DURATION OF THE PRE-CAMBRIAN ERA 39 



THE LONG EARLY HISTORY OF THE EARTH 



As its most general result, absolute dating has made us aware of 

 the extremely long early history of the earth, that part of its history 

 which has been only very meagerly documented. Only when life on 

 earth had developed forms with hard parts, such as skeletons, did 

 abundant fossilisation become possible. This began, more or less 

 simultaneously, in a number of different zoological phylae at the 

 beginning of the Cambrian system. This is the first system of the 

 Paleozoic era, and every geologist working with fossils feels that 

 there is a clean break in the history of the earth at that time. There 

 is the later history of the earth, from the Cambrian system onwards, 

 in which one can nicely date findings with fossils, and then there is 

 that vague, earlier period, almost without fossils, the pre-Cambrian. 



This break, occurring at the base of the Cambrian system, has 

 now been dated at about 600 million years ago. But the oldest 

 crustal rock so far for which a radioactive date has been established 

 is 3,300 million years old. 



So all the eras of the Paleozoic, the Mesozoic and the Cenozoic 

 together, with their 600 million years' time-span, take up less than 

 one fifth of the time during which our earth had already developed 

 an outer crust more or less similar to the crust we live on now. 



It is at some time during this very early period of the earth's 

 history that the origin of life took place. 



Tables II and III give a recent geological time-scale (Kulp, 1960) 

 in which this difference between 'normal' and earlier geologic history 

 is well expressed. The first table gives the ages and the duration of 

 the eras and systems since the beginning of the Paleozoic, the time 

 during which fossils were more or less abundant. The second table 

 includes all earlier datings and consequently immediately focusses 

 attention on the long duration of earlier geologic history and on the 

 long duration of the pre-Cambrian. 



Figs. 35 and 36 also clearly show this difference. Fig. 35 schemat- 

 ically shows the development of life on earth over the last 600 mil- 

 lions of years. Fig. 36 presents more schematically the distinction 

 between the time of the origin of life on earth, a couple of billion 

 years ago, and its later development since the beginning of the 

 Cambrian system and the Paleozoic era. 



