The human neocortex's size and surface area are critical for intellectual ability. Recent studies show that the development of the gyrated human neocortex involves a lineage of neural stem and transit-amplifying cells forming the outer subventricular zone (OSVZ), a proliferative region outside the ventricular epithelium. This process expands the neocortex by increasing neuron number and modifying neuron migration trajectories. Comparing these features to other mammals and known molecular regulators of the mouse neocortex suggests evolutionary mechanisms for this development. The neocortex in mammals evolved to enable higher cognitive functions, with variations in shape, size, and neuron number among species. Rodent neocortex is small and nonfolded, limiting its ability to model larger, folded neocortex. Evo-devo approaches, using comparisons of living species, suggest that small evolutionary changes in key regulators can significantly impact species development. Differences in brain development mechanisms across species may reflect evolutionary variations of ancestral mechanisms. Human, nonhuman primate, carnivore, and marsupial neocortex development reveals how neural progenitor cell populations vary in size and shape. Expansion of progenitor cells in the OSVZ during development increases neocortical volume and surface area, particularly in humans. The radial unit hypothesis, proposed by Pasko Rakic, describes neocortical development through proliferative units translating into ontogenetic columns and proto-maps. This hypothesis was later refined by studies showing that radial glia (RG) cells generate neurons through asymmetric divisions and radial fiber migration. These studies also revealed that intermediate progenitor (IP) cells, which undergo symmetric divisions, contribute significantly to neuron production. The OSVZ contains progenitor cells that can be RG-like or IP-like, with the latter undergoing multiple rounds of cell division before generating neurons. This process is crucial for neocortical expansion, as it allows for increased neuron number and surface area. The expansion of the OSVZ is influenced by factors such as cell proliferation and death rates, with genetic studies showing that the TBR2 gene is critical for SVZ neurogenesis. The OSVZ in primates is larger and more complex than in rodents, with a distinct inner and outer region. This region contains progenitor cells that express markers of RG and IP cells, contributing to neocortical expansion. The expansion of the OSVZ is associated with increased progenitor cell proliferation and the generation of neurons. Studies in ferrets and humans show that the OSVZ contains progenitor cells that can self-renew and generate neurons. These cells, called oRG cells, have unipolar fibers and can divide asymmetrically to produce bipolar daughter cells. The expansion of the OSVZ is linked to the increased number of progenitor cells and their ability to generate neurons. The radial unit hypothesis has been revised to include the role of the OSVZ in neocortical development, with oRG cellsThe human neocortex's size and surface area are critical for intellectual ability. Recent studies show that the development of the gyrated human neocortex involves a lineage of neural stem and transit-amplifying cells forming the outer subventricular zone (OSVZ), a proliferative region outside the ventricular epithelium. This process expands the neocortex by increasing neuron number and modifying neuron migration trajectories. Comparing these features to other mammals and known molecular regulators of the mouse neocortex suggests evolutionary mechanisms for this development. The neocortex in mammals evolved to enable higher cognitive functions, with variations in shape, size, and neuron number among species. Rodent neocortex is small and nonfolded, limiting its ability to model larger, folded neocortex. Evo-devo approaches, using comparisons of living species, suggest that small evolutionary changes in key regulators can significantly impact species development. Differences in brain development mechanisms across species may reflect evolutionary variations of ancestral mechanisms. Human, nonhuman primate, carnivore, and marsupial neocortex development reveals how neural progenitor cell populations vary in size and shape. Expansion of progenitor cells in the OSVZ during development increases neocortical volume and surface area, particularly in humans. The radial unit hypothesis, proposed by Pasko Rakic, describes neocortical development through proliferative units translating into ontogenetic columns and proto-maps. This hypothesis was later refined by studies showing that radial glia (RG) cells generate neurons through asymmetric divisions and radial fiber migration. These studies also revealed that intermediate progenitor (IP) cells, which undergo symmetric divisions, contribute significantly to neuron production. The OSVZ contains progenitor cells that can be RG-like or IP-like, with the latter undergoing multiple rounds of cell division before generating neurons. This process is crucial for neocortical expansion, as it allows for increased neuron number and surface area. The expansion of the OSVZ is influenced by factors such as cell proliferation and death rates, with genetic studies showing that the TBR2 gene is critical for SVZ neurogenesis. The OSVZ in primates is larger and more complex than in rodents, with a distinct inner and outer region. This region contains progenitor cells that express markers of RG and IP cells, contributing to neocortical expansion. The expansion of the OSVZ is associated with increased progenitor cell proliferation and the generation of neurons. Studies in ferrets and humans show that the OSVZ contains progenitor cells that can self-renew and generate neurons. These cells, called oRG cells, have unipolar fibers and can divide asymmetrically to produce bipolar daughter cells. The expansion of the OSVZ is linked to the increased number of progenitor cells and their ability to generate neurons. The radial unit hypothesis has been revised to include the role of the OSVZ in neocortical development, with oRG cells