Eocene Epoch

55-34 million years ago


The nervous systems of ancestral anthropoids evolved a number of features which are shared among modern anthropoids.

In monkeys and apes, the trigeminal nucleus is larger, The somatosemsory cortex 1 has a complete mirror image. The auditory cortices of humans and other primates are similar. Lesions in the auditory cortex can interfere with the ability of primates to distinguish between other primate vocalizations (or interfere with speech comprehension in humans). All anthropoid primates have a primary auditory field and surrounding secondary fields represented in their auditory cortex. In all anthropoids, there are connections between the auditory cortex and the frontal lobe. Speech involves a number of brain regions including Broca's area, basal ganglia, inferior frontal cortex, dentate nucleus, and lateral cerebellar hemispheres, all of which are well developed in non-speaking primates.

In the ancestors of anthropoid primates (and tarsiers) 4 cell layers developed in the thalamic geniculate body, the lateral geniculate nucleus became more specialized, and the orientation of lateral geniculate nucleus became more ventral. The septum pellucidum developed, there was a greater development of gyri, and large integration areas developed in all 4 lobes of brain.

In anthropoid primates, the optic areas were highly developed and the olfactory areas were reduced. The importance of the VNO was reduced. In the retina, cones outnumber rods and anthropoid primates (and tarsiers) no longer possess a tapetum lucidum. Anthropoid primates developed stereoscopic vision and an enhanced sense of touch.