MadSci Network: Evolution

Re: what are the first animals to have different brain parts?

Date: Thu Jun 17 18:49:40 1999
Posted By: N. Bradley Keele, PhD, Assistant Professor, Dept. Neuroscience, Baylor University
Area of science: Evolution
ID: 925689350.Ev

Hi Molly.

In general, to begin to define brain areas as cerebrum or cerebellum, it it necessary for an organism to show a high degree of encephalization, compartmentalization of function, and a 3 (or 5) lobe developmental brain structure. Given these constraints, we are limited to discussing brains of chordates, namely vertebrates. (My apologies to all who study the nervous systems of anelids, mollusks, and arthropods). Chordates are defined as having (among other characteristics) a thin, rod-like structure, the notochord, at some time during their development, and a nerve cord positioned dorsal to the gut (in humans, the spinal cord). Evolutionarily, the brain of vertebrates developed by the accumulation of nerve cells at the cephalic end of the nerve cord, contributing to the brainstem (hindbrain). Early in the evolution of vertebrates, a special sensory system became associated with each major part of the brain: the olfactory organs with the forebrain, the eye with the midbrain, and the ear and related organs with the hindbrain. Each of the three sections, furthermore, developed dorsal outgrowths of gray matter forming, respectively, the cerebrum, the midbrain roof, or tectum, and the cerebellum. With these developments the three-part brain stem was then transformed into a brain of five regions: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

Comparative neurobiology suggests that the cerebellum first arises in fish, as a specialized structure of the lateral-line system (used for orientation of the organism). Primative fish, such as hagfish and lampreys lack cerebellar structures. However, bony fish (teleosts) have a well-definded cerebellum. Somewhere between these two types of fish, perhaps cartilaginous fish such as sharks, a structure anatomically and functionally similar to mammalian cerebellum first appears.

The question regarding cerebrum is a bit more difficult to answer, based partly on a lack of uniformity of terms. An evolutionary precursor to the cerebrum is the pallium. The pallium is further divided into the archipallium, paleopallium and neopallium, based on when it appears in evolutionary development. Fish have mostly archipallium, which may be a precursor to mammalian hippocampus; amphibians have archipallium and paleopallium and a group of basal nuclei, which are precursors to mammalian basal ganglia, such as the striatum. Neopallium first appears in advanced reptile brains, such as crocodiles. In mammalian brains, neopallium becomes greatly enlarged and is termed neocortex. The cerebrum in mammals is comprised of neocortex.

Unfortunately, our understanding of the evolutionary development of specific brain structures is hindered by a lack of comprehensive comparative studies. This may in part be due to the nature of brain tissue - it doesn't fosilize! Thus, we're left doing some speculation as to when structures of lower vertebrates anatomically and functionally resemble their mammalian counter parts.

I hope this has given you some help.

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