EARLY AND RECENT PRIMITIVE BRAIN FORMS

The quest for a primitive vertebrate brain-one in which fundamental neuronal pathways can be studied relatively unobscured by the multiplicity of complex circuits present in the mammalian organ-has led comparative zoologists and clinical neurologists t o explore the nervous system of fishes. It was among these animals, logic suggested, that brains might exist that would offer a clue t o the early steps in vertebrate encephalization. The literature supports the supposition t o a degree: it contains a considerable amount of descriptive material, much of it traceable t o the interest in vertebrate structure generated by the Darwinian controversy. However, those hoping for insight into the nature of the primitive brain and its further evolution soon recognize from the variety of the brains classed as piscine that finding a simple, primary vertebrate brain will not be easily done. Under the heading of fishes, research workers provincial in their terrestriality have lumped animals that differ widely from one another, belong t o lines that have been genetically separate from each other for hundreds of millions of years, and possess brains that are visibly dissimilar. Furthermore, these lower vertebrates are not all primitive. They lack the structures that make subaerial life possible, but they have evolved along several divergent paths far from the state that characterized the first members of the vertebrate assemblage. Comparative neurologists who are curious about a particular type of fish brain may study it for its own sake, but investigators seeking information applicable in some way t o the brains of higher terrestrial forms are obliged to acquaint themselves with the vertebrates’ family tree. The jawless, limbless hagfishes and lampreys, for instance, which are often labeled the most primitive living vertebrates, are not truly surviving representatives of the first vertebrates in anything like their original state. Hagfishes and lampreys may display some traits that are primitive; but they have also specialized, unique characters and some apparently simple structures that have evolved through retrogressive change. Their connection with the basal vertebrate stock may be indirect, through descent from an extinct assemblage of armored forms called ostracoderms, which were already diversified and specialized when they appeared in the fossil record nearly 500,000,000 years ago. To suggest that any aspect of the brain unique t o one of these creatures foreshadows the appearance of an apparently similar modification of the higher vertebrate brain is t o misinterpret the evidence. Even the earliest fishes with jaws and paired fins-the extinct acanthodians and placoderms-do not supply brains acceptable as primitive. These fishes too had evoked adaptively , developing certain peculiar traits in the process. Rather than looking for primitive brains, the investigator must discern those characteristics in the brains of lower vertebrates which may be primitive-that is, structural and functional patterns that seem t o have been carried, conservatively, in several piscine lines, while other, more labile traits were molded in highly special ways. The same kind of discrimination is called for o n the part of the investigator