Figure 1. Gene expression variation in the PFC and CBC.

(A) Principle components of the two brain regions' transcriptomes based on all expressed genes (CBC:12,853; PFC:12,447). Each individual is represented by a circle; colors represent species (red, human; blue, chimpanzee; green, macaque), and lighter shades indicate younger age. The proportion of variance explained by each component is shown in the axes labels. In both regions, the first component corresponds to macaque-hominid divergence, the second to age differences, while the third component (z-axis) separates humans and chimpanzees. (B) Examples of the three types of divergence in the PFC dataset. Each point represents an individual (red, human; blue, chimpanzee; green, macaque), while lines show cubic spline curves. The x-axis shows individuals' age in log₂ scale. Gene names are shown on top of each panel. (C) Numbers of genes assigned to the three divergence types (Table S2). (D) Example neighbor-joining (NJ) representing constitutive and pattern divergence, based on expression profiles of PFC type II and type III genes. Trees were constructed per gene, using Euclidean distances between expression-age trajectories between each pair of species (Text S1). The figure reflects the mean divergence per gene set. (E) Distributions of log₂-transformed human-chimpanzee branch ratios estimated from NJ trees across gene sets. Ratios <|10| are only shown for illustrative purposes. See Table S3 for additional analyses on the distributions.