Exp Brain Res (2009) 192:553–560

DOI 10.1007/s00221-008-1491-9

REVIEW

Mental time travel and the shaping of language

Michael C. Corballis

Received: 24 February 2008 / Accepted: 27 June 2008 / Published online: 19 July 2008© Springer-Verlag 2008

Abstract Episodic memory can be regarded as part of amore general system, unique to humans, for mental timetravel, and the construction of future episodes. This allowsmore detailed planning than is aVorded by the more generalmechanisms of instinct, learning, and semantic memory. Tobe useful, episodic memory need not provide a complete oreven a faithful record of past events, and may even be partof a process whereby we construct Wctional accounts. Theproperties of language are aptly designed for the communi-cation and sharing of episodes, and for the telling of stories;these properties include symbolic representation of the ele-ments of real-world events, time markers, and combinato-rial rules. Language and mental time travel probably co-evolved during the Pleistocene, when brain size increaseddramatically.

Keywords Memory · Episodic memory · Mental time travel · Language · Evolution

Introduction

“Life’s more interesting phenomena,” he replied,“probably always have this Janus face towards thepast and the future, are probably always progressiveand retrogressive in one. They reveal the ambiguity oflife itself.”Thomas Mann, Doktor Faustus

Tulving (1985) introduced an important distinctionbetween knowing and remembering. I am privileged toknow Giovanni Berlucchi, and appreciate his stature as sci-entist, leader, and family man. Of all the scientists I know,he most epitomizes the ideal of scholar and gentleman. ButI also remember Giovanni, and see him in my mind’s eyescurrying from his oYce to the library in the Institute atVerona to fetch an article he thinks I should read, or escort-ing me across a street near the lab to buy me a drink in abar. I have many images of his genial company.

The distinction between knowing and remembering isalso that between semantic and episodic memory (Tul-ving 1972). Semantic memory is our vast storehouse offacts about the world, the combined dictionary and ency-clopedia of the mind. Episodic memory is the memoryfor events, the mind’s personal diary. To some extent thetwo must be related. Tulving (2002) has argued, forinstance, that the storage of episodic memories mustdepend on semantic memories that are already in place,but are then related to the self in subjectively sensedtime. This allows the experience of an event to be storedseparately from the semantic system. Yet there is alsoevidence that semantic and episodic memory can bedoubly dissociated. In most cases of amnesia, episodicmemory is lost while semantic memory remains largelyintact (e.g., Tulving et al. 1988). Conversely, peoplewith semantic dementia, a degenerative neurologicaldisorder that aZicts some people in late adulthood, showsevere decline in semantic memory, but their episodicmemories remain remarkably and surprisingly intact(Hodges and Graham 2001).

In this essay I propose to build on the concept ofepisodic memory to encompass mental travel into thefuture as well as the past, and the evolution of syntacticlanguage.

M. C. Corballis (&)Department of Psychology, University of Auckland, Private Bag 92019, Auckland 1042, New Zealande-mail: m.corballis@auckland.ac.nz

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Mental time travel

“It’s a poor sort of memory that only works back-wards,” the Queen remarked.·Lewis Carroll,Through the Looking Glass.

The notion that episodic memory might be part of amore general mechanism to travel forwards as well as back-wards in time was developed by Suddendorf and myself;we called it mental time travel (Suddendorf and Corballis1997, 2007). Perhaps the Wrst to suggest this notion,though, was the physiologist David Ingvar, a pioneer in theuse of regional cerebral blood Xow to measure brain activ-ity, who concluded that activation in the prefrontal cortexwas especially important in providing the internal connec-tion between past and future. In a doubly premonitory pas-sage, he wrote:

On the basis of previous experiences, represented inmemories, the brain—one’s mind—is automaticallybusy with extrapolation of future events and, as itappears, constructing alternative hypothetical behav-ior patterns in order to be ready for what may happen(Ingvar 1979, p 21).

More recent brain-imaging studies show that remember-ing past events and imagining future ones activate the samecore network in the brain (Addis et al. 2007), and peoplewith amnesia following hippocampal damage have as muchdiYculty imagining new experiences as remembering pastones (Hassabis et al. 2007). The transition from past tofuture is of course not entirely seamless, since we are gen-erally, though perhaps not always, aware of which eventswere in the past and which were Wgments of an imaginedfuture. Nevertheless it seems clear that both directions ofmental travel call on largely overlapping cortical mecha-nisms.

According to this wider perspective, then, episodicmemory is not so much a memory system as part of a sys-tem for constructing future events, based on vocabularies ofevents from the past. Episodic memory is in fact notori-ously fragile and incomplete. With the exception of thatarising from semantic dementia, amnesia typically aVectsepisodic memory rather than semantic memory. In one dra-matic case, for example, a patient with extensive damage tothe frontal and temporal lobes was unable to recall any spe-ciWc episode from his life, yet retained semantic knowledge(Tulving et al. 1988). Even without brain injury, peopleprobably remember only a tiny fraction of actual past epi-sodes (Loftus and Loftus 1980), and events are oftenremembered inaccurately, even to the point that people willclaim with some certainty to remember events that did notin fact happen (Loftus and Ketcham 1994; Roediger andMcDermott 1995).

Mental time travel, then, is an act of construction. This isclearly true of imagining the future, but we also constructour pasts, often embellishing the true facts. As Neisser(2008) put it, “Remembering is not like playing a tape orlooking at a picture; it is more like telling a story” (p. 88).In this sense, episodic memory evolved not to provide afaithful record of the past, but rather to provide the materialfrom which to construct detailed scenarios for the future.Instinct, learning, and semantic memory also allow foradaptation to future contingencies, but episodic memoryprovides an extra degree of detail that enables us to planwith greater precision and speciWcity than is possiblethrough more general adaptive mechanisms. In the cause offuture planning, episodic memory need not even place ahigh premium on truth, and human culture is imbued withother sources, such as history, gossip, and Wction, that pro-vide extra material upon which to build our lives. Stories inthe form of novels, plays, movies, or television soaps are allavidly consumed, and no doubt have an adaptive function.And I shall suggest below that language itself evolved toallow us to share our episodes and add richness to our epi-sodic vocabularies.

Mental time travel may also have enabled the sense oftime itself. Just as we humans are obsessed with stories, sowe are obsessed with time, in the form of clocks, calendars,appointments, schedules, and taxes. The very concept ofself may derive from the sense that we have experiencesthrough time, past and future—the self, in other words,transcends the present. The sense of time leads inevitably tothe understanding of death, and may be at least partlyresponsible for the emergence of religions that encouragethe comforting—although occasionally damning—belief inlife after death. The sense of time no doubt helps us adaptmore eYciently to the complexities of human existence, butit is at times (sic) a mixed blessing.

Uniquely human?

He said “What’s time? Now is for dogs and apes!Man has Forever!”—Robert Browning, A Grammar-ian’s Funeral.

Tulving (1972) argued that episodic memory wasuniquely human, and Suddendorf and I proposed more gen-erally that only humans have the capacity to travel mentallythrough time (Suddendorf and Corballis 1997, 2007). Ofcourse other animals do have memories. A dog may recallwhere a bone is buried, and some birds have prodigiousmemories of where they have cached food. Clark’s nut-crackers seem to be among the most proliWc, storing seedsin thousands of locations and recovering them with high(but not perfect) accuracy (e.g., Kamil and Balda 1985).The question, though, is whether these feats depend on

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episodic or on semantic memory. That is, do the birdsremember actually caching the seeds or do they simplyknow where they are cached?

It has been suggested that an animal might be consideredto demonstrate episodic memory if it can be shown toremember what happened, where it happened, and when ithappened. This has been dubbed the www criterion (Sud-dendorf and Busby 2003). This criterion, it has beenclaimed, has been met in scrub jays, which can select thelocations of food they have previously cached not onlyaccording to the type of food that is stored there, but alsoaccording to how long it has been stored, implying that theyremember when it was stored. For example, they willrecover recently cached worms in preference to nuts, sincefresh worms are more palatable, but if the worms have beencached for too long they will retrieve nuts, because theworms will have decayed and become unpalatable (Claytonet al. 2003). If another jay observes them caching food,they will later re-cache it, presumably to prevent theobserver stealing the food. They will only do this, however,if they have themselves stolen food in the past—it takes athief to know a thief (Emery and Clayton 2001). Claytonet al. (2003) conclude that scrub jays can not only remem-ber the what, where, and when of past events, but can alsoanticipate the future by taking steps to avoid future theft.

Meadow voles, it has been claimed, also have the capac-ity to remember what, where, and when. In an experimentby Ferkin et al. (2008), male voles were Wrst allowed toexplore two chambers, one containing a pregnant female24 h pre-partum, and the other containing a female that wasneither lactating nor pregnant. Twenty-four hours later,they were again given access to the chambers, which werenow empty and clean, and spent more time exploring thechamber that had contained the previously-pregnant femalethan the one that had housed the other female. This sug-gests that they had remembered the pregnant female, andunderstood that she would now be in postpartum estrus, astate of heightened receptivity. In another condition theyWrst explored a chamber containing a female in postpartumestrus and another containing a female that was neither lac-tating nor pregnant, and was not in estrus. Twenty-fourhours later they were again allowed to explore the cages,now clean and empty, and showed no preference for thechamber that had housed the female in estrus. This suggeststhat they realised the female would not longer be in a stateof heightened receptivity.

These studies are ingenious, but perhaps do not provethat the animals—scrub jays or voles—actually remem-bered the episodes. Associative memory might be suYcientto link an object with a location, and a time tag or “use-by”date might then be attached to the representation of theobject to update information about it. More generally, thewww criterion may not be suYcient to demonstrate true

episodic memory anyway. I know where I was born, when Iwas born, and indeed what was born, but try as I might I donot remember it; this is semantic memory, not episodicmemory. Our semantic memories are replete with knowl-edge about events in the past, but this does not mean thatwe can mentally relive these events. A recent study alsosuggests that rats can learn to retrieve food in a radial mazeon the basis of how long ago it was stored, but not on whenit was stored, suggesting that “episodic-like memory in ratsis qualitatively diVerent from episodic memory in humans”(Roberts et al. 2008, p. 113).

Similarly, there is a subtle distinction between mentaltime travel into the future, and planning on the basis offuture knowledge. We can know that the sun will risetomorrow, that an aunt is coming to stay, that the weatherwill soon get colder, and we can plan accordingly. But thisis not the same as mentally constructing a future episode, orperhaps several versions of a future episode, so that theymay be compared and evaluated. Thus the scrub jay that re-caches the food when a likely predator watched the originalcaching may be mentally anticipating a future theft, or itmay simply have learned through association that re-cach-ing in this situation leads to increased likelihood of success-ful retrieval. Nevertheless the question of whether non-human species are capable of true mental time travel,whether forward or backward in time, is by no meansclosed, and the experiments so far conducted provide usefulleads. An especially diYcult aspect to test in non-humananimals is the subjective component, although it is conceiv-able that neurophysiological evidence may eventually bebrought to bear on this issue. In humans, the subjectivesense of recollection seems to depend on activation in thehippocampus and posterior parahippocampal gyrus (Dianaet al. 2007), although the amygdala rather than the parahip-pocampal gyrus seems to be involved in emotional memo-ries (Phelps and Sharot 2008). Recordings fromhomologous areas in non-human animals may provide awindow into their subjectivity.

One aspect of human episodic memory that is not evi-dent in any of the animal studies so far carried out is gener-ativity. This is perhaps not a property of the memorysystem itself, but is nonetheless critical to our ability toconstruct episodes based on remembered information, andas Ingvar (1979) anticipated may depend critically on thefrontal lobes (see also Wheeler et al. 1997). Our memoriesfor episodes are made up of combinations of people,actions, objects, places—along with qualities such as timeof day, weather, season, mood, emotional states, and thelike. Imagined future events are similarly constructed, andwe may in fact compose diVerent scenarios depending per-haps on the weather, or on whether or not Aunt Myrtle orUncle Fred will show up. This combinatorial structure ofepisodic memory leads naturally to language, itself a

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faculty generally regarded as uniquely human, at least in itsproperties of recursiveness and generativity (e.g., Hauseret al. 2002).

Language

Language, in its nature successive, is part of the fallenworld, the world of time.—Sir Frank (Kermode2001), Palaces of Memory.

Language is exquisitely designed to express “who didwhat to whom, what is true of what, where, when andwhy,” as Pinker (2003, p. 27) put it, in an extravagance ofws. This is precisely what is needed to recount episodicmemories—or indeed to tell Wctional stories. Pinker mightalso have added future events—who will do what to whom,or what will happen to what, where, when and why. Whenconsidering the future, the conditional may also be impor-tant—if it rains, then X will happen; if it does not we mayenjoy Y. To a large extent, then, the stuV of mental timetravel is also the stuV of language.

If the experiences of events are to be shared, the timedimension places a much greater burden on the communi-cation channel than if events are experienced only in thepresent. Events in the present are shared by mutual experi-ence, and it may take only a few signals to direct attention,or to convey the importance of some components ratherthan others. Animals will sometimes use simple signals todraw attention to events that their conspeciWcs may not beable to see, as when chimpanzees use pant hoot calls to sig-nal the discovery of food (Goodall 1986), or vervet mon-keys use diVerent calls to warn of diVerent predators(Cheney and Seyfarth 1990), but no syntax or combinato-rial structure is necessary. The time dimension vastlyincreases the mental canvas, since reference to diVerenttimes generally involves diVerent places, diVerent actions,diVerent actors, and so on. In order to represent or refer toepisodic elements that are not available in the present, weneed very large vocabularies of concepts, as well as ofwords to represent them. And we need rules to represent theway in which the elements of an event are combined, andcorresponding rules to convey these combinations to othersin the form of language.

The rules of language must depend to some extent on therules by which episodes are mentally constructed, but theyare not identical to them, since they are also constrained bythe communication channel. Speech, for example, requiresthat the information be “linearized,” squeezed into asequence of sounds that bears little resemblance to the orig-inal episode. This inevitably requires that the ingredients ofthe episode be represented by symbols that are largely arbi-trary. The linguist Charles Hockett put it this way:

… when a representation of some four-dimensionalhunk of life has to be compressed into the singledimension of speech, most iconicity is necessarilysqueezed out. In one-dimensional projections, an ele-phant is indistinguishable from a woodshed. Speechperforce is largely arbitrary; if we speakers take pridein that, it is because in 50,000 years or so of talkingwe have learned to make a virtue of necessity (Hock-ett 1978, p. 274–275).

The sequential nature of speech also means that lan-guage does not map precisely onto the action in an episodeitself; for example, simultaneous events must be recountedsequentially, and language devices must be invented toexpress simultaneity.

Signed languages, which are no less Xexible or sophisti-cated than speech (Emmorey 2002; Neidle et al. 2000), canallow representations that are more iconic, and that conveyepisodes more directly, since manual signs are themselvescomposed in four-dimensional space. It has been estimated,for example, that in Italian Sign Language some 50% of thehand signs and 67% of the bodily locations of signs stemfrom iconic representations (Pietrandrea 2002). But even insigned languages, signs tend to become less iconic andmore arbitrary over time, in the interests of speed andeYciency. This process is known as conventionalization(Burling 1999).

If it is to convey information about episodes, languagemust include some mechanism for conveying when epi-sodes occurred, or will occur. In many languages this isaccomplished by tense markers. In English, for example,verbs describing actions and states are endowed withtense to indicate diVerent points in time, as well as dis-tinctions between conditional and unconditional, continu-ous and non-continuous, and so on. Thus the words walk,walked, and walking, along with auxiliaries (e.g., willwalk, might have been walking), refer to diVerent times ortiming conditions to do with a perambulatory event. Somelanguages have no tenses as such, but have other ways ofindicating time. In Chinese, for example, the time of anevent can be indicated by adverbs, such as tomorrow, andwhat are called aspectual markers, as in a sentence thatmight be roughly rendered as He break his leg before (Lin2005).

The structure of language probably also reXect the com-plexities and requirements of culture. For example, the lan-guage spoken by the Pirahã, a tribe of some 200 people inBrazil, has only a very primitive way of talking about rela-tive time, in the form of two tense-like morphemes whichseem to indicate simply whether an event is in the presentor not. Pirahã also includes a few words which indicatesuch temporal markers as night, day, full moon, and so on(Everett 2005). The Pirahã are said to live largely in the

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present, with no creation myths, no art or drawing, noindividual or collective memory for more than two genera-tions past. Their language also has no numbers or system ofcounting and no colour terms. The grammar itself is simple,with no embedding of phrases, and the system of pronounsis the simplest yet recorded. The Pirahã have remainedmonolingual despite more than 200 years of trading withPortuguese-speaking Brazilians and speakers of othernative languages. One might be tempted to believe that theysuVer from some genetic defect, but this idea is Wrmlyrejected by Everett.

This work is understandably controversial (see the cri-tique by Nevins et al. 2007 and the response by Everett2007). Nevertheless it is perhaps unlikely that the Pirahãare unusual, and languages from other oral cultures maywell exhibit similar features; for example the Iatmul lan-guage of New Guinea is also said to have no recursion(Karlsson 2007). Tomasello (2003) suggests that theoriesof language have been unduly inXuenced by the character-istics of written language, and remarks that “there arevery few if any speciWc grammatical constructions ormarkers that are universally present in all languages”(p. 5). The sheer variety of diVerent language structuresargues against Chomsky’s (1975) notion of an innate“universal grammar;” and suggests instead that languagemay be a product of what Locke and Bogin (2006), afterMarler (1991), called an “instinct for inventiveness” thatmay go beyond language per se. Languages are probablyshaped through a process of “grammaticalization”(Hopper and Traugott 2003), whereby the structure isstreamlined and rendered progressively more eYcient.Any commonality may be due to the fact that peopleseverywhere exchange stories, whether about their ownremembered lives or future plans, or derived from historyor sheer imagination. Time may well be coded diVerently,depending on its importance, how it is perceived, andvariations that are due simply to the vagaries of humaninvention.

My proposal, then, is that grammatical language evolvedprimarily to communicate episodes, thus greatly enlargingthe vocabulary of real-world entities for the construction ofpersonal futures (see also Corballis and Suddendorf 2007).People everywhere have a thirst for stories, history, andgossip, and language is the primary medium allowing theseto be shared. Of course language is also used to communi-cate semantic information, but the initial impetus, I suggest,came from the emergence of episodic memory, mental timetravel, and the sense of time. And although episodic memo-ries may require a foundation of semantic memory, as sug-gested by Tulving (2002), semantic memory in turn is nodoubt shaped and extended through the accumulation ofepisodic memories.

Co-evolution of language and mental time travel

Although mental time travel may have set the initial stagefor language, the two must also have coevolved. Thus Gär-denfors (2004) writes that, in his view, “there has been aco-evolution of cooperation about future goals and sym-bolic communication” (p. 243). Language itself adds to thecapacity for mental time travel, since it provides a meansby which people can create the equivalent of episodic mem-ories in others, and therefore contributes to their episodicthinking. By telling you what happened to me, I can eVec-tively create an imagined episode in your mind, and thisadded information might help you adapt more eVectively tofuture conditions. And by telling you what I am about to do,you may form an image in your own mind, and work out aplan to thwart me.

Just when these capacities emerged is of course as matterof conjecture, but the most likely clue is brain size. Mentaltime travel adds vastly to storage requirements, since itbecomes necessary to form representations of objects,actions, and so forth that are not immediately present.Pinker (2007) suggests that the average speaker has avocabulary of some 50,000 words, and with the exceptionof function words, which serve grammatical functions,these words are linked to stored concepts. We also needrules to combine concepts to make up episodic memories orplan future episodes, or indeed to create stories, and werequire somewhat diVerent rules to translate these episodesinto language. Although we probably store only a smallfraction of the actual episodes in our lives, the combinedrequirements of concept memory, language, and episodicformation place huge demands on storage.

The human brain is some three times larger relative tobody size than that of the great apes (Wood and Collard1999). When considering the absolute mass of brain tissueover that predicted for an average mammal of the samebody size, humans have about 1.1 kg, or Wve times that ofchimpanzees, in excess (Whiten and Suddendorf 2007).The increase in brain size was not evident in the early hom-inins, and did not really begin until the emergence of thegenus Homo some 2 million years ago, some 4 millionyears after the split of the hominins from the line leading tomodern chimpanzees and bonobos. The earliest species tra-ditionally included in the genus were Homo habilis and H.rudolfensis, whose brain capacities ranged from around500 cc to about 750 cc, which is also signiWcantly largerthan that of the chimpanzee at around 393 cc and the larger-bodied gorilla at around 465 cc (Martin 1992). H. ergasteremerged around 2 million years ago, and by 1.2 millionyears ago boasted a brain capacity as large as 1,250 cc.Migrations out of Africa began around 1.8 million yearsago, and the Asian counterpart to ergaster, known as

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H. erectus, showed a similar increase. Thus in a space ofabout 750,000 years, brain size more than doubled.

Brain size appears to have reached a peak, not withH. sapiens, dating from about 170,000 years ago, but withthe Neanderthals. Analysis of Neanderthal DNA suggeststhat the most recent common ancestor we share with theNeanderthals dates from about 700,000 years ago, and ourancestral populations split from about 370,000 years ago(Noonan et al. 2006), so the increase in brain size may havetaken diVerent trajectories. In some individual Neander-thals, brain capacity seems to have been as high as1,800 cc, with an average of around 1,450 cc. Brain size inour own species, H. sapiens, is a little lower, with a present-day average of about 1,350 cc (Wood and Collard 1999).

Another feature of the genus Homo is the prolongationof brain development. To conform to the general primatepattern, human babies should be born at around 18 months,not 9 months (Krogman 1972), but as any mother willknow this would be impossible, given the size of the birthcanal. The brain of a newborn chimpanzee is about 60% ofits adult weight, that of a newborn human only about 24%.This feature probably emerged in evolution with theincrease in brain size, and there is evidence that it was pres-ent in H. erectus by 1.6 million years ago (Brown et al.1985). Our prolonged childhood means that the humanbrain undergoes most of its growth while exposed to exter-nal inXuences, and is therefore more Wnely tuned to its envi-ronment.

Like the chimpanzee, the early hominins seem to haveundergone two phases of ontogenetic development, knownas infancy and juvenility, before reaching adulthood ataround age 12. Beginning with H. habilis, a third stage,known as childhood, appears to have been squeezed inbetween infancy and juvenility, and adulthood was delayedby about a year. Childhood and juvenility were progres-sively lengthened in H. erectus, with the age of adulthoodcreeping up to around 15. In H. sapiens, it has been sug-gested, another stage, known as adolescence, was insertedbetween juvenility and adulthood. According to this sce-nario, then, humans undergo four developmental stages:infancy from birth to age 2½, childhood from 2½ to about7, juvenility from 7 to 10, and adolescence from 10 to about17 (Locke and Bogin 2006). It is during the period of child-hood, unique to Homo, that both mental time travel (Busbyand Suddendorf 2005) and syntactic language (Pinker1994) emerge.

The event that triggered these developments, and theensuing emergence of language and mental time travel, wasprobably climate change. The epoch prior to the arrival ofthe genus Homo was known as the Pliocene, beginningabout 5.3 million years ago. Toward the end of the Plio-cene, the earth became colder, and the ensuing Pleistocenegave rise to a series of crippling ice ages. The Pleistocene is

formally dated from 1.81 million years ago to 11,500 yearsago, although it has been argued that it should be datedfrom as early as 2.58 million years ago (Suc et al. 1997).The harsh conditions of the Pleistocene brought about thechange from forest to the more open and exposed savannain Africa and Asia, and may have at diVerent times cut oVvarious hominin populations from one another. Our ownspecies, H. sapiens, arose some 200,000 years ago, or later,in Africa, where cold episodes created extreme dryness,forcing our forebears to be more resourceful and coopera-tive to survive.

An especially dangerous feature of the savanna was thepresence of large carnivorous animals, including at least 12species of saber-tooth cats and 9 species of hyena (Foley1987), whose numbers peaked in the early Pleistocene. Ourpuny forebears had previously been able to seek cover fromthese dangerous predators in more forested areas, and per-haps by retreating into water, but such means of escapewere relatively sparse on the savanna. Not only did thehominins have to avoid being hunted down by these killers,with their sharp teeth and claws, and immense speed andstrength, but they also had to compete with them for foodresources. Given their arboreal heritage, the hominins couldnot have competed physically with the aggressive cats ofthe savanna, nor escaped with the speed and agility of theantelope. Their survival must have depended in part onwhat might be termed the third way, which was to establishand occupy a so-called “cognitive niche” (Tooby andDeVore 1987. It was a question of survival of the smartest.

As our forebears gained control over the ecological chal-lenges of the Pleistocene, they may have encountered a fur-ther threat—themselves. Paraphrasing Humphrey (1976),Alexander (1990) even suggested that “the real challenge inthe human environment throughout history that aVected theevolution of the intellect was not climate, weather, foodshortages, or parasites—or even predators. Rather, it wasthe necessity of dealing continually with our fellow humansin social circumstances that became ever more complex andunpredictable as the human line evolved (p. 4).” Futureplanning is probably as often concerned with overcominghuman competitors as with cooperative enterprises. Para-doxically, mechanisms for group cohesion may have beenenhanced by competition between groups. Adaptation prob-ably depended increasingly on the understanding and pre-diction of human action, made possible through memoryfor events and shared stories about human activities.

According to the view developed here, language andmental time travel evolved over the past 2 million or soyears, and may have been equally developed in the large-brained Neanderthals as in H. sapiens. What this does notexplain is why the Neanderthals were forced into extinc-tion, and why there was a “human revolution” (Mellars andStringer 1989), apparently restricted to our own species,

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comprising sudden and rapid advances in technology, cul-ture, art, bodily ornamentation, and the like (Mellars 2004).Some have suggested that this heralded the late emergenceof syntactic language itself (e.g., Bickerton (2002). Analternative that is more in keeping with the gradual evolu-tion of language through natural selection (Pinker andBloom 1990) is that language evolved as a system of man-ual gestures, with autonomous speech becoming the domi-nant mode only with or after the emergence of H. sapiens.The spectacular developments toward so-called “modernhuman behaviour” may then have resulted, not from lan-guage per se, but from the freeing of the hands, and the par-allel use of the hands for technology and the voice forexplanation—and gossip. But that is another story (Corbal-lis 2004).

Summary and conclusion

Episodic memory can be considered a special case of thecapacity to construct future episodes as well as recall pastones. The imagining of both past and future events, knownas mental time travel, appears to depend on the same corenetwork in the brain. Mental time travel in either directionis essentially a constructive process, typically involvingdiVerent combinations of overlapping pools of elements. Isuggest that syntactic language evolved primarily (if notexclusively) to convey episodes, whether past or present, orindeed Wctional, to others. Thus we can reap the beneWts ofthe experiences of others. Language is exquisitely adaptedfor the telling of stories, involving large vocabularies ofconcepts and corresponding labels, combinatorial structure,and markers of place and time.

Language and mental time travel probably co-evolvedduring the era known as the Pleistocene, when forest gaveway to savanna, and our forebears were forced to dependon greater social cohesion and planning in order to survivea hostile environment. Group cohesion may also have exac-erbated intergroup conXict, leading to further accumulationof communicative and planning skills. Co-evolution of lan-guage and mental time travel may have driven the dramaticincrease in brain size, the prolongation of postnatal growth,and the insertion of critical periods of development, duringthe Pleistocene.

The idea that language may be dependent on mental timetravel stands in opposition to the notion that language is anencapsulated system, with its own rules, dependent on aninnately given “universal grammar” (e.g., Chomsky 1975).Instead, the approach taken here is more in line with whathas come to be known as Cognitive-Functional Linguistics,whereby language is regarded as part of the more generalcognitive system (e.g., Tomasello 2003). Of course, mentaltime travel may not be the only cognitive capacity underlying

language, but I suggest that it is a critical one. If you livedentirely in the present, what would you have to talk about?

Acknowledgments I thank Thomas Suddendorf, who introduced meto the idea of mental time travel, and an anonymous referee for his/herhealthy disbelief.

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