EARLY HISTORY OF LIFE 



29 



cathode). A solution containing ions will 

 conduct an electric current and is called an 

 electrolyte. Atoms will unite to form mole- 

 cules depending on the number of electrons 

 they can deliver or consume in their ionic 

 state. For example, the hydrogen atom has 

 one electron; when it loses this it becomes 

 a hydrogen ion, a naked proton. The oxy- 

 gen atom lacks two electrons in its outer 

 shell; when these are gained it becomes an 

 oxygen ion, an anion (Fig. 2-6). When the 

 hydrogen and oxygen ions are present in 

 the same system, they are brought together 

 because "unlike charges attract"; since oxy- 

 gen requires two electrons to complete its 

 outer ring, two hydrogen atoms are re- 

 quired to do the job. The result is a mole- 

 cule of water, a stable compound, essential 

 in all living things. Likewise, thousands of 

 other molecules are formed from atoms, 

 many of which play important roles in bio- 

 logical systems. 



Water forms only a very few hydrogen 

 and hydroxyl ions (H+ and OH"), that 

 is, only a very small proportion of the total 

 number of molecules break up into these 

 ions. Since water is the main constituent of 

 living material it might be expected that 

 any increase or decrease in these ions would 

 be detrimental to life. When acids break 

 up into ions, large numbers of hydrogen 

 ions are produced. That is why they are 

 called acids. Likewise, bases produce large 

 numbers of hydroxyl ions. Because they do 

 produce such large numbers of these ions, 

 neither strong acids nor bases are tolerated 

 by living things. 



Of all the atoms in living things, carbon 

 certainly is the most important. This may 

 be due in part to its physical make-up. It 

 possesses just one half the maximum num- 

 ber of electrons in its outer shell, which 

 means that it does not lose or gain others. 

 It unites with a large variety of other atoms 

 by simply sharing its electrons. This ar- 

 rangement permits combination with other 

 carbon atoms to produce long chains or 

 rings which may then join up with a large 



variety of other atoms to produce immense 

 molecules. It was undoubtedly this nature 

 of carbon that made it the central atom 

 around which life was built. We find it 

 permeating all biological systems and play- 

 ing important roles, not only in the con- 

 struction of living materials, but also in 

 storing and releasing energy which is 

 essential in life processes. 



From the foregoing account we see that 

 life is a complex system of protons, neu- 

 trons, and electrons, combined into atoms 

 and molecules, all committed by natural 

 laws to follow specific patterns of behavior. 

 Out of this have come all of the living 

 things on the earth today, from amoeba to 

 man. Our next step would logically be to 

 find, if possible, when, where, and how the 

 first living thing appeared on the earth. 



THE FIRST LIVING THINGS 



With the physical world settled down to 

 a relative stable condition the stage was set 

 for the beginning of this most remarkable 

 drama, the inception and subsequent un- 

 folding of life in all of its variety and com- 

 plexity. The initial steps were almost im- 

 perceptible, extending over hundreds of 

 millions of years, but once underway, life 

 gathered momentimi, spreading out in all 

 directions over the extremely thin outer 

 shell of the earth. This reirion, which ex- 

 tends only a few feet above and fewer feet 

 below the crust itself, has become thor- 

 oughly inhabited except for small areas like 

 tlie regions near the poles which possess 

 such adverse conditions that life has never 

 gained a foothold. 



Some people have speculated that life 

 might have come to earth from some other 

 planet in the form of spores (capsules ca- 

 pable of withstanding unfavorable condi- 

 tions) through interstellar space. This 

 seems unlikely because of the intense heat 

 it would have to endure en route. Bits of 

 inorganic matter occasionally fall to earth 

 in the form of meteorites; when they reach 



