One of the main reasons why we are so interested in the other primates is that by looking at them we can obtain some ideas of what our ancestor must have been like a few millions years ago. Even though, we are not descended from any modern-type monkey or ape, our lineage does appear to have gone through stages in which we were a medium-sized, reasonably intelligent creature with good binocular vision, hands that were good at manipulation and the ability to climb trees. An evolutionary trend in primates involves the development of offspring both before and after birth and their integration into complex social systems. Another trend in primate evolution has been toward a more elaborate brain. In addition to brain size and gestation periods, social organization also demonstrates and plays an important role in primate evolution due to its complexity and hierarchy.
Brains size does matter
The larger the size of the brain the more intelligence the species, and throughout primate evolution size of primate brain has grown in size. Most mammals display some asymmetry between the right and left side of the brain in size and morphology. According to Prince Charles Lucien Bonaparte who in 1837 divided mammals into two groups namely Educabilia and Ineducabilia. One Group, with brain surface that must be reflective of at least some intelligence, Bonaparte called Educabilia. The second and obviously inferior group he referred to as Ineducatbilia. Bonaparte's Educabilia consisted of the carnivores, ungulates, manatees and other sirenians, whales and of course primates. Educabilia was characterized by having the large portion of the brain, the cerebrum, subdivided by a crease of fissure into two or three lobelike "segments." The other group had only a single-lobed, undifferentiated cerebrum.
Not only do the large hominoids have large brains, their brains differ from those of other primates in having more distinct asymmetries to them, where features of the right and left cerebral hemispheres are unequal in size or extent of expression. Humans have long been known to develop extreme cerebral asymmetries, which are supposed to be correlated with language and handedness of information from the different senses. The brain of anthropoids is larger, relative to body weight, than that of prosimians and is characterized by a complicated pattern of folds and fissures on the surface. The neocortex that has the functions to receive, analyzes, and synthesize is the most recently evolved brain region, does what we regard as the clever stuff: learning, language and making sense of the world. It takes up about 80% of the human brain but less than 60% of a New World monkey's brain. Humans have long been known to develop extreme cerebral asymmetries, which are supposed to be correlated with language and handedness. The researchers believe that this could reflect an adaptation for living in larger groups with more complex social interactions.
The reproductive cycle of copulation, gestation, birth, and lactation occupies the higher female primates for a year or more. The female does not usually come into estrus again until the offspring of the previous pregnancy is weaned. Primate infants are generally born fully furred and with their eyes open. At birth, the infant embarks upon what will be a long period of growth and development in the care not only of its mother, but also often in that of other relatives or members of the same social group. The complex and relatively stable societies of primates provide, a degree of protection that enables the young to have a long period of learning. Except in the case of man, chimpanzee, and gorilla, the newborns are able to cling to the mother's fur and need no support. Physical dependency ends when the young are weaned, but it is followed by an extended period of psychological maternal dependency lasting from 2 1/2 years in lemurs to 14 years or so in human. This educational period is crucial...
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