43 



IV. THE NATURAL EVIDENCE 



In chapter III we have seen how the rock record provides infor- 

 mation about life on the early Earth. And we have seen that this 

 record leaves us a gap in time from about 3.8 b.y. ago to the time of 

 Earth's origin at 4.5 to 4.6 b.y. ago. We have also discussed in chap- 

 ter I the essential elements of life. But what were the conditions 

 under which life originated? What was the early Earth like? To 

 understand this we seek additional knowledge: we must know how 

 the solar system formed and how the Earth formed from it. In addi- 

 tion, we must know the chemical behavior of carbon and its com- 

 pounds in the prebiological environments. Only then will we have the 

 knowledge necessary to bridge the gap and understand how life came 

 to be. 



THE BIRTH OF THE SOLAR SYSTEM 



Before there were any stars there was only gas, and this gas was 

 essentially just a mixture of hydrogen and helium. But by the time 

 the Sun appeared, several previous generations of stars had added 

 other heavier elements to this interstellar gas as a result of the syn- 

 thesis of elements in stellar interiors, and in the catastrophic explo- 

 sions of supernovae. We find evidence for this change in composition 

 by examining the various types of stars in the galaxy. The oldest stars 

 we find today (having formed at an earlier epoch) contain a smaller 

 amount of heavy elements than does the Sun. The age of the galaxy 

 is estimated to be close to 12 b.y., while the oldest stars with heavy- 

 element concentrations similar to the Sun's have ages of only 6 to 

 7 b.y. The Sun itself is about 4.6 b.y. old. 



Today we can examine environments where stars form and then 

 try to reconstruct the conditions that preceded the appearance of the 

 Sun and its retinue of planets. We find these environments among the 



The solar magnetic field (white spiral lines) as it might have appeared 4.5 b.y. 

 ago when created by the high spin rate of the Sun. This would have produced 

 melting of rocks in the Asteroid Belt and elsewhere in the Solar System. 



