CHAPTER 36 



The Evolution of Man 



317. Primate Evolution 



The line of evolution that led from the ostracoderms to the primates 

 was traced in Chapters 22 to 24. Although the fossil records of horses, 

 elephants, camels, and many other mammals are quite good, those of 

 the primates are regrettably fragmentary. Most of our primate ancestors 

 lived in tropical forests, where fossils are not likely to be preserved. 

 However, there are representatives of several primitive groups of pri- 

 mates alive today from which we can get some idea of what our ancestral 

 primates might have looked like. The earliest placental mammals were 

 small, tree-dwelling, insect-eating animals; from these insectivores have 

 evolved all the kinds of placental mammals alive today. The primates 

 remained mostly arboreal and are relatively unspecialized. 



There are three groups (suborders) of the primates: the lemuroids, 

 which includes the tree shrews, lemurs and lorises; the tarsioids, the 

 tarsier; and the anthropoids, monkeys, apes and man. The primates are, 

 in general, rather unspecialized mammals; the specializations they do 

 have are adaptations for arboreal life; grasping hands and feet (with 

 opposable thumbs and great toes); some or all of the fingers and toes 

 with flattened nails; very flexible, mobile arms and legs; well developed 

 brains (especially the cerebrum); and binocular vision. 



The primate line appears to have begun with the tree shrews, which 

 are intermediate between the primitive insectivores and the primates. 

 There are fossil tree shrews known from the Oligocene, and some tree 

 shrews, such as Tupaia (Fig. 24.16), which still survive in the forests of 

 Malaya and the Philippines. The tree shrew looks a bit like a squirrel 

 with a long snout and tail, but has opposable first toes. During most of 

 primate evolution, the trend was toward greater adaptation for an 

 arboreal life. Only in some of the larger apes and man has this trend 

 been reversed. 



318. The Lemurs 



The lemurs are believed to represent the next stage in the evolution 

 of the primates. These are small nocturnal, arboreal animals, with long 

 tails, long, flexible limbs, and grasping hands and feet (Fig. 36.1). Lemurs 

 are found today in the tropics of Africa and Asia, but especially on the 

 island of Madagascar. Fossil lemurs have been found in deposits from 



738 



