Restless Creatures: The Story of Life in Ten Movements

Introduction

Then God said, ‘Let us make man in our image, after our likeness …’ So God created man in his own image.

—Genesis 1:26–27 (RSV)

That’s as much as the Bible has to say about the origin of man and why we are as we are: if we take the above verses literally, God just felt like making a species that looked like him. As explanations go, it’s pretty thin – one wonders, for instance, why God happened to be humanoid in the first place. Not that the rest of the living world fares much better. The only vague allusion in the first pages of Genesis to a possible link between an organism’s form and its way of life is the description of birds as winged. Such reticence is hardly surprising, given that curiosity is denounced as the mother of all sins a couple of chapters later. Righteousness apparently requires that we take everything for granted. Fortunately, we ended up ignoring this prescription: the first rule of a post-Darwinian, evolutionary worldview is that when it comes to life, we should take absolutely nothing for granted. Living things are as they are largely as a result of the process of adaptation: the gradual accumulation of favourable mutations over countless generations by the action of natural selection.

Now, some would have it that natural selection is therefore the one and only answer to any question about life. But while this claim is sort of true, at least as far as the adaptive features of organisms are concerned, pointing at a single element in a causal chain hardly amounts to an intellectually satisfying explanation. However, we’d be forgiven for thinking that this is about as far as we can go. It’s all very well accepting the authority of natural selection as the agent of adaptive change, but the genetic mutations that create the variation upon which natural selection acts occur by chance. Furthermore, the eventual fate of a mutation hinges on the particulars of the environment in which a population happens to find itself. Are we really any the wiser? Is evolutionary history just one damned thing after another – a long sequence of specifics and contingencies? Have we succeeded only in replacing God with Fortune?

Some would argue that this is indeed the case. Ernest Rutherford – widely regarded as the father of nuclear physics – once said that ‘physics is the only science; all else is stamp collecting.’ And while he would never have sanctioned such scornful language, evolutionary biologist Stephen Jay Gould was really on the same page when he declared that any rerun of the tape of evolution would produce a living world utterly alien to the one we know. In saying this, Gould was tacitly agreeing that, on the grand scale, evolution is an incomprehensible, unruly beast – opaque to the order-seeking searchlight of science. In this book I offer a wholly different view, for I believe that, contrary to first impressions, there is a way of making deeper sense of ourselves and other living things. Life, you see, despite its overwhelming diversity, has a single overriding theme – one that has dominated evolutionary possibility from the very outset. That theme is locomotion – the apparently simple act of moving from one place to another.

It was pterodactyls that showed me the light. These were the animals that attracted my attention when I first became a research zoologist. My choice of subject was partly driven by the usual childhood dreams of dragons and lost worlds not quite snuffed out by a scientific education (thankfully), but it was also a pragmatic one. Flight is not something to be tackled lightly – after all, we humans cracked it only one hundred years ago – so I reasoned that natural selection must have had a particularly tunnel-visioned concern when it came to the pterodactyls. The unforgiving physical demands of flight would surely have dominated their form and behaviour completely, as is the case today for bats, birds, and, indeed, man-made aircraft. Such stringent limitations on evolutionary possibility are an absolute godsend for a palaeontologist. If I bore these constraints in mind, I thought, then even with limited help from the fossils, and obviously no chance to observe the animals’ behaviour directly, I could still go a long way towards reanimating the objects of my affection.

I’m happy to report that my faith was justified. With aerodynamics as my guide, even the most basic data became a rich source of information. Although my later work involved virtual reconstructions and wind tunnel tests, the first thing I did was take my chosen species – a magnificent beast called Anhanguera – and estimate its weight and wing area from its fossil remains. Anhanguera’s wings were enormous – spanning nearly 5 metres, with a 1.2-square-metre shadow – but the creature was also surprisingly light, weighing in at a mere 10 kilograms, give or take. Now, simple physical considerations dictate that weight must be balanced by the force of lift in steady flight, and aerodynamic theory tells us that the amount of lift depends on wing area and airspeed, to a first approximation. With its big wings and lightweight build, Anhanguera could generate enough lift to stay airborne at a remarkably low cruising speed. Which was just as well: those enormous wings weren’t suitable for the vigorous flapping that would be required to make up for a substantial airspeed shortfall.

That was just the beginning. The trouble Anhanguera had with flapping meant that it needed gravity’s help to get up to speed, along with rising warm air currents – thermals – to maintain altitude. It must therefore have roosted on cliffs near tropical seas, whose waters are balmy enough to sustain the thermal fields. In this regard, Anhanguera was a lot like today’s frigate birds, but the resemblance may have gone further than simple habitat choice. Frigate birds are notorious for their aerial piracy – they plunder fish from other birds on the wing. What many people don’t realize is that this objectionable behaviour is born of the birds’ locomotory ‘design’. Because they need the assistance of gravity to take off, frigate birds cannot risk alighting on the water to feed. Attacking other birds in mid-air is a perfectly reasonable response to this difficulty. Who knows – maybe Anhanguera and its kin were the analogous scourge of the Cretaceous skies.

All that information came from just two numbers: weight and wing area. With a little physical know-how I was able to take some petrified bones and return a fully functional animal, placed more or less securely in the ecology of its time. This experience was to be my epiphany – I would never look at the world in the same way again, for once accustomed to a locomotory point of view I realized that flight is not unique in its power to shape adaptation. On the contrary – I began to see the guiding hand of locomotion everywhere I looked. Thanks to Anhanguera I had stumbled upon life’s big secret, hiding in plain sight. Locomotion is perceptually immediate – one doesn’t need a telescope or microscope to discern it; neither does one have to wait generations to see it in action. It’s happening all the time, all around us. My way was clear: I resolved to lay bare the restless heart of the living world. This book is the culmination of my quest.

There are, I think, two reasons for the pervasive influence of locomotion on the design of living things. First, getting from place to place effectively and efficiently is often one of the most important determinants of how many healthy offspring a creature begets, which as far as natural selection is concerned is ultimately the only thing that matters. Survival and reproduction both require that organisms seek out fuel and raw materials for the purposes of growth, repair, and baby-making, while ideally avoiding competitors or any hungry creature that might make a meal out of them. Sexual reproduction often requires that organisms approach each other, and whether sexual or asexual, one’s offspring must eventually fly the nest, so to speak, if they’re not to become entrenched rivals of their parents or each other. All of which means that locomotion tends to enjoy a high priority in the eyes of natural selection.

The second reason that locomotion leaves such obvious marks on living things is of a more physical nature. It doesn’t matter whether we’re considering a corkscrewing bacterium, a climbing ape, a sprinting cheetah, a spinning sycamore fruit, a soaring albatross, a burrowing worm, a swimming swordfish, or a strolling human: everyone without exception must defer to the same underlying physical reality. Organisms are physical objects, after all, and when moving around, they must obey Newton’s laws of motion, along with a few other rules and regulations concerning levers, the behaviour of fluids, and such like. Given the high selective premium placed on efficient and effective movement, these rules typically impose tight constraints on the shape and behaviour of locomotory creatures.

At this point, you might be tempted to employ your own locomotory abilities (metaphorically at least) to run away as fast as possible, but you’ll miss out if you do. The laws of which I speak are really not that scary, and the great importance of locomotion to an organism’s fitness means that the form and behaviour of living things are often attuned to the same physical principles. If there were any doubt as to the truth of this statement, you have only to consider the innumerable cases of convergent evolution that pepper the history of life. Whales and dolphins have been shaped so thoroughly by the demands of efficient underwater locomotion that for a long time they were regarded as fish. The three groups of flying vertebrates – bats, birds, and pterodactyls – have been brought to strikingly similar anatomical destinations by the unbending physical needs of flight. The diversity all rests on one beautifully simple foundation.

An important question may by now have occurred to you. If all organisms are contending with the same physical reality with the same underlying rules, why don’t they all look and behave the same? Why is the living world so astonishingly diverse? There are two principal answers to this question, one more intuitive than the other. First, of course, different organisms inhabit different physical environments where (to put it in prosaic terms) the values of certain variables in our equations of motion aren’t the same: they may dig through soil, swim through water, fly through air, or move on an interface between these realms, each technique requiring different anatomical and behavioural traits to pull it off effectively. On similar lines, a creature’s size has an enormous impact on its physical experience. The largest locomotory entity – the blue whale – is one thousand million million million times bigger than the smallest – a Mycoplasma bacterium. The physical changes across so vast a size range have all kinds of locomotory consequences. An elephant can’t bound around, for instance, because its legs would need to be impractically thick to withstand the enormous forces involved. A mouse, on the other hand, rarely does anything but bound. Air, whose physical presence is barely noticeable to us in most situations, is positively treacly to the tiniest flying insects, so they can get away with wings that are just tufts of bristles. Such a design is not recommended for a Boeing 747.

The second, less intuitive answer to the diversity question concerns the impact of the past on a creature’s present. Evolution is usually a gradual process, for there are strict limits to the extent of viable change from one generation to the next. While big changes are possible (witness the occasional two-headed mutants), they tend to cause catastrophic losses of fitness, and so are quickly eliminated from the gene pool. Future evolutionary pathways are therefore heavily constrained by the present state of an evolving population. By extension, one needs to know a creature’s evolutionary past to fully understand why it is as it is now. For no walk of life is this proviso more important than locomotion. Two creatures moving in the same environment may face the same physical challenges, but their adaptive solutions may be wholly different, just because their ancestors came at the problem from different angles. The flying vertebrates again provide a useful illustration. Their wings are superficially similar, but not identical: birds use feathers, bats stretch a skin-like membrane between their elongated fingers, and pterodactyls, despite also carrying membranous wings, supported each with only one finger. These distinct takes on flight in the three groups trace back to differences in their respective ancestors, some subtle, some not so subtle, when they began to test the air.

The history of locomotion may thus be conceived as a 4-billion-year dance between the physical rules of propulsion and the logic of natural selection, with each step dependent on the one that came before. In this book I will retrace this long dance, and in so doing will show how the need to move has shaped the living world. I begin in Chapter 1 with ourselves. Human locomotion is wonderfully amenable to personal exploration and experimentation, making us the ideal platform for learning the ropes of biological propulsion. As a species, we’re also surprisingly capable movers by the standards of our close ape relatives. This is something we usually take for granted – when asked to say what makes us special, most people talk about our superior mental faculties. Yet I’d guess that a similar majority would grant far higher status to elite athletes than to Nobel Prize winners. The subconscious high regard in which we hold our locomotory skills is also betrayed by the extent to which the terminology of movement pervades the language of achievement: when we do well, we’re ‘going places’, ideas need to ‘get off the ground’, we ‘chase’ goals, ‘get up to speed’ at a new task, make ‘leaps’ of understanding, and ‘jump at the chance’ to try something new. And while we don’t often consciously consider the value we place on locomotion, we certainly miss it if it’s gone – our locomotory freedom is one of our most prized attributes.

In considering human movement we’re going to come face-to-face with our first evolutionary puzzles, the most obvious being our use of two legs rather than the standard mammalian four to get ourselves from place to place. This mystery will serve as a springboard for our journey back through the evolutionary history of locomotion, for it’s only by peeling away the layers of recent adaptations that we can hope to find an answer to this curious anomaly. However, in doing so, we’re going to uncover yet more locomotory puzzles that will inevitably take us back even further, until we eventually reach the very origin of locomotion itself. Given our interest in understanding ourselves, the focus of our backward trek through time is our own ancestral lineage, but that doesn’t mean that the story is ours alone, for the deeper we go, the more widely shared are the ancestors we encounter. That means that the more we learn about our own locomotory past, the more we’ll come to understand the broader canvas of the living world, and the more obvious the universal signature of locomotion will become.

To mark the journey down our ancestral lineage I have chosen as way stations a number of key locomotory transitions, each of which forms the central story of a chapter. These shifts in the tempo and meter of the dance of life all granted access to fruitful new ways of moving, and so have special significance in the grand evolutionary narrative of locomotion. In Chapter 2 – the first step on our historical journey – we will explore how our tree-dwelling ancestors became two-legged and eventually left the forests behind. Then, in Chapter 3, we will briefly divert from our ancestral lineage to turn our attention to the skies, and to the various origins of the lucky flying animals that now roam therein. In Chapter 4, we will dive beneath the waves, to examine how natural selection for swimming caused the appearance of the vertebrate backbone, before moving on in Chapter 5 to our closer fishy ancestors, who turned their fins into limbs and crawled onto the land. In Chapter 6, as our journey takes us ever deeper, we will learn how the demands of locomotion shaped the fundamental anatomical blueprint of the animal kingdom, with its clearly defined fore-to-aft axis and left/right symmetry. Chapter 7, on the other hand, will look into how animals ended up controlling the locomotory movements of their finely honed bodies, thanks to the origin of the nervous system. Through these major locomotory transitions we will learn how our ancestral lineage was forged and reforged by the demands of movement, and how biological locomotion works in different environments and on different scales.

Many aspects of the transitions I cover on these first six legs of our evolutionary journey are very obviously related to locomotion – such as the origin of flight, of human two-leggedness, and the transformation of fins to limbs. Other changes have a relationship with movement that might seem a little more surprising. Our much-lauded opposable thumbs, for instance, originally had nothing to do with tool use – the digit’s realignment was a climbing adaptation. The famous Cambrian explosion – the relatively rapid diversification of animal body types that began about 545 million years ago – was kick-started by crawling adaptations, as we’ll see in Chapter 6. Perhaps most striking of all, as discussed in Chapter 7, the brain and sensory organs were originally nothing more than a guidance system – a computer – to coordinate the body’s movements to and fro. The evolution of locomotion is about far more than legs, wings, and fins – indeed, the deeper we dig, the more apparent it will become that few if any aspects of an animal’s being aren’t related in some way to its present or past adaptations for movement.

Chapter 8 takes a different tack, by exploring the various occasions when locomotion was abandoned – or rather, ostensibly abandoned, for we’re going to find that the lifestyles of the static owe much to their history of motion. Indeed, I hope to convince you that, strange as it may seem, locomotion has actually dominated the evolution of plants almost as much as it has that of animals. Although plants themselves usually can’t move around, their seeds and pollen must for the purposes of sexual reproduction and dispersal. These imperatives have impacted not only on the design of the dispersal agents (witness the helicopter-like fruit of sycamore trees) but also on the form of the stationary plant that releases them. Height is an obvious dispersal-assisting quality, and flowers are so good at ensuring pollen gets delivered exactly where it needs to go (via insect couriers) that one could regard them as indirect locomotory organs.

The various narrative twists that we’re going to encounter on our long journey show that a creature’s evolutionary history shouldn’t be regarded as a mere straitjacket. Adaptations can open doors to future possibility as well as close them, and this is never more likely than when those adaptations have a locomotory impact. If a creature acquires a new way of moving, through its travels it may end up exposing itself to a whole new set of selective pressures: pressures that might push its descendants in an entirely unexpected evolutionary direction. After all, an organism must experience a new environment before it can adapt to it. Consider flight again: it’s a fantastically effective way to get around, but only those creatures that move in the complex world of the forest canopy are ever likely to stumble upon the selective pressures that might eventually make it possible. But this power of locomotion to unlock new ways of living doesn’t just apply to the colonization of new environments, as will become very obvious once we embark on the final leg of our journey back in time. In Chapter 9, we’re going to see how the adaptive refinement of locomotion in single-celled creatures laid the groundwork for the great multicellular kingdoms that were to come, before we arrive at last at the most important locomotory transition of all: the beginning of locomotion itself. Judged on its evolutionary consequences, this was undoubtedly the most significant transition in the history of life since its origin. Before locomotory powers evolved, life was little more than unusually complex chemistry. Once organisms started to move around, however, they began to encounter each other, opening the doors to predation, parasitism, sex, and symbiosis. In other words, it was thanks to locomotion that life took on its essential character, and it’s had the leading role in the unfolding drama of evolution ever since.

The book will end where it began – with ourselves, or rather our mind, for it turns out that we have locomotion to thank for more than our body alone. In Chapter 10, we will see that our curiosity, our joy, even consciousness itself all owe their intangible existence to propulsion. This shaping of our mind by the dance of natural selection and locomotion gives added significance to our search for self-understanding. We’ve been gifted with an insatiable desire for movement, but this wish has brought us to dangerous territory in recent years, with our locomotory technologies now threatening the health of both our bodies and our minds. Appreciating how we’ve been built by life’s long locomotory dance is therefore no mere academic concern – it may in the end be our best chance of finding a way to live more healthy, meaningful, and fulfilling lives.

Let the journey begin.

1

Just Put One Foot in Front of the Other

In which we immerse ourselves in the oft-underestimated magnificence of getting around on our own two feet

Know thyself.

—ancient Greek aphorism

Iwonder if you wouldn’t mind trying something out. If it’s safe and convenient, I’d like you to take a few steps. For more of a challenge, try turning a corner, or should the local lie of the land permit, go up or down a slope or a staircase. By all means, break into a run if you’re able and feeling sufficiently energetic. For most of us, unless hampered by injury, disease, or old age, all of this is so easy that we barely need to think about it. If we get the urge to go somewhere, we just go – rarely if ever do we need to devote any attention to the ins and outs of making the journey happen. But in taking our locomotory skills for granted like this, we seriously under-appreciate what is in fact a movement machine of dazzling sophistication. Our engineers have built spacecraft that can land on comets, and our computers have beaten grand masters at chess, but we have yet to see a robot whose movements come even close to the elegance, ease, and flexibility of human walking and running.

So, why not try those few steps again, but this time, think about exactly what you’re doing. How do you initiate and terminate movement? How do you avoid falling over, even when the ground is uneven? How do you turn, or shift up a gear into a run? Why, indeed, switch from walking to running at all? And how are you doing this all with such fantastic fuel efficiency – ten times that of top-of-the-range walking robots, such as Honda’s ASIMO?¹ I realize, of course, that these are difficult questions to answer, for many of the processes that bring about self-propulsion happen beneath the level of our conscious awareness. But we cannot possibly begin our time-travelling, locomotory tour of life on Earth without first turning our enquiring eyes onto our own locomotion: we need to familiarize ourselves with the evolutionary destination before working out how we got here.

THE SIMPLE ANSWER

For something as commonplace as locomotion, it took us an awfully long time to even begin to understand how it works. Aristotle, a Greek philosopher we’ll meet properly in Chapter 4, was one of the first people to thoroughly ponder the problem. His observations and musings led him to conclude that all motions fall into one of two categories. The first – natural motions – are what an object or material does without being forced: what he deemed the ‘heavy elements’, water and earth, naturally fall; whereas the ‘light elements’, air and fire, rise. His second category – violent motions – covered those movements imposed on an object by an applied driving force, which would include locomotion. Take the force away, so he thought, and motion ceases. These ideas accord well with common sense: we need to push or pull a stationary object to shift it, and if we stop manhandling it, the object usually comes to a halt soon afterwards.

About 2,000 years later, Italian physicist-astronomer-philosopher Galileo realized that there was something deeply wrong with Aristotle’s thinking. While Galileo initially accepted that motions could be natural or violent, he reasoned that if an object’s natural tendency is to move directly towards the centre of the Earth, only movement in the exact opposite direction – that is, upwards – could be regarded as purely violent. What about objects moving horizontally? Galileo construed that those motions – directed neither away from nor towards the planet – must occupy a third category, which he called neutral motions. His great insight was to realize that once external impediments were removed, it would take only a small force to send an object into neutral motion, and once moving, it would take an impediment – friction, for instance – to stop it.

That may sound familiar. Galileo’s thoughts, encapsulated in his law of inertia, are essentially identical to the first law of motion formulated by mathematician-physicist Isaac Newton (1642–1727):

Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.²

That’s not to say that Newton’s thoughts on movement were simply a rehash of Galileo’s. In his second law of motion he extended the concept of inertia, stating that the acceleration (a) of an object caused by the application of a force (F) is in direct proportion to the magnitude of that force, but in inverse proportion to the object’s mass (m). In other words, F = ma. Furthermore, unlike Galileo, Newton realized that there was no physical basis for granting special status to natural motions: a fall, like any of Aristotle’s violent motions, must be caused by an applied force. Taken together, Newton’s insights made complete sense of Galileo’s famous free-fall experiment in which, as legend has it (some maintain that this was only a thought experiment), he dropped two balls of different mass from the Leaning Tower of Pisa. According to Aristotle, the heavier object should have fallen considerably faster, but in fact they struck the ground at nearly the same instant.³ The heavier ball’s greater mass meant that the force drawing it to the ground was larger, but greater mass also entails greater inertia, so the resulting acceleration was identical to that of the smaller ball. That constant acceleration (roughly 9.8 metres per second per second at sea level) was the result of the pull of gravity, and is now denoted g; the force – an object’s weight in the strict sense – can be found by multiplying that figure by the object’s mass.

The relationship between force, mass, and acceleration uncovered by Newton is clearly of great importance to locomotion. But where does the force that propels living things from place to place come from? This is where Newton’s third and final law of motion enters the picture. The third law is the really famous one, which states that every action has an equal and opposite reaction, but it’s also the least intuitive. It isn’t immediately obvious that when pushing on an object, the object simultaneously pushes back on you, but with hindsight it’s plain that this must be the case. If no force were pushing back, you wouldn’t be able to feel the object you were shoving. Similarly, when a falling ball bounces off the floor, there must be a force pointing upwards to enable the reversal of direction. Most critically as far as locomotion is concerned, the downward-pointing force of weight that we’re all subject to must be balanced by an equal and opposite force directed upwards, ultimately derived from the tiny forces acting between the atoms and molecules of the ground, or else we’d sink through the floor. That ground reaction force is the key to locomotion. Push back on the ground and it pushes forwards on you, accelerating you towards your chosen destination.

Our muscles are responsible for generating the push, though their action is necessarily indirect. That’s because muscles can only pull, so skeletal levers are required to convert the motion. The propulsive action of the human leg, for example, is brought about by its extensor muscles. The calf muscles, which attach to the heel bone via the Achilles tendon, swing the foot down and back about the ankle joint; the bulky quads, which run down the front and sides of the thigh to the tibia (the shinbone), straighten the leg at the knee joint; finally, the gluteus maximus (the buttock muscle), and the hamstrings, which run down the back of the thigh, swing the entire leg backwards about the hip joint. When the foot is planted on the ground (and as long as there’s sufficient friction between the two), the overall backward push of the leg brought about by the contraction of these muscles causes the forward acceleration of the body. That can’t continue indefinitely, of course, and because muscles can’t actively lengthen, the extensors must be reset by the contraction of their so-called antagonists – the flexor muscles – which each attach on the opposite side of a joint to its corresponding extensor. The principal leg flexors are: the tibialis anterior, which runs along the shin and inserts on top of the instep – this is largely responsible for raising the foot and thereby re-lengthening the calf muscles; the hamstrings, which bend the knee⁴ and stretch the quads; and the iliopsoas muscles, which run from the lower back and pelvis to the top of the femur (thigh bone) – they pull the leg forwards at the hip and stretch the buttock muscles. It goes without saying that the foot must be off the ground during these movements, otherwise we’d push forwards and end up back where we started. Fortunately, nature has provided us with a second leg, which can take over the support and propulsion duties during the reset.

1-1: The principal flexor and extensor muscles of the human leg (a) and a simplified picture of their actions (b, c). The tibialis anterior, hamstring group, and the iliopsoas group (of which only the lower, iliacus muscle is shown here, running from the inside of the pelvis to the femur) flex the ankle, knee, and hip, respectively (b), while the calf muscles, quadriceps group, and the gluteus maximus antagonize these actions by extending the same joints (c). When a muscle is activated (indicated by the darker tone in b, c), its relaxed antagonist (pale tone) is re-lengthened, thanks to its attachment on the opposite side of the relevant joint. Note that some of these muscles, such as the hamstrings, are bi-articular – they cross two joints – and so have alternative actions, depending on the activation state of other muscles or whether the leg is supporting any weight; the hamstrings, for instance, can extend the hip as well as flex the knee.

So, there we have it – the essential character of walking locomotion, with each leg alternately supporting the body during its stance phase and preparing for the next in its swing phase. Being a walk, there’s no unsupported aerial phase, so both legs are on the ground for at least 50 per cent of their respective strides (a stride encompasses one stance and one swing). This duty factor can get as high as 70 per cent in very slow walking, declining to about 55 per cent if we really need to get a move on but can’t quite bring ourselves to break into a run.

That’s all well and good, but the picture we’ve painted so far is pretty crude. Nothing we’ve seen yet couldn’t be applied to robots, with servos and motors taking the place of muscles, so we’ve little indication as to what makes our version so elegant and efficient by comparison. And there’s nothing to tell us what makes running different from walking, aside from the reduced duty factor that gives it its characteristic airborne stage. There must be more to it than that. Indeed there is, but uncovering the nuances of human locomotion was never going to be easy. Even the most leisurely of strolls involves moment-by-moment changes in the disposition of our limb segments that happen too fast for even the most dedicated observer to fully grasp, to say nothing of all the behind-the-scenes actions in the body that bring about these movements. What we needed was a way to slow down time and lift the bonnet on our locomotory engine.

THE TIME LORDS

The man usually credited for ushering in the modern study of locomotion is the brilliant photographer Eadweard Muybridge. Born Edward Muggeridge near London in 1830, he immigrated as a young man to San Francisco, where he made something of a name for himself as a landscape photographer. His locomotory calling came in 1872, when railroad tycoon and former California governor Leland Stanford invited him to his stock farm in Palo Alto, supposedly to settle a $25,000 bet that a horse periodically becomes airborne when galloping.⁵ Muybridge was at first sceptical that photographic technology would be up to the task, but he gave it a go, and soon showed that, even when trotting, a horse does indeed lift all its hooves off the ground for a split second in each step cycle. His success was the turning point of his life: from that moment, capturing the movement of animals became Muybridge’s obsession. He worked intermittently at Palo Alto for several years, where he hatched an ingenious plan. He placed a set of cameras at regular intervals along a track, each rigged so that its shutter was activated by a trip wire stretched across the course. When Stanford’s horse Sallie Gardner galloped past, it therefore took a series of photographs of itself. The result was a breathtaking sequence of images that showed for the first time every intricate detail of an animal’s locomotory movements. Among other revelations, these pictures proved that the contentious aerial phase occurred not when the legs were at full stretch as many had supposed, but when the forelimbs and hindlimbs were at their closest approach.

1-2: Muybridge’s images of Leland Stanford’s horse ‘Sallie Gardner’.

Muybridge’s horse photographs won him widespread acclaim, and in 1884 he was offered a job at Pennsylvania University to apply his technique to a range of other animals, from baboons to lions. Significantly, he was also asked to photograph human movement, and he duly obliged, producing many sequences of men and women, not just walking and running, but jumping, boxing, somersaulting, dancing, even getting into bed. These works were beautiful and enthralling, and indeed remain so to this day. But in scientific terms, they really only scratched the surface. To work out exactly how we move, much more was needed than a simple series of freeze-frames. Fortunately, at about the same time that Muybridge began to uncover the