Defining primates like other taxonomical groups on the basis of morphological or anatomical characteristics is extremely difficult as

• Primates lack distinguishing bodily specializations like other mammalian groups
• Primates exhibit amazing variety of forms and grades of organization – Size, Habitat, Behavior etc..

Though there are ten trends, there are related to three principle areas…

1. Limbs and extremities (1-3)
2. Head- eyes, brain and dentition (trends 4-8)
3. Life cycle-reproduction, growth, and longevity (trends 9 & 10)

• These are trends and not characteristics. Hence are not uniformly present, but are expressed to varying degrees among the members of the order.
• As a rule, except for the first one, these trends are less well developed in the living prosimians and in fossil forms than in the more advanced, modern Higher Primates.
• They are all interrelated and adaptive to a mode of life which has been the outstanding factor in primate evolution-the arboreal habitat. Other mammals have taken up arboreal life but none have done it in exactly the same way
• Many of man’s characteristics are simply refinements of a basic primate pattern that evolved millions of years ago in response to the necessities of arboreal living.

Evolutionary Trend

The series of evolutionary trends given by Sir Wilfred E.LeGros Clark are:

1. The preservation of a generalized structure of the limbs – like primitive pentadactyly, and the retention of certain elements of the limb skeleton (such as the clavicle) which tend to be reduced or to disappear in some groups of primates. Single bone in upper segment, paired bones in lower segment to permit some degree of rotation with five digit (pentadactyl) extremities for grasping, clavicle or collarbone retained in shoulder gridle to permit upper limb to have greater reach and range of movement in several directions.
2. An enhancement of the free mobility of the digits, especially the thumb and big toe (which are used for grasping purposes). It is indicated by the ability to move the digits (like thumb and great toe) independently. But unlike primates, in Humans the foot has become weight bearing organ rather than a grasping organ.
3. The replacement of sharp compressed claws by flattened nails, associated with the development of highly sensitive tactile pads on the digits. Evolution of flattened nails from compressed claws and development of highly sensitive tactile pads on the digits. These modifications were for facilitating a secure grip in the trees.In primates the palms and soles are naked of hair for better grip.
4. The progressive abbreviation of the snout or muzzle- Due to increase in the overall shape of the skull, there is a tendency for facial portions of the skull like snout area to become relatively small. This trend is supported by behaviour trends where the higher primates use eyes and hands to do things that a prosmian would use his snout and mouth for.
5. The elaboration and perfection of the visual apparatus with the development to varying degrees of binocular vision. It is due to enlargement of visual areas of brain, change in structure of retina and nerves connecting it with brain for better coordination between sensory stimuli and muscular response, moving forward of eyes in head to permit stereoscopic vision. ex: the eyes of living tree shrews are located on the sides of head and in higher primates they are directly forward.
6. Reduction of the apparatus of smell. Decrease in sense of smell due to decrease in olfactory areas of brain.
7. The loss of certain elements of the primitive mammalian dentition (3:1:4:3), and the
   preservation of a simple cusp pattern of the molar. Reduction in number of teeth due to reduction in face and jaws. preservation of simple cusp pattern of the molar teeth

   Dentition
   Dental Formulae & Number of Teeth in Various Living Primates
   Group	Dental Formula	No. of Permanent Teeth
   Lemuridae	2-1-3-3/2-1-3-3	36
   Indridae	2-1-2-3/2-0-2-3	30
   Lorisoidea	2-1-3-3/2-1-3-3	36
   Tarsioidea	2-1-3-3/1-1-3-3	34
   Cebidae	2-1-3-3/2-1-3-3	36
   Callithricidae	2-1-3-2/2-1-3-2	32
   Cercopithecoidea	2-1-2-3/2-1-2-3	32
   Hominoidea	2-1-2-3/2-1-2-3	32

8. Progressive expansion and elaboration of the brain, affecting predominantly the cerebral cortex and its dependencies. Progressive expansion and elaboration of brain resulting in more accurate sensory perception and greater variety of behavioral responses to environmental factors. The changes are more predominant in cerebral cortex which has become expanded and its surface folded.
9. Progressive and increasingly efficient development of those gestational processes
   concerned with the nourishment of the fetus before birth. In primates the mother gives necessary nourishment to the young through placenta, mammary glands. This association enables the young to learn various forms of behaviour which promote their survival.

   Comparative Data on Growth & Development
   Species	Gestation	Age at sexual maturity	Length of growth period
   Rhesus Monkey	5 V2 months	2-3 years	7-8 years
   Chimpanzee	7 V2 months	8-9 years	11-12 years
   Man	9 months	13-15 years	20-21 years

10. Prolongation of postnatal life periods.There is a well-marked trend for a lenghtening of the period of growth and development and delay of maturation. This resulted in longer gestation period, increasing immaturity of infant at birth, longer postnatal growth period and later attainment of sexual maturity. The female primates usually have one infant at a time and thus the evoultion is characterized