All Old World monkeys, apes, and humans share this 2.1.2.3 dental formula.  This not only sets us apart from New World monkeys and prosimians, but it also reflects the evolutionary closeness of the Old World anthropoid species.  By comparison, the general placental mammal dental formula is 3.1.4.3.

Female hamadryas baboon in estrus (note the sexual skin)

In some species of Old World monkeys and apes, adult females have sexual skins or swellings, which are nearly hairless large swollen patches of skin around the genital area that become very prominent when they are in .  These areas swell with fluids and turn bright pink or red due to hormonal changes that occur in preparation for ovulation.  The sexual skin also produces odors that excite males of the species. They become highly attentive to the females at this time.  In the case of olive baboons, males are most attracted to females with the largest sexual skin bulge.  Those females tend to have babies more often and subsequently pass on the genes for this impressive signal of sexual readiness.