The overall objective of this project is to find and define the processes in the evolution of the nervous system and to determine the relationship between the evolutionary history of a structure or system to normal and abnormal development. The hope is that insights into the evolutionary mechanisms will also elucidate interspecific variability of brain organization, which in turn may provide structure-function correlation. More importantly, the studies may elucidate what evolutionary and ontogenetic constraints determine what synapses are possible between neurons and which are not. Since evidence about the evolution of circuitry cannot be obtained from fossils, Dr. Ebbesson designed an approach to study brain evolution 20 years ago that involves a systematic examination of a given neural system in the broadest possible spectrum of vertebrates with the hope that principles of evolution would emerge from the pattern of the data. The initial objectives have now been reached in that the basic organization and variability of several sensory systems, including the ascending spinal and the visual systems in representative samples of most major vertebrate groups, are now known. The many pathways discovered and described during the last 20 years by a number of investigations point to several operational evolutionary strategies. One of these has been described as the parcellation theory which states that diffuse, undifferentiated, overlapping neural systems existed in the beginning of vertebrate evolution and that during evolution of complex behaviors, and analytical capacities related to these behaviors, a range of neural systems evolved, not by de novo appearance or one system invading another, but by a process of segregation and isolation (parcellation) that involves the formation of more isolated subcircuits by the selective loss of connections. The resultant daughter systems individually lack one or more connections of their ancestors. The evolutionary process is thought to be reflected in the normal ontogenetic development of a given system where a similar segregation- isolation process take place. The objective of Dr. Ebbesson for the next three years is to test and define this theory by defining connections more precisely in the sensory pathways in species from three vertebrate classes that are likely to have extensive overlap of systems; i.e., unparcellated systems. These animals include the bowfin, the nurse shark, and the tiger salamander.
