参考答案

正确答案：evolved at least 1.6 million years ago
evolved at least 1.6 million years ago 解析：由题干中的stone tools定位到第9段头两句Dr．Suddendorf argues that these overlapping networks for mental time travel evolved...此处需要填入动词性成分。题干中的primitive people与原文中的，primitives属同义转述；human episodic memory与前一句these overlapping networks for mental time travel相对应。另外题干中的primitive people carried stone tools with them是对Dr．Suddendorf所举事例的注解。综上可知答案。