THE NATURAL HISTORY OF MAN 75 



general, signify that the jaw is too small. Likewise the fact 

 that the third molars and lateral incisors are variable in de- 

 velopment and frequently fail to appear indicates a tendency 

 to a reduction in the number of the teeth. A comparison of 

 man's dentition with that of a typical mammal having forty- 

 four teeth, shows that man has probably already lost twelve 

 teeth during his long period of evolution. The projecting chin 

 which man alone of all animals possesses is the result of the 

 shrinkage of the tooth-containing part of the jaw. This can 

 be well illustrated by a comparison of the fossil jaw of the 

 Heidelberg or Neandertal man with that of modern man. 

 'Tooth decay also is not found so extensively in the primitive 

 races as in the civilized and is apparently absent in the fossil 

 races, with the notable exception of the Rhodesia man recently 

 found in the Broken Hill mine. It is clear that the usual 

 prophylaxis against tooth decay is merely temporizing with 

 the real problem. Since the present civilization apparently 

 carries a distinct threat to the teeth, a radical change in diet 

 and habits is necessary to preserve them. What such change 

 should be we are awaiting our nutrition experts to show us. 



ANATOMICAL VARIATIONS 



Finally, further evidence as to the ancestral history of man 

 can be obtained from a study of structural variations. No 

 two people are exactly alike in bones, muscles, or vessels, and 

 even the nerves, the least variable structures in the body, show 

 differences. Some of these variations are probably fortuitous, 

 others are to be considered as a reversion to an ancestral form, 

 and still others progressive. 



Ordinarily man has twelve pairs of ribs but occasionally a 

 thirteenth rib appears. This is known as a cervical rib and is 

 located in the neck just above the thorax. It is generally 

 reduced in length but may be long enough to reach the sternum. 



