Final answer:
During the Oligocene epoch, some monkeys began to adapt to life on the savannah, although the shift to upright bipedal walking is more associated with later hominids in the Pliocene and Pleistocene epochs.
Step-by-step explanation:
During the Oligocene epoch, monkeys evolved one more time, shifting some of their members from arboreal lifestyles in the trees to life on the savannah. It is important to note that while New World monkeys remained primarily arboreal, some Old World monkeys started to adapt to life on the ground. The transition to a life on the savannah where upright walking on two legs is advantageous is more accurately associated with the later hominids, particularly during the Pliocene and Pleistocene epochs, culminating in the emergence of bipedal creatures such as Australopithecus afarensis, known as Lucy, who lived about 3 million years ago.