Extended Data Fig. 1. Birthdates of initial classes of INs in macaque.

a. Drawings of the lateral view of developing macaque brains across stages surveyed in this study and estimated comparable human and mouse stages based on the translating time model⁴² of cortical neurogenesis. b. Spatiotemporal distribution of new-born neurons, as determined by RNA velocity latent time, from each class as a proxy for birthdate. Bars represent the proportion of cells from each class in each region, at each timepoint (columns sum to 1). Cajal-Retzius neurons, MGE-derived cortical interneurons, and LGE-derived projection neurons first appeared early in development, starting at PCD40, followed by the later appearance of immature CGE-derived cortical interneurons and LGE_MEIS2/PAX6 neurons, consistent with broad patterns of temporal ordering in mouse^51,55,56. c. UMAP showing which cells are new-born in red (latent time < 0.5 quantile of latent time for each class) or maturing (>0.5 quantile) in blue. Clearly cycling progenitors are not included. d. Kamada-Kawai graph visualization of Pearson correlations between a gene pair’s expression highlight the emergence of initial class gene co-expression patterns during macaque neuronal differentiation. Edges shown are Holm-Šídák corrected q value < 0.05 calculated by bootstrap, with thickness and color representing correlation. e. Stacked bar chart showing number of macaque cells collected at each timepoint, colored by region from which the cells are derived. Note undersampling of VMF structures between PCD50 and PCD100. f. Normalized stacked bar chart showing regions from which each macaque initial class is derived across the whole dataset. g. Violin plot showing the distribution of genes detected per cell for each macaque batch. h. Violin plot showing the distribution genes per cell detected for each mouse dataset.