CHAPTER XXIX 



MAN'S RELATIVES: THE PRIMATES 



The order of mammals that interests us most is the Primates, for we 

 ourselves belong to this order. Unfortunately the fossil record of primate 

 evolution is very fragmentary; primates have been chiefly inhabitants of 

 tropical forests, and this environment is not one in which fossils are likely 

 to be preserved. On the other hand various primitive groups of primates 

 still have living representatives to which we can turn for help in inter- 

 preting the fossils. 



The origin of primates. In early Cretaceous times the lands were 

 covered with forests, but there was little development of a ground vegeta- 

 tion. This may explain why the early placental mammals, like the early 

 marsupials, were tree dwellers. Their arboreal habit is not directly evident 

 from the Cretaceous fossils, which are mostly skulls and teeth. Instead it 

 is deduced from the fact that in all the stocks descended from the 

 Mesozoic insectivores the earliest types had the first toe set apart from 

 and differently hinged than the other toes. This condition indicates that 

 they came from ancestors possessed of grasping feet, an arboreal adapta- 

 tion. During the late Cretaceous a ground flora began to appear, and at 

 this time many mammalian stocks forsook the trees and took to the 

 ground. From these the clawed carnivores and the hoofed herbivores 

 developed. Some of the insectivores, however, remained in the trees, and 

 among these were the ancestors of the primates. 



The beginnings of the primate line are found in a group called the tree 

 shrews — small arboreal mammals intermediate between typical insecti- 

 vores and true primates. Only a single fossil tree shrew is known, from the 

 Oligocene of Asia, but the group fortunately still survives in the Malayan 

 area. These creatures are squirrel-like in appearance. The foot has a 

 grasping first toe, and claws instead of nails (except on the hind foot of 

 the Oligocene fossil). The skull is long and low, with an eye orbit which 

 is partly or completely closed behind by a bar of bone. The teeth are 

 similar to those of the insectivores, with molars suited for mincing and 

 slicing rather than grinding. The brain is intermediate in size between 

 that of typical insectivores and the lowest of the primates. 



From such ancestors the further evolution of the primates has been 



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