Figure 6.

Summary of temporal change of gene expressions among mouse, marmoset and macaque neocortex (A) and hypothetical model of tool use associated changes (B) and brain expansion of primates (C). Tool use induces gene expressions (e.g. SPON2 and ADAM19) in efferent neurons of the SII area. Their gene products function in the intraparietal sulcus (IPS) and induce cortical changes. In turn, this makes subsequent gene expression changes (e.g. WIF1) in the neurons or oligodendrocytes of the IPS area. Reciprocal interaction of these gene products may cause plastic changes such as axogenesis, myelination or synaptic changes (B). Acquisition of activity‐dependent gene expressions might have accelerated brain expansion by both innate and behavior‐dependent changes simultaneously acquired (C).