IX 



of the most ancient rocks known, and before, if we can. The winds 

 and the waters, the volcanic fires and the slumping sands, the lava 

 pours and the rolling stones — those were not much different from 

 today. But in fact we do not even know whether human beings — 

 time travelers could have breathed that otherwise commonplace 

 breeze. Was it oxygenated? Could the old algal mats already use 

 oxygen and sunlight to build their substance in the open air? Or had 

 they not yet made that invention, so that they subsisted in a very 

 different atmosphere from ours, perhaps not yet even making good 

 use of solar photons? We cannot be sure. We know that many bac- 

 teria today are fit for vigorous life oxygen-free. We know, moreover, 

 that all the oxygen of today's atmosphere is turned over very rapidly 

 by the green-plant world. But just when that ability first arose has 

 not yet been fixed. It is in fact the molecular facts we still seek, with 

 much difficulty, there in the most ancient rocks. For it is on that 

 level that the mechanisms of life must have begun, about 4 b.y. back. 

 But the rocks are steadily reworked, buried, heated, and reheated in 

 the Earth's fiery mantle. We have to look hard indeed to find rocks 

 older than Isua in Greenland, which may — or well may not — bear 

 the crucial evidence we seek. Certainly, that search will go on, and 

 with sharper analyses we will find more clues in the ancient, dis- 

 turbed, and enigmatic samples. 



THE DEPTH OF SPACE 



Perhaps, we can work our way forward in time to life, forward 

 from the date when the Sun and its planets were somehow con- 

 densed jointly out of the interstellar gases. We know that date rather 

 well. The surface of the Moon, a large collection of meteorites that 

 have been trapped from orbit by their chance encounters with Earth, 

 and the strongly supported inferences from the dating of long series 

 of Earth rocks, all lead to the conclusion that our planet was 

 assembled by a complex set of processes just about 4.5 (±0.1) b.y. 

 ago. In the time between 4.5 and 3.8 b.y., we know Earth became 

 the round sphere it is, the rocks of the crust grew solid and were well 

 sorted, and the processes of ordinary geology approached the famil- 

 iar. This stormy period, with an epidemic of heavy impacts on plane- 

 tary surfaces, is under detailed study which includes the hints given 



