The applicant proposes to perform experiments on the brains of lungfishes that will determine if they are the closest living relative to the ancestor of all land vertebrates, including humans. Brain organization, particularly the organization of the forebrain, provides valuable clues to the biology of an animal and to taxonomic relationships among animals. Further data on brain organization in lobe-finned fishes is critical, as some early member of the lobe-finned fishes gave rise to the first land vertebrates. Living lobe-finned fishes include the coelacanth and the lungfishes; most are relatively rare, almost all are threatened; and coelacanths are impossible to obtain on demand. Experimental studies comparing coelacanth brains are therefore not feasible. Current opinion holds that lungfish brains are highly aberrant, but the applicant has generated a new model of forebrain organization in the African lungfish. This model suggests that forebrain organization in lungfishes is far more similar to that in land vertebrates than has been recognized. He now proposes to test three hypotheses inherent in this model by using biological tracers to reveal certain features of lungfish brains. He will compare these features with those in amphibians (the first land vertebrates). If his hypotheses are valid, then lungfishes are the living lobe-finned fishes most closely related to the ancestor of all land vertebrates, and their forebrain organization can be used to infer forebrain organization in the common ancestor of lungfishes and land vertebrates. This work will provide insights into how vertebrate nervous systems have subserved new mechanisms of behavior and change. Data from the project are expected to reveal a robust adaptive strategy for the central nervous system, extending back to the earliest land vertebrates. The data will also provide valuable new information about the first land animals for neurobiologists, evolutionary biologists, paleontologists and general vertebrate biologists.