A primate jawbone found in Namibia in southern Africa has been identified by anthropologists as that of an ape that lived between 10 million and 15 million years ago. Researchers generally agree that such ancient primates lived only in dense forests. Consequently, the dry, treeless expanses now dominating the landscape in and around Namibia must have replaced an earlier, heavily forested terrain. The argument assumes which one of the following? ['There were no apes living in the area that is now Namibia prior to 15 million years ago.', 'The ape whose jawbone was found lived in or near the area that is now Namibia.', 'The ancient primates were numerous enough to have caused severe damage to the ecology of the forests in which they lived.', 'Modern apes also tend to live only in heavily forested terrain.']