Primitive primates had more primitive nervous systems
(including smooth cerebrums depicted in two species given above). Many
features did evolve in the nervous systems of ancestral primates which
are shared among modern primates.
Of the five fossil mammals with a foramen magnum relatively smaller than
any modern placental mammal, 4 are of fossil primates (3 adapids and a
lorisid; the fifth was an Eocene carnivore). (This measure is an indirect
measurement of the size of the spinal cord.) The cervical and thoracic
regions of the vertebral canal in the fossil primates Notharctus and Smilodectes
were smaller than in any modern primate while the size of the vertebral
canal in the lumbar region was comparable to those primates alive today.
The ability to move the hand with fine, skilled movements evolved before
primates and is present in a number of mammalian lineages, such as mice.
Primates brought the control of skilled movements under the control of
the visual system. Primates increased the extension of corticospinal tract
in spinal cord and its size. In primates, more ventral spinal cord gray
matter receives corticospinal input than in other mammals. Digital dexterity
and the size of the corticospinal tract increase gradually throughout
In primates, the relative size of the cerebellum is increased, the nucleus
intercalates is divided into a nucleus emboliformis and a nucleus globosus,
and there is an increase lamellation of cerebellum.
The hypoglossal nucleus is subdivided and the ramus descendens hypoglossi
originates from the spinal cord rather than the hypoglossal nucleus. Primates
possess a pregeniculate nucleus and the red nucleus extends into the diencephalon.
In primates, the cerebral hemisphere extends over part of olfactory bulb.
Prosimians have areas of the fronto-parietal cortex homologous to those
found in higher primates, although some prosimian areas are further divided
in higher primates. Prosimians possess areas which represent the entire
body and other areas which represent only partial areas. A comparison
of the brains of a prosimian and Old World monkey found homologs of the
4 somatosensory regions and 10 motor regions of the prosimian in Old World
monkeys. Primates possess a corticospinal projection from the mouth field
of the agranular cortex. While cats and primates have multiple visual
areas, they appear to have evolved independently. Primates share multiple
sensory representation areas for each sense, including 3 auditory fields
with primary-like characteristics that receive input from the thalamus
and have a developed granule layer 4. Most therians have some degree of
separation between the claustrum and the cerebral cortex. In primates
these structures are completely separated.
In primates, about half of the tectopetal projections are ipsilateral
unlike the ancestral state in which all projections are contralateral.
Broca's area is a region of the left motor cortex which controls the muscle
movements which are essential for human speech. In nonhuman primates,
stimulation of Broca's area moves muscles of larynx.
Primates possess a central sulcus which may or may not appear in other
placental groups. Other gyri and sulci in primates include the calcarine,
paracalcarine, temporalis superior, temporalis inferior, postlateralis
(lunatus), and interparietalis (lateralis). Primates develop a fovea where
cones are concentrated in the retina.