Evolution of the Brain
Ontogeny recapitulates phylogeny?
Ontogeny of brain is rapid.
– Mirrors what we see in the evolutionary
history of the vertebrates.
– Begins as an expanded neural tube
– Has Olfactory, Optic, and Otic lobes.
correspond to the
Olfactory, Optic, and
Otic portions of the
Evolution of the Brain
Notice, there are several flexures in the
The basic nerve tube (primordial brain)
has the basic afferent and efferent
Components above the ‘tube’ generally
are association centers.
Association neurons can be complex
and operate on several levels.
Some re-organization in the brain
– Basic motor control of locomotion in lower
– Still present in mammals, but mediated by
higher control centers.
Brain is bilaterally symmetric, with
‘wiring’ between halves.
Lower vertebrates: essentially comparable to
anterior section of spinal cord.
Contains cranial nerves III, IV, V, VI, VII, IX,
In Higher vertebrates, the efferent and
afferent columns are broken down into
Contains reflex circuits for sensory reception
and responding effector organs of the head
and gill region.
Coordination and regulation of activities
and the maintenance of posture.
Equilibrium and body orientation.
In mammals, plays an important role in
– Primary inputs are 1) acoustico-lateralis
and 2) proprioceptors.
– Skink, optic, and olfactory inputs.
Midbrain and Diencephalon
All vertebrates – center of nervous correlation
Higher vertebrates – way station between
lower brain areas and cerebral hemispheres.
Includes the tectum and tegmentum.
Floor of Diencephalon contains optic chiasma
and pituitary gland (hypophysis cerebri)
Fish and amphibians – tectum is major brain
Reptiles and birds – also important.
Mammals – significantly reduced.
Initially, they were just loci of olfactory
In early tetrapods, they were centers of
In mammals, they are association
Why begin as olfactory?
– Olfaction was perhaps the most important
sense in early vertebrates.
– Olfaction is still crucial even to most
mammals (think of dogs).
– Olfaction is less important in primates, but
In fish and amphibians, the cerebral
hemispheres can be divided into
– Paleopallium (olfactory lobe)
– Archipallium (hippocampus)
– Basal nuclei
Primitively, the cerebral hemisphere is
merely an olfactory lobe.
– Fibers go either to the tectum, or to the
– Fibers to the basal nuclei (corpus striatum
in mammals), the thalamus, and the
Gray matter moves to the surface to
become the pallium (cloak).
– The paleopallium is still primarily olfactory.
– The archipallium is antecedent to the
hippocampus of mammals. It is a
correlation center in all tetrapods, and is
related to emotional behaviors.
Note – modern teleosts do not use
extensive olfaction, and not surprisingly,
the basal nuclei and paleopallium in
teleosts is displaced.
– Also have reduced sense of smell
– Minimal development of neopallium, but
extensive basal nuclei (hence, bird brain).
– Monotremes and Marsupials