12 The History of Life

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FIGURE 12.4 Radiometric Dating Using Carbon-14Radiometric dating uses the natural decay rate of unstable isotopes tocalculate the age of a fossil. One-quarter of the original 14 C remains. Radioactive decay of 14 C is shown inFIGURE 12.4. Carbon-14 dating can be used to date objects only up to about45,000 years old. If the objects are older than that, the fraction of 14 C will betoo small to accurately measure. Older objects can be dated using isotopeswith longer half-lives.Determining Earth’s AgeScientists have used radiometric dating to determine the age of Earth.Because Earth constantly undergoes erosion and rock recycling, rocks onEarth do not remain in their original state. Unlike Earth, meteorites—whichare mostly pieces of rock and iron that have fallen to Earth’s surface fromspace—do not get recycled or undergo erosion. Meteorites are thought tohave formed at about the same time as Earth. Therefore, meteorites providean unspoiled sample for radiometric dating. Uranium-to-lead isotope ratiosin many meteorite samples consistently estimate Earth’s age at about4.5 billion years.Summarize Why are meteorites helpful for determining the age of Earth?12.1 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What types of evidence of ancientlife can be preserved as fossils?2. Why is a uranium isotope oftenused rather than 14 C in radiometricdating to determine the ageof Earth?CRITICAL THINKING3. Apply Considering that millions ofspecies have lived on Earth, why arethere relatively few fossils?4. Contrast Explain the differencebetween relative dating andabsolute dating.Connecting CONCEPTS5. Earth Science When mountainsform, the order of rocklayers can be disturbed. Howcould radiometric dating beused to sort out the relativeages of such rock layers?Chapter 12: The History of Life 363
CHAPTER 12MATERIALS• 10 pennies• graph paper• 3 pencils of differentcolorsPROCESS SKILLS• Analyzing• Interpreting Data• ModelingINVESTIGATIONRadioactive DecayIn this lab, you will model how scientistsdetermine the age of a fossil. Whereas thescientist pictured is measuring the decay ofcarbon from a sample of bone, you will modelthe decay of a fictitious radioactive isotopecalled “tailsium.”PROBLEM How is the half-life of an unstableisotope used to determine the age of a material?PROCEDURE1. Arrange your pennies so that they are all tailsside-up.These pennies represent 10 atoms of“tailsium,” a radioactive isotope.2. Pick up all 10 tailsium atoms and drop them on the table. The pennies that fallheads-side-up represent atoms of a more stable element, “headsium.” Put theheadsium atoms off to the side. Count the number of tailsium atoms. Record thatvalue in a table under toss 1.3. Pick up only the tailsium atoms and drop them again. Put the newly formedheadsium atoms off to the side with the other headsium atoms. Count thenumber of tailsium atoms and record this value in your table.4. Repeat step 3 until there is no more tailsium, or until you have run 10 tosses.5. Share your results with your class. Using the class data, determine the averagenumber of tailsium atoms that remained after each toss.ANALYZE AND CONCLUDE1. Graph Data Graph your group results along with the average for the class on thesame sheet of graph paper. The number of tosses should be on the x-axis, andthe number of tailsium atoms remaining should be on the y-axis.2. Calculate Using the class average, calculate an average half-life for tailsium. This isthe number of tosses it took for half of the pennies to “decay” to headsium.3. Analyze Below is the ideal rate of decay for tailsium. Graph these data on thesame axes as your other graphs, using a different colored pencil to differentiatebetween the data sets. Which data set was closer to the ideal rate of decay, yourgroup’s data or the class average? Explain.TABLE 1. IDEAL RATE OF TAILSIUM DECAYTime Start 1 2 3 4 5 6 7 8 9 10# tailsiumatoms 10 5 2.5 1.25 0.625 0.313 0.156 0.078 0.039 0.019 04. Apply Assume that there are 20 years between tosses. According to your datafrom the penny lab, how old would a material be that had 3 tailsium atoms and 7headsium atoms? How old would it be according to the class data?364 Unit 4: Evolution
Page 1 and 2: CHAPTER12The Historyof LifeKEY CONC
Page 3 and 4: 12.1 The Fossil RecordKEY CONCEPT F
Page 5: Connecting CONCEPTSChemistry of Lif
Page 11: 12.3Origin of LifeKEY CONCEPT The o
Page 14 and 15: FIGURE 12.11 RNA AND DNARNA, in the
Page 16 and 17: MAIN IDEAEukaryotic cells may have
Page 18 and 19: DATA ANALYSISGeneration Times of Ba
Page 20 and 21: The middle of the Paleozoic era was
Page 22 and 23: 12.6Primate EvolutionKEY CONCEPT Hu
Page 24 and 25: Walking UprightMany hypotheses have
Page 26 and 27: FIGURE 12.20 Examples of Hominid Sk
Page 28 and 29: INVESTIGATIONUnderstanding Geologic
Page 30 and 31: Chapter AssessmentChapter Vocabular
Page 32 and 33: For more test practice,go to ClassZ
Page 34 and 35: Staph InfectionsThese athletes were

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