140 INTRODUCTION TO EVOLUTION 



carbon 14 date of 9,883 years, indicating that these early Americans lived 

 about 10,000 years ago (Sellards, 1952). 



Other physicochemical methods will doubtless be developed to supple- 

 ment the carbon 14 method. One such method already being employed de- 

 pends upon the rate at which fluorine becomes incorporated into bones 

 during fossilization. Although variables in the process have yet to be ex- 

 plored, the test gives promise of usefulness. Utilization of this method con- 

 tributed significantly to proof that the "fossils" called "Piltdown man" 

 were a hoax (Weiner, Oakley, and LeGros Clark, 1953). 



Visualizing Geologic Time 



Unavoidably our ideas of time are conditioned by the length of the hu- 

 man life span and its subdivision into periods (infancy, youth, etc.) and 

 years. The term "one million years" is so far outside our experience as to 

 be meaningless to us. Multiples of a million years are, if anything, even 

 less meaningful. We may have the vague impression that a million years 

 is "a very long time," and that a thousand million years is "a very, very 

 long time." But in other connections a thousand years also seems "a very 

 long time." Indeed, all periods longer than a human lifetime or two have a 

 tendency to fade into "a-very-long-time" vagueness for us. 



But we can grasp the meaning of the length of a year and of its subdivi- 

 sions into months, weeks, days, hours, minutes, and seconds. Consequently 

 James C. Rettie (1950) rendered a signal service by picturing geologic time 

 in subdivisions of a year. He imagined a moving picture taken of earth by 

 inhabitants of another planet, using a super-telephoto lens and a time- 

 lapse camera. This imaginary film was taken at the rate of one picture per 

 year for the last 757 million years. When it is run in a projector at normal 

 speed (twenty-four pictures per second), twenty-four years of earth his- 

 tory flashes by each second. Since the author has the film run continuously 

 twenty-four hours a day, about two miflion years of past history are shown 

 on the screen each day. To show the entire 757 million years requires run- 

 ning the film continuously for one full year. The author starts the show at 

 midnight of one New Year's Eve and runs it without interruption until mid- 

 night of the next New Year's Eve. 



For many fascinating details of this movie readers are referred to the 

 original article or to the reprint of it in Coronet magazine (March, 1951 ). 

 We have space for but a few high spots. 



Throughout January, February, and March the movie runs on without 

 showing any signs of life upon the earth. Single-celled organisms appear 



