Eocene Epoch

55-34 million years ago


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 the primates.
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.