220 INTRODUCTION TO EVOLUTION 



Briefly, we recall that the sequence, subsequent to attainment of vertebrate 

 status, was as follows: crossopterygian fishes, to labyrinthodont amphib- 

 ians, to cotylosaur reptiles, to therapsid reptiles, to primitive mammals 

 of group Pantotheria, to mammals of Order Insectivora. 



Order Primates 



We have noted that the Insectivora constitute the group of mammals 

 from which the other orders of placental mammals are believed to have 

 arisen. Among these other orders is Order Primates, the order to which 

 man belongs, in company with tree shrews, lemurs, tarsiers, monkeys, 

 and apes. These forms are grouped together in one order because they 

 possess in common a number of anatomical features. Yet the order is not 

 characterized by any one big specialization as are many other orders of 

 mammals. Thus, members of Order Artiodactyla (e.g., deer, antelope) 

 are specialized for running on two toes of each foot, members of Order 

 Carnivora (e.g., cats, dogs) for flesh eating, members of Order 

 Chiroptera (bats) for flying, members of Order Cetacea (e.g., whales, 

 porpoises) for swimming, and so on. Members of Order Primates exhibit 

 a trend toward freeing of the forelimbs from locomotor duties, making 

 possible their employment for other purposes, notably the grasping and 

 handling of objects. The lower primates show only the beginnings of this 

 trend, which reaches its culmination in man. The trend was doubtless of 

 highest importance in human evolution, being closely connected with 

 development of the upright posture which forms one of man's most 

 distinctive attributes. This emancipation of the hands from duties of 

 locomotion was probably an essential prerequisite for the great develop- 

 ment of the brain mentioned above as man's crowning achievement. 



The trend toward emancipation of the forelimbs from locomotor duties 

 and employment of these limbs for grasping and handling is reflected in a 

 number of bodily features characterizing members of Order Primates. 

 Thus, the thumb and great toe are more or less opposable to the other 

 digits; flattened finger nails have largely replaced the claws possessed by 

 other mammals; and the eyes are directed anteriorly instead of laterally. 

 Obviously, anteriorly directed eyes are in much better position to see 

 and examine objects held in the hands than are laterally directed eyes. 

 As the forelimbs have lost locomotor duties the hind limbs have 

 assumed more and more complete responsibility for body support and 

 locomotion when primates walk on the ground. This fact is reflected in 

 the plantigrade walking position of the feet, with the sole of the foot 

 nearly flat on the ground for its entire length. 



