PHYLOGENY 



Finally, two lower jaws of mid-Pleistocene age, found in Algeria in 

 1954, have been described under the name Atlanthropus. While one of 

 these is larger than the typical Pithecanthropus jaw, they are otherwise 

 quite similar to the latter; and so it appears probable that Pithecanthropus 

 was widely distributed in the Old World in mid-Pleistocene times. 



These fossils also illustrate the major differences between man and the 

 great apes. These may be summarized in two categories: adaptations to 

 erect posture, and the development of the brain and associated changes 

 in the skull. Of these, enlargement of the brain has lagged behind in hu- 

 man evolution, as shown by the study of australopithecines and Pithecan- 

 thropus. 



The adaptations to erect posture may have begun as early as Oreopithe- 

 cus, were quite far advanced in Australopithecus, and were largely com- 

 plete in Pithecanthropus. In order to show how many lines of evidence may 

 bear upon seemingly so simple a character, some detail may be given on 

 this subject. The ankle bones bear the main weight of the body while 

 erect. In man, these are large, while in the apes they are small. Corre- 

 spondingly, the toes of the apes are long and freely movable, the great toe 

 being opposable to the others. In man, the toes are short, and the great 

 toe is held in line with the others, thus making a more rigid support for 

 the weight of the body. As already pointed out, the leg bones are longer 

 than the arm bones in man, while the reverse is true in the apes. The trunk 

 of man is short relative to the legs, while the opposite is true in the apes. 

 The human situation is obviously mechanically more stable for erect pos- 

 ture. The hip bone of the apes is long and narrow, while that of man is 

 broad, thus giving optimum support to the viscera when in the erect 

 position. The curvature of the spinal column of the apes is a single, sweep- 

 ing, outward curve, like that of any four-footed mammal. This tends to 

 throw the animal off balance when standing erect. But the curvatures of 

 the spinal column of man alternate in direction, thus averaging out to a 

 straight line. In the upright position, the human knee and hip joints are 

 held straight, while those of the apes are flexed slightly. Finally, the occip- 

 ital condyles of the apes are near the posterior end of the skull and are 

 directed backwards, while those of man are near the center of the base 

 of the skull and are directed downward. Thus the human adaptations to 

 erect posture affect every part of the skeleton, as well as the viscera in 

 many ways which have not been discussed. These adaptations appear to 

 be largely completed in the most primitive men known. 



The most important change in the skull has been the increase in size 

 of the brain case. The largest brain case known in apes has a capacity of 

 685 cubic centimeters. The average of the Javanese fossils is about 900 

 cubic centimeters, while that of modern man is about 1350 cubic centi- 

 meters. This has been accomplished by an increase in the height of the 

 vault of the skull, and by an increase in the diameter of the skull above 

 the ear line. In the apes and in Pithecanthropus the greatest width of the 

 skull is at the level of the ears. In later human races, the skull lias bc^'ome 

 wider in the parietal region. As a result, the shape of the skull has become 

 more nearly globular. This change started later than the adaptation to 



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