Mental Time Travel and Human Evolution

Discovery of the Fourth Dimension:
Mental Time Travel and Human Evolution

Thomas Suddendorf

A thesis submitted in partial fulfilment of
the requirements for the degree
of

University of Waikato
January 1994

Abstract

This article considers the role of mental time travel in human evolution. A central
thesis is that other primates, although having memory and expectation, do not possess
the same ability to live in the past or in the future. The first half of this paper argues
that reconstructive access to the past (i.e. episodic memory) is dependent on other
advanced cognitive capabilities (e.g. self-awareness and meta-representation) and
focuses on the results of recent `theory of mind' research in order to evaluate the
thesis. Mental simulation is the proposed underlying mechanism for the development
of both mindreading and mental time travel. The second half contrasts flexible
awareness of possible futures with other forms of `anticipatory behaviour' and reviews
evidence about how far other primates may think ahead. The phylogenetic history of
mental time travel and its adaptive and exaptive relationships to other features are
discussed. Mental access to the fourth dimension is essential for many of the
distinctive characteristics of our species.

Preface

"If martians have been observing the development of the blue-green planet called
Earth, they would have noticed the extraordinary, exponentially growing changes
during the last millennia. One component of this planet, the species Homo sapiens
sapiens, would have been easily recognized by the martians as the cause of these
changes. Even more astonishing to the hypothetical martians than the tremendous
effects humans have on the environment, would have been the human capability to
commit global suicide" (Suddendorf, 1992, p.4). Hence, martian scientists might ask
themselves what distinguishes humans from all the other creatures this planet
produced? Most of humans' closest relatives, the other great apes, continue to behave
calmly. So, what enabled and motivated humans to change the face of the Earth?
These are the great questions that puzzled me, and many before me, for a long
time. The last two years I have tried to research and evaluate our knowledge in
respect to a potential answer that so far was mainly neglected. The answer I advocate
in this thesis is that humans, unlike other animals, developed a mental access to the
fourth dimension: our awareness of past and future. The argument comprises recent
findings in cognitive psychology, comparative psychology, developmental
psychology, evolutionary psychology and primatology, and touches on many
neighbouring fields such as archaeology and neurophysiology. Most issues raised are
worth separate essays, but in the light of the scope of this paper and the variety of
issues relevant, many topics can be illuminated only briefly (and some could only be
pointed out in form of footnotes). This may be the price one has to pay if one does
not focus on a specific detail but rather tries to bring together interdisciplinary
knowledge in pursuit of the `larger picture'.
This picture is nonetheless confined to the `mental time travel' aspect of human
evolution and I would like to urge those who try to reconstruct an even broader picture
of human evolution to include this aspect. The argument is written in article style and
indeed, has been submitted for publication. Because I did not want to disrupt this
theoretical contribution, I decided to add the preliminary results of a survey I started

in April 1993 in form of an Appendix (A). Furthermore, in Appendix B I suggest an
experimental paradigm that may be able to put some of the ensuing questions to test.
Both Appendices (A & B) are a substantial part of the thesis work and I would like to
encourage readers to go through those in addition to the main body.
Questions that were once philosophical are now beginning to yield to scientific
inquiry, and I hope that, even if the material requires some mental effort, some of the
excitement of this enterprise is conveyed in this thesis.

Acknowledgements

I would like to thank Alison Annals, Richard Aukett, Barbara Gerding, Pam Oliver
and my parents for their help and support. I am indebted to the people in the
Department of Psychology at Auckland University, for generously hosting me as a
guest student for the last year. Most of all I have to thank Mike Corballis for so many
reasons that there is no point in listing them all. He has been the best supervisor
possible for my project and academically speaking he alone was worth travelling half
around the world from Germany to New Zealand. His admirable modesty will surely
let him hate this acknowledgement.

Introduction

We humans have long attributed to ourselves special qualities of mind, spirit
or morality that are denied all other creatures. Religions seem to play a special role in
this by bestowing uniquely upon us an immortal soul or by tracing our origins to some
divine act of creation. [Corballis, 1991, p.3]

What is the essential difference between humans and other creatures on planet
Earth? Even after the general acceptance of the theory of evolution, when humans
acknowledged having common ancestors with all other species, we put ourselves at
the top of the ladder, standing in solitary splendour far above all other species.
This conceit may of course be regarded as a `false consensus bias' (Ross, Green &
House, 1977) created by western scholars who were raised under the influence of the
Christian tradition, which perpetuates an unbridgeable gap separating humans from
animals. There are, however, other perspectives and influential philosophical and
religious traditions that emphasize the continuity rather than discontinuity of human
evolution. Hinduism, for example, views animal and human minds as stages that
differ in merely quantitative fashion in the continued progression towards Nirvana.
Features that the western scientific enterprise assumed to be uniquely human only a
few years ago (e.g. symbolic thinking, tool use, self-awareness) have been shown to
exist, at least to a degree, in nonhuman great apes, too (e.g. Greenfield & Savage-
Rumbaugh, 1990; Goodall, 1986; Gallup, 1983)1. In order to uphold the belief in an
unbridgeable gap separating humans from animals, researchers of the last decades
have been continuously forced to pose increasingly more restrictive (or less elegant)
definitions of humanity or of supposedly uniquely human qualities, such as language
(Gibson, 1990). An example of this trend has to do with the one-time belief that only
humans use tools. In the face of increasing evidence to the contrary, it was then
proposed that only humans manufacture tools, and more recently still that only
humans use tools to manufacture tools (e.g. Beck, 1980)2.
Recent evidence suggests that some great apes, in contrast to monkeys, may have
at least a rudimentary3 `theory of mind' (Premack & Woodruff, 1978; Premack,
1988), may be able to use pedagogy in both laboratory (Fouts, Fouts & van Cantfort,
1989) and field (Boesch, 1991) contexts, may show empathy and compassion
(Boesch, 1992), may be able to truly imitate (Byrne, in press; Meador, Rumbaugh,
Pate & Bard, 1987), and may have the ability (possibly underlying all those above) to
imagine other possible worlds (Byrne & Whiten, 1992). These `discoveries' reveal
more and more a picture of apes as the `missing link' bridging the gap between the
animal kingdom and humanity. One could be even tempted to re-locate this `gap', so
that it separates the great apes, rather than only humans, from all other animals.
Nevertheless, it can still be argued that there is a substantial gap between humans
and the other great apes, if only because of the extraordinary impact we have had on
the environment. As Passingham (1982) puts it:

Our species is unique because, in only 35000 years or so, we have
revolutionized the face of the Earth. We have created entirely new environments for
ourselves, have changed the lives of animals, and have the power to threaten the
existence of life on our planet. [p.21]

The present article will elaborate on this `something extra' that the great ape `homo'
exhibits. I expect that the debate about whether it constitutes a continuity or
discontinuity in phylogeny will eventually become immaterial, in the way the nature-
nurture debate turned out to be. Depending on the philosophical emphasis implicit in
the theoretical filter through which one views it, one may classify human evolution
either as a discontinuity or a continuity, although the two may not be mutually
exclusive.
Apparent discontinuities or the impression of qualitative differences can result
from continuous gradual changes in phylogeny, in the way that H2O changes
`qualitatively' from ice to water to gas as temperature continuously and quantitatively
increases. These types of changes might be best described by the term metamorphosis
(Bischof, 1985). The crucial evolutionary concept of exaptation (e.g. Gould, 1991;
Gould & Vbra, 1982) might have been the underpinning of such a metamorphotic
change in human evolution: A gradually evolved adaptive feature may have had a
variety of `side' characteristics that were not directly relevant to the survival of the
organism, but at a later point in phylogeny these characteristics may have become the
basis for new strains of adaptation. In their theory of "punctuated equilibrium"
Eldredge and Gould (1972) argued that major phylogenetic changes may occur in
short periods of time producing an apparent discontinuity in the fossil record and
among living species. The seemingly qualitative differences between humans and
other animals might have been the result of a metamorphotic change that gave new
fitness value to a variety of phylogenetic older features. This may have rapidly
changed humans some time during the last 5 to 8 million years, after the phylogenetic
split from the line that led to the modern chimpanzees.
In this article I want to re-introduce into the debate an aspect of human thought,
rather than morphology, whose phylogenetic emergence might have had such a
profound significance that it could have been a prime mover in human evolution: the
discovery of the dimension of time.
Wolfgang Koehler (1917/1927) anticipated with his work many of the `recent
discoveries' about the mentality of apes. Contrary to the prevailing Zeitgeist, Koehler
emphasized the relatively sophisticated mental capacities of chimpanzees, but he also
noted an important limitation:

`The time in which the chimpanzee lives' is limited in past and future"
[Koehler, 1917/1927, p.272].

We humans, by contrast, seem to be able to concern ourselves with issues that are not
limited in past or future. Events as remote as the crucifixion of Christ can be very
important to us, and we even tackle questions about the extent of time itself by
developing religious or scientific concepts like `genesis', `big bang' or `judgment day'.
Indeed, most of what is written or talked about refers to something that has happened
in the past or could happen in the future; the present appears to be just a brief segment
passing from the future into the past. While life is always happening in this present,
our cognition, emotion, motivation and behaviour is largely influenced by what we
believe has happened or may happen. Humans can `mentally travel in time'; we can
use our imaginations to represent events of past and future and reflect upon them.
Clearly, animals have some sort of memory and expectations, but their ability to
mentally travel in time may be significantly limited.
The human ability to mentally travel in time is central to the interpretation of
human evolution that is presented here. In particular, I will argue that our ability to

anticipate the future, based on our access to the past, has changed human motivation,
emotion, cognition, and behaviour, and was one of the major forces that led us to
change the very face of the Earth. Perhaps we need to improve this ability even more
if we are to survive the rapid changes we have produced recently.
The purposes of this article are to highlight mental time travel as an important
human condition, to determine whether it is uniquely human, to propose a possible
underlying mechanism, to assess its phylogenetic and ontogenetic development and to
discuss its adaptive or exaptive relationship to other human characteristics. First I
discuss mental time travel into the past and then mental time travel into the future.

Mental Time Travel into the Past

It seems reasonable to argue that in order to imagine a past that lies before their
own lifetimes, people must have evolved the means to represent (remember) events of
their own past. Many scientists have argued for fundamental differences between
animal and human memory (e.g. Aristotle; Bischof, 1985; Gardner, 1975 cited in
Marshall, 1982; Marshall, 1982; Tulving, 1983). But since animals can obviously
learn from past experiences, it might be thought that human and animal memory differ
only in degree. However, research in different areas (e.g. psychology, philosophy,
artificial intelligence; see Polster, Nadel & Schacter, 1991, for a recent historical
review) suggests that memory consists of multiple systems that may be functionally
and structurally distinct (although alternative views exist, see for example Roediger,
1990), and this raises the possibility that one or more of these memory systems may
be an acquisition unique to humans.

A Uniquely Human Memory System?

Research on the phenomena of amnesia, in particular the extensive studies of the
patient H.M., forced psychologists to distinguish between different kinds of memory,
because one type of memory can be impaired while another continues to function
normally. Despite his memory loss, H.M.'s behaviour can be influenced by the past
without him being aware of it (see Ogden & Corkin, 1991, for a recent review). The
spared learning or memory abilities, as reflected in learned skills, classical and
operant conditioning, priming, habituation and sensitization, affect performance
without affording an `image/picture of the past' (Koehler, 1917/1927), a `recollective
experience' (Gardiner, 1991) or `access' to the experience that is affecting current
performance (Zola-Morgan & Squire, 1990). While there is ample evidence that
animals rely extensively on information stored in this implicit fashion, there is
considerable debate about whether animals possess the memory systems that are
impaired in global amnesia. These are semantic and episodic memory (see Ogden &
Corkin, 1991; Squire, 1992). Tulving (1972, 1983), who proposed the dissociation
between semantic and episodic memory, saw these as functionally separate although
interacting systems. Semantic memory comprises context-free knowledge or facts
about the world and episodic memory comprises personal experiences and events
(Tulving, 1983). Consolidation of both kinds of information appears to be dependent
on the hippocampus and related structures (Squire, 1992). Rats, monkeys and humans

seem to have similar hippocampus-dependent memory systems (Squire, 1992), but
this similarity might be due to a shared ability to store semantic facts, rather than
episodes.
Ridley (1992) argued that semantic memory exists in animals such as vervet
monkeys, because these animals apparently represent not-perceptually-present facts
about their social world. The distress call of an isolated infant, for instance, leads the
mother to look to her infant whereas other mothers look to the mother (see Cheney &
Seyfarth, 1990, for further examples). Evidence for episodic memory, the memory of
personal experiences associated with past points in time, seems to be more difficult to
find for animals, and Tulving (1983) himself went so far as to suggest that episodic
memory may be uniquely human.
This suggestion did not stay unchallenged, however. Olton (1984) and earlier
Roitblat (1982) pointed out that animals display certain behaviour, as in a trial of a
delayed conditioned-discrimination task, or in foraging where an animal must
remember not to go to the same flower twice to obtain nectar, which indicates that
"remnants" of a previous experience allow it to affect later behaviour. Thus, the
animal "represents" a past event and, according to Olton (1984), therefore possesses
episodic memory.
However, Dretske (1982) argued that this inference is not unambiguous. If event
A leads to the change B in the cognitive apparatus of an animal and B affects
behaviour C at a later point in time, then B does not necessarily carry any information
about A itself, and may therefore not be a true representation of A. The mediator B
might be `causal' rather than `informational'; "that is, that the memory trace of the
stored event only contains instruction for future behavior, without any information
that would permit the reconstruction of the past" (Tulving, 1984, p.258). The fact that
animals can `recognize' objects that they have seen only once before could be a result
of a feeling of familiarity rather than of a remembrance of that event (Ridley, 1992).
In his reply to Olton's critique Tulving accepts that animals may have a form of
episodic memory that serves at least as a causal mediator, and in fact Olton's examples
meet many and violate none of the criteria Tulving (1983) laid out for episodic
memory. However, Tulving maintains that animals may not be able to "mentally
travel back in time to recollect and reminisce the way humans do" and illustrates his
position by rhetorically asking "was Aristotle wrong when he said that, `Many
animals have memory and are capable of instruction, but no other animal except man
can recall the past at will'...?" (Tulving, 1984, p.258).
If one accepts Dretske (1982) and Tulving's (1984) argument that learning from
single events does not constitute evidence for representations of the past, it appears as
yet impossible to show that animals travel mentally into the past even if they actually
do so. It is however possible to further analyze the possibility of mental time travel in
animals indirectly by considering related mental capacities.

Mental Capacities and Mental Time Travel

Tulving (1985) argued that different kinds of consciousness characterize different
memory systems. Procedural (here termed implicit) memory implies anoetic
(nonknowing) consciousness, semantic memory implies noetic (knowing)
consciousness, and episodic memory implies autonoetic (self-knowing)
consciousness. Although it is not clear whether these types of consciousness are, as
Tulving argued, properties of the memory systems themselves (Schacter, 1989, for

example, argued for separate memory and consciousness modules), they appear to be
interlinked. Episodic memory contains information about past states of one's self and
of the world. On the one hand, in providing autobiographical information about one's
own past, memory of past events (episodic memory) provides the basis for one's
personal identity. On the other hand, in order to attribute representations to
experiences of self at an earlier point in time one may need to be aware of one's self in
the present (cf. Howe & Courage, 1993), an ability that only chimpanzees (e.g.
Gallup, 1970), orangutans (e.g. Suarez & Gallup, 1981) and a gorilla (Patterson,
1991)4 have demonstrated through self-recognition in a mirror5. Monkeys and even
elephants can learn how a mirror works, but, in contrast to the great apes, they cannot
locate markings viewed in a mirror if these markings are on their own bodies (Gallup,
1983; Povinelli, 1989). If one cannot consciously (autonoetically) recognize the self
in the present (e.g. in the mirror), how could one possibly recognize the self in the
past? In order to understand that current mental images can represent one's own
earlier experiences, one needs to have a concept of self that allows for such inferences.
It might be objected that it is not necessary to postulate the need for metacognitions
such as the inference that representations are experiences of the self in the past,
because memories of past events could reveal themselves without them. But, as I will
show in the following paragraphs, memory for past episodes, unlike memory for facts,
does imply metacognitions in that it involves active reconstruction and attribution,
and is therefore as much a function of the present as a record of the past.
The term memory is often associated with a fixed databank (e.g. library) but this
metaphor appears to be more appropriate for semantic knowledge (memory for facts)
than for episodic memory. In contrast to the retrieval of facts, retrieval of past
episodes usually recodes (`updates') the stored information (Tulving, 1984)6.
Retrieval of past episodes appears to be more than `opening and reading a file'; the
past episodes need to be reconstructed.
Almost a century ago Freud (1895, cited in Marshall, 1982) noted that even
memories that reveal themselves as images require a story grammar if remembrance is
to be distinguished from random hallucinations. However, the storyline is often
reconstructed on the basis of one's general knowledge rather than what actually
happened (e.g. Bartlett, 1932) and may therefore not be part of the memory trace7.
The order of past events in time seems not to be a property of memory. After
reviewing the evidence Friedman concluded recently that

[i]n spite of the common intuition that chronology is a basic property of
autobiographical memory, the research reviewed demonstrates that there is no single,
natural temporal code in human memory. Instead, a chronological past depends on a
process of active, repeated construction [Friedman, 1993, p.44]8.

The reconstruction of time and storyline presupposes that one is aware of the
`pastness' of the current representations. In other words, in order to reconstruct
episodes one needs to be able to make the basic distinction between representations of
the past (memories) and representations of the present (e.g. hallucinations,
perceptions). But even this basic quality appears not to be inherent in memory, that is,
memories seem not to be marked as memories. On the one hand, there are plenty of
examples of confabulation, the experience of remembering without the existence of
corresponding memory representations (see, for example, Bowers & Hilgard, 1986).
We frequently `remember' events that did not happen (or at least not in the way we
recall them). On the other hand, it has been shown that "[h]aving - and even using - a

memory representation of a prior event is not sufficient to insure the subjective
experience of remembering" (Jacoby, Kelley & Dywan, 1989, p.417). A feeling of
`pastness' appears not to be a property of the memory representation, but has to be
inferred and attributed (or, in the case of confabulation, misattributed) to the current
representation (Jacoby, Kelley & Dywan, 1989). However, it is this subjective
experience of the `pastness' of representations, not the objective validity of memory
(as usually investigated in human and animal research), on which the ability to
mentally travel into one's own past rests. Thus, what Tulving (1983, 1984) called
episodic memory is as much dependent on present mental abilities as on memory
storage of the past. What appears to be required is a concept of self and the ability to
form meta- or second-order representations of one's own knowledge.
In order to travel mentally back in time, i.e. to attribute representations to
experiences of the self in the past and reconstruct these representations into episodes,
one needs to have access to the content of one's own mind (cf. Ridley, 1992). In
addition to the primary representation (e.g. I am in a park), one has to represent this
representation as a memory. Other primary representations that are represented as
memories (e.g. I go shopping and I play ball) can be reconstructed into a past episode
(e.g. I was in a park, played ball and then went shopping) which may afford further
metacognitions. The ability to voluntarily (selectively) choose what events of the past
are internally generated is a characteristic of human mental time travel that even more
strongly demands flexible access to one's own mind.

The Mentality of Primates

Do animals have an awareness about the contents of their own minds? Cheney and
Seyfarth (1990) argue that monkeys do not recognize and represent their own
knowledge. Monkeys do not entertain metacognitions of their own states of mind.
Just as people with `blindsight' do not know (are not consciously aware) that they
have vision, monkeys may not know what they know, or even that they know (e.g.
Gallup, 1983; Humphrey, 1986). Since the subjective experience of remembering
seems not to be evoked by the memory trace itself, some animals may have
representations of past events without the awareness (knowing, representing) that
these representations are past experiences and consequently without the possibility to
actively reconstruct these representations into narrative episodes.
But how can we know whether or not animals form second-order representations of
their own mental states? We can only infer from observable behaviour. Behavioural
available only for chimpanzees and perhaps the other great apes (Whiten and Byrne,
1991). This evidence comprises observations indicating behaviour such as pretend
based on the ability to form meta-representations.
The question whether or not animals can attribute mental states such as
remembering to themselves might be best assessed by examining the growing body of
data concerning the attribution of mental states to others (cf. next section). Since
Premack and Woodruff's (1978) original article on whether chimpanzees have a
`theory of mind', research on animals' and children's conceptions of mind has boomed.
I will review some of the research results that are important for mental time travel.
A `complete' theory of mind, such as that possessed by adult humans, is expressed
by being able to attribute mental states to one's self (e.g. do I desire, intend, believe or,
according to the previous argument, remember X?) and to others (e.g. does she desire
or believe X or Y?) even when there is a discrepancy between one's own and other's

knowledge (Cheney & Seyfarth, 1990). A complete theory of mind does not evolve in
a single step, whether in phylogeny or in ontogeny, but is better described by gradual
differences between species and between developmental stages. The ostrich that
buries its head in the sand is apparently intellectually unable to take the visual
perspective of others. However, Kummer, for example, observed a female baboon
apparently hiding parts of her body (i.e. her hands that were grooming a subadult
male) from the male leader, implying that she was able to mentally take the male's
visual perspective (Kummer, 1990, record 56 in Byrne & Whiten, 1990). This does
not necessarily imply an attribution of a mental state resulting from seeing. But an
understanding of that another's visual perspective differs from one's own might be
viewed as a phylogenetic step into that direction (cf. Whiten, 1991).
There appears to be little evidence indicating that monkeys attribute mental states,
i.e. that they are aware of others' or their own intentions, beliefs or knowledge
(Cheney & Seyfarth, 1990). Rhesus monkeys, for example, fail to comprehend what
another individual knows as a result of seeing. They choose randomly between the
advice of those trainers who saw the baiting of one of several containers that were
invisible to them and those who could not have seen it (Povinelli, Parks & Novak,
1991). Chimpanzees, however, learn to pick the individual who can help solve the
task (Povinelli, Nelson & Boysen, 1990; Premack, 1988). Besides understanding
visual perspective in this case it appears necessary to attribute resulting states of mind
(knowledge).
Similar discrepancies between the performance of chimpanzees and monkeys in
the realms of deception (Byrne & Whiten, 1990, 1992), teaching (Boesch, 1991; Fouts
et al., 1989) and imitation (Byrne, in press; Meador et al., 1987) may also be due to
chimpanzees' superior skills in attributing mental states (Byrne, in press; Cheney &
Seyfarth, 1990). Imitation, for example, is not evident in monkeys in spite of the
popular belief that monkeys are notorious imitators. Recent reviews attribute
monkeys' social learning to `lower level' mechanisms such as stimulus enhancement
and social facilitation, while they accept the evidence for `true' imitation in great apes
(Byrne, in press; Meador, et al., 1987). Monkeys may not truly imitate because they
cannot impute motives (Cheney & Seyfarth, 1990) and because they may not be able
to mentally take other's roles (Byrne, in press). In a cooperation task devised by
Povinelli, Parks and Novak (1992) monkeys learned their part without gaining
knowledge about the contingent role of the other participant. This can be inferred
from the observation that in a role-reversal condition the monkeys did not show
positive transfer effects from their prior experience in the other role. By contrast,
chimpanzees were able to assume the other's role when they were reversed (Povinelli,
Nelson & Boysen, 1992).
These and other observations substantiate the view that only chimpanzees (and
perhaps the other great apes) have some meta-representational understanding of, or
awareness about, the nature of mind (see Suddendorf, 1993, for a comprehensive
review of the evidence). Sceptical reviews (e.g. Heyes, 1993) nevertheless maintain
that none of the recent efforts has provided convincing evidence for mental state
attribution in animals. Heyes (1993) argues that the observed behaviour of apes could
be explained by learning processes that do not entail the attribution of mental states.
If Heyes is right, then there would be no reason to believe that any animal can
reconstruct past episodes, because no species has provided evidence for the required
awareness about contents of their own or other minds. But the consistent discrepancy
between the performances of monkeys and great apes on varied measures has
convinced many scholars that our closest relatives seem superior at attributing mental

states, rather than merely at learning (e.g. Byrne & Whiten, 1992; Cheney & Seyfarth,
1990; Povinelli, 1993; Premack, 1988; Suddendorf, 1993).
Yet, the consistent discrepancy between the performances of great apes and adult
humans also ensures that even if chimpanzees can make attributions, these attributions
are limited in a number of respects (Premack, 1988). There is experimental evidence
suggesting that chimpanzees, in contrast to most other animals, can understand that
others may differ in what they see (Premack, 1988), what they intend and desire
(Premack & Woodruff, 1978) and what they know (Povinelli, Nelson & Boysen,
1990). However, there is no evidence suggesting that chimpanzees can
simultaneously represent their own knowledge and different knowledge of others (see
Cheney & Seyfarth, 1990; Premack, 1988). The understanding of false beliefs (see
Wimmer & Perner, 1983) seems to indicate a marked conceptual shift in children's
(three and a half to four years) understanding of mind (e.g. Gopnik, 1993; Wellman,
1991; see below). Chimpanzees have not provided evidence for reaching this level of
mental attribution, but investigations are scant (see Suddendorf, 1993, for a review).
As yet, we have to assume that chimpanzees cannot represent others' false beliefs, i.e.
they cannot represent knowledge that is in opposition to their own (Premack, 1988,
Premack & Dasser, 1991; Whiten, 1992).

Knowing Yourself and Knowing Others

If we assume that apes do have some ability to attribute mental states, may the
deficits in mindreading capacity that are nonetheless apparent bear any constraining
impact on their potential mental time travelling ability? I will risk a speculation that
an inability to represent mental states opposed to one's own present mental states
applies not only to the mental states of others, but also to one's own earlier mental
states. This inference is empirically supported by recent findings in child psychology.
In human development two, three and four-year-olds progressively master the
attribution of desires, then beliefs, and finally false beliefs (i.e. simultaneously
representing knowledge that is contrary to their own) (see, for example, Gopnik, 1993;
Wellman, 1991; Whiten, 1991; Wimmer & Perner, 1983). Consistent with the above
speculation, the acquisition of attributing opposing mental states to others coincides
with, or follows, the acquisition of attributing opposing mental states to one's self in
the past (Gopnik, 1993). Three-year-olds fail to understand that their current
knowledge, for example that there are pencils and not smarties in the candy box, may
not be available to others and wrongly predict that another child believes pencils to be
in the candy box (Perner, Leekam & Wimmer, 1987). Asked what they themselves
believed to be in the candy box before they were shown, they apparently fail to
remember their own previous false belief and say that they originally thought pencils
were in the box (Gopnik & Astington, 1988). In contrast to changes in belief, changes
in the physical world were remembered by the subjects. Children younger than four
years seem to have problems remembering information referring to an opposing past
state of themselves as much as they fail to represent contrary mental states of others.
This appears to be as true for desires as for the later understanding of intentions and
beliefs. Having eaten enough to satiate a desire, still one third of three-year-olds
reported not having been hungry before: after eating four portions of mousse the
children insisted that they had not had a desire for the mousse before (Gopnik, 1993).
Full understanding of opposing-to-own-present mental states of others or past self (of
desires, intentions and beliefs) is not reached before the average age of three and a

half to four years.
The inference, then, is that if chimpanzees cannot attribute mental states to others
that oppose their own, then they should not be able to attribute past mental states to
themselves that oppose their present ones. We have seen that recent findings in
developmental psychology, while not providing proof, do support this idea. However,
would this inability restrict chimpanzees' ability to mentally re-experience past
episodes? Does this inability restrict three-year olds' episodic memory?
While implicit memory (e.g. conditioning, priming) appears to be functional very
early in infancy, most studies indicate that memory for past episodes develops
between age three and four (see Pillemer & White, 1989 for a review). Two and a
half year olds may recall some fragments of remote past events, but only between the
ages of three and four do children seem to begin to mentally travel into their past, i.e.
to mentally reconstruct past episodes as organized narratives. Around the same age
(between three and four, Loftus, 1993; Pillemer & White, 1989; Sheingold & Tenny,
1982) childhood amnesia begins to cease. That is, adults' accessible memory for past
episodes usually begins from that point9.
At around the same time children acquire gradually the abilities to mentally
reconstruct past episodes and to attribute mental states to others and to themselves.
This may not be mere coincidence. Children who fail to recall their opposing-to-
present past mental states may not have acquired a fully established episodic memory
system because past episodes are defined by the past state of the world and the past
state of self. Although the children seem to some extent to be able to remember past
states of the world, they are apparently unable to recall their own past mental states, or
at least those aspects of their past mental states that are contradictory to the present
one. A full picture of a past episode, however, requires that one represents one's
former states of mind (e.g. desires, intentions and beliefs) in order to understand and,
more fundamentally, reconstruct the past interaction of self and the world.
That children have difficulties with the essential self-referencing aspect of episodic
memory is further substantiated by the finding that three-year-olds have problems
recalling the source of their own current knowledge (Gopnik & Graf, 1988; O'Neill &
Gopnik, 1991). Although it may have happened only minutes ago, they apparently
fail to reconstruct the episode during which they have acquired their current
knowledge. A fully-fledged episodic memory system that allows for mental time
travel back to (or, reconstruction of) the experience of self at a past point in time
demands the ability to represent opposing-to-present mental states. Only at the age of
three and a half to four years does the child fully master this, and only then may it
therefore establish an episodic memory system comparable to the adult human one.
Later still, between four and eight years, the child acquires an explicit knowledge
about the culturally dependent time patterns (e.g. weeks, months, years) that assist the
structuring of one's own past experiences (Friedman, 1991, 1992).
If chimpanzees cannot represent opposing-to-present mental states of their past
selves, one may conclude that chimpanzees cannot have a fully established episodic
memory system. While they may have an elementary memory for past events of the
world comparable to that in children under three and a half years, the reconstruction of
one's own past experiences requires the representation of opposing-to-present mental
states. Without such reconstruction mental time travel into one's own past is
drastically limited.

A Model for the Development of Mental Time Travel and Mindreading

Because of the hypothesized link between mindreading and mental time travel I
will propose a model for mental time travel that is based on one of the explanatory
concepts that have been put forward for the development of `theory of mind'.
Simulation theory is a model proposed for children's mindreading development
(Gordon, 1986; Harris, 1991; Humphrey, 1986; Johnson, 1988), and it may also serve
as a conceptualization for mental time travel. According to this model the child does
not hold an actual `theory' of mind but acquires knowledge of another person's mind
by internally simulating that person's situation (i.e. his or her self-world
constellation)10. This idea is based on evidence that two- to three-year-olds can
imagine having a mental state (e.g. belief or desire) that they do not have and that they
can imagine different worlds (Harris, 1991). Both abilities appear to be evident in the
development of children's pretend play. During early childhood children improve
their ability to reason from pretend premises (see Harris, 1991) which appears
necessary for more advanced mindreading based on the analogy from imagined to
real. Harris (1991) postulates that imagined events are produced against a background
of default settings which correspond to the current state of self and of the world.
Between two and four years of age the child learns to become flexible and accurate in
altering the default settings in order to imagine what it would be like being in
somebody else's position. The child has to change the default setting of the state of
the world (as known to the child) to the state of the world as known to the other and
the intentional stance of self to the intentional stance of the other in order to make a
correct simulation of the self-world constellation of the other.
According to this approach, the typical failures of younger children result from an
insufficient ability to set aside their own current knowledge of the state of the world
and/or to detach from their own intentional stance (Goldman, 1993; Harris, 1993).
Their own present states interfere with the simulation. Escaping from the influence of
one's own mental states is learned first in respect to intentional stance and then in
respect to knowledge and belief. By age four the child is able to accurately simulate
the mind-world relationship of others. Only later still, I suggest, may these simulation
processes lead to the formation of an actual theory of mind, i.e. a semantic set of rules
that describe the nature of mind and that allow for understanding and prediction of
another's mind and action without requiring an internal simulation. The ability of
mental perspective taking and simulation is, however, not lost to adult humans. While
we can quickly infer that someone is jealous, based on our knowledge about the
circumstances and our semantic theory of the nature of minds, we can also try to
imagine what it would be like being in the other's position. Such a simulated
`episodic' process may result in better, more empathic understanding and better
prediction of the other person's feelings, thoughts and actions.
The ontogeny (and perhaps phylogeny) of our access to our own past might be
quite similar to the development of mindreading outlined above. The mental
reconstruction of past episodes may also be based on using one's imagination to
represent other possible worlds and states of self. A semantic concept (theory) of the
past may develop only later in life. Instead, children may first have to learn to
mentally simulate (reconstruct) past episodes. Instead of imagining what it would be
like, one may simulate what it was like being in a specific situation. The
reconstruction of past episodes may be understood as an internal simulation of past
states of the world and of the self. This requires the basic capacity to attribute current
representations to experiences of one's self at a past point in time, i.e. to recognize
them as memories. Both processes are fostered in human development by the

guidance of parents who usually ask for and support the verbal report (reconstruction)
of past episodes. However, as for the mindreading capacity, children have to acquire
the ability to accurately alter default settings, i.e. to set aside their current mental state
(intentional stance to and knowledge about the world) when attempting to simulate
past episodes11. Only at about age four may they be able to mentally travel back in
time and simulate (re-experience) their own past state and the state of the world as it
was known to the past self. Later still, this ability to re-experience (re-present) past
episodes may result in an abstract understanding of the past which may allow for
quick access to relevant information of past episodes without involving internal
simulation and re-experiencing of the complex episode. As with mindreading, the
earlier system of simulation remains functional in later life. We can remember that
we have seen Paul this afternoon without simulating the past episode, and we can also
travel mentally into the past, `picture' the situation, reminisce (re-experience) the
encounter and empathize with our own or Paul's position. Instead of proposing a
sharp distinction between the systems it may be more reasonable to view them as two
extremes of a continuum whereby the more abstract conscious access grew out of the
ability to mentally simulate past episodes12.
Can other animals use mental simulation for the reconstruction of past episodes?
Chimpanzees (and perhaps the other great apes) fulfil the mental simulation
prerequisite of being able to imagine other possible worlds (Byrne & Whiten, 1992).
In contrast to monkeys, chimpanzees seem to be self-aware and appear to engage in
`true' pretence, i.e. they form second-order representations about the world (Whiten &
Byrne, 1991). Reasoning in this imagined world appears to be possible for
chimpanzees and proper transference between real and imagined world also appears to
occur. In order to solve Koehler's (1917/1927) raking problem by insight the
chimpanzee Sultan seemed to form a mental representation of the situation,
manipulate components in his imagination until he hit upon a solution, and then enact
the solution in the real world. Chimpanzees seem to be able to attribute at least some
mental states to others (Premack, 1988), to imitate (Meador et al., 1987) and to take
other's roles (Povinelli, Nelson & Boysen, 1992), which may mean that they can use
mental simulation processes. Perhaps chimpanzees are only limited by the `small' but,
according to the outlined model, significant step of escaping their present state of
mind in order to fully travel mentally into their own past and reminisce the way we
do. I have argued above that chimpanzees' presumed inability to simultaneously
represent opposing-to-own mental states of others may also indicate their inability to
represent opposing-to-present mental states of their own past, and in this respect they
may resemble three-year-old children13. A completely functional episodic memory
system may require the ability to detach from the present, to attribute representations
to the past and to use the imagination to reconstruct them into episodes with a
storyline for the interaction of both, states of the world and states of self. Awareness
of and access to the contents of the dimension of time may be strongly dependent on
the full development of all mental abilities involved in such a system. An overview of
the model is provided by table 1.

Table 1. Proposed Development of Mindreading and Mental Time Travel into
One's Own Past

Mental stage Ontogeny Phylogeny

2nd-order representation self-awareness 2 yearsa great apes
imagine other possible worlds gorilla?

Attribution of reconstruction of 3 years great apes?
mental states but past events but
interference of interference of
own present state own present state
in simulation in simulation

attribution of time travel into 4 years homo erectus?b
mental states; past; simulation
simulation without without
interference interference

abstract theory of abstract concept 5 years homo sapiens
mind of past onwards

a
age is not a causal agent but allows for roughly averaged categorization
b
see section `when did mental time travel emerge' below

Why Travel Mentally into the Past?

What adaptive advantage could it have to represent one's former state of mind in
the context of past events, i.e. to mentally travel back in time, if phylogenetic older
forms of memory already allow for learning from a single event? We humans seem to
care for our memory of past events as we do for a personal treasure, as expressed by
the phrase: no one can take away your memories. These memories seem to be valued
for their own sake. We can tell stories about our past, which certainly has the
adaptive advantage of transmitting knowledge to others who do not have to
experience those events themselves in order to learn from them. However, this
advantage could only have evolved after the emergence of language and is of benefit
mainly to the receiver. It appears more reasonable to search for selective advantages
in the individual actually doing these time travels. In fact, we do not merely repeat
past situations mentally, we also reflect on them, which constitutes a significant
advantage in that it permits us to learn over and over again from a single experience
and thereby to increase semantic knowledge. One picture may contain information
requiring thousands of words to describe, and similarly one episode may contain
thousands of semantic facts that may only become accessible if one can repeatedly

look at it, either in reality or in imagination.
It is clear that a superior semantic knowledge about the world (e.g. facts about the
physical, chemical and biological reality) was necessary for our species to change the
world in the way it has (e.g. through technology, agriculture, science etc.). Some
scholars have taken this as evidence for the idea that semantic memory must have
evolved, whether in phylogeny or in ontogeny, after the evolution of episodic memory
(e.g. Donald, 1991; Seamon, 1984). However, other animals appear to possess
semantic memory (see above, p.8), which supports Tulving's (1983, 1984, 1985)
perspective that episodic memory must have evolved later. Nevertheless this
development of a memory for past events may have had the great adaptive advantage
of boosting the older semantic memory system. The ability to mentally travel back in
time, to simulate past experiences, enables the individual to extract far more semantic
knowledge then could have been extracted at the moment of the experience itself.
Episodic memory potentiates the access to information utilizable for the generation of
abstract theories. Conversely, Kinsbourne and Wood (1975, cited in Kinsbourne,
1989) showed that the absence of episodic memory slows down the acquisition of new
knowledge. Using one's imagination to reconstruct episodes of the past is an
evolutionary catalyst that boosts the ability to acquire sophisticated semantic
knowledge. Causal chains can be analyzed by putting (reconstructing) an event in
broader contexts, relationships and rules can be extracted and the same (simulation)
system can be used to change aspects, permitting the individual to test alternatives
without having to face real-life consequences14.
Although these abilities clearly increase the fitness of the organism, they may not
have evolved for these reasons. In recent years the idea that intelligence or even
consciousness has emerged in order to deal with social problems has become popular
(e.g. Byrne & Whiten, 1988; Humphrey, 1976, 1986; Jolly, 1966). This hypothesis of
so-called `Machiavellian intelligence' (Byrne & Whiten, 1988; de Waal, 1982)
suggests that the need to become a good natural psychologist was the selective
pressure that produced intelligence in primates. Clearly, if one knows what is on the
mind of another individual, one is better at predicting and manipulating behaviour, at
cooperating and planning, and at imitating and teaching. If the Machiavellian
intelligence hypothesis is correct, then our `self intelligence', our knowledge of
present and past self, might be an exaptation: it may have emerged from the ability to
understand others. Humphrey (1986) argued along these lines when he claimed that
the human desire to gather a variety of experiences emerged because it allows us to
understand others in similar situations. But searching for a functional or temporal
order in the interdependent complexity of evolution is difficult. A selective pressure
that would favour social intelligence would have inevitably favoured self intelligence,
but the reverse pattern may equally be true. Social and self intelligence seem
inextricably interwoven. Chimpanzees that have been reared in social isolation seem
not to be able to recognize themselves in a mirror (Gallup, McClure, Hill & Bundy,
1971), but if one can recognize one's self in a mirror, every conspecific may, at least
to a degree, appear to be a mirror image of one's self (Bischof, 1978). The ability to
understand others and one's (past) self probably co-evolved. But whether or not one
ability emerged merely as a by-product of the other remains debatable.
Regardless of whether the origins of mental time travel into one's own past are
adaptational or exaptational, once evolved, it provided the basis for a definition of
one's personal identity. The imagination can be used to re-present and re-analyze the
past and based on (or extrapolated from) this past it may also be used to pre-present
change, was necessary for the evolution of a consciousness like ours (Ingvar, 1985;

Popper & Eccles, 1977), and has had a possibly metamorphotic impact on many and
varied aspects of life.
But another important ability is deeply involved in this evolutionary process. It is
the most sophisticated tool humans have developed for reading the minds of others
and expressing one's own: language. It is difficult to disentangle the evolutionary
sequence of reading the minds of others, mental time travel and language. All appear
to be interdependent. In ontogeny, early language training seems to precede the
emergence of the other abilities and may indeed facilitate internal meta-representation
and enhance narrative reconstruction of past episodes. However, there is a reason
why language probably emerged late in the phylogenetic sequence. The crucial aspect
of human language is its infinite flexibility (e.g. Corballis, 1991). In order to think or
talk with infinite flexibility humans must have something of infinite flexibility to
think or talk about. The ability to mentally travel in time may have provided just that,
creating mental access to the virtually unlimited content of the fourth dimension and
therefore an open-endedness of thought that required an open-endedness of language
for adequate expression.

Summary

The question whether or not animals can mentally travel back into their own past
cannot be answered directly, because in the absence of introspection or language we
can infer only from observable behaviour what underlying mental mechanism may be
involved. Animals can learn from single events, but this is not sufficient evidence for
the existence of the ability to conjure up episodes of the past. Animals may have
limited access to their past, not because they lack the necessary capacity to store
information, but because of insufficient cognitive abilities.
Remembering past episodes seems to involve active reconstructive processes.
Memory representations appear not to be `marked' as memories; instead, their role
needs to be inferred. Mental time travel into one's own past appears to require meta-
representations and access to the content of one's own mind. Currently available data
suggest that only the great apes have such mental capacities. The same picture
emerges when one considers the interdependency between episodic memory and self-
awareness: only the great apes (with the possible exception of gorillas) seem to be
able to recognize themselves in a mirror. Monkeys fall short on both accounts,
leaving the great apes as the most likely, if not only, candidates for potentially being
able to mentally travel in time the way we do.
Nevertheless, even the great apes may suffer cognitive limitations, especially with
respect to `theory of mind', that may prevent mental time travel. "There is very little
evidence that chimpanzees recognize a discrepancy between their own states of mind
and the states of mind of others", write Cheney and Seyfarth (1990, p. 254).
Chimpanzees' failure to solve tasks that demand the simultaneous representation of
opposing-to-present states of mind (of knowledge and belief) may indicate that they
are unable to represent their own past states of mind if those are opposed to their
present ones. This also appears to be the case for children under the age of four. Only
after this is overcome, may it be fully possible to travel mentally into one's own past.
Drawing from simulation theory of mindreading (e.g. Harris, 1991), I suggested
that mental time travel might be understood as a reconstruction of memory traces into
episodes that takes place in the imagination of the individual. An ability to set aside
the current state of mind would be necessary for the simulation of past episodes.

While chimpanzees seem to fulfil most cognitive prerequisites for mental simulation,
they may not be able to escape the influence of the present mental state, which may
hinder the establishment of a fully fledged episodic memory system. Further research
may prove this wrong, but if not, and if Koehler (1917/1927) is right in his opinion
that even chimpanzees' access to time is limited, then we might infer that mental time
travel emerged after the phylogenetic split from apes, and was a critical factor in
human evolution.

Mental Time Travel into the Future

In Greek mythology it was Prometheus who stole fire from heaven for the use of
humans. "The name Prometheus literally means foresight, the ability to look ahead
and in imagination to experience events that lie in the future" (Coan, 1987, p.44). So,
according to the ancient Greeks, it was foresight that gave us some of the powers of
the gods: the ability to see, to create, to control; the power that make us stand between
the worlds of the animals and those of the gods. Does this ancient wisdom bear a
grain of truth for the processes that determined the course of human evolution?
Among the adaptative advantages of developing the ability to travel mentally into
the past I briefly mentioned that this ability also provides the basis for the ability to
look into the future. This might have been a crucial consequence indeed. Predicting
the future is a fundamental human capacity that needs to be considered by any theory
attempting to illuminate the reasons for the distinct impact our species has had on
planet Earth.
The most crucial questions we can ask, according to Humphrey, are:

"Where have we come from? What are we? Where are we going? ... They are
not really separate questions, but one big question taken in three bites. For only by
understanding where we have come from can we make sense of what we are; only by
understanding what we are can we make sense of where we are going" [Humphrey,
1986, p. 174].

The dependency of an understanding of future on an understanding of past and present
is obvious: unlike the past and present, the future is a pure figment of our
imagination. Mental states of past self and present others are attributed on the basis of
actual stimuli (memory traces and perceptions), but attribution of future mental states
is not a reaction to stimuli from the future, but is extrapolated from past and present.
The same applies to representations of the world. The future has to be voluntarily
imagined. Animals who lack the ability to reconstruct past episodes may also lack the
ability to simulate future ones.
The level of understanding past and present determines the possible level of
understanding the future. Non-conscious effects of the past, as produced by classical
conditioning, can result in same-level anticipations of future (e.g. Pavlov's dogs
salivated before meat powder was present). Conscious awareness of past episodes,
however, can result in an awareness of potential future episodes. Mental time travel
into the future may be achieved through extrapolation from similar past episodes
(prediction by analogy) or, more important for the understanding of humans'
extraordinary impact on the world over the last 10,000 years, through the application

of semantic knowledge (prediction by theory) about the laws that govern nature. As
with the development of mindreading and mental time travel into the past, the more
abstract theory-based predictions may derive from the more basic ability to mentally
simulate future episodes.
`Instinctual' versus `Intelligent' Anticipatory Behaviour

It is highly adaptive for an organism to be able to act not only in order to ensure
present survival, but also in order to increase future survival chances. Anticipations
have an apparent value for survival: if one knows what will happen one can act now
in order to prevent harm or maximize profit. Anticipatory behaviour, however, can
result from underlying mechanisms other than mental time travel (see also Appendix
A). Learning (e.g. operant conditioning) is inherently prospective, but the future need
not to be mentally represented by the individual. Similarly, insight-free instincts, such
as hibernation, are another way organisms adapt a priori to recurring environmental
changes (e.g. seasons) without the necessity for individual representation of future
situations. Hibernators prepare for the winter even if they have not experienced that
season before. The relatively inflexible anticipatory behaviour and its occurrence in
only narrow contexts appears to distinguish what is labelled instinctual from what is
labelled intelligent.
However, this dichotomy may be another qualitative distinction that may be
viewed as a metamorphosis deriving from mere gradual differences. Recently, Gibson
(1990) has argued that intelligence and instinct may be viewed as the two ends of one
continuum of `mental constructional ability'. Based on connectionist models of the
interconnectivity to the degree of hierachicalization of behaviour and therefore to the
degree of flexibility that determines how instinctual or intelligent behaviour appears
to be. In this perspective, the relatively inflexible and context-specific anticipatory
behaviours of relatively small-brained hibernators are at the one end of the continuum
of mental constructional ability, while the flexible, context-independent mental time
travels of adult humans represent the other end. The constructional capacity to
mentally simulate, or `picture', future events seems not to be required for anticipatory
behaviour such as the hoarding of nuts by squirrels. Even the apparently foresightful
behaviour of satiated leopards wedging half-eaten carcases in a tree does not require
the representation of future hunger and feast, because it is a typical behaviour for
members of that species and evolved as an adaptive behaviour for specific
circumstances (F. Reynolds, personal communication, June 19, 1993).
Cheney and Seyfarth (1990) refer to a kind of "laser beam" intelligence that
animals often display in a single domain but not in others. This inability to apply the
knowledge possessed in one domain to contextually different problems might be due
to an inaccessibility of the knowledge. It may be the awareness about knowledge,
knowing that and what one knows - or in short, metacognition - that allows us to
apply our knowledge to different domains in a flexible and generative manner (cf.
Cheney & Seyfarth, 1990).
The ability to form meta-representations of one's own knowledge might be a
development at one end of Gibson's `mental constructional ability' continuum, like a
metamorphosis, resulting in flexible behaviour of a qualitatively new kind. As
mentioned earlier, evidence for some form of such second-order representations is as
yet available only for the great apes (Cheney & Seyfarth, 1990; Suddendorf, 1993;
Whiten & Byrne, 1991). Mental time travel, however, whether into the past or into
the future, requires this ability.
The use of one's imagination to mentally simulate future situations may require the

same, and perhaps even more, cognitive abilities as mental simulation of one's past.
In contrast to monkeys, great apes have provided evidence for the advanced cognitive
features of second-order representation, imagination of other possible worlds, self-
awareness and mindreading (which all appear interlinked). Great apes may therefore
have the basic cognitive capacities required for mental simulation. Instead of
imagining what it was like (as in simulation of past episodes) or what it would be like
(as in mindreading) the simulation process may also be used to answer what it will be
like being in a future situation. As for the other two applications of the mental
simulator, it appears necessary for the individual to detach from the current mental
state in order to imagine other states (of others, past self or future self) that may be
opposed to the present one. Mental simulation of the future may be restricted in
chimpanzees by their seeming inability to represent opposing-to-present future mental
states and, additionally, by their limited access to past experiences which may limit
the ability to infer future states of the world and self.
Despite these potential limitations, great apes' advanced cognitive abilities should
allow for flexible problem solving with an eye to the future. In fact, Doehl (1970)
showed that the chimpanzee Julia was able to look several steps ahead in a sequential
problem-solving task. Julia had to choose between two keys in a transparent box
which opened further boxes with keys until arriving at the final box containing either
a food reward or nothing. Only by choosing the right key in the beginning was reward
obtainable. Julia learned to act not by chance but by determining the route leading to
the reward before choosing the initial key. Each trial was of course a different
arrangement of keys and boxes so that simple chaining explanations can be ruled out.
Julia was able to look five steps ahead in pursuit of her final goal.
The different tool cultures of chimpanzees provide further examples of
`forethought'. The chimpanzees at Gombe, for example, manufacture tools at one
place to use them later for termite fishing at another place out of sight (Goodall,
1986). In that a branch is trimmed for use as a probe, such behaviour may be based
on the high `mental constructional ability' of second-order representation. Whiten and
Byrne (1991) argue that besides the primary perception of the branch as a branch, the
individual has to generate a meta-representation of it as a probe. Similarly to these
observations in the field, the experimental observation of Sultan's problem solving by
insight (Koehler, 1917/1927; see above, p.28) seems to indicate that chimpanzees can
use their imagination to mentally construct (`picture') possible future realities.

Anticipating Near versus Remote Futures

Despite this evidence for chimpanzees' capacity to imagine the future, Koehler
(1917/1927) argued that it was restricted. He suggested that it is "of theoretical
importance that the clearest consideration of a future event occurs when the
anticipated event is a planned act of the animal itself" (Koehler, 1917/1927, p. 272).
The reason for this has to do with the motivational connection between the animal at
present and the anticipated event. Clearly, Sultan imagined `the future' as an attempt
to get the bananas that could satisfy his present hunger. Similarly, Julia's performance
was driven by her desire for the food reward and the Gombe chimpanzees'
manufacture of sticks is motivated by an appetite for termites. However complex
these anticipations are, they are concerned with a relatively near future. Koehler
viewed this as the `dynamic essence of drive behaviour' and as belonging to the

present; the anticipations do not go beyond the actual context of one behavioural unit
or gestalt. In other words, one could say that animals appear to be bound to the
present. This notion is expressed in many writings about the restrictedness of animal
thought compared with that of humans. Recently, Donald wrote for example that
apes' "behavior, complex as it is, seems unreflective, concrete and situation bound"
(Donald, 1991, p. 199). And Stebbins (1982) and Eccles (1989) refer to `time-
binding', i.e. simultaneous access to past and future, as a unique human feature.
Bischof (1978, 1985) and Bischof-Koehler (1985) make this point more explicit and,
based on Koehler's (1917/1927) writings, suggest a limit to the extent to which
animals can represent the future. They claim that animals cannot anticipate future
needs or drive states and are therefore bound to a present that is defined by their
current motivational state. Only humans, they argue, have acquired the ability to
imagine a remote future that lies beyond the current needs. I will call this idea the
Bischof-Koehler hypothesis (this name refers to all three researchers involved:
Wolfgang Koehler (1917/1927), Norbert Bischof (1978; 1985) and Doris Bischof-
Koehler (1985)).
None of these authors provided a clear definition of drive or need, whether
anticipated or current. They seem to rely on a common sense understanding of the
terms. A review of the long debate about motivation, drives and needs goes beyond
the scope of this paper. However, Bischof (1985) illustrates his point with the
example of the homeostatic motive, thirst. When an animal is thirsty it tries to find a
way to get something to drink: perception is focused on key stimuli that indicate
access to water (e.g. certain plants that grow only close to lakes and rivers), memory
is searched, and so on. To begin these procedures animals must first experience the
thirst; humans need not. While a full-bellied lion is no threat to nearby zebras, a full-
bellied human may well be. Clearly, humans anticipate future needs very often as
expressed in that we collect (buy) food even if we are not hungry or in that we carry
(possess) tools, even if we do not need them to satisfy any current needs, because we
can anticipate their usefulness for the satisfaction of future needs. Business, for
instance, is to a great extent dependent on anticipations of one's own and others' future
needs.
The Bischof-Koehler hypothesis appears to be consistent with the idea, outlined
above, that animals may be unable to escape the influence of the present mental state.
While chimpanzees fulfil most of the cognitive requirements necessary to use the
setting aside their own current mental state in order to imagine opposing-to-present
future (past) mental states. The emphasis that the Bischof-Koehler hypothesis places
on the representation of future needs is justifiable. What adaptive advantage would
there be in developing the capacity to imagine remote futures, if this forethought is
conferred to serve only the present needs? If all one cares about is related to the
current needs because one cannot imagine future ones (or cannot set aside the present
one), what aid does one receive from imaginations of remote futures? Only if one can
realize that one will have different future needs does it appear to make sense to invest
in further capacities to represent aspects of remote futures. While many animals may
have concurrent drives and needs with varying degrees of urgency, anticipating future
needs appears to be special. This ability seems crucial for the evolution of `unlimited'
mental time travel.
However, chimpanzees appear to have problems with representing opposing-to-
own mental states of knowledge and belief, but not necessarily with the attribution of
contrary desires and intentions. Chimpanzees can attribute desires and intentions to
others (Woodruff & Premack, 1978), but it is not clear if they can do so when these

states actually oppose their own current state. In order to demonstrate this, one might
seek evidence that completely satiated chimpanzees, for example, can attribute hunger
to others (but see also the experiment suggested in Appendix B). If chimpanzees can
attribute opposing-to-present states of needs to others, then problems with setting
aside the current state of needs could hardly account for the postulated inability to
represent own future needs. Rather than being caused by an inability to
simultaneously represent opposing (future) needs, chimpanzees' limited access to a
remote future may be due to their limited access to their past. In order to mentally
infer future states of the world and self, one has to extrapolate from one's knowledge
of the past. The voluntary imagination of future needs evoked by an imagined remote
future environment may demand more access to the past (e.g what environmental
circumstances produce(d) what needs) than chimpanzees possess.

Can Apes Travel Mentally into Remote Futures?
Reviewing the Evidence

Although Griffin (1978) pointed 15 years ago to the importance of acquiring
knowledge about animals' sense of a remote future and urged cognitive ethologists to
specifically study this realm, as yet little has been published on the topic. The few
experimental investigations of animal forethought are generally concerned with the
ability to anticipate near futures (e.g. Washburn & Rumbaugh, 1992). As yet we need
to rely on anecdotal data in order to assess the validity of the Bischof-Koehler
hypothesis.
When Jane Goodall (1986) asked to what extent chimpanzees can plan ahead, she
chose an example of a male chimpanzee called Satan: "[w]hen Satan followed a
female in estrus until she nested, then slept close beside her, was he planning the early
morning getaway? Or did he simply take advantage, each time, of the favorable
circumstances he found himself in the morning?" (Goodall, 1986, p.588). No matter
what the answer, it seems to be apparent that Satan, even if he had planned the
situation, was acting according to his present sexual drive. The anecdote would
therefore constitute a single, although extended, entity of `dynamic drive behaviour';
that is, it would still not extend into the `future' in Koehler's sense.
Bischof (1985) points to a general evolutionary pattern progressively increasing the
gap between drive and action. Great apes display quite extensive gaps. They are able
to postpone the immediate enactment of their current drive, producing the intention to
receive gratification at a future point in time. De Waal (1982), for example, reported
an instance that took place in the Arnhem Zoo. The researchers hid grapefruits in the
chimpanzee enclosure by burying them in sand. Once outside, the chimpanzees
searched enthusiastically but unsuccessfully for the hidden treat, although several,
including Dandy, passed over the spot. Only later in the afternoon did it become
apparent that not all chimpanzees had failed to find the spot. Unnoticed by the others,
Dandy went straight to the hiding place, dug up the fruits and enjoyed the treat having
avoided competition. Other examples of this kind can be found in Byrne and Whiten's
(1990) database of tactical deception in primates. However, while postponing the
enactment of a current drive may be a necessary prerequisite and a step towards
future-need anticipation and consideration, it is not equivalent.
Chimpanzees have been observed carrying stones over long distances to open nuts
at a place where no suitable `stone tools' can be found (Boesch & Boesch, 1984).
However, even this extreme instance of apparent forethought seems to be induced by

the current drive. "What is imagined is the resonance of current needs in a future
environment" (translated from Bischof, 1985, p. 541). The chimpanzees that pick up
the stones and carry them seem to do this to satisfy the already present motive of
having an appetite for these special nuts. Thus, it could be argued that the future
environment can be anticipated, but only with the inducement of the current drive,
which remains unchanged and outlines the instance as one behavioural unit.
As yet, only one reported anecdote appears to suggest that chimpanzees may
anticipate future needs:

It is November and the days are becoming colder. On this particular morning
Franje collects all the straw from her cage (subgoal) and takes it with her under her
arm so that she can make a nice warm nest for herself outside (goal). Franje does not
do this in reaction to the cold, but before she can have actually felt how cold it is
outside. [de Waal, 1982, p.192]

Apparently, the chimpanzee Franje anticipated the future coldness and the resulting
future desire or need for warmth. But since the above citation is the entire
information published about this case, many question marks remain. As with many
other anecdotes, alternative, more parsimonious, explanations cannot be ruled out and
are indeed plausible. Experimental research is needed (in Appendix B I propose a
possible experimental paradigm). A single anecdote, in contrast to a single proven
case, cannot falsify the Bischof-Koehler hypothesis, i.e. that animals are present or
situation-bound because they cannot anticipate own future needs.
However, absence of evidence for future-need anticipations in animals is not
equivalent to evidence for absence. The Bischof-Koehler hypothesis appears to be
congruent with our current knowledge about animals (see Appendix A for the results
of a survey of animal foresight). But the Zeitgeist of science in the first 70 years of
this century did not allow much anecdotal evidence for animal intelligence (such as
forethought) to surface and experimental studies did not focus on `mentalist' concepts.
While the widespread and relatively uncritical use of anecdotes led 19th-century
scientists to radically overestimate the mentality of animals (Lindsay, 1880, for
example, concluded that animals engage in criminal activities and commit suicide,
cited in Bernstein, 1988), most 20th-century scientists appear to have made the
systematic bias of underestimating animals' cognitive capacities. `Lloyd Morgan's
canon' destroyed the acceptability of the anecdotal method and the emergence of `the
principle of parsimony' demanded that one should attribute the behaviour of an animal
to no higher, or more complex, level of mental ability than is strictly necessary. It
remains debatable, however, whether our decision about what constitutes higher and
what constitutes lower is correct (cf. Griffin, 1981; Suddendorf, 1993). To find data
that could potentially falsify the Bischof-Koehler hypothesis among the database
deriving from this period appears unlikely even on a priori grounds. However, the
research climate is changing. Speculations about the evolution of mental abilities in
recent studies (e.g. Cheney and Seyfarth's study on vervet monkeys) seem to place
more emphasis on what level of mental ability is optimal rather than what level is
minimal (Figueredo, 1992). And the anecdotal method has been successfully
reintroduced for studying primate deception (Whiten & Byrne, 1988; Byrne &
Whiten, 1990, 1992), a phenomenon completely ignored under the earlier paradigm.
Instead of naive overestimation of animal mentality without experimental evidence
and instead of underestimation of animal mentality by depriving us of valuable
anecdotes (the phenomenon of infanticide, despite its social and evolutionary

importance, was unrecognized until anecdotes were `accepted'), we now seem to be
entering a phase of more balanced inquiry. Experimental studies have shown many
advanced mental abilities to exist in animals (see above, p.2, 3, 15-19) and systematic
gathering of anecdotal observations enriches the realms of further inquiry (see
Appendix A).
Before discussing the adaptive advantages of mental time travel into the future yet
another type of apparent forethought needs to be mentioned.

It often happens that human beings discover the goals of their behaviour only
in retrospect. During adolescence, for example, we stand up against our parents,
provoking and challenging them. Later we may explain this behaviour by saying, `I
wanted my independence', but remember that we did not start the generation conflict
with this motive explicitly in mind. It was an unnamed, unconscious motive. [de
Waal, 1982, p.193]

De Waal put forward this example as a possible explanation for the strategic
intelligence displayed by the ex-alpha male Yeroen of the Arnhem chimpanzee
colony. After losing his alpha position to Luit, Yeroen cooperated with the third male
Nikkie which eventually brought Yeroen, as Nikkie's right hand, back to power. This
success occurred months later after being initially negative. Not all behaviour that
turns out to be smart in retrospect was necessarily intended with the future goal in
mind.
Similar explanations could account for apparently forethoughtful behaviour such as
the acquisition of mental maps for future use. Gorillas and chimpanzees seem to
acquire an extensive knowledge about their territory, permitting the individual to take
the shortest route to desired fruiting trees or, in the case of chimpanzees of the Tai
forest, to stones for opening nuts (C.E.G. Tutin, personal communication, June 25,
1993; Boesch & Boesch, 1984). Whether this knowledge is acquired with intention,
that is, having in mind the usefulness of this knowledge for the future, is questionable,
however. Many animals learn more than would be actually necessary for current
demands. Learning as such is inextricably interwoven with control and
anticipation15. Information is stored for future use. Indeed, animals may store
information for future use only; not for the reconstruction of the past (see above, p.7-
36). It is a well known fact that rats, for example, learn where food is hidden in a
maze even if they are not hungry. Being hungry a day later, they go straight to the
food when being put into the maze. This so-called latent learning requires that the rats
have acquired a mental map for potential future use. There is no reason, however, to
assume that the rats have mentally travelled into the future, anticipated the potential
future hunger and decided that it is worth remembering where the food was placed.
Neither is there evidence that the mental maps acquired by apes require such
forethoughtful mental processes. In spite of their advanced mental capacities,
chimpanzees have as yet not provided evidence for awareness about remote futures
(cf. Appendix A).
It may therefore be conjectured that only humans can mentally travel in time in a
virtually unlimited way.

Why Travel Mentally into the Future?

As outlined in the previous sections, organisms evolved a variety of means to
enable them to act in ways that increase future fitness. Mostly innate, species- and
domain-specific, anticipatory behaviour and basically all forms of memory and
learning are of this kind. While all great apes may have developed the mental
constructional ability to meta-represent their own knowledge, i.e. to know what they
know, only humans may have discovered a new dimension of knowledge (time) that
they are aware of. What adaptive advantage could it have to evolve mental time travel
into the future additionally to the phylogenetic older forms of `considering' the future?
In the following I will only touch on some of the crucial consequences. Outstanding
among those potential advantages is the individual flexibility of anticipations and the
consequential increased degree of flexibility and generativity of behaviour.
In contrast to phylogenetic earlier mechanisms, mental representation of possible
futures allows for anticipation of virtually anything and for flexible adaptation of
current behaviour in consideration of this future. The fitness of this new mechanism
depends mainly on the accuracy of relevant mental predictions. Mental simulation of
likely futures can be achieved by extrapolating similar past episodes, by generating
and applying heuristics and, at perhaps the most sophisticated and recent level, by
induction or deduction of actual theories about the world (including the mind). In
short, it depends on one's access to the past and the knowledge one has been able to
extract about the `laws' of nature, such as causal relations. The immediate fitness
value of mental time travels into the future is the increased degree of flexibility in the
ability to act now for the future. Once on this phylogenetic track, strong selective
pressure favours better anticipation of the future, i.e. better acquisition of experience
and knowledge and its utilization for prediction, and better manipulation of the future
(see below).
In this process, behaviour became more and more dependent on what is mentally
represented. Apes may to some degree read the minds of others in order to predict
and manipulate behaviour, but for humans, with their mental access to time, this
becomes essential. Although the behaviourists tried, human behaviour can often not
be understood or predicted without considering what is on the individual's mind. In
particular, one has to take into account that human behaviour can be driven by
intentions that derive from mentally represented goals that lie in the remote future,
well beyond the satisfaction of the current needs. Understanding and prediction of
human behaviour is further hampered by the obscure complexity of the not-directly-
observable mental world of social interaction. A far more sophisticated
`Machiavellian intelligence' emerged to deal with these problems: a narrative mind
able to understand and predict the world in an event-filled rather than abstract time
(cf. Carrithers, 1991). First of all, however, new mechanisms of motivation must have
emerged with mental time travel.
Classic theories of motivation, generalized to animals and humans, emphasized
innate forces and learned stimulus-dependent factors. With mental time travel into
remote futures humans acquired a quite different set of influential factors. The
anticipation of future needs, as pointed out above, might have been a milestone in
evolution. Simulating future environment-need constellations (e.g. a dry area will
evoke thirst) can affect current motivation and behaviour (e.g. one may decide to carry
water when walking into a particular area) even when this is contradictory to the
present environment-need constellation (e.g. plenty of water and no thirst) (cf.

Bischof, 1985). With mental time travel into the future a third component of
motivation needs to be considered because imagined future needs can be converted
into current motivators. Long-term goals can be generated and current behaviour can
be adjusted to serve these aims.
To make these processes functional within the cognitive apparatus, I postulate the
emergence of a cognitive motive organizer, or CMO, which fulfils two
phylogenetically new tasks: (1) to represent most likely and significant anticipations
and/or remember relevant earlier anticipations (cf. `memory of the future', Ingvar,
1985) and (2) to evaluate and coordinate these anticipations and the current
environment-need constellation. New and differentially weighted motives can be
generated and complexly organized. In recent years motivation theorists have come to
appreciate the importance of humans' concern about the future. Bandura (1991)
summarized those studies and theories of motivation that include cognitive motivators
(expectancy-value, goal and attribution theory). He concluded that "[c]ognitive
regulation of motivation relies extensively on an anticipatory proactive system rather
than simply on a reactive negative feedback system" (Bandura, 1991, p.150). Earlier
he pointed out that "even in the so-called biological motivators, human behavior is
extensively activated and regulated by anticipatory and generative cognitive
mechanisms rather than simply impelled by biological urges" (Bandura, 1991, p.70).
This proactive anticipatory system, or CMO, demands an executive and decision-
making authority. This may be the origins of what we believe to be our `freedom of
will'. Our intentions, motives and goals appear, at least to a degree, to be up to us.
Animal behaviour is driven by innate, learned or homoeostatic factors which may not
be under the voluntary control of the individual because the organism cannot represent
alternative future drives. In contrast, the CMO allows humans to alter and even create
new motives based on access to the future. Individuals became responsible for their
own drive management16. An intrinsic by-product of this new element of choice is
that one may be `wrong' in one's decisions. This, of course, is the fundamental basis
of morality. Instead of being driven, humans can put themselves in the driver's seat,
having to take responsibility for their own intentions and actions. Nature let one of its
creatures off the leash, as it were.
However, perceived control, whether the control is real or not, is a dimension of
reinforcement for humans and animals. But mental time travel results in a new kind
of potential control over one's own destiny. This refers not only to one's intentions
and goals but also to one's influence upon the future. Humans must have inevitably
learned that their future anticipations are based on guesses and inferences, not on
clairvoyance. On the one hand, better predictions may be achieved through greater
knowledge, but on the other hand they may also be achieved through greater control
of the future. Attempting to direct the future through proactive manipulation appears
to be a natural consequence of an awareness about possible futures. The perceived
control may be achieved through `religious' or `natural' technologies (see Festinger,
1983)17. The required generativity and flexibility, perhaps underlying both
technology and language (Corballis, 1991; Greenfield, 1991), could only have been
achieved through mental access to the infinite content of the dimension of time.
All these processes that I have touched on are, of course, far more complex than
could thoroughly be discussed in this paper and involve many factors that are
multidimensionally interconnected (see also Bischof, 1985; Suddendorf, 1992).
However, it should have become clear that our awareness of time is deeply involved
in many crucial human developments. The scientific neglect of this important human
capacity needs to be overcome if we ever want to understand our nature, our evolution

and our development. A final important consequence remains to be discussed, which
will clearly illustrate the importance of our awareness of time for ourselves.
As Humphrey (1986) pointed out, the crucial questions of where we come from,
what we are and where we are going are essentially one. Only with an awareness
about time can we ask these questions and perhaps find answers, through which we
can define our personal identity. In addition to the self-awareness displayed at present
in other great apes, humans can form a concept of self over time. Although
fundamental psychological and physical changes take place in our individual
development from infancy to old age our capacity for mental time travel allows us to
view any of these stages as part of our personal identity. Furthermore, this self
experiences a certain control over its own drive management, by being the authority
that is making decisions about goals and, if necessary, even about overriding the basic
instincts (e.g. hunger in strike or fasting) in pursuit of these goals. Our impression of
`freedom of will' and our personal identity over time are consequences of mental time
travel that have profoundly affected the human ego.
Mental time travel into an unrestrictedly remote future must have confronted
humans with what according to Freud is the most frightening of all conceivable facts:
one's inevitable death. The perception of continuity of one's personal identity over
time might have resulted in the belief in a continuing identity after death: a
continuing `soul', whether in heaven or hell, in this world or another, in the same body
or in a different one. This belief may be applied, not only to those who can form such
mental concepts (i.e. humans; discontinuity philosophy), but also to those who cannot
(i.e. all living beings; continuity philosophy). To deal with the insecurity and lack of
control about one's destiny after death, socially shared after-life concepts may have
emerged: the core of religions. Goals and needs that lie beyond one's own lifetime
were anticipated. The ancient Egyptians or Chinese, for example, believed in the
after-life need for goods while people who believe in heaven and hell may anticipate
the need to have a `clean slate'. Current life became strongly affected by the answers
to the inevitable questions mental time travel provoked. I agree with Gould (1991)
that a single exaptive argument for the origin of religions cannot explain all
multifaced and complex aspects of the evolution of these institutions, but the
emergence of the ability to become aware about time and one's personal future surely
played a fundamental role.
When human's mental horizon broadened by the dimension of time, the course of
human evolution changed significantly. Changes in behaviour, cognition, motivation
and emotion must have taken place, justifying the simplifying label `prime mover'.

When did Mental Time Travel Emerge?

In order to investigate the history of a living organism or of some of its
characteristics, one can use information about currently living species and about
remains and artifacts from the past. Recent advances in genetics have changed our
concept about the relationship of today's species. In particular for many it was
surprising that, in spite of the apparent differences, humans and chimpanzees (Pan
troglodytes) seem to be genetically more closely related than chimpanzees and gorillas
(Gorilla gorilla) (Miyamoto, Slightom & Goodman, 1987; Gibbons, 1990)18.
The common ancestors of humans and chimpanzees lived about 5 to 8 million
years ago. Since the human-ape branch grew out of the primate tree some 30 million
years ago, humans share about 22-25 million years of evolution with chimpanzees that

humans and chimps do not share with monkeys. It should therefore not surprise that
in many respects chimpanzees differ more from monkeys than they differ from us.
However, the reviewed evidence suggests that chimpanzees do not share the
apparently important human capacity of unrestricted mental time travel and one may
therefore conjecture that this characteristic developed after the phylogenetic split from
chimpanzees19. What evidence do we have about the emergence of mental time
travel in the last 5 to 8 million years of evolution?
The archaeological record provides us only with limited and selected information
about human evolution. Important cognitive developments, such as mental time
travel, often do not manifest in physical evidence. We are therefore likely to commit
type II errors in our archaeological analysis; that is, we are likely to accept the null
hypothesis that our forebears did not possess some cognitive ability when in fact they
did. Conversely, the likelihood of committing type I errors, i.e. to reject the null
hypothesis and accept the hypothesis that some cognitive ability was present, even
though it was not, is reduced. However, type I errors are also likely because of
possible misinterpretations of the few data that we have. With these considerations in
mind, let us now consider the evidence for mental time travel in our ancestors.
Stone tools - perhaps merely due to their durability - are the oldest known artifacts
and have often been viewed as the earliest evidence for real humans (this is why
Leakey labelled the 1.8-million-year-old body remains `Homo habilis' or `handy man'
in contrast to the australopithecines). However, the oldest stone tools, the so-called
Oldowan tools (datings vary from 2.4 to 1.6 million years old), associated with Homo
habilis20, seem to be within the competence of modern chimpanzees (Wynn &
McGrew, 1989; see also Toth et al., 1993) and the stone tool culture of tai
chimpanzees (e.g. Boesch & Boesch, 1984), although not involving manufacture of
stone tools, has been evaluated as representing a similar state of development (Wynn
& McGrew, 1989). The production of an Oldowan tool may require some mental
picture of the finished product and the use to which it will be put, but is not
convincing evidence for mental time travel beyond the current needs because it might
have been manufactured merely as a means to satisfy a current drive (just as appears
to be the case for the tai chimpanzees). Although we may commit a type II error,
current evidence does not support the idea that mental time travel beyond the current
drive was present in H. habilis, even though the capacity to think ahead might have
improved.
Evidence for mental time travel into a remote future is more convincing for Homo
erectus. With the emergence of H. erectus (about 1.6 million years ago) a more
sophisticated, so-called Acheulian, tool culture occurred. One of these tools was the
biface handaxe whose production included a somewhat symmetrical removing of
flakes from two sides of the stone core so that the tool became sharper and more
pointed. The manufacture of costly bifacial stone tools leads one to assume that they
were not intended for one time use only, but were kept for future use, which would
imply that the manufacturer has anticipated future needs for which this tool might be
helpful. In turn, the anticipation of multiple future uses could have been an incentive
for the more time-consuming manufacture of more sophisticated, versatile and long-
lasting tools. However, H. erectus' Acheulian tool kit showed little further refinement
for over a million years. Perhaps H. erectus represents an intermediate stage of the
evolution of mental time travel, in that simulation of future and past episodes was
possible, but the generation of abstract semantic concepts and theories from this
knowledge was still in its infancy (see table 1. above).
The earliest evidence for a consequential increase in flexibility and generativity is

Mental Time Travel and Human Evolution

Mental Time Travel and Human Evolution

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