have found fossils in it.Many fossils, such as dinosaur bones and impressions of fern fronds,are obviously the remains of once-living things. Less-obvious evidenceof life are the microscopic spheres and rods found in ALH 84001.


Summary

Vocabularyasteroid (p. 578)meteorite (p. 579)zircon (p. 578)

VocabularyCanadian Shield

(p. 581)differentiation

(p. 580)Laurentia (p. 582)microcontinents

(p. 582)Precambrian shield

(p. 581)

Vocabularybanded iron

formation (p. 586)

cyanobacteria (p. 585)

red bed (p. 587)stromatolite (p. 585)

Vocabularyamino acids (p. 590)Ediacara fauna

(p. 592)eukaryote (p. 591)hydrothermal vent

(p. 591)prokaryote (p. 591)Varangian Glaciation

(p. 592)

Main Ideas• Geologists have used radiometric dating to show that Earth

must be at least 4.2 billion years old.• Because the solar system formed all at the same time, Moon

rocks and meteorites that are approximately 4.6 billion years oldsuggest that Earth is 4.6 billion years old too.

• The early Earth was a very hot place because of abundant radioactive isotopes, bombardment by meteorites, and gravitational contraction.

Main Ideas• Earth’s early crust formed by the cooling of the uppermost

mantle. This early crust weathered and formed sediments.• Sediment-covered slabs of this early crust were subducted and

generated magmas that contained granitic minerals.• During the Archean, microcontinents collided with one another

throughout the Proterozoic and formed the cores of the conti-nents. By the end of the Proterozoic, the first supercontinent,Rodinia, had formed.

Main Ideas• Earth’s early atmosphere and the oceans formed mainly by the

process of outgassing.• Nearly all of the oxygen in the atmosphere is a result of

photosynthesis.• Certain minerals oxidize, or rust, in the presence of free oxygen.

Proterozoic red beds are sedimentary rock deposits that containoxidized iron. They are the evidence that there was free oxygenin the atmosphere during the Proterozoic.

Main Ideas• All the ingredients were present on the early Earth to form pro-

teins, the building blocks of life. Amino acids, the molecules thatmake up proteins, were likely abundant on the surface of theearly Earth.

• Prokaryotic cells are generallly small and contain no nuclei.Eukaryotic cells contain nuclei and are generally larger and morecomplex than prokaryotic cells.

• The first evidence of multicellular animals are fossils of 2.1 bil-lion year old eukaryotic algae.

SECTION 22.1

The Early Earth

SECTION 22.2

Formation of theCrust andContinents

SECTION 22.3

Formation of theAtmosphere andOceans

SECTION 22.4

Early Life onEarth

Study Guide 597


1. What is the commonly accepted age of Earth?a. 4.6 million years c. 4.6 billion yearsb. 46 million years d. 46 billion years

2. Which of the following was not a source of heatfor the early Earth?a. meteor bombardmentb. gravitational contractionc. radioactivityd. hydrothermal energy

3. What are small asteroids called?a. comets c. cratonsb. meteoroids d. microcontinents

4. What is the process by which a planet becomesinternally zoned when heavy materials sink toward its center and lighter materials accumulate near its surface?a. photosynthesis c. accretionb. dewatering d. differentiation

5. Where is most of the North AmericanPrecambrian shield exposed at the surface?a. Canada c. Wisconsinb. Minnesota d. Michigan

6. What mineral can be used to radiometrically dateEarth’s age?a. zircon c. hematiteb. quartz d. feldspar

7. Refer to Figure 22-6. What name is given to the core of the modern-day North Americancontinent that formed in the Proterozoic?a. Baltica c. Grenvilleb. Yavapai d. Laurentia

8. What is the name of the first supercontinent,which formed near the end of the Proterozoic?a. Laurentia c. Rodiniab. Grenville d. Pangaea

Understanding Main Ideas 9. What volcanic process most likely formed Earth’s atmosphere?a. differentiation c. crystallizationb. outgassing d. photosynthesis

10. Why is ozone a necessary component of Earth’satmosphere?

11. Why is Earth’s atmosphere rich in nitrogen (N)and carbon dioxide (CO2) today?

12. Rearrange the following phrases to create a cycle map that describes the formation of Earth’s early crust.

Applying Main Ideas

KEEP A CLEAR MIND When you take atest, each new question should be a clean slate.Once you have read a question, considered theanswers, and chosen one, put that questionbehind you. Don’t let one or two troublesomequestions distract you while you’re working onother questions.

Test-Taking Tip

magma with new chemical

composition formed

sediments drawn down into subduction zones

partial melting of sediments

and subducted slab

granitic crust formed as

magma crystallized

subductioncontinued forming

more magma

upper mantle cooled and

crystallized

crust weathered

sediments formed

Assessment 599

1. Which of the following is NOT a likely sourceof the Precambrian Earth’s heat?a. radioactivityb. asteroid impactc. increased solar activityd. gravitational contraction

2. What does orogeny refer to?a. the drifting of microcontinentsb. the building of mountain rangesc. the formation of volcanic islandsd. the breaking apart of the supercontinents

3. Which of the following was NOT a source of information about the early presence ofoxygen on Earth?a. red beds c. stromatolitesb. banded iron d. meteorites

formations

13. Explain how geologists have determined the ageof Earth.

14. Discuss the relationships among the formation ofthe continents, the atmosphere, and the oceans.

15. What is the geologic significance of banded iron formations?

16. What geologic evidence suggests that free oxy-gen was accumulating in Earth’s atmosphere during the Proterozoic?

17. What is the difference between prokaryotes andeukaryotes? Which appeared first in the fossilrecord?

18. What characteristics of continental crust allow it to“float” higher on the mantle than oceanic crust?

19. Why are orogens deformed?

20. What is the significance of the Ediacara fauna?

21. Discuss the evidence that suggests that mostmembers of the Ediacara fauna were immobile.

22. Explain how the production of oxygen throughphotosynthesis by cyanobacteria affected thecomposition of the atmosphere and the develop-ment of other organisms.

23. A rock sample from Mars is reported to containfossil evidence of life. What kind of fossil wouldyou expect it to be? Explain your answer.

24. Where in North America would you look if youwanted to find evidence of Archean life? Explainyour answer.

25. When making a map of geologic age provinces,as you did in the Mapping GeoLab in this chapter,why did you draw the lines between the datapoints instead of connecting them?

26. How might Earth’s surface be different if watervapor had not been a product of outgassing?

Thinking Critically

Test Practice

INTERPRETING SCIENTIFIC ILLUSTRATIONSUse the diagrams to answer questions 4 and 5.

4. How do members of Group A differ frommembers of Group B?a. They belong to the Kingdom Plantae.b. They can be found in Proterozoic fossils.c. They contain no nuclei.d. They are all unicellular.

5. Where did members of Group B probablyoriginate?a. glaciers c. Australian faunab. hydrothermal vents d. oil deposits

Group A Group B

Documents

Chapter 22: The Precambrian Earth - Sierra Vista Unified ... · PDF filedescribe what happened to the col- ... For about the first 4 billion years of Earth’s 4.6-billion-year existence,