Evolution. 1. Collectively, those specialized areas of the human brain which evolved during the primate adaptation a. to diurnal (i.e., daylight) living, and b. to a life in trees. 2. Specifically, those modules of the forebrain which process color, eye-hand coordination, facial recognition, grasping, and 3D navigation by sight.
Usage: With the primate brain, nonverbal communication takes a dizzying turn toward complexity. Many signs (see, e.g., COLOR CUE, EYE CONTACT, EYEBROW-RAISE, FACIAL EXPRESSION, and PRECISION GRIP) depend on its neural circuitry. The primate brain, which developed from modules and paleocircuits of the mammalian brain, began its arboreal evolution ca. 65 m.y.a. in the Paleocene.
Hand signals I. With agile digits designed for climbing, our primate ancestors extended their forelimbs to reach for and to grasp insects, fruits, and berries. Manual dexterity (through advances in motor, premotor, supplementary, and association areas of the neocortex) led to the use of leaves, sticks, bones, and stones as tools (see ARTIFACT). (N.B.: These modular areas of neocortex managed the hand-and-arm movements our species turned a. to the manufacture of chipped-stone hand-axes, and b. to the use of conceptual hand-signals called mime cues.)
Hand signals II. The primate brain enabled voluntary movements of the hands and arms, to achieve goals beyond locomotion (seeWALKING) and standing on all fours. Sophisticated motor-control centers permitted new movements, such as reaching, grasping, andgrooming with the fingertips (which also could be used as gestures, i.e., as body movements to convey information about intentions andmoods).
Eye signs. By ca. 35-to-40 m.y.a. in the earliest apes, the primate brain dedicated distinct modules of visual cortex a. to the precise coordination of hand-and-eye movements, and b. to the recognition of faces. (In the living apes, dedicated nerve cells of the lower temporal lobe respond to hands and faces exclusively [see, e.g., Kandel et al. 1991:458-59].) "Marler  and Van Hoof  agreed that in most species of primates the face . . . is the most important part of the animal" [Izard 1971:38]).
Climbing cues. Visual learning is the hallmark of the primate brain. Foraging in trees (and using sight rather than scent) to find colorful fruits and berries went hand-in-hand with remembering where and what to pick. Unlike birds which fly directly to food spotted in trees, primates must chart a clever route through labyrinthine vines, limbs, and leaves. Mentally, they must navigate from point A to point B by decoding the branchways from many angles. (N.B.: In their 3D world, primates became skilled arboreal navigators. Today's monkeys, e.g., have sharp color vision, depth perception, and enhanced memory to recall the location of edibles scattered among forking branches and twisting vines.)
RESEARCH REPORTS: 1. "Over half of the neocortex in [living] nonhuman primates is occupied by visual areas ['At least 25 visual areas beyond the primary visual cortex . . . ']" (Sereno et al. 1995:889). 2. The primate's inferotemporal cortex is thought to be essential for object recognition (Wang et al. 1996:1665).
Neuro-notes I. Primates have prehensile hands with which to grab tree branches, fruits, and insects. Deliberate grasping is mediated by a region of the frontal neocortex called the supplementary motor area. This module programs complex muscle contractions to open and close the hand on purpose. The supplementary area also helps coordinate arm postures required to support the hand movement itself. At the same time, the primary motor cortex regulates the force with which moves are exerted. Instructions from these areas descend through the corticospinal tract directly to spinal-cord circuits below, which instruct muscles in the forearm to open and close the hand (deliberately: see Neuro-notes IV).
Neuro-notes II. The primate brain's premotor cortex controls the proximal movements which project an arm to its target. The premotor cortex, which receives visual input from the posterior parietal cortex, sends fibers to the brain stem's medial descending systems, as well, notably, to the (not-so-deliberate, i.e., reflexive) reticulospinal tract, which links to spinal circuits which control our proximal and axial muscles.
Neuro-notes III. The decision to grasp comes from a variety of areas in the primate brain. Sensory circuits, e.g., may advise a slipping hand to tighten around a branch. The basal ganglia may influence hand-over-hand movements of the climbing sequence itself. The limbic systemmay excitedly close a hand over a plump red berry. In such cases, the decision routes through reflexive circuits standing by in the brain stem: these instruct the spinal cord to close the hand.
Neuro-notes IV. A novel feature of the primate brain is its ability to grasp deliberately--i.e., to grasp on purpose--through the corticospinal tract (thus bypassing older brain-stem circuits altogether). This more advanced nerve tract, which began its evolution in the mammalian brain, elaborated in the primate brain. (N.B.: The corticospinal tract adds precision and voluntary control to our grasping gestures.)
Neuro-notes V. A region of the primate brain's posterior parietal cortex (Brodmann's area 5) processes information received from the primary sensory cortex (Brodmann's areas 1, 2, and 3), relating it to the position of the reaching arm. (N.B.: Special arm projection neurons fire when a monkey reaches for a nearby food item, e.g., but not if the arm reaches out merely for the sake of reaching.)
Neuro-notes VI. Area 5 receives input from the inner ear's vestibular sense, as well, regarding the head's orientation in space. It also hears from premotor areas of the frontal neocortex, which govern the motor plans for reaching, and from the mammalian brain's limbic system (the latter's cingulate gyrus, e.g., keeps area 5 updated on the primate's emotional state of mind).
Neuro-notes VII. ". . . using a dedicated monkey PET scanner at Hamamatsu Photonics in Hamakita, Japan, Hirotaka Onoe's team at the Tokyo Metropolitan Institute for Neuroscience last year discovered a new site of color processing in the monkey visual system" (Barinaga 1998:1397).
Neuro-notes VIII. 1. Studies show that the cerebellum of apes and human beings is proportionately larger than that of monkeys, perhaps due to adaptations, in the former primates, for bipedal walking and brachiation, as well as for monkey-like climbing (Rillinga and Inselb 1998). 2."Hence it is interesting that a species with one of the largest positive cerebellar residuals in our study (Hylobates lar) is among the most versatile, with climbing, bipedal walking and running, leaping, bridging, and brachiating all in its repertoire (Hollihn, 1984). The cerebellum has also been implicated in motor planning (Ghez, 1991). In contrast to humans and chimpanzees, baboons apparently lack 'presyntactical motor planning', the ability to modify current movements based on awareness of movements to follow (Ott et al., 1994). Thus, the larger relative cerebellar volume of apes compared with monkeys might reflect an increased cognitive representation in the cerebellum of hominoids" (Rillinga and Inselb 1998).
Illustration from Cambridge Encyclopedia of Human Evolution (copyright 1992 by Cambridge University Press)