138 INTRODUCTION TO EVOLUTION 



mined by occurrence of major geologic revolutions, such as extensive 

 mountain formation. 



Most of the eras are subdivided into periods (second column), separated 

 from one another by geologic phenomena of less magnitude than those 

 which mark the ends of eras. The oldest period within an era is placed at 

 the bottom of the sequence of periods comprising the era. 



Dating the Past 



Table 7.1 contains no statement of lengths of time involved. A recent 

 estimate of the length of time which has elapsed between the present and 

 the beginning of each of the last three eras is as follows (Kulp, 1961 ) : 



Beginning of Cenozoic Era 63 million years ago 



Beginning of Mesozoic Era 230 million years ago 



Beginning of Paleozoic Era 600 million years ago 



We must recognize that these figures are rough approximations only. 

 The difficulties of estimating geologic time in terms of years are so great 

 that wide differences of opinion are inevitable. Yet within the last few 

 years more accurate and quantitative methods than any previously available 

 have been developed. These methods depend upon "clocks" which the 

 rocks themselves contain in the form of radioactive elements (isotopes). 

 One of the most important, and the first to be utilized, is uranium. At a 

 constant rate, uranium emits helium and is transformed into lead. The age 

 of some rocks can be determined by comparison of the proportions of un- 

 decayed uranium and of lead present in the rock. The method is applicable 

 only to rocks which contain uranium, of course; unfortunately the sedi- 

 mentary rocks richest in fossils usually do not contain this element. Ura- 

 nium tests indicate that the earth may be as old as 4500 million years 

 (Patterson, Tilton, and Inghram, 1955). The oldest known fossils are those 

 of algae found in rocks some 2600 million years old (Holmes, 1954). 

 Hence for nearly two billion years of its existence the earth was probably 

 not populated with living organisms. Recently other physicochemical meth- 

 ods of dating fossils and archeological discoveries have developed so rapidly 

 as to give great promise for the future of accurate dating of prehistoric 

 life. Of these the most accurate method depends upon the determination 

 of the rate at which radioactive "heavy carbon" atoms in a given sample 

 of material are losing their radioactivity. Heavy carbon has an atomic 

 weight of 14, instead of the "normal" 12. Atoms of carbon 14 are formed 

 in the earth's upper atmosphere by the action of cosmic rays on nitrogen 



