THE GROUPS AND ANCESTRY OF ORGANISMS 



107 



material goes through a cycle of decay from one 

 mineral to another. For example, uranium changes 

 to a unique kind of lead that can be distinguished 

 from other lead, and radioactive carbon goes to non- 

 radioactive carbon. The life of any radioactive ma- 

 terial is expressed in terms of its "half life," the time 

 necessary for one-half of the substance to decay com- 

 pletely. For uranium the half life is five billion years; 

 for carbon it is 5570 years. However, it takes much 

 more than two half lives for any radioactive material 

 to decay completely. At the end of one half life there 

 is one-half of the original material left'; at the end of 

 two half lives, one-fourth; at the end of three, one- 

 eighth; etc. 



Many radioactive substances are utilized in the 

 extremely critical techniques of dating. For example, 

 uranium may be used to determine the age of rocks in 

 millions of years, and radioactive carbon may be used 

 for dating fairly recent materials. Although these and 

 other radioactive materials can give accurate dating, 

 generally within 5 per cent of the true age, procedures 

 must be meticulous or considerable error is possible. 

 For example, only an expert technician is capable of 

 selecting the materials for dating. Test rocks must be 

 unweathered. Also there are analysis difficulties, as 

 in uranium dating where primary lead often is mixed 

 in the sample with the lead produced by the radio- 

 active decay of uranium; other radioactive materials 

 may also be present. Therefore, for reliable esti- 

 mates, many separate determinations, if possible on 

 different radioactive samples, must be made. Al- 

 though these difficulties exist in all radioactive dating, 

 there is an apparently reliable estimate of the age of 

 the oldest known rocks at over three billion years! 

 This would mean that our planet must be at least four, 

 and probably more, billion years old; many geologists 

 consider four and one-half billion years a reasonable 

 approximation of the age of the earth. 



FOSSILS 



Fossils give direct or indirect evidence of past life. 

 A fossil can be actual remains of an organism — soft 

 parts, hard parts, or waste products; any impressions 

 or replacements of original substances, such as molds 

 or casts of a once-living creature; or any indication of 

 past life no matter how remote, such as tracks, trails, 

 or burrows. In general, the more recent the fossils the 

 better their condition and the more numerous they 

 are. Naturally, an organism that has just died (tech- 



nically a fossil) is relatively easy to identify. All the 

 hard and soft parts are present; there is really little 

 basic difference between the dead creature and a liv- 

 ing representative of the same species. However, the 

 older a fossil the less it looks like a once-living crea- 

 ture. In the case of many animals, older fossils pro- 

 gressively may consist of deffeshed bones in a life-like 

 position, well-preserved but scattered bones, par- 

 tially broken bones, bone fragments, and hardly rec- 

 ognizable pieces. This tendency for progressive de- 

 struction is directly associated with geological proc- 

 esses. Because the amount of geological activity is 

 directly related to time, the older the fossils generally 

 the poorer their condition. From a related point of 

 view, the older the rocks the fewer the indications of 

 past life. 



Recency of fossil formation is not the only impor- 

 tant aspect of fossil permanence. Different organisms 

 vary in their likelihood of remaining as fossils. It 

 should be no great surprise that the fossils most 

 frequently found are of plants or animals with hard 

 parts. Remains such as bones, shells, or coral de- 

 posits are more likely to persist long enough to be 

 preserved and are less likely to be removed either by 

 decay or by geological activities. On the other hand, 

 soft parts are susceptible to complete loss. Soft parts 

 and time worked together to cause the paucity of 

 archaic fossils. The oldest fossil-containing rocks 

 have few fossils because they represent the time of the 

 most simply constructed organisms, species com- 

 posed almost entirely of soft parts. For example, the 

 first three billion years of earth history included the 

 dawn of life but left hardly any easily recognizable 

 fossils. Rather, this record contains mostly hydro- 

 carbons and other chemical compounds that prob- 

 ably were formed by living creatures. For the next 

 one and one-half billion years of life (Proterozoic Era) 

 fossils still are poor. These fossils are often frag- 

 mentary; however, there are recognizable sponges, 

 corals, worms, mollusks, algae, and other groups. 

 Some of these fossils give good evidence for the pres- 

 ence of most to all animal phyla, algae, protistans, 

 and monerans. 



A rather recent contribution to the history of life is 

 the study of minute remains such as microscopic re- 

 productive structures (e.g., plant pollen) and micro- 

 organisms. Such plant remains, according to some 

 botanists, definitely establish the vascular plants as 

 being with the Proterozoic Era fossils. In fact, the 

 various kinds of fossil evidence allow the generaliza- 



