Mesozoic fear circuits are those that human mammals share with many other mammals. These circuits are thus posited to be hardwired in extant humans primarily by wild-type alleles driven to fixation by Mesozoic selective sweeps with no remaining allele polymorphisms (as defined by lack of allele variants with a frequency over 1%)

Neuroevolutionary Time-depth: ∼ 140 million years (Mesozoic)