80 



the meteorites originated or the organic matter synthesized were the 

 same as those on the primitive Earth. However, the complex suite of 

 organic molecules found in carbonaceous meteorites provides a new 

 basis for our understanding the phenomenon of chemical evolution. 



As mentioned in chapter IV, the widespread occurrence of 

 organic compounds — not produced by any life form — in the 

 cosmos and within our solar system confirms the expectation, based 

 on elemental abundances in the stars, that organic chemical evolution 

 is one natural consequence of the evolution of matter in the universe. 

 But organic chemical evolution is inextricably intertwined with the 

 evolution of environments, be they interstellar clouds, meteorite 

 parent bodies or planets; its progress toward the origin of life may be 

 terminated at different stages depending on the physical and chemi- 

 cal constraints imposed by the environment. 



As noted in previous chapters, the Earth's atmosphere, oceans, 

 and crust during the first 500 million years are difficult to define in 

 any detail. Too little is known to fix the actual time of accretion, the 

 heterogeneity of the accreting materials, the state of the Sun, and the 

 quantity of short-lived radioisotopes that could have influenced the 

 thermal structure of the early Earth. 



In many respects our knowledge of the early Earth is much like 

 our knowledge of the early solar system. It is model-dependent and 

 relies on the reconstruction of an environment by extrapolation from 

 a record preserved in (but deciphered only in fragmentary fashion 

 from) lunar rocks, meteorites, and remotely discernible features of 

 Venus and Mars and of very ancient rocks and sediments of the 

 Earth. As more of the record is unveiled, new evidence is revealed, 

 new interpretations arise, and models undergo revision. Thus, all that 

 can be done at present is to define a rather wide range of possible 

 compositions for the early atmosphere and oceans and to suggest the 

 implications of atmospheres and oceans within this range for the 

 problem of the origin of life. These models have provided the basis 

 for experimental simulation studies. 



THE EXPERIMENTAL ERA 



Laboratory efforts have been extensive and have provided much 

 of the insight we have gained into the origins of life on Earth. 



