PREHISTORIC MAN 493 



only on materials containing carbon derived from the carbon dioxide of the 

 atmosphere. Such materials include all organic remains, and limestones of recent 

 origin. In 1946, it was discovered that a small but constant fraction of the carbon 

 in the atmosphere is the radioactive isotope Cm (radio-carbon) instead of the 

 ordinary C12. The radio-carbon is formed in the upper layers of the atmosphere 

 by bombardment of nitrogen by cosmic rays. It becomes uniformly dispersed in 

 the air and is built into the bodies of all living things through their dependence 

 upon photosynthesis. Tests prove that the same percentage of d 4 is present in 

 the bodies of animals and plants the world over. When organisms die the accumu- 

 lation of carbon ceases, and the radio-carbon originally present in their bodies 

 gradually disappears through radioactive decay of the Cu atoms. The rate of this 

 disintegration is known, its half life being 5,568 ± 30 years. Therefore the age of 

 a carbon-containing fossil or rock that is not too old can be determined within 

 narrow limits by measuring the amount of radioactivity remaining in a standard 

 sample by means of electronic counters. 



The first extensive series of radio-carbon dates was published in 1950-1951. 

 It included age determinations on various historic and prehistoric objects made 

 by man and on fossil bones, shells, charcoal and wood. The date of the last (Man- 

 kato) ice advance of the fourth glacial age in America was found (by tests made 

 on buried trunks of spruce trees) to be about 9,000 B.C. or 11,000 instead of the 

 previously assumed 25,000 years ago. This casts doubt on the Milankovitch 

 theory, since there is no minimum at this date on the radiation curve. Where the 

 radio-carbon dates can be checked by historical records or tree-ring dates, they 

 generally show close agreement, although some unexplained discrepancies occur. 



Certain other methods of dating, applicable to postglacial and late Pleistocene 

 times, require only brief mention. The tree-ring record has been carried back some 

 3,000 years, giving a graph of climatic variation, chiefly that of rainfall. Analysis 

 of the pollen content of bog layers has revealed much of the vegetational and 

 climatic history of the glacial and periglacial regions during the past 15,000 

 years. Measurement of the thickness of the annual layers of sediment (varves) 

 deposited in lakes and bays that received glacial melt-water has given an equally 

 long record of postglacial temperature variations and the rate of ice recession. 

 Other time correlations for the late Pleistocene are based upon Baltic Sea levels 

 and similar phenomena. Finally, where other means of dating are unavailable, 

 sometimes the fauna! assemblage itself gives a clue to the climatic conditions 

 under which it lived, by means of which it can be tentatively assigned to a par- 

 ticular age or subage. 



FOSSIL MEN 



Our knowledge of prehistoric man comes from two sources -his fossil 

 bones, and remains of the things he made and did. From bis bones the 

 physical anthropologist reconstructs his bodily characteristics; from his 

 tools, weapons, ornaments, dwellings, and even his refuse the archeologist 

 reconstructs bis way of life. The first of these studies is concerned with 

 the evolution of man's body: the second with the evolution of his mind 

 and culture. They tell separate stories which run parallel and merge only 



