EVOLUTION OF LIFE ON THE EARTH 7 



We are thus induced to explore the approximate age of the 

 earliest living things. Light has been thrown on this ex- 

 tremely difficult problem by the science of geology through 

 studies of rocks and petrifications. 



About 40 different ways have been suggested for estimat- 

 ing the age of the earth. Earlier estimates were based on 

 the rate of the loss of heat of the sun and the earth, on the 

 rate of erosion of land, on the rate of formation of sediments 

 in the ocean, also on the amount of salt in the ocean, as- 

 suming that the ocean was originally fresh and gradually 

 became salty from the rivers that flowed into it. After 

 numerous surveys this method led to the assumption that 

 the age of the oceans of the earth is about 100 million 

 years, but this figure is much too low as compared with the 

 results of other calculations. In recent studies, A. C. 

 Spencer and K. J. Murata of the United States Geological 

 Survey found that some of the salt carried into the oceans is 

 again removed by clay and held as a deposit on the sea floors. 

 After correcting the old figures for this removal of salt, the 

 age of the ocean is reestimated at 500 to 700 million years. 



In order to determine the approximate age of the earth, which 

 is of course considerably older, we use the modern and more 

 accurate methods based on studies of radioactivity. Ura- 

 nium, a radioactive metal and the heaviest chemical element, 

 disintegrates at a slow, constant rate; in one year one gram 



of uranium produces gram of lead. The age 



1 1,250,000,000 



of certain rocks can be calculated from the ratio of lead to 

 uranium in them as determined by chemical analysis. 

 These results can be checked by several independent meth- 

 ods all of which are based on transformations of one chem- 

 ical element into another by means of radioactivity. The 

 results of these different methods check as near as can be 

 expected. The final result is an estimate of about 2 billion 

 years for the earth's age. 



