492 THE CHANGING GENERATIONS 



Before we can interpret the fossil record of human evolution, we need 

 to know the relative ages of the fossils and their relation to the events of 

 the Pleistocene. There are various ways of establishing a Pleistocene 

 chronology and of dating the fossils. 



The basic chronological data are geological and are derived principally from 

 the glaciated regions and adjoining (periglacial) areas. Each advance and melting 

 back of the ice sheets, and each warm interglacial interval, left its traces, though 

 many of the earlier evidences were destroyed by later glaciations and subsequent 

 erosion. The ice sheets left around their borders deposits of boulder clay and 

 outwash from glacial streams; in the periglacial zone wind-blown dust formed 

 characteristic loess deposits; stream terraces in unglaciated territory record 

 changes in elevation and climate; coastal terraces show the various levels of the 

 sea; and soil layers retain evidence of the climates under which they were formed. 

 From such data the sequence and relative duration of the glacial and interglacial 

 ages and of their subdivisions have been worked out. Fossils found in association 

 with identifiable geological features can be assigned relative though not absolute 

 ages on this evidence. 



The so-called "astronomical" dating is based upon a hypothesis as to the 

 cause of glaciation which was elaborated by Milankovitch in 1930. This hypoth- 

 esis holds that the temperature changes responsible for ice ages are due to the 

 periodic variations in the amount of solar radiation received in one hemisphere, 

 caused by precessional changes in the earth's axis and the known cyclic variations 

 in the path of the earth around the sun. It assumes that the cold maxima will 

 produce glaciation in the affected hemisphere (northern or southern) only when 

 continents stand high and oceanic circulation is impeded, as during a geological 

 ♦ revolution. Milankovitch's laboriously calculated radiation curve for the northern 

 hemisphere purports to give the temperature fluctuations over the 600,000 years 

 which according to this hypothesis is the length of the Pleistocene. The as- 

 tronomical theory has been widely accepted in Europe and was made the basis 

 for a detailed world-wide Pleistocene chronology by Zeuner. 1 It is not, however, 

 the only theory as to the causes of glaciation 2 and, as noted below, is not sup- 

 ported by the radio-carbon dating of the last glacial advance. 



Radio-carbon dating is one of the most remarkable scientific achievements of 

 recent years. It promises to give accurate and reliable dates up to a maximum 

 of about 35,000 years past. It has no geographical limitations; but it can be used 



1 In a most interesting and ambitious book, Dating the Past, Longmans, Green & 

 Co., Inc., New York, 1951. 



2 A second widely held hypothesis assumes that ice ages are caused by variation in 

 the amount of radiation emitted by the sun, or, alternatively, by variation in the 

 amount received by the earth because of passage of the solar system through cosmic 

 dust clouds. A third assumes ptirely terrestrial causes for ice ages, suggesting that 

 locally produced glacial nuclei may set off a runaway reaction and grow into conti- 

 nental glaciers through the siphoning of water by evaporation from the oceans to the 

 glacial centers. The recent demonstration by radio-carbon dating of the simultaneity 

 of the late glacial warm intervals in North America (Cary Mankato) and in Europe 

 (Allerod), suggests an extraterrestrial cause. 



