Mental time travel, language, and evolution.

Tulving (1985) was probably the first to use the term "mental time travel" to describe the human capacity to imagine personal events from the past, as well as to envisage possible future ones. He and others have also claimed that this capacity is unique to humans. One likely reason for this is that mental travel into the past, or episodic memory, is part of what has been termed declarative memory-or memory that can be declared. This implies a critical role for language itself, also generally regarded as unique to humans. I argue here that language probably did evolve to enable explicit reference to the nonpresent, a property known as displacement. This need not imply, though, that mental time travel is itself unique to humans. Behavioral evidence from a number of different species, including birds, mammals, and apes, increasingly implies the ability to recall specific events from the past, and even to imagine possible future ones. Brain recordings from both rodents and humans also show the hippocampus to be active in both replay of past events and the "play" of events that did not actually occur, including imagined future ones, suggesting an evolutionary continuity. The wider hippocampal/entorhinal system, with likely involvement of other cortical areas, has a generative capacity, underlying spatiotemporal imagination in both humans and nonhuman animals, which may underlie the generativity of language itself. As a communication system, though, language evolved late as a means of sharing imaginative explorations, or what Dor (2015) termed "the instruction of imagination."