Prehistoric chimpanzee species used tools similar to those used by prehistoric humans; prehistoric tools recently found in East Africa are of a type used by both species. The area where the tools were found, however, is a savanna, and whereas there were prehistoric humans who lived in savanna habitats, prehistoric chimpanzees lived only in forests. Ttherefore, the tools must have been used by humans rather than by chimpanzees. Which one of the following is an assumption on which the argument depends? ['The prehistoric ancestors of modern chimpanzees were not capable of using tools more sophisticated than those found recently in East Africa.', 'Prehistoric humans did not carry their tools with them when they traveled from one place to another.', 'Prehistoric humans never ventured into areas of the forest that were inhabited by prehistoric chimpanzees.', 'The area where the tools were found was not a forest at the time the tools were in use.']