Reading Essentials for Biology Glencoe (794133), страница 37
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SC.D.1.4.4 The student knows that Earth’s systems andorganisms are the result of a long, continuous change over time. Also covers SC.D.1.4.2This section discusses how Earth itself contains a record of life. The evidence is found in rocks. Inthe space below, describe what evidence you think the rocks provide about life.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnEarly History of EarthCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What was early Earth like?Some scientists think that early Earth was very hot.
Earth’s surface may have been hit hard and often by meteorites. These activities would have heated the surface. The inside of Earth was alsounder a lot of pressure. It became so hot that volcanoes violentlygave off lava and hot gases that relieved the pressure. The gaseshelped to form the atmosphere, but it was not an atmosphere youcould breathe. This early atmosphere did not have oxygen.About 4.5 billion years ago, Earth cooled enough for the watervapor in the atmosphere to condense. This probably led to millions of years of thunderstorms.
Enough water fell to createoceans. Some scientists think that some time after this, between3.9 and 3.5 billion years ago, life formed in the oceans.Make Flash Cards Makingflash cards is a good way tolearn. As you read the material, think of questions ateacher might ask on a test.Write each question on oneside of a flash card. Thenwrite the answer to the question on the other side. Use theflash cards to quiz yourselfuntil you know the answers.History in RocksCan scientists be sure how Earth formed? No, they cannot.Because rocks are constantly destroyed and new rocks formed,scientists cannot study any rocks that were created in Earth’s earliest history.
The oldest rocks found on Earth are about 3.9 billionyears old. But rocks are still an important source of informationabout the many forms of life that existed on this planet.1. Why can scientists notbe sure how Earth wasformed?________________________________________________________________________READING ESSENTIALSChapter 14151NameDateClassSection14.1The Record of Life, continuedWhat do fossils tell us?2. Infer How might fossilteeth reveal an animal’sdiet?________________________________________________________________________Some rocks contain fossils.
A fossil is evidence of an organismthat lived long ago and that became preserved in Earth’s rocks.From fossils, scientists have discovered that the millions of plantsand animals living today are just a small part of all the species ofplants and animals that have ever lived. In fact, scientists thinkthat about 95 percent of all the species of plants and animals thatever lived on Earth are now extinct.What do paleontologists study?How do fossils form?Often, fossils formed when plants or animals died in mud, sand,or clay. They became covered with more mud, sand, or clay. Overtime, the area where the living thing died became compressed orpacked down.
It hardened into a type of rock called sedimentary(sed uh MENT uh ree) rock. In a similar way, fossils are formingtoday at the bottoms of lakes, streams, and oceans. Fossils are alsofound frozen in ice or hardened into a tree sap called amber. Mostfossils, though, are found in sedimentary rock.How are fossils dated?When scientists discover a fossil, they want to know how old itis. There are several ways to figure out the age of fossils. One is amethod called relative dating. Sedimentary rocks form in layers,with new or younger layers added on top of older layers. The figure on page 153 shows an example of sedimentary layers. Fossilsin lower layers are thought to be older than fossils in higher layers.152Chapter 14READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Paleontologists (pay lee ahn TAHL uh justs) are scientists whostudy ancient life.
They are a lot like detectives. They use fossilsas clues to things that happened long ago. A fossil can show thekinds of organisms that lived long ago. Fossils can even showsomething about the behavior of ancient animals. For example,fossil bones and teeth provide clues about the size of an animal,how it moved around, and what it ate.Paleontologists also learn about ancient climate and geography.When they find a fossil of an ancient plant that looks like a plantwe see today, they might conclude that the ancient plant neededthe same type of climate as today’s plant.
If they find fossils of seacreatures in an area that is currently a desert, they might realizethat the desert used to be an ocean.NameDateClassSection14.1The Record of Life, continuedThis does not tell scientists the exact age ofthe fossil. However, it does tell scientiststhe fossil’s age in relation to other fossilsabove it or below it.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are radiometric datingtechniques?To find the specific age of a fossil orrock, scientists use radiometric dating techniques. These methods measure the rate ofdecay, or breakdown, of radioactive isotopeswithin the rocks.
Sedimentary rocks cannotbe dated this way. For that reason, scientistsuse other kinds of rocks that are near thesedimentary rocks.In an earlier chapter you learned that a radioactive isotope isan atom with an unstable nucleus. The nucleus breaks down, ordecays, over time, giving off radiation. When the nucleus is completely decayed, a new radioactive isotope has formed. Scientistsuse the decay rate like a clock. Scientists try to find out the age ofa rock by comparing the amount of radioactive isotope it containsand the amount of new isotope it forms after decaying.
Let’s saythere is a rock that contains a radioactive isotope that decays tohalf its original amount in one million years. That means that thehalf-life of that particular isotope is one million years. If the rockhas equal amounts of the original isotope and the new isotope, itmust be one million years old. The two most common isotopesthat scientists use to date fossils and rocks are potassium-40 andcarbon-14. Both are radioactive isotopes.3.
Explain relative dating offossils.________________________________________________________________________4. List the two most common radioactive isotopesused by scientists to daterocks and fossils.A Trip Through Geologic Time________________________By looking at fossils and by figuring out the age of rocks,scientists have come up with something like a calendar of Earth’shistory. This calendar is called the geologic time scale. The scaleis divided into four sections called eras. Each era represents avery long period of time.
The four eras are:1. Precambrian (pree KAM bree un)________________________2. Paleozoic (pay lee uh ZOH ihk)3. Mesozoic (me zuh ZOH ihk)4. Cenozoic (se nuh ZOH ihk).READING ESSENTIALSChapter 14153NameDateClassSection14.1The Record of Life, continued5. What is a mass extinction?________________________________________________________________________Each era is subdivided into periods. Scientists selected the divisions based on information from fossils.
Each division of the timescale is based on the kinds of organisms that lived during thattime. The fossil record also shows several times when a massextinction took place. A mass extinction is an event that happenswhen many organisms disappear from the fossil record at thesame time.
The figure below shows the four eras of Earth’s geologic time scale.Precambrian EraPaleozoic EraAmphibiansReptilesJellyfishesSponges4.6 billionyears ago245 millionyears agoFerns544 millionyears agoMesozoic EraDinosaursFishes245 millionyears agoCenozoic EraFloweringplantsBirdsSmallmammalsHumansLarge mammals66 millionyears agopresentIt is hard to understand the huge amounts of time that havepassed since the early history of Earth. If we compare the geologic time scale to an imaginary calendar, it will be easier.What life existed during the Precambrian era?In your imaginary calendar, January 1 is the day Earth formed.The earliest fossils have been found in Precambrian rocks.
Precambrian is toward the end of March on our calendar. Scientistsfound these fossils in the deserts of western Australia. They havefound similar fossils on other continents. The fossils look like a154Chapter 14READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.One-celledlife formsNameDateClassSection14.1The Record of Life, continuedmodern species of photosynthetic cyanobacteria, which you willlearn about later. These early fossils show that during the Precambrian era, living things were mostly one-celled organisms. Bythe end of the Precambrian era, however, multi-celled organismssuch as sponges and jellyfishes were present.
The Precambrian eramakes up about 87 percent of Earth’s history, which is the middleof October.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What life appeared during the Paleozoic era?During the next era, the Paleozoic era, the number of differentkinds of organisms increased greatly. In fact, there was such avariety of living things that the Paleozoic era has been dividedinto six different periods.During one of the periods, the Cambrian, so many new typesof living things appeared that paleontologists often refer to an“explosion.” During this time, many different types of animalssuch as worms, sea stars, and arthropods lived in the oceans.
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