106 



CLASSIFICATION, PHYLOGENY, AND EVOLUTION: 



chemical reactions to complete evolution of a virus- 

 like condition are relatively simple. "Living" viruses 

 are chemically similar to a hereditary unit, or gene, 

 of more complex life forms; but probably are not 

 modern descendants of a virus-like stage. Anything 

 causing ancient virus-like creatures to combine with 

 one another would produce organisms like the 

 simplest, present-day bacteria. This archaic moneran 

 stage, the first definite life, was not far from evolu- 

 tion into unicellular creatures, the Protista. For ex- 

 ample, the living sulfur bacteria may be similar to the 

 ancestral Monera. These sulfur bacteria have a pig- 

 ment quite similar to chlorophyll, which is capable of 

 carrying on a kind of photosynthesis. From such 

 organisms, it is a small step to the protistans, and 

 from protistans, a small step to the origin of the plant 

 or animal kingdom. 



GEOLOGICAL HISTORY 



Relationships among organisms would not be 

 above the level of wild speculation if it were not 

 for the record of life upon our planet. It is the pur- 

 pose here to summarize fossil history and the physical 

 history of the earth. However, to appreciate fully the 

 geological time table, one must first define a fossil and 

 examine how remains are dated. 



DATING PAST EVENTS 



The history of earth's past physical and biological 

 events is pieced together from information gained 

 from the rocks. In any one locality it might not be too 

 difficult to determine the sequence of rock layers and 

 life; however, when one attempts to summarize the 

 earth's history on a world-wide basis, it becomes 

 necessary to relate remote localities. Also, time be- 

 comes of prime importance. Once a sequence of 

 events is determined, one wants to know how long the 

 world was influenced by certain conditions and how 

 long it took certain animals to evolve into others. 



Correlation. There is a definite relationship be- 

 tween time and the correlation of isolated localities. 

 If it were possible to date every layer of rock in every 

 locality, one would then have a record of correlated 

 structures; that is, the distribution of all rocks formed 

 at each particular time. However, such dating is be- 

 yond the possibility of human accomplishment; there 

 is time only to date a minute sample of the earth's 



layers of rocks. Therefore, the relating of geological 

 events must be based on some simpler, less time- 

 consuming procedure. The historical geologists call 

 the procedure they now use correlation. 



Geological correlation is done in three main steps. 

 First, the geologist looks for continuity. He traces the 

 distribution of a particular layer of rock and assumes 

 that all of it was formed at the same time and under 

 similar conditions. This layer he calls a formation. 

 Second, he looks for structural similarity among iso- 

 lated rock layers. If isolated rocks are similar to one 

 another, the geologist considers separately the layer 

 found directly above and the one directly below. 

 Then, if all the upper rock strata seem to be a homo- 

 geneous appearing unit, or formation, and so do all 

 the lower layers, it is concluded that each isolated but 

 similar group belongs to one formation. This same 

 general procedure is used in even more complex de- 

 termination of formations. Finally, the geologist tries 

 to correlate fossils. If these indications of past life 

 are essentially the same in isolated, similar rock 

 layers, he concludes that the remote strata belong to 

 one formation and are approximately of the same age. 

 Correlation does no more than provide a sequence 

 of past events; it does not tell how long each event 

 lasted or how long ago each started. Therefore, other 

 techniques are required to provide a geological 

 calendar. 



Dating. About fifty methods have been used to 

 measure geological time; however, they can be sum- 

 marized under four main groupings. The first type 

 attempts to date the age of the earth by estimating 

 the past rate of heat loss from the sun or earth and 

 gives an age of about forty million years. However, 

 this age is no longer considered valid, because it is 

 based upon the false assumption that the earth and 

 sun are simple cooling bodies. The second type 

 evaluates erosion rates and provides a similar esti- 

 mate of the earth's age. Although erosion rate is a 

 fairly accurate technique for approximating the age 

 of recent events, it is little better than the first type 

 for long-range events. The third type uses the rate of 

 deposition of eroded materials. Although no longer 

 acceptable because of unequal rates of deposition at 

 different times, it does imply that the earth is one 

 hundred million years old. The final type, the present 

 one, is by calculation of the amount of radioactive 

 decay of rocks. Age approximation, not only of the 

 earth but also of the features formed since the origin 

 of the earth, is possible because each radioactive 



