564 - Heredity and Evolution 



other grounds, the fossils of this stratum pro- 

 vide an invaluable clue for the identifica- 

 tion of the corresponding stratum in other 

 regions of the earth. 



Eras and Periods of Geological Time. The 

 earlier geologists were able to identify five 

 well-defined series of strata, but found con- 

 siderable gaps between the series, and smaller 

 gaps within each series. On the basis of these 

 gaps, the strata were divided and subdivided 

 into the eras and periods of geological time, 

 as is shown in Table 29-1. Subsequently the 

 study of the strata has progressed consider- 

 ably, extending to ever wider regions of the 

 earth. Thus many of the gaps have been filled 

 in, and the lines of demarcation between the 

 eras and periods are becoming less sharply 

 and arbitrarily defined. 



The relative duration of the eras and 

 periods of geological time is quite plainly 

 indicated by the thickness of their strata, 

 although considerable variation must have 

 occurred in the rate at which sedimentary 

 rock was deposited in different areas and at 

 different times. In fact, until recently there 

 was little hope of obtaining reliable estima- 

 tions as to the absolute duration of the geo- 

 logical periods. Now, however, physicists 

 have found that the proportion of radio- 

 active elements contained in certain rock 

 provides an excellent index of the absolute 

 age. Uranium, for example, gradually trans- 

 forms into lead by a series of radioactive 

 disintegrations, and the rate of this disinte- 

 gration is independent of temperature and 

 the other variables of the environment. Spe- 

 cifically, it requires 4.5 billion years for half 

 of a given sample of uranium to degrade into 

 lead. Therefore, in rocks that are devoid of 

 lead except for that which has been derived 

 from uranium, the lead: uranium ratio gives 

 an accurate measure of the absolute age. 

 These age measurements are in excellent 

 agreement with the most recent and reliable 

 data based on sedimentation rates. Conse- 

 quently the durations of the eras, as specified 

 in Table 29-1, can be accepted with a fair 

 degree of confidence. 



Brief Survey of the Fossil Record. The 



main outlines of the history of pre-existing 

 life are shown in Figures 29-10 and 29-1 1 and 

 Table 29-1; and the ensuing account will at- 

 tempt little more than to exemplify a few of 

 the major evolutionary trends. 



No recognizable fossils are found in the 

 Archeozoic strata, but all these rocks are 

 badly metamorphosed and eroded. However, 

 the presence of graphite and other materials 

 of presumably organic origin indicates that 

 the very earliest forms of life had their first 

 beginning quite early in the Archeozoic 

 period. 



In Proterozoic rocks, fossils are still rela- 

 tively scarce and poorly preserved, but quite 

 a few are plainly recognizable. The very 

 earliest fossils are identified as primitive 

 Schizomycophyta; and most biologists agree 

 that the earliest cellular organisms were col- 

 orless plants somewhat similar to the modern 

 bacteria (p. 601). In the upper Proterozoic 

 strata there are unmistakable fossil remains 

 of algae (akin to the modern Thallophyta), 

 numerous Protozoa, a few sponges (Porifera), 

 and — near the very top — a few other in- 

 vertebrate groups (Fig. 29-10). 



A considerable gap exists between the 

 strata ol the early Paleozoic and the late 

 Protero/oic eras; that is, no sedimentary rocks 

 formed during this interval have been found 

 on the land surfaces as the}' exist today. In 

 fact the geological evidence indicates that 

 most of the land areas of today were also 

 land areas toward the end of the Proterozoic 

 period. The earliest Paleozoic (Cambrian) 

 strata contain a rich fossiliferous record, with 

 an abundance of algae and some representa- 

 tives of all the principal invertebrate phyla. 

 However, the Cambrian strata show few, if 

 an)', true vertebrates, no plants higher than 

 the Thallophyta, and, in fact, no terrestrial 

 organisms of any kind. All life at this time 

 was still aquatic. Moreover, Cambrian ani- 

 mals while belonging to the same phyla as 

 our modern species, were generally quite dif- 

 ferent. On the whole they were more primi- 

 tive than the present species, and in many 



