Final answer:
Old World monkeys expanded into the ecological niches left by the decline of the Great Apes in Africa, adapting to a wide range of environments due to their dietary and behavioral flexibility.
Step-by-step explanation:
During the decline of the Great Apes in Africa, it was the Old World monkeys, with their adaptability and wide array of species, that moved in to fill the vacant ecological niches. Old World monkeys, which include those in the family Cercopithecoidea, evolved to occupy both arboreal and ground habitats, adapting to a variety of ecological roles previously held by apes. These species, showing greater diversity in size, form, and behavior than the apes, have successfully thrived in the changing environment of Africa.
Even as many ape species began to decline, either for environmental reasons or due to competition, monkeys diversified and gradually took over niches left open, using their agility, smaller size, and often social structures to exploit new ecological opportunities. Some of the more adaptable and widespread Old World monkeys include the baboons and macaques. Their varied diets and behavioral flexibility have allowed them to flourish in areas where certain ape populations have dwindled.