Figure 1. Comparison of brains and circuit organization across vertebrates.

(a) Simplified phylogeny and brain evolution in amniotes. Left: Schematic diagram depicting phylogenetic relationships across major amniote lineages; branches containing bird lineages are highlighted in blue. Only select major extant groups are depicted; dashed line indicates dinosaur lineage extinction. Red asterisks indicate species/groups that possess the trait of vocal learning. Right: Schematic diagrams of representative vertebrate brains highlighting the relative sizes of pallial (orange) and subpallial (pink) areas in representative synapsids and sauropsids. (b) Schematic generic representation of the ‘nuclear’ vs. ‘layered’ hypothesis of pallial organization in avian (top) and mammalian (bottom) brains (after Karten, 1991). Color coding indicates analogous (and possibly homologous) elements between lineages. (c) Schematic diagrams of specialized pathways for birdsong (top) and speech (bottom). Top: Vocal control circuitry of the adult male zebra finch, depicted in the parasagittal plane; some structures have been omitted for clarity. The system consists of a direct motor pathway for song production (in black) that includes connections from HVC to RA, and from RA to brainstem nuclei involved in vocal-motor and respiratory control (nXIIts, RAm), and an anterior forebrain pathway for song learning (in white) containing a cortico-striato-thalamo-cortical loop with projections from pallial LMAN to striatal Area X, from Area X to thalamic DLM, from DLM to LMAN, and from LMAN back to Area X. Additional connections are shown in grey. Bottom: Proposed vocal control circuitry in humans consisting of a direct motor pathway for speech production (in black), and striatal brain regions (Cd, Pt) that have been proposed to be involved in speech learning (in white), adapted from Pfenning et al. 2014. The descending projections from RA and dLMC to brainstem vocal nuclei are thought to be convergently evolved in songbirds and humans. Abbreviations: Am, nucleus ambiguus; Cd, Caudate nucleus of the striatum; dLMC, dorsal part of the laryngeal motor cortex; DLM, medial part of the dorsolateral nucleus of the anterior thalamus; GP, Globus pallidus; HVC, HVC proper name; LMAN, lateral magnocellular nucleus of the anterior nidopallium; nXIIts, tracheosyringeal portion of the nucleus of the hypoglossal nerve (XII); PAG, periaqueductal grey; Pt, Putamen; RA, robust nucleus of the arcopallium; RAm, nucleus retroambiguus medialis; X, Area X.