Evolution of the Chordate Telencephalon

Abstract

The dramatic evolutionary expansion of the neocortex, together with a proliferation of specialized cortical areas, is believed to underlie the emergence of human cognitive abilities. In a broader phylogenetic context, however, neocortex evolution in mammals, including humans, is remarkably conservative, characterized largely by size variations on a shared six-layered neuronal architecture. By contrast, the telencephalon in non-mammalian vertebrates, including reptiles, amphibians, bony and cartilaginous fishes, and cyclostomes, features a great variety of very different tissue structures. Our understanding of the evolutionary relationships of these telencephalic structures, especially those of basally branching vertebrates and invertebrate chordates, remains fragmentary and is impeded by conceptual obstacles. To make sense of highly divergent anatomies requires a hierarchical view of biological organization, one that permits the recognition of homologies at multiple levels beyond neuroanatomical structure. Here we review the origin and diversification of the telencephalon with a focus on key evolutionary innovations shaping the neocortex at multiple levels of organization.

Artificial mosaic brain evolution of relative telencephalon size improves cognitive performance in the guppy (Poecilia reticulata)

“…Phylogenetic comparative analyses on vertebrate telencephalon evolution are not uncommon. For instance, the neocortex makes up the majority of the telencephalon in primates (Briscoe & Ragsdale, 2019). Group social complexity, diet and cognitive performance are positively associated with neocortex size in this taxon (Shultz & Dunbar, 2010; Reader, Hager & Laland, 2011; DeCasien & Higham, 2019).…”

Distribution of the cholinergic nuclei in the brain of the weakly electric fish, Apteronotus leptorhynchus: Implications for sensory processing

“…In the light of recent studies that revealed several homologies between pallial and subpallial areas in fish and mammals (Briscoe & Ragsdale, 2019; Ganz et al, 2012; Giassi, Duarte, et al, 2012; Giassi, Ellis, & Maler, 2012; Giassi, Harvey‐Girard, et al, 2012; Harvey‐Girard et al, 2012; Nieuwenhuys, 2009; Wullimann & Mueller, 2004), it is worth mentioning that, in areas ascribed to the dorsal telencephalon (Giassi, Duarte, et al, 2012; Giassi, Ellis, & Maler, 2012; Giassi, Harvey‐Girard, et al, 2012; Harvey‐Girard et al, 2012), no ChAT‐positive cells were detected. This situation is similar to that described for the pallium of most other fish species (Lopez et al, 2012; López et al, 2013; Morona et al, 2013; Mueller et al, 2004).…”

Genetically-identified cell types in avian pallium mirror core principles of excitatory and inhibitory neurons in mammalian cortex

“…The vertebrate pallium is considered essential for advanced cognitive abilities. Unlike other sectors of the vertebrate brain, the cytoarchitecture and elaboration of pallial domains vary immensely across vertebrate classes, and even within closely related taxa 1 . In mammals, for example, dorsal pallium develops into a six-layered structure called neocortex, the elaboration of which is associated with advanced cognitive abilities 2,3 .…”

“…In mammals, for example, dorsal pallium develops into a six-layered structure called neocortex, the elaboration of which is associated with advanced cognitive abilities 2,3 . In cartilaginous fishes, some taxa have a layered pallium, while in other species the organization of pallium is not at all obvious, and difficult to separate from subpallial structures 1 . Large swaths of bird pallium are unlayered 4,5 (Fig.…”