10 



that we find copiously in space. The familiar liquid water is not 

 known to us with certainty anywhere in the universe save on Earth. 

 We suspect it was in the bodies where the carbonaceous meteorites 

 formed, probably some class of asteroids. We might have seen signs 

 of it in the margins of the Martian polar cap; maybe it is present 

 underground on Mars. The point, of course, is that liquid water is a 

 fleeting substance; it can persist only within a limited range of tem- 

 perature at reasonably high pressure. Such a regime is from a cosmic 

 viewpoint intermediate in temperature; 10 or 20 times above the 

 temperature of the cold gases of space, but 10 or 20 times below the 

 temperature of a star surface. To realize liquid water, the pressure 

 must stay rather high, compatible almost certainly only with the 

 surface gravity of a modest, cool, planet-sized body. Thus, liquid 

 water and life as well seem to be phenomena of high density. The 

 near-vacuum of space cannot keep liquid water, and the atomic 

 collisions which allow sequential reactions there are haltingly infre- 

 quent, even though quite numerous within the clouds in space. So 

 familiar an "organic" compound as ethyl alcohol is well detected in 

 such interstellar clouds. The total amount present there is huge; a 

 single cloud contains more alcohol than all life on Earth has made 

 over all its history, but it remains more dilute than a laboratory 

 vacuum. It is so dilute there that any buildup to truly complex 

 molecules is painfully slow. 



Water, the medium of life, dominates life today. Ninety percent 

 of evolutionary time had passed before life could emerge from water 

 (or perhaps take water along) to populate the land. Until that epoch, 

 life was to be found only below the water's surface, or near that 

 boundary, or, at the driest surface locked within damp enclosing soil. 

 Land life now has elaborate and specialized devices to avoid dryness. 

 It remains true that the biopolymers themselves depend on water-like 

 chemical bonds for their very existence. The hydrogen bonds in 

 which a proton forms a positive electrostatic link between two nega- 

 tively charged electron clouds, is the chemical bond of intermediate 

 stability that lends the big polymers their subtle mix of stable and 

 labile properties. If it were not water within which life grew it must 

 all the same have been some other hydrogen-rich medium. 



These logical inferences confirm our present findings: Life on 

 Earth must have begun in or near water. That much seems sure. 



