14 Arboreal Remnants

There is a deep irony about our success as a species, which is not I think sufficiently recognized. A few million years ago (estimates vary), our not-so-distant ancestors were expelled from their ancient habitat in the trees and driven down to the ground. This was not a case of voluntary expansion outward, like conquering heroes. It is not that they had become unquestioned masters of the trees and then added the ground to the empire of their conquests: this journey into the uncharted was not confident imperialist expansion. Rather, they could no longer live in the safety and plenty of the trees, probably because of climate change (perhaps in concert with other factors); they were forced to displace themselves and try to scratch a living from the more barren and dangerous flatlands. They were not well adapted to this new habitat. The descent (from heaven to hell?) must have been experienced as traumatic, catastrophic—more like forcible exile than exultant conquest. The mortality rate probably took a steep rise. Times must have been hard. Mere survival became paramount. The ground predators alone were a constant danger. Our ancestors, though hardy, may have been brought to the brink of extinction following the compulsory relocation. All of our hominid relatives did go extinct (e.g., the Neanderthals) with only Homo sapiens surviving into the long term. We alone had the magic touch, or the brute luck of the draw. We (our ancestors) made it through the wilderness, but just by the skin of our teeth. I picture this fraught period as one of fear, desperation, and discomfort. Our ancestors had to adapt rapidly, or die out. And adapt they did, furiously so: tools, society, and language were the major adaptations. These were in the nature of desperate expedients—like using crutches when your legs have been broken. We were like refugees in a wretched shantytown, compared to our previous life of arboreal luxury. Other ape species stayed in the trees, or around them, where they belonged, and are still happily frolicking there. Instead of picking fruit and enjoying the shelter of the canopy, existing in small intimate family groups, we had to hunt and scavenge in packs, on two legs no less, accommodating strangers, roaming far and wide, enduring many dangers, suffering the beating sun and the drenching rain, and having to find new modes of protection against predators and the elements. We just weren’t built for the new earthbound vagabond lifestyle. And those ground predators were truly terrifying—we were totally outmatched. It was a tough life being a human in the post-arboreal wasteland. So far from being conquering heroes, our ancestors were more like bedraggled victims of cruel and peremptory nature, hanging on by a thread.

But, against all the odds, those makeshift measures—tools, society, and language—began to demonstrate an unexpected potency. After millennia of precarious existence, living on the edge, with life nasty, brutish and hellishly short, we started to assert ourselves. Our peculiar type of intelligence began to pay off in the struggle to survive. Who knew? Our tools got bigger and better. We became weaponized. We grew ever more numerous, ever less fearful. We started to gaze confidently out over the plains, instead of trembling at the edge of the forest. We spoke a richer and more useful language. Even the dominant predators started to be wary of us. This ascendancy has now reached the point where we can be described as the most successful species on the planet: secure, populous, fearless, and powerful. We have mastered nature, where once it threatened to destroy us. And herein lies the great irony: biological catastrophe led in the end to biological triumph. We snatched victory from the jaws of defeat. It is as if a wretched refugee population, driven from its homeland, managed to ascend to a position of world domination. What looked like rickety measures of last resort—crude artificial organs (i.e., tools) and hand puppetry (i.e., language)—turned out to be the most powerful adaptations ever contrived by nature! They could outdo the ferocity and speed of a lion or the venom of a snake or even the size and strength of an elephant (or mammoth: now extinct, probably as a result of rampant human predation). Amazing! We became the ultimate predators—as the end result of our initially feeble efforts to ward off the predators that confronted us on the ground. From a certain point of view, being made homeless was the best thing that ever happened to us. But it was touch-and-go, seat-of-the-pants, by no means a foregone conclusion. Our tiny twitchy restless hands are what saved us, surprising as it may seem. The great irony of human evolution, then, is that a manifest tragedy turned into a blessing in disguise: a major threat to species survival became the reason for survival. Stopgap measures (tools, language) became, improbably enough, awesome weapons of survival.

That is the irony that is not sufficiently appreciated. We tend to have a rather “entitled” attitude to our own existence and domination, as if it were a smooth and predictable ride to world conquest, guided by our adventurous spirit and innate intellectual gifts. Once nature had produced us, we vainly feel, the die was cast: we were destined come out on top—it was only a matter of time. But that is far too self-congratulatory and unrealistic. It certainly would not have seemed that way to our struggling ancestors out there on the African plains.¹ In fact, our survival was a matter of pure luck: the occasion of our imminent extinction became, surprisingly, the origin of our huge success. If our ancestors had stayed in the trees, courtesy of a pleasant climate, they would probably have survived as they were; but by being driven from the trees, they set the stage for the most dramatic evolutionary development in the history of the planet—us. A short-term tragedy turned into a long-term triumph.

Think of it like this. Suppose tigers lost all their teeth because of some virulent new bacterial infection that eats through them in hours and even prevents them from passing them on to their offspring. The tiger species must now survive without teeth. Their extinction looks guaranteed, and indeed much suffering ensues, with the tiger population terribly depleted. Then a few brainy tigers have the idea, admittedly desperate, of inserting small stones in the mouth to substitute as teeth. It doesn’t work very well—tigers often come off badly in their hunting efforts. But it works well enough to keep the tiger species in existence, just barely (maybe the bacteria happen to produce an adhesive substance that makes the stones stick to the gums). The tigers get by, just about, relying on their shabbily designed false teeth. But then, in the fullness of time, this desperate “technology” starts to show unexpected side benefits—it encourages greater cooperation, which leads to expansions in tiger intelligence, and ultimately to language—and eventually tigers become the dominant species. Ironic, is it not? Who would have predicted it? What a turn up for the books! Catastrophe turns into triumph. Well, our tools and language are like the tiger’s false teeth (remember that early human tools are just bits of chipped stone, and language is just a few crude hand gestures).

It would not be surprising if a species with this kind of past showed some inkling of its status. The shadow of its past might still fall over it. A certain fear and anxiety might attend its daily existence. For the toothless tiger, there would be a fear that the stones might run out or the natural adhesive no longer work—then what? For the treeless human, there would be a fear that our technology might be taken from us, by natural catastrophe or theft—then what? We are defenseless without our tools: our natural state (a “bare forked animal,” as King Lear says) is one of total vulnerability. What if we were all struck dumb one day—or words began to seem like mere noises in the air (a virus invades the speech centers in the brain)? What if species-wide aphasia developed? We would be finished. Finished. We still need our post-arboreal crutches, but they could be taken from us. Thus we live with a constant background hum of anxiety—rational enough, given the facts. What would it be like to be returned to the wilderness, like our ancestors, with only our bare hands to protect and feed us, and not even language to communicate with? We wouldn’t last a week. We need the safety net of our technology, and we need the linguistically mediated cooperation of others. We are constitutionally unable to fend for ourselves without these support systems, and we know it. It is as if we have a species memory of our distant parlous origins and dread being returned to that primordial terror. We know quite well what it would be like to lose our tools and our talk—those recent evolutionary acquisitions. We didn’t always have them, and no natural necessity guarantees that we shall keep them. And they are all that protect us from the jaws of the hungry wolves of the world.²

I am speaking here of the psychological effects of our species past—of what formed us into the species we are today. In what follows I am going to speculate further about other remnants of our evolutionary history. To what extent is the present psychological condition of man shaped by his evolutionary past? This is a familiar enough question, much discussed in evolutionary psychology, but I think my speculations will be less familiar. In particular, I want to explore the psychological remnants of our arboreal past. It is often remarked that the human body bears remnants of our past as tree dwellers: our hands, arms, shoulders and trunk are those of a natural brachiator.³ The lower body is now fairly well adapted to bipedal locomotion on the ground, but the upper body still lives partly in the trees. Thus most people (barring obesity or infirmity) can hold their body weight while suspended by their hands, and can even swing a bit. The hands can grip tight, the shoulders rotate, and the back muscles support the weight. Some people can do a few pull-ups in this brachiating position. We still have the brachiating knack, to some extent. But that is certainly not the limit of human brachiating skill: consider trapeze artists, gymnasts, and climbing children. The capacity is evidently still there in dormant form, awaiting the right training. A skilled gymnast on the high bar is a supreme brachiator; indeed, he or she arguably outperforms even the best brachiator among primates (probably the gibbon). Clearly the human body is still capable of a high level of brachiation performance. I am surprised the fitness industry has not latched onto this fact about humans and launched “brachiation training” programs, promising to exploit the natural abilities of the body and lead us back to our ancient ways.⁴

But might there not be more subtle remnants of our arboreal heritage? Might there not be odd quirks and tastes, preferences and urges, that reflect our tree-lined past?⁵ Here we can only be speculative, but a number of modern human tendencies would seem to bear out the hypothesis of arboreal preservation. What we know is that our ancestors still lived in trees about four million years ago. This is not a long time in evolutionary terms. Their brains will have been adapted for tree living and this would be coded in their genes. They would be hard-wired for the arboreal life-style. When they were driven down from the trees, their brains would not have altered much, retaining the same basic psychological profile—the tree-dwelling mentality. Even as they began to evolve to conform to a terrestrial lifestyle, eventually becoming bipedal tool users and speakers, the old brain circuits would have persisted, existing alongside new cerebral machinery. Natural selection would not destroy the old circuits; it would merely add machinery for the new behaviors. Evolution always works by ancestral preservation, and in this case what is preserved is the tree living mentality—though now obsolete and largely dormant. There is every reason to believe that the old tree-dedicated brain circuits would persist to this day, folded somewhere into our cortex, idling. These circuits, however, may still be activated in some fashion in our new habitat, revealing themselves in odd aspects of our psychological life. So we should be prepared to find psychological remnants of our ancestors’ arboreal brain; it would be surprising if we did not. That brain formation had existed for many millions of years before the descent, and it would not be snuffed out in a blink of evolutionary time.

Consider, then, our attitudes and emotions toward trees. We like trees; we like them a lot. We like to have them around, to gaze at them, to touch them, sometimes to climb them. We find them soothing, beautiful, and vaguely spiritual. We admire them, we fret about their survival; it hurts to see them cut down. We have them in our gardens, our parks, and our city streets. We visit woods and forests. We feel that a life without trees would be a lesser life. No question, we have a thing for trees. We also love wood, the look and feel of it. We make our houses out of it, our furniture, and our musical instruments. We feel that wood is authentic and somehow good (we feel differently about metal and plastic). We are also drawn to tree imagery: pictures of trees, tree diagrams, tree analyses (logic, linguistics, biology). The form of a tree appeals to us, as if we have a cognitive module for trees. Maybe we have an innate idea of a tree (surely a gibbon does). The platonic form of a tree is embedded deep in our psyche. This is all just what you would expect given our arboreal history: our basic bodily design was forged in trees, our ancestors spent their entire lives in trees, our brains took shape surrounded by trees.⁶ Thus we are haunted by trees. Again, I am surprised the therapy industry has not latched onto this, with “arboreal therapy” and “tree getaways” and “forest restoratives.” You might actually get some peace of mind by shinning up a tree and hanging out there for a while. Supervised tree talk while stretched out on a branch or hammock might be quite therapeutic. Tree psychology could be a popular course of study (I can imagine “findings” like: “People who spend 50% of their leisure time in the presence of trees have been found to score at a significantly higher level on contentment tests than those who have no exposure to trees: see the Journal of Arbo-Hedonics, September 2014”). A weekend of vigorous brachiation training and intense arboreal therapy might be just what the doctor ordered—and there is a sound scientific basis for it in the science of human evolution. We might recover some of the tranquility that characterized our days as full-time denizens of the sheltering trees (living arboreal primates always look as if they are having a terrific time up there).

Consider, too, our attitudes toward heights. We have a healthy fear of them, as if schooled in the high-wire lifestyle. Living in trees, that would probably be your major fear. But we also have a fondness for heights—we seek them out. We enjoy views from high up, of open vistas, as if from the upper reaches of a tree. We build our houses in multiple stories, so that we can look out on the world from a safe and soothing height. We like our beds to be elevated, not at ground level. We enjoy balconies and turrets. Stairs appeal to us. Escalators and elevators are curiously thrilling. We endeavor to “go up in the world.” We sometimes feel “down.” We don’t want to be “grounded.” We want to “climb,” professionally and socially. Onward and upward! The ground is where the dirt is; the sky is where heaven is. Hell is even depicted as under ground.⁷ All this churning human affect is consistent with the arboreal remnant hypothesis. I do not say this is all solid scientific fact, only that it is suggestive and worth pondering, in light of the likelihood that our arboreal past still survives in our inherited brain. I doubt that elephants have these preferences—they seem quite content on the solid ground. They do not yearn for “higher things” or despise the dust and mud. Tree climbing holds no allure for them. Elephants have robustly terrestrial souls. The human soul, however, still hankers for arboreal heights, though in a muffled and sublimated way. The branches of trees are where the human heart still dwells—our ancient hearth and home. Crocodiles dream of mud and murky water, birds long for the open air, reptiles adore a patch of rock in the warming sun. We like to sit in the shade of tree and imagine climbing it.

I am suggesting, in effect, that we have a kind of tree-obsessed unconscious persisting in the crevices of our brains, as a relic of our previous mode of existence. I call this the Darwinian unconscious. It is structurally similar to the Freudian unconscious, in that it is a background psychic reality that shapes what we consciously feel and do, but it differs as to content and etiology. It does not originate in childhood sexual trauma and repression, but in the adaptations of our distant progenitors, genetically transmitted to us down through the generations, courtesy of ancestral preservation. The arboreal brain is buried inside our modern human brain, still ticking quietly away. We do not actively repress this unconscious, as Freud would claim, but it comes to us in disguised and modified forms, so that some “psychoanalysis” is necessary to interpret our feelings and actions. The meaning of our feelings and actions may not be immediately apparent, because they are a confusing blend of the old and new—as a typical town is an architectural blend of the old and new. These are psychological remnants, not anatomical ones, and they form a complex mental stew. The Darwinian unconscious is a repository of ancient psychological traits that have been modified and redirected, but not extinguished—just as the body is a repository of modified ancient traits. We thus exhibit regressive tendencies, as Freud also postulated; but here our regression is to our species childhood, not our individual childhood. We are constantly harking back, as if trying to recover a vanished past, tugged at by our evolutionary history. Our species consciousness is thus suffused with feelings of regret or loss or nostalgia—a kind of ingrained existential discontent. We have never quite adjusted to our recently adopted terrestrial lifestyle. We still hunger for the Up.⁸

It is interesting, in this connection, to consider our attitudes toward birds—our erstwhile brothers and sisters in the trees. Our ancestors will have spent their days in close proximity to birds. Human beings have a distinct affection for birds. Do they remind us of times past? Quite possibly we have an ancient cognitive module for birds, buried deep in our arboreal brain. We also admire and envy birds—they are capable of an even greater freedom from the pull of the Earth than we once enjoyed. We (our ancestors) could brachiate, which is close to flying, but they really can fly. They have not been driven down to the Earth, forced from their natural element, rendered flightless. Moreover, we resemble birds in a number of ways: we are both bipedal, with specially adapted forelimbs, both vocally indefatigable, nest building, restlessly migratory, often traveling in groups. The bird stands upright on its hind legs and it sings—to an enraptured audience to boot. We feel a natural affinity with birds, a sense of camaraderie. Some birds can even talk in human language. And their taste in plumage can rival our own sartorial excesses. This felt affinity is itself likely a remnant of our erstwhile tree-living days, like other forms of species recognition.⁹

But where our paths really cross is in the flightless species. These birds have descended from the heights too, and they have the vestigial wings to prove it. The wings are remnants of earlier times, like our brachiation-ready hands. Flightless birds may use their truncated wings to swim or swat off flies, repurposing an earlier adaptation, but their remnant status is obvious. The penguin and the ostrich are thus our biological brethren: they belong to the rare zoological category of the recently modified habitat-switcher. Their evolutionary history is written into their bodily form, and they have not yet cast off what is no longer adaptive. They are transitional beings—animals at the crossroads of evolution. They are neither one thing nor the other. In them we glimpse our own transformative history. It is thought that an absence of predators leads to flightless birds, and that it may occur quite abruptly. In principle, the wings could be repurposed for a new use, but in practice they often wither away. Perhaps in some species they will eventually disappear completely. Our tree-adapted hands might have withered away too, once climbing and brachiating were no longer called for; but in fact they were redeployed, and necessarily so.¹⁰ Humans and ostriches are both newly minted, compared to most species, and hence a work in progress—as witness the perils of bipedalism and the diameter of the newborn human’s head. We are treeless apes, as the ostrich is a flightless bird—a kind of zoological oxymoron or paradox. We are not so much the “naked ape” (we apparently have as many hairs on our body as the average ape, but finer¹¹) as the “grounded ape.” Like the ostrich, we are still feeling the repercussions of an evolutionary upheaval. We are both still in the process of adjusting. We are still finding our feet, biologically speaking. Our current body type might be strictly temporary. We have not yet reached a state of evolutionary equilibrium.

It is thought that our distant past involved not just hunting but also scavenging—indeed, for some period scavenging may have been the main type of food gathering for humans.¹² We weren’t very good at killing big prey, so we went for corpses left by more formidable predators. This strikes me as poetically correct, because it is less flattering than the usual fearless hunter stereotype (since when have humans ever been fearless?). Does scavenging have any remnants in the modern human? Well, we don’t tend to wander around searching for the leftovers of big cats any more, but perhaps we exhibit homologous behaviors. Some people, maybe out of necessity, do seem to like roaming around picking up what others have discarded—scrap metal, bits of wood, old furniture, seashells, and so on. This seems like a natural thing to do—a pleasant and harmless diversion (except when it leads to excessive “hoarding”). But is there any more general behavior that looks like scavenging? What do we do that involves traipsing around from place to place, picking stuff up, looking it over, deciding whether to take it home or not? Our legs may be tired at the end of the day, but we feel the effort was worthwhile. Shopping, of course. There is no mortal danger in shopping, you don’t need to cooperate with a pack of other acquisitive individuals and share the spoils, and you might end up with something you like—food, clothes, tools, junk of all sorts. Shopping is the modern civilized form of scavenging.¹³ There is something slightly shameful about it, as there is with scavenging—because you have not done the real work—but it gets the job done. It beats having to bring down the prey all by yourself. The modern department store is a scavenger’s paradise, with everything within tolerable walking distance, snacks if you need them, and no problem with the weather. Here again our Darwinian unconscious is directing the proceedings, as the activity taps into ancient brain circuits honed long ago on the African plains. People will “shop till they drop” because in bygone days they needed to be determined scavengers in order to survive. Modern advertising thus appeals to the inner scavenger in all of us.

Someone may object that this is all too fanciful. Am I not just making it up as I go along? Does every form of human activity and affection have this kind of evolutionary explanation? Consider our general desire to live by expanses of water, sometimes even immersing ourselves in it: is this plausibly viewed as a remnant from our days as fish? True, our distant ancestors were fish, so should we suppose that our liking for water reflects ancestral preservation from the brains of our fish ancestors? Isn’t this better explained in other ways, such as the functional utility of water and an aesthetic preference? I would agree that our interest in water is not well explained by ancestral preservation of fish characteristics in the human brain, though I would not rule out in principle possible remnants in the human psyche stemming from our fish ancestors. In this case the ancestors existed an extremely long time ago and there has been enormous change in the brains of the creatures at the origin of the ancestral line. Psychological remnants are much less likely with this kind of evolutionary distance. Also, the human affinity for living near water has a far more immediate evolutionary explanation, namely that humans need water in order to live. That is, water has great biological utility for us. It all depends on the case, which is a matter of detail and judgment. Some psychological traits may reflect ancestral preservation, while others may not—you have to examine the cases on an individual basis. I would say that our fascination with trees betrays some clear indications of remnant status, rather like our fear of snakes, but unlike our interest in water. In the case of snakes, our African ancestors would have needed a strong aversive reaction to them, given their prevalence on the African planes and in the bush. This aversion would be genetically inherited. It now exists in city-dwelling people who are in no danger from snakes at all, as a remnant from the past, not as a valuable contemporary adaptation. So, methodologically, we should not rush uncritically into remnant-style explanations of current psychological traits, but equally we should be open to the possibility of such explanations, proceeding on a case-by-case basis. I submit that the hypothesis of arboreal preservation is a plausible example of this type of explanation, as is the scavenger theory of modern gathering behavior (including shopping). At any rate, there is a component of these contemporary traits that looks very much like a relic from the past.

Our psychological makeup is multilayered, like geological strata over which other strata have formed. That is the Darwinian picture of the mind. As one form of psychological reality succeeds another over the millennia, according to the prevailing conditions of life, the old psychology remains lurking beneath the surface. This has been true throughout evolutionary history. In our case the ancestral psychology was forged principally in the trees and adapted to that niche, though it was modified and to some degree replaced by a psychology more suited to the ground. But it has not been totally discarded and lives on, shaping a psyche no longer ensconced among branches and leaves. Our habitat may be post-arboreal, but our minds are not. An arboreal stratum lies beneath all the subsequent accretions.¹⁴