The telencephalon, which provides the highest forms of nervous activity in humans and other animals, is one of the most important innovations of vertebrates. Although this part of the brain has been described in all living vertebrates, its evolutionary origin is still poorly understood. This article discusses one of the possible approaches to studying the expression and functional properties of genes that regulate the early development of the forebrain in cyclostomes (lampreys) as the most archaic representatives of vertebrates. The results of studies of genes such as Anf, FoxG1, and genes of the Noggin family are described.