Culture and its transmission from generation to generation is the defining feature of humanity. It is perhaps the best candidate for the thing that separates us from other beasts. Though there are other species that have been shown to hand down accumulated knowledge – including chimps, who show some evidence for cultural transmission of tool-use – no other animal approaches our ability to layer breakthrough upon breakthrough in such a complex way, and certainly no other animal does it with the conscious intent to lift future communities beyond the achievements that came before. That is a human distinction if there ever were one.
Why has no other animal evolved this demonstrably useful ability? There are lots of intelligent animal candidates, but most have some straightforward biological reality holding them back.
Take the octopodes: famously intelligent, and with a rich visual communication system in their incredible instantaneous colour changes. But their very short lifespans, coupled with rapid senescence and death after a single mating and clutch of eggs, leaves little time for a parent to hand on knowledge to offspring. Most fish and reptiles face a similar challenge – though they do not die after reproducing like octopodes, they do tend to produce large clutches of eggs that hatch and mature without much, if any, parental input, relying on the statistics of very large clutch sizes to ensure a few offspring reach adulthood.
Birds are the real quandary. The philosopher and biologist Peter Godfrey-Smith points to birds as the expected place to find another species treading the same path that we humans have, not least because they already come with several of the important adaptations that made cultural sharing possible for us: complex brains, long lives, strong parental care of offspring in most species, and robust communication. With all of those advantages, why don’t birds have complex culture like we do? Why do they not write technical manuals and make art and argue over economic policy? Why do they not have a market economy, with not only goods for trade, but luxury goods whose value relies on concepts rather than raw usefulness. Why don’t birds drive Bentleys?
The last time I spoke to Godfrey-Smith about this, he asked if I had any ideas why birds hadn’t produced a highly cultural species. I didn’t then. But I do now.
His question set me cogitating about birds and what makes them different from humans. This was a good counter-exercise for me – I’ve written a book about how much birds and humans share, from long childhoods to powerful brains to monogamous parenting. But the question of why no species, not even among the fantastically complex and intelligent large parrots, has developed complex, human-like culture is especially compelling in the context of those similarities.
To answer it, we need to stop and consider how natural selection works. Natural selection (and, by extension, evolution) is a force with no foresight. It responds to the challenges that a species is currently facing. It does not, and cannot, ‘see’ broad sunlit uplands on the horizon and move toward them. This is because selection is a game of elimination – it happens when individuals die without reproducing, or having reproduced less than their neighbours. In each generation, that which is unsuccessful is culled by natural selection, and that which is successful endures. Occasionally, a random mutation or fortuitous combination of genes produces a family offshoot that is even more successful than its ancestors and cousins, resulting in higher rates of reproduction, or longer lives (which allow more reproduction), or higher offspring survival (who in turn reproduce more). Even in these cases, though, selection is not a positive force; those individuals live on a planet of scarcity. Their success comes at the expense of others that were just scraping through before the new mutations emerged. Those others stop scraping through and die, thanks to the success of the new mutation-bearing family.
Think of lifespan versus number of offspring produced per year as our two spatial dimensions
This is important, because it means that natural selection can affect only what currently exists, not what could exist. Even if what could exist would be better or more successful. The consequence of this is that most evolutionary change must happen as a result of a ‘push’ rather than a ‘pull’ – a species’ traits change because they are currently inadequate, and being eliminated by selection, rather than being pulled toward a better alternative.
It is easier to understand this if we use the common metaphor of an evolutionary landscape. Think of this as a three-dimensional surface full of peaks, valleys, troughs and flats. The height of those peaks and valleys represents how evolutionarily successful an organism is – its fitness. (There are different ways to judge fitness, but number of surviving offspring is most typical.) The other two dimensions are metaphorical, and represent all the different traits an organism could have – obviously, there are many more than two traits to consider, but the three-dimensional landscape is a useful metaphor, and we can imagine considering just two traits in a useful way. Think of lifespan versus number of offspring produced per year as our two spatial dimensions. We could imagine one peak of fitness at a short lifespan with a high rate of reproduction (like the octopus), and another peak of fitness at a long lifespan and a slow rate of reproduction (like humans, parrots or whales). Both peaks are successful strategies, while alternative combinations would be deep valleys of low fitness. Middling combinations might be a flat plain in the middle of the landscape.
Imagine natural selection as a blind mountaineer looking for peaks. He can feel the slope under his own feet (ie, where a species currently is – its current combination of traits), and try to move uphill, avoiding the abyss of low fitness. But he is blind – he cannot look across the plane to the peaks far away and move deliberately in that direction. He can get there only by climbing uphill from where he is, bit by bit, using just the information below his feet. This is why evolution can get ‘stuck’ at the local maximum – the blind mountaineer will never go ‘down’ in the service of getting to a higher peak on the other side of a valley, because he doesn’t know the peak is there; he senses only that in that direction lies a valley of death.
While the evolutionary landscape works well as a metaphor, the ‘climbing’ aspect of natural selection doesn’t quite capture what a passive and inconsiderate force it is. That’s why I prefer to flip the model on its head.
Rather than fitness being the peaks, let’s make it the troughs. Rather than a mountain climber actively trying to move up, natural selection is a totally passive ball on an undulating plane, naturally coming to rest when it has rolled all the way down.
Another way to think about it is that the height of the landscape represents how much effort an organism has to exert to go on living and reproducing. Highly fit organisms are evolutionarily optimised, and survive with relatively lower effort; they are deep troughs in the landscape. Organisms that are less fit have to put in more effort to get by; they are further up the valley walls, inexorably pulled downward by evolution’s desire to maximise reproductive fitness at the lowest possible cost.
This is a much better metaphor for evolution, and it means that the steepness of the slopes in our landscape becomes a metaphor for the selection pressures being exerted on an organism. The steeper the slopes, the stronger the pressure to fall down into the trough, developing a trait that must be very useful indeed to so spectacularly reduce the effort of survival and reproduction.
This model, with optimised combinations of traits pulled into deep wells by passing objects, looks much like gravity
In essence, evolution is a system under pressure. And our metaphor of the ball rolling about on an evolutionary landscape helps us to understand when evolution happens. A species that has ‘rolled’ down to a local minimum comes to rest there, and stops moving. At the bottom of the trough, there is low selection pressure exerted on the species – life is pretty good. The effort the species has to expend to survive and reproduce is low, and the species flourishes, but doesn’t change much. On the other hand, a species up the slope in our landscape is trying to move down. It isn’t optimised for its local conditions yet, and is changing more quickly as selection pressure eliminates those individuals with traits on the higher side of the slope. The species ultimately moves down the slope, responding to the pressure.
Any species that comes to rest at a local minimum might seem eternally trapped. But there are two ways it can escape this fate. First, the conditions in the real world can change, which in turn changes the shape of the evolutionary landscape. Imagine a highly adapted freshwater fish living in a lake that suddenly floods with saltwater. The fish’s evolutionary valley suddenly becomes a peak – its survival rate in salt water is drastically lower than in freshwater, and the species either becomes extinct or evolves damned quick to favour saltwater. More likely the former than the latter (unless it is a molly, Poecilia sphenops.)
The other means of escape is a dramatic genetic change – like a beneficial mutation – that moves the species out of its valley of doom – perhaps over the crest toward a different local minimum. Evolutionary biologists call this ‘saltation’ – basically meaning big jumps in evolution, and it is (debatably) the likely way new species come about (as opposed to variation within a species, which occurs by more gradual shifts in traits over time).
To anyone interested in astrophysics, this model, with optimised combinations of traits pulled into deep wells by passing objects, looks a great deal like gravity – where heavy planets and stars bend spacetime around them, pulling in passers-by. It’s not wrong to think of highly adapted combinations of traits as having a gravitational pull on organisms that approach them in evolutionary space. Much like the gravity of a massive object, the pull affects only those objects that get close enough to come within its sphere of influence; and different sources of gravity can both pull on a passing object, just as all evolutionary outcomes involve a trade-off.
Keep gravity in the back of your mind as we return to the difference between humans and birds.
Humans obviously exist at a highly optimised state of evolution. We have conquered the planet. We reproduce prolifically and we live much easier lives with more abundance of basic resources than any other animal. Our constellation of traits combines to create a deep valley in the evolutionary landscape where reproductive outcomes are high (so high that we try to consciously control reproduction) and effort is low.
A great deal of our success results from being a cultural species. To take only the brusquest example of this, our complex culture enables things like specialisation, commerce, transportation and trade; thus, in developed countries, only a small percentage of humans are involved in sourcing food for the whole rest of the species. The average human spends a trivial amount of his or her life in pursuit of food. This is an unheard-of level of plenty, compared with even our closest relatives. For a species like ours, the valley is deep and wide, driving increasingly capable brains and increasingly complex sharing of information – and presumably, creating enough gravity to draw other species in.
That hasn’t occurred.
For reptiles, fish, amphibians, cephalopods and other groups, it is not terribly surprising that they haven’t fallen down the well of complex culture in the evolutionary landscape. Their constellation of traits actually places them rather far away, in their own local minima, with the gravitational effects of human-style culture felt only distantly, if at all. The leaps and bounds it would take to get close are large enough that simple probability means it likely won’t ever happen. They have sufficient success where they are – and a series of evolutionary lottery wins would be needed to jump repeatedly toward the collection of traits that mammals have, let alone humans.
Despite the old ‘bird brain’ slur, birds’ small heads actually contain very large numbers of neurons
Birds, though, are different. They are not nearly so far away. If you think about the prerequisite traits to develop complex culture, you might come up with a list something like this:
- large brains with strong intelligence and learning capabilities;
- long lives, so that understanding of the world can be learned and refined over time;
- highly overlapped generations, so that information learned can be transmitted through time;
- meticulous rearing of offspring, for the same reason; and
- refined intra-species communication, so that high-fidelity transmission of information can be communicated.
Some of these similarities are particularly striking. Despite the old ‘bird brain’ slur, birds’ small heads actually contain very large numbers of neurons, thanks to adaptations that have shrunk the size of their individual neurons to fit in a smaller package. As a result, their forebrains (the thinking part) have neuron numbers to rival primates.
And birds, like some primates, do have simple cultural transmission, just enough to tempt us to consider why they don’t have more complex capabilities. In one study, scientists wearing a distinctive, grotesque mask trapped and released American crows. The crows quickly learned to associate the mask with danger, responding with alarm calls and mobbing when they saw it again – even years later. More intriguingly, crows that were never trapped but had observed others respond with alarm also learned to fear the mask. This response was observed across years and appeared in new generations, suggesting that the information was socially transmitted, likely through both observation and vocal cues.
Indeed, birds have traits that suggest the emergence of true culture far more consistently than many mammals. Birds live longer and are far more likely to attentively raise their offspring than comparably sized mammals, and their cognitive abilities and communication are easily on par.
This causes some biologists to talk about birds as a ‘near miss’ at highly complex culture – and they are right to do so. If we extend our gravity metaphor, we can imagine their evolutionary trajectory as swinging closely past the massive pull of a star, curving and sailing away into the clear, rather than settling into a consistent orbit.
But I think we can extend the gravity metaphor just one step further. With all of the above traits that make them prime candidates to develop culture, how is it possible that birds did not end up sucked into the same orbit as humans?
The answer: they were sucked in by a much, much more massive, steep well in the evolutionary landscape. One from which escape may well be impossible.
It’s so obvious that it’s trivial, but it is the most important thing about birds. They fly. We can say it even though there are a few species that don’t, because flight is the ancestral state for birds. The flightless species lost flight in particular peculiar contexts, rather than never having it to begin with. The defining feature of birds is that they fly.
We know it from childhood, so we fail to confront how important it is that birds fly. Flight has evolved only four times: insects, pterosaurs, bats, and birds. Insects are small, fragile and relatively short-lived. Their flight is limited by size and lifespan. Even migratory species like the monarch butterfly achieve long-distance flight only over the course of multiple generations.
Pterosaurs were the largest-ever flying animals, but we now think they may have needed to take off from high outcroppings to really make the most of their wings – they might not have been able to leap to the sky at whim, like birds and bats.
Birds and bats are really the only groups of animals that have unencumbered freedom of the skies (and most of what I write about birds also applies to bats, of which there are simply many fewer species). It is a very rare trait, evolutionarily speaking. The only reason it doesn’t seem rare is that there are so very many species of birds – twice as many as there are mammals.
It opens up completely vacant niches that can be exploited only by fellow flying species
Flight is an incredible adaptation. It opens an entire third dimension of free movement to species capable of flying. First and foremost, this is game-changing for a prey animal. Being able to escape a predator by travelling in a direction the predator is incapable of going, namely, up, is a huge selection advantage. A flying animal is at risk of consistent predation only from other flying animals, which are fundamentally rare. Moreover, flight limits size and weight, so a bird at the higher end of the flying spectrum is unlikely to have many flying predators large enough to cause her much of a problem.
The reduced risk of predation is in turn why birds live so long. It creates a virtuous cycle where each subsequent year of the bird’s life has a higher potential reproductive value than it would if they didn’t fly, which in turn drives the evolution of robust repair mechanisms in the body, of slower but more attentive reproductive strategies, and of a longer natural lifespan. Together, these are a suite of traits that biologists call ‘K-selected’ – basically, the ‘live slow, die old’ strategy.
Flight also provides access to resources not available to the landbound. All manner of food sources and safe habitats become available when an animal can fly – from tall flowering trees’ nectar and fruits, to inaccessible cliffside perches, to stronghold-like tree hollows for nesting. It opens up completely vacant niches that can be exploited only by fellow flying species.
All of this is to say that evolving the ability to fly removes a huge amount of selection pressure from the species that have it. It makes life easier. It is itself a deep, low well in the evolutionary landscape. It is a very, very massive gravitational object pulling in species and keeping them in its sphere.
Sticking with our metaphor, flight could be the most massive and dense ‘object’ in the evolutionary landscape. And what do we call the most dense and massive gravitational objects?
Black holes.
Flight is an evolutionary black hole. It is a gravitational well with no bottom, a trait so powerfully effective at improving survival and reproduction that it plunges a species into a well of easy life and high fitness from which there is no escape. Or, to return to more conventional evolutionary language, it relieves an incomparable amount of selection pressure that might drive a species to alternative traits.
I think this is a better explanation for the lack of complex culture in birds than the ‘near miss’ metaphor. Unlike objects flying through space, an evolving species isn’t rapidly whizzing by in a straight line – it is wibbling and wobbling about in evolutionary space, taking a drunkard’s walk in the general vicinity of its current landscape. If birds were simply almost-cultural, with all the predispositions they have, they wouldn’t ‘miss’ cultural evolution and sail off into the distance, they would continue to wobble about the rim, always one small deviation away from falling in to join us in the valley of cultural plenty.
Instead, they fell into a black hole called flight. Once birds as a group had flight, their future was sealed – flight would be their defining trait, and would delimit the futures available to them. Flight resolved a huge amount of the pressure that drove humans to seek cultural efficiencies in feeding themselves, fighting predators, hunting prey, transmission of information, and all of the other complicated things that we do to be successful. Birds, by comparison, live on Easy Street – they fly away. They fly away from predators, they fly away from food shortage, they fly away from environmental change. There is no pressure on them to evolve the means to establish agriculture. They can fly away instead.
For birds, flight is the dense core of the black hole, the singularity
And yet flying did not deny birds the opportunity to diversify into myriad different and varied forms. The black hole of flight has room inside it for a gargantuan number of different strategies – as long as they all involve flight.
This is something like our understanding of a real, physical black hole, with a boundary called the event horizon – the point of no return, beyond which nothing, not even light, can escape the gravitational pull. According to an interpretation by the physicist Roger Penrose, once you cross the event horizon to the interior of the black hole, time and space switch places: there is actually infinite space inside, but finite time. For birds, there is an almost infinite evolutionary space inside the black hole, with room for predators like falcons and raptors, prey like quails and pigeons, huge swans and tiny hummingbirds, monogamous parrots and polygamous turkeys, brainy crows and showy birds-of-paradise. But all of them fly. Flight is the dense core of the black hole, the singularity. Flight is their inevitable destiny, providing room for limitless variation as long as flight is preserved.
For those birds that do not fly, their escape was not their own. Whether the flightless kakapo of New Zealand, or the dodo, or the ostrich, or emu, these are niche cases where the evolutionary landscape shifted beneath them, and the black hole itself disappeared. On islands without predators, for example, the huge evolutionary benefit of flight does not exist in the same way, so it can be lost relatively easily. But these are exceptions. No matter how useful cultural capabilities might be for birds, unless they evolve their way to a version of those traits that exists inside the black hole of flight, it will never happen, because you can never leave a black hole.
Yet if the black hole of flight is infinite, why isn’t there space inside for culture, too? Why are there no birds that, yes, happen to fly, but also build machines, ascribe meaning to desirability, have Veblen goods, and train their offspring to maintain the family’s honour?
The reason is likely that human culture is a hard evolutionary trait to acquire. It emerges from the harmony of several other traits that are, in and of themselves, not terribly helpful, and may even be a hindrance, in a vacuum. Big brains, handy though they are, also demand huge energy resources and very long, vulnerable childhoods to grow.
Everything that is true about flight’s incredible selection benefit is true about human culture
Bipedalism that frees up hands to make machines and to write comes at the cost of evolving odd, long feet and an upright posture that causes back pain and makes childbirth dangerous. And, of course, forelimbs hosting handy hands become unsuitable for evolving into wings for flight. Intelligence, sociability and many of the other traits we rely on for our cultural abilities are expensive, in energy and survival terms. In evolutionary terms, that means they need to be providing a lot of benefit to be worth that cost.
In order for that to be the case, there has to be a lot of room for improvement. There has to be a lot of evolutionary pressure. Down in the bottom of the black hole of flight, the pressure is just too low for the trade-offs to be worthwhile. Humans, being slow, flightless, hairless, clawless apes, had a huge amount to gain by evolving the tools of cultural transmission. Birds, flying, living long lives of relative ease, do not feel the same pressure.
It is probably the case that our cultural abilities are also a black hole in evolution. Everything that is true about flight’s incredible selection benefit is true about human culture. We have also fallen down an impossibly steep slope of selection to arrive at the incredible complexity of human life today. I cannot fathom what set of circumstances would cause us to evolve away from this complexity. But at the bottom of our two black holes, we and the birds are separated.
We will both meander about the infinite space of our black holes but will not leave them. They will not play canasta, and we will not fly. Our futures are expansive, but point interminably to our respective singularities – theirs to flight, and ours to culture. For each to have the other would be splendid, but evolution doesn’t aim at splendid. It rolls, unthinkingly, away from pressure, and our respective pressures have been released by our respective all-defining traits.
And that is why birds do not drive Bentleys.
