We first asked whether recently evolved proximal promoters are primarily found in ancestral DNA sequences older than 100 Ma (Figure 6A, Figure S7). To our surprise, we discovered that across four orders of mammals, the recent evolution of promoters occurred within evolutionarily younger DNA segments (i.e., not shared with other study species) about three to four times as often as occurred by exaptation of ancestral DNA. For instance in mouse, 1,400 recently evolved promoters occurred in DNA sequences present only in this species (i.e., not shared even with rat); in contrast, only 260 recently evolved promoters were found in ancestral DNA.