542-488 million years ago

jawless fish


The brain of the first fish was larger and more differentiated than that of their chordate ancestors. The new features which had evolved include cerebral hemispheres, olfactory pathways equivalent to those of vertebrates, a semicircular canal, additional cranial nerves, features of the autonomic nervous system, and the ability to taste sour, bitter, and salty tastes.

The medulla developed a choroid plexus (Hardisty, p. 312-3). The thalamus developed a habenular complex (Butler, 1996, p. 303). There is disagreement over whether modern hagfish possess a cerebellum; the development of this brain region reached a more definitive stage by the evolution of the vertebrates (Butler, p. 184). The midbrain included a nucleus of the superior raphe and an interpeduncular nucleus (Butler, 1996, p. 207, 213). There was significant development in the cerebrum in the first craniates. Cerebral hemispheres developed (Ariens, p. 1251) and the major regions of the cerebrum were present: the pallium (both the medial/hippocampal region and the dorsal & lateral/cortex region) and the subpallium (both the striatum and septum) (Kardong, p. 646). There were distinct nuclei in the tegmentum (Butler, 1996, p. 216-7), paired olfactory placodes which formed olfactory bulbs (Ariens, p. 1244), and a preoptic nucleus (Ariens, p.1247).

Although early craniates possessed a spinal cord, its primary function was for the control of simple reflexes operating locally. There were very few fibers which traveled either to or from the brain--only a few fibers from midbrain and medulla pass through the spinal cord (Ariens, p. 278). The dorsal and ventral roots of spinal cord joined to form mixed nerves (although this condition would be absent in some jawless fishes such as lampreys; Ariens;Romer p. 547) and collateral ganglia developed (Kardong, p. 628).

The early craniates possessed rudiments of the sensory systems found in higher vertebrates. Olfactory pathways connected to the hippocampus. (Hardisty, p. 316). Visual pathways connected the retina to the optic tectum (Butler, 1996, p. 244). One semicircular canal had developed (Romer, p. 526). The first craniates possessed all basic vertebrate tastes except for sweet (Ariens, p. 157).

As the head became more important, the cranial nerves better developed. The nuclei of cranial nerves V through X were located in the medulla. The trigeminal nerve developed large maxillo-mandibular and ophthalmic branches (Ariens). The vestibulocochlear nerve developed 2 main branches, the anterior and posterior rami (Ariens, p. 500; Hardisty, p. 312-3).

The early craniates possessed features of the autonomic nervous system which would be retained throughout the vertebrates. They possessed chromaffin cells which could secrete epinephrine to mediate responses to stress, although these cells would be located in the heart prior to the evolution of gnathostomes (Hardisty, p. 359). Autonomic innervation regulated the activities of the gut just as in higher vertebrates: ACh stimulated the gut while NE inhibited it (Hardisty, p. 360). Autonomic fibers were included in the vagus nerve (Romer p. 547).