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Readers who take an interest in animal cognition may be aware of recent claims that orangutans are capable of planning their travel itinerary a full day in advance, and are also capable of “mental time travel,” i.e. the ability to mentally re-experience past events and to plan for future events. What I shall argue instead is that while some non-human animals are capable of planning for anticipated events and even communicating their intentions regarding these events, the ability to plan for the future is a uniquely human ability. And while some non-human animals are capable of remembering and mentally drawing upon events that occurred in their past, only humans are capable of mentally traveling back in time to the past at will. Thus the scientific evidence to date is fully consistent with the view that only humans are capable of having an autobiographical concept of time, with past, present and future events laid out along a single axis. It is this concept of time which is required for rational planning – which is why certain kinds of planning are beyond the capabilities of non-human animals, as I’ll argue below.
The latest evidence
A recent report in Science Daily (September 11, 2013), titled, Orangutans Plan Their Future Route and Communicate It to Others, claims that orangutans can make plans for the following day:
Male orangutans plan their travel route up to one day in advance and communicate it to other members of their species. In order to attract females and repel male rivals, they call in the direction in which they are going to travel. Anthropologists at the University of Zurich have found that not only captive, but also wild-living orangutans make use of their planning ability…
Orangutans generally journey through the forest alone, but they also maintain social relationships. Adult males sometimes emit loud ‘long calls’ to attract females and repel rivals. Their cheek pads act as a funnel for amplifying the sound in the same way as a megaphone. Females that only hear a faint call come closer in order not to lose contact. Non-dominant males on the other hand hurry in the opposite direction if they hear the call coming loud and clear in their direction…
In extreme cases, long calls made around nesting time in the evening predicted the travel direction better than random until the evening of the next day. Carel van Schaik and his team conclude that orangutans plan their route up to a day ahead. In addition, the males often announced changes in travel direction with a new, better-fitting long call. The researchers also found that in the morning, the other orangutans reacted correctly to the long call of the previous evening, even if no new long call was emitted.
“Our study makes it clear that wild orangutans do not simply live in the here and now, but can imagine a future and even announce their plans. In this sense, then, they have become a bit more like us,” concludes Carel van Schaik. (Emphases mine – VJT.)
The study by Carel P. van Schaik, Laura Damerius and Karin Isler, on which the Science Daily report was based, is titled, Wild Orangutan Males Plan and Communicate Their Travel Direction One Day in Advance (PLoS ONE, 2013; 8 (9): e74896 DOI: 10.1371/journal.pone.0074896). The concluding paragraph reads as follows:
In conclusion, regardless of the mechanisms used, wild orangutan males indicate future travel directions through their long calls over a span of about one day. This broadcast information is used in turn by their audience to adjust their own range use. These findings strongly suggest the presence of the ability to plan for the future, as demonstrated in captive experiments with the same species. Such an ability is probably adaptive in many ecological and social conditions. Among orangutans, the combination of a patchy, scarce diet (trees with ripe fruit), high energy requirements due to large body size and an expensive arboreal locomotion style [49], and a dispersed social system suggests major benefits for high travel efficiency and thus a long-term planning horizon. We therefore do not expect such a planning ability to be limited to orangutans, but rather to exist also in other apes and perhaps other large-brained animal taxa. However, only the fact that orangutan males happen to announce their plans to conspecifics allowed us to recognize its usage in the wild.
To my mind, the authors of the paper are making a huge logical leap here. The fact that orangutans make calls indicating their intended travel directions over a span of one day need not imply that they envisage following that route as an event lying one full day into the future. All that they need to possess is the relevant intention to follow that route. Likewise, the animals that decode these calls need not understand them as referring to the following day. All they need to do is update their mental maps of where the dominant male is heading.
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Indeed, it is extremely unlikely that orangutans have a concept of “tomorrow,” or anything remotely analogous to that term. For if they do, then either they are capable of communicating the concept to one another or they are not.
If they are, and there is some special call of theirs that specifically means “tomorrow,” then the question naturally arises: how do young orangutans learn that this is what the call means? The idea that body language could fix the meaning is simply risible: as readers who are familiar with the philosopher W. V. O. Quine‘s famous gavagai example will be aware, the problem of indeterminacy of translation makes any decisive identification impossible. Alternatively, one could suppose that orangutans are somehow neurologically hard-wired to tweak to the fact that a certain call by the dominant male denotes “tomorrow,” but this is an ad hoc supposition which explains nothing, and which additionally seems to impute bizarre psychic properties to matter: presumably, there would have to be laws of Nature which relate certain behaviors to the acquisition of certain temporal concepts. Unless we are willing to ascribe to these apes a fully structured language, with a grammar of its own, which enables them to construct and express meaningful propositions, there seems to be no way for them to express the concept of “tomorrow.” But once we make such a supposition, we have entered “Dr. Dolittle” territory.
The only alternative, then, is to suppose that orangutans are not capable of communicating the concept of “tomorrow,” but that they still possess it. But this is a very odd hypothesis, which makes sense only if we imagine that each orangutan has his/her own “private language” for entertaining rarefied, non-shareable concepts such as “tomorrow.” But if this “private language” is in principle incapable of being shared with other individuals, then why should we suppose that orangutans possess such a language?
The other recent item of news relating to orangutans is a report in Science magazine (18 July 2013), attributing to them the capacity for mental time travel (Apes Capable of ‘Mental Time Travel’). The report, by science writer Virginia Morell, is commendably more balanced than the Science Daily report quoted above: the author strives to be balanced, and present supporting and dissenting points of view. First, the new findings:
A single cue — the taste of a madeleine, a small cake, dipped in lime tea — was all Marcel Proust needed to be transported down memory lane. He had what scientists term an autobiographical memory of the events, a type of memory that many researchers consider unique to humans. Now, a new study argues that at least two species of great apes, chimpanzees and orangutans, have a similar ability; in zoo experiments, the animals drew on 3-year-old memories to solve a problem. Their findings are the first report of such a long-lasting memory in nonhuman animals. The work supports the idea that autobiographical memory may have evolved as a problem-solving aid, but researchers caution that the type of memory system the apes used remains an open question…
To see if chimpanzees and orangutans have autobiographical memories that can later be triggered with a cue (as were Proust’s by eating the pastry), Martin-Ordas and two other researchers devised a memorable event for the apes at the Leipzig Zoo. In 2009, eight chimps and four orangutans individually watched Martin-Ordas place a piece of a banana on a platform attached to the outside of a caged testing room. The apes could get the treat only by reaching through a slot with a long stick. The researcher then hid two sticks, only one of which was long enough to reach the banana…
Each ape took the test four times. “We set it up to see if cues—like Proust’s madeleine — would trigger a memory event for them,” Martin-Ordas says. But instead of using a single cue like a scent or a taste, the researchers offered the apes “a constellation of cues: me, the room, and the specific problem,” Martin-Ordas says…
Over the next 3 years, the chimpanzees and orangutans took part in many other tasks… But they never experienced the same exact events as they had during the four tests — until one day in 2012.
Then, in a sort of déjà vu, they were faced with precisely the same setup with the researcher that they had encountered in 2009. Apparently, the combination of cues triggered something like a madeleine-moment for the apes because every ape, except for one orangutan, instantaneously remembered exactly what to do and solved the problem.… The seven apes in a control group had not taken the original test and did not find the tool.
A summary of the study may be found here (Memory for Distant Past Events in Chimpanzees and Orangutans, by Gema Martin-Ordas, Dorthe Berntsen and Josep Call, in Current Biology, Volume 23, Issue 15, 1438-1441, 18 July 2013). The study’s lead author believes that the findings constitute evidence for autobiographical memory in chimpanzees and orangutans. But as Virginia Morell points out in her report, other scientists are far from convinced:
Together, the experiments reveal that at least two species of great apes “can remember specific events and retrieve this memory to solve a particular problem,” something never shown before in great apes, says Jonathon Crystal, a comparative psychologist at Indiana University, Bloomington…
But Crystal and others are not convinced that this experiment demonstrates autobiographical memory. “Is there evidence here for ‘cued recall,’ as the authors argue? No,” says Martin A. Conway, a memory researcher at City University London. Conway does think the apes have “moment-by-moment episodic memory, but they are not saying to themselves, ‘I can’t believe it. I’m back in this stupid lab with this stupid test.’ Believe me, that’s not what they’re doing.”
Once again, it appears that the authors of the study have made an unwarranted logical inference. The fact that chimpanzees and orangutans are capable of retrieving memories of events from their past does not imply that the have a concept of the past, let alone of “three years in the past.” In the absence of such a concept, the use of the term “autobiographical memory” to describe the animals’ capabilities is unwarranted: the authors should have chosen a less loaded term.
Can animals remember when an event occurred, in the past?
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During the past decade, several researchers have ascribed a memory of “what, where and when” to apes and corvids (crows and their close relatives), and more recently, to the black-capped chickadee (pictured above), a bird with a remarkable memory for caches where it has stored food in the past. To the lay reader, this terminology may convey the misleading impression that these animals can remember how long ago an event occurred (in days, weeks, months or years). In fact, a careful reading of the studies performed on these animals reveals that all they can do is remember roughly what time of day an event occurs, and distinguish between a long interval of time (e.g. a day) and a short one (e.g. an hour). There is absolutely no evidence whatsoever that these animals have a concept corresponding to “three hours ago”; their memory for “when” is entirely relative. In those cases where they remember the time of day, their memory is relative to some point in their circadian cycle, while in cases where they distinguish a long interval from a short one, their memory merely involves a comparison of two intervals.
In an article titled, Mechanisms of What-Where-When Memory in Black-Capped Chickadees (Poecile atricapillus): Do Chickadees Remember “When”? (Journal of Comparative Psychology, 2011, Vol. 125, No. 3, 308-316), Miranda C. Feeney, William A. Roberts, and David F. Sherry, argue that black-capped chickadees exhibit an integrated what-where-when memory, under certain conditions:
Integrated what-where-when memory, sometimes called ‘episodic-like’ memory, has been shown to occur in a number of animals, including food-storing birds and rodents. It is not always clear in these
studies, however, what aspect of “when” is remembered. We examined memory for what, where, and when in black-capped chickadees Poecile atricapillus in a procedure designed to dissociate memory for elapsed time (how long ago) from memory for a point in time, the sense of “when” implied in most discussions of episodic or episodic-like memory. Chickadees searched for food in multiple trials that each involved a two-phase procedure. In Phase 1, birds found preferred mealworms in half the sites on one side of an aviary and less preferred sunflower seeds in half the sites on the other side of the aviary. In Phase 2, following a retention interval, birds searched in the aviary with fresh sunflower seeds placed in the sites
that held seeds in Phase 1 but either fresh or degraded mealworms in the sites that held mealworms in Phase 1. Whether a site held fresh or degraded worms depended on either when during the day Phase 1 had occurred (Group When), how long ago Phase 1 had occurred (Group H[ow] L[ong] A[go]), or both (Group When + H[ow] L[ong] A[go]). Chickadees in all three groups were able to discriminate where and when palatable worms were to be found, but there were temporal limitations on the use of HLA and When + HLA cues that were not found for the use of When cues.
However, it turns out that the “How Long Ago” testing merely required the birds to discriminate between an interval of one hour and an interval of 28 hours, and to use this ability when searching for food. This is hardly an ability that requires a rich concept of the past – or indeed, any concept at all. Birds in the “When” group had to distinguish between two times of day: 9 a.m. and noon. But any animal with a circadian rhythm (a 24 hour body clock, which has been observed in in plants, animals, fungi and cyanobacteria), could, in theory, make such a distinction.
Is there any good evidence that animals plan for the future?
In a recent article titled, Evidence for future cognition in animals (Learning and Motivation 43 (2012): 169-180. doi: 10.1016/j.lmot.2012.05.005), William A. Roberts handily reviews the best evidence to date that animals have an awareness of the future:
Perhaps the best evidence to date for the planned use of tools by animals appeared in research carried out with bonobos and orangutans by Mulcahy and Call (2006; also see Suddendorf, 2006). The apes learned to use tools to obtain a reward in two different problems. In one problem, a plastic tube had to be used to retrieve grapes from a cylinder, and in the other problem, a hook had to be used to obtain a bottle of grape juice. On test trials, an ape was allowed to see the problem (and reward) but was blocked from accessing it by a transparent Plexiglas barrier. Several tools were available outside the barrier, including the useful tool and several non-useful tools. A subject was allowed to select tools and carry them to a waiting room. After 1 hr, the subject returned to the test room and could bring any of the tools it had taken. The apes carried tools out of the test room on 70% of the trials and chose the useful tool for the problem shown significantly above chance probability. Two of the ten apes initially tested (one bonobo and one orangutan) were further tested with the delay between problem exposure and test extended overnight for 14 hr (Experiment 2). The orangutan chose the suitable tool on 11/12 trials, and the bonobo chose the suitable tool on 8/12 trials. On most of the trials, the apes also returned to the test apparatus with the appropriate tool to gain access to the reward.…
In a somewhat similar series of experiments, Osvath and Osvath (2008) demonstrated tool selection for future use by two chimpanzees and an orangutan. In their initial experiment, these subjects were given one demonstration and one opportunity to use a hose to suck in a highly favored fruit soup. In subsequent tests given in a different setting, they chose between the hose and three other distractor objects. The ape then carried the object chosen back to the group enclosure, where it stayed for 70 min. It was then allowed to enter the reward room where it could use the hose, if it had selected the hose, to obtain fruit soup. During testing, one chimpanzee and the orangutan chose the hose on 14/14 trials, and the other chimpanzee chose the hose on 13/14 trials…
In his article, Roberts argues that the most parsimonious interpretation of these findings is that the animals possess some sort of future cognition:
A first consideration is whether there is enough evidence to be convinced that future-oriented cognition can be found in animals. This author feels that there is, based on both field and laboratory reports that come from different species and different paradigms and procedures… Apes select appropriate tools needed for future retrieval of food, retain these tools, and carry them to a test site when given an opportunity to put them to use. Choice of even novel but appropriate tools is preferred over inappropriate objects previously associated with food reward. In field observations, tayras cached not-yet-ripe plantains in trees for later consumption when ripe, and a chimpanzee carefully stored caches of stones in an adventitious location for later use as missiles to throw at zoo visitors. Although a variety of lower-level mechanisms might be invoked in an attempt to explain these various observations, a more parsimonious view is that they represent future cognition or MTT [mental time travel – VJT] in animals.
Now, if Roberts is merely arguing here for the existence of “future-oriented cognition” in apes, then he is on fairly solid ground. The apes mentioned do appear to be selecting tools in order to satisfy their anticipated needs. At the very least, they must be capable of keeping in mind these goals and selecting the best means to achieve them, while doing so. But it does not follow that the animals are envisaging the realization of these goals as events occurring in the future, or even that they have a concept of “the future” as such. The term “future-oriented cognition” is dangerously ambiguous: in a weak (and fairly non-controversial) sense, it connotes awareness of anticipated goals, while in a stronger (and more controversial) sense, it means awareness of goals in the future. Only if apes possess the latter kind of cognition can they be said to plan for the future.
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In their article, Tool use, planning, and future thinking in children and animals (in McCormack, T., Hoerl, C. and Butterfill, S. (eds.),
Tool use and causal cognition. Consciousness and self-consciousness, Oxford University Press, 2011, pp. 129-147, ISBN 9780199571154), authors Teresa McCormack and Christopher Hoerl point out that the behavior of the apes, described above by Roberts, satisfies a test for future cognition proposed by memory researcher Endel Tulvig, known as the spoon test. If Tulving is right, then it would seem after all as if the apes are acting in order to satisfy future needs:
…Tulving’s (2005) idea of a “spoon test,” introduced in the context of an attempt to characterize a situation in which future thinking is required. Tulving describes an Estonian tale involving a girl who dreamt that she went to a party and found that she could not eat dessert because she did not have a spoon with her. Anticipating that she might dream that she attended the party again, she then took a spoon with her to bed. What is meant to be critical about this example is that the girl took the spoon to bed not because she needed it given the current state of the world and her current goal, but in order to satisfy a future need in an environment quite different from how things were right now.
However, in their essay, McCormack and Hoerl argue that “mature human planning and future thinking involves a particular type of temporal cognition,” and they maintain that there are good reasons for doubting whether Tulving’s spoon test properly assesses this kind of cognition:
One argument to the effect that demonstrating that an animal can pass a version of the spoon test is not sufficient for showing that it is capable of thinking about the future can be found in Roberts and Feeney (2009a). They argue that in Osvath and Osvath’s (2008) study, the apes “could have chosen the hose because they knew its functional value for sucking up fruit soup without anticipating its use 70 mins in the future” (p. 275). Their point here is not that the animals were choosing the hose because of a current rather than a future need, as Suddendorf et al. (2009) argue. Rather, their objection is that there is no evidence that the animals have any idea of when in the future they will need the tool. What they suggest is required is a demonstration that animals show behavior that indicates that they can discriminate between different points in the future.…
McCormack and Hoerl are particularly unimpressed by the feats of Osvath’s stone-throwing chimpanzee, arguing that at most, they demonstrate that the ape had an understanding of what time of day the visitors he threw stones at would arrive:
Take, for example, the much-discussed case of Osvath’s (2009) stone-throwing ape. This chimpanzee calmly gathered stones daily before its zoo opened, which it subsequently hurled angrily at visitors later in the day. In fact, the chimpanzee not only gathered such stones but, when denied stones in its compound, created its own weapons out of the fabric of its enclosure to stockpile for the same purpose, which could be (generously?) interpreted as a form of tool manufacture. There seems to be something that does not quite ring true about the idea that, while gathering the stones, the chimpanzee was thinking about the future. This does not, or not just, come down to the fact that the chimpanzee had ample opportunity to learn the usefulness of the stone-gathering behavior over the course of its time spent at the zoo, as some commentators have suggested. Rather, what seems to be missing from this example is any sense that the animal distinguishes one stone-gathering and throwing episode from the next, or, we could say, that the animal can think episodically. The stone-gathering allows the animal to prepare, yet again, for the recurrent point in the day that the visitors arrive. The point here is not that the animal has no sense at what specific point in the day the visitors will arrive (Roberts & Feeney, 2009b). The chimpanzee could well, on each occasion, be highly sensitive to the amount of time that remains before they arrive, but there is no reason to believe that its temporal sensitivity involves being able to tell one day apart from the next.
McCormack and Hoerl invoke a distinction between cyclic and chronological time, suggested by John Campbell, in order to explain why apes may not possess a concept of future times, after all:
In his extensive discussion of animals’ temporal abilities, John Campbell (1994) makes an important distinction between temporal frameworks that can represent events’ position within a periodic cycle and temporal frameworks that represent events as lying along a linear dimension. He describes this distinction in terms of the idea of a difference between two ways of being “oriented” in time: temporal orientation with respect to phase and temporal orientation with respect to particular times. Animals, he argues, do not have a concept of particular times, but may be able to represent events’ locations relative to recurring time patterns – i.e., represent the location of events with respect to the phase at which they occur in a repeating cycle. Representing an event as occurring at noon daily involves a type of temporal sensitivity, but does not involve representing that event as occupying a unique, unrepeatable, position in time. It is precisely the ability to represent events as occupying such unique positions in time, though, that seems to be at the heart of our mature concept of time, and our notion of the future. As Roberts and Feeney (2009a, 2009b) describe, we do think of the future as stretching out in front of us, with temporal locations forming a unique sequence. This conception of time is linear: the sequence never repeats itself.
I would argue that it is precisely this linear concept of time that is essential to the human understanding of free will: the notion that when one makes a choice, one stands at a decisive “fork in the road”: a fork at which one may have to make an irrevocable commitment. Or as Robert Frost put it in his poem, The Road Not Taken:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.
Because they have a linear concept of time, human beings also understand that there’s no going back and altering the past. By contrast, an animal with a cyclic concept of time, argue McCormack and Hoerl, “has no way of properly distinguishing between events that can and cannot be affected by their behavior merely due to the passage of time.”
The authors argue that a cyclic concept of time is cognitively limiting in another vital respect: it means that there is no general procedure for determining whether one event precedes or succeeds another:
Campbell (1994, 2006) has also drawn out another limitation in the temporal thought of animals that only possess temporal orientation with respect to phase. On a linear conception of time, time is a single dimension that stretches into the past and the future, and all possible times fall somewhere on this dimension. The upshot of this is that we can potentially think about the temporal order relations that obtain between any two points in time: we will always be able to at least consider for any two events A and B, even if they are entirely unrelated, whether A happened/will happen before or after B. We may not always know the answer to this question, but the question itself is never meaningless to us. This is not the case for an animal that has only orientation with respect to phase…
The point is that if events are located with respect to different temporal frameworks, the animal may have no way of considering whether an event A, located in time with respect to one temporal framework, occurred before or after another event B, located in time in some other way.
This notion of ordering is vitally important when we plan for the future. When we do so, we construct a narrative – the story of our own lives:
Campbell (1994) has pointed out that when we think about the use to which we put our own ability to represent events as occurring at particular times, we find that this happens primarily in the context of what he calls a narrative grasp of events. There are various different ways in which the term “narrative” might be understood here, but in this context, perhaps the simplest one would be that we put our notion of particular times to work in the context of our connected story of our own lives, as they stretch out into the past and future… Thus, we have autobiographical narratives about how our past has unfolded that involve representing particular points in our lives at which things happened (e.g., “that day when I passed my driving test”), and long-term plans for how we want the rest of our lives to unfold that involve representing points in our future life that by their very nature will never repeat (e.g., “by the time I’m 50 I want to have paid off my mortgage”).
McCormack and Hoerl contend that the ability to construct a life-narrative is a distinctively human trait, and that the tool use studies performed on animals are incapable of demonstrating the occurrence of narrative thinking on their part:
What thinking along these lines shows, though, is that on the view that a grasp of the notion of particular times is tied to a narrative grasp of events in the sense we have introduced (see also Hoerl & McCormack, 2005), the sorts of tool use studies described above are never going to establish future thinking in animals. Surely narrative thought, at least as understood in the way we have sketched, is something that is distinctively human…
In his 2012 article, Evidence for future cognition in animals, cited above, Roberts draws attention to further limitations in animals’ future cognition, noting in passing that there’s no evidence to date that animals can formulate and choose between alternative future plans. Nor is there currently any evidence that animals are capable of grasping that different events will occur at different points in time:
If we assume that future cognition is found in animals, a further theoretical question is “what is the nature of future anticipation and planning in animals?” There is virtually no evidence that animals have an ability to form several alternative future plans and choose among them, as people do. However, absence of evidence is not evidence of absence. Perhaps future experiments will be able to address this question of flexibility in animal future planning.
A more basic question is whether animals have a sense of future time as a dimension with a number of locations on it. The discovery of WWW [what-where-when] memory in animals has been of major importance because it suggested that they could remember when events occurred at specific points in past time. A similar question can be asked about future time. Do animals anticipate that different events will occur at different points in future time? The alternative to this possibility is that animals might only have a representation of a generalized future in which events will happen. Such a non-temporally specific representation of the future would be more like semantic memory [memory for facts, rather than experiences – VJT] than like episodic future memory.
Finally, Roberts suggests an ingenious experiment which could resolve the question of whether apes possess a concept of future future time as a dimension with a number of locations on it:
Suppose apes are exposed to fruit soup, with no tube with which to drink it, at 10 am and to a jar of honey, with no stick to dig it out, at 2 pm for several days. At 8 am on a test day, the subjects are given a choice among tools that consist of a hose, a stick, and two other distractor objects and can only choose one object. If chimpanzees and orangutans can anticipate a future in which availability of fruit soup and honey occur at different times, one nearer the present time than the other, they should choose the hose. If they were given the same choice at 12 pm, however, they should choose the stick. Such experiments might begin to shed some light on whether animals can view different points in future time.
I’ll give the last word to well-known cognitive researcher Thomas Suddendorf, who has long argued that mental time travel is a uniquely human ability. Suddendorf adheres to a Darwinian view of biological evolution, but has the courage to stick his neck out and declare that on the psychological level, there are genuine discontinuities between humans and other animals. As he puts it in a recent letter to Trends in Cognitive Sciences (Volume 17, Issue 4, 151-152, 07 March 2013) titled, Mental time travel: continuities and discontinuities:
Therefore, I do not think it is useful to resurrect Darwin’s blanket statement that differences in mind between humans and animals certainly are one of degree and not of kind. The picture is more complicated [9]. Even when we find evidence for apparent differences in degree, this need not rule out that differences in kind are also entailed: as temperature gradually increases, the properties of H2O change in kind as it transforms from a solid, to a liquid, to a gas. By the same token, continuous increase in, say, working memory capacity, may well lead to discontinuous changes in what a mind can possibly conceive of [10]. I see no reason why mental time travel should not have evolved gradually through Darwinian descent with modification. However, continuity over evolutionary time (e.g., from Australopithecines to Homo) should not be confused with a need to postulate an absence of gaps in the distribution of traits among extant species [9]. As transitional forms go extinct, vast qualitative differences can certainly emerge. On current evidence, it still appears that human mental time travel is profoundly special. There are few signs that animals act with the flexible foresight that is so characteristic of humans.
Suddendorf’s book, The Gap: The Science of What Separates Us from Other Animals (Basic Books) is currently in press. It promises to be very exciting reading. In the meantime, here are my thoughts on the subject of what separates us from other animals: vive la difference!