10
A NEWTON OF THE GRASS BLADE?

IN THE FIRST SENTENCE OF On Growth and Form, D’Arcy Thompson enlists the eighteenth-century German philosopher Immanuel Kant in his cause, quoting Kant’s assertion that without mathematics a discipline may be a science, but it cannot be Science.

Kant certainly thought that natural philosophers should seek unities. He complained that Linnaeus’s classification system for species focused too much on the details and ignored the whole from which life sprang. His thinking was a major influence on Alexander von Humboldt. But Kant, writing in his Critique of Judgement, doubted that living things would give up their secrets:

[I]n terms of merely mechanical principles of nature we cannot even adequately become familiar with, much less explain, organized beings and how they are internally possible. So certain is this that we may boldly state that it is absurd for human beings even to attempt it, or to hope that perhaps some day another Newton might arise who would explain to us, in terms of natural laws unordered by any intention, how even a mere blade of grass is produced.

Kant believed that biology and physics occupied different intellectual worlds. This book has been the story of some of the scientists who



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In the Beat of a Heart: Life, Energy, and the Unity of Nature 10 A NEWTON OF THE GRASS BLADE? IN THE FIRST SENTENCE OF On Growth and Form, D’Arcy Thompson enlists the eighteenth-century German philosopher Immanuel Kant in his cause, quoting Kant’s assertion that without mathematics a discipline may be a science, but it cannot be Science. Kant certainly thought that natural philosophers should seek unities. He complained that Linnaeus’s classification system for species focused too much on the details and ignored the whole from which life sprang. His thinking was a major influence on Alexander von Humboldt. But Kant, writing in his Critique of Judgement, doubted that living things would give up their secrets: [I]n terms of merely mechanical principles of nature we cannot even adequately become familiar with, much less explain, organized beings and how they are internally possible. So certain is this that we may boldly state that it is absurd for human beings even to attempt it, or to hope that perhaps some day another Newton might arise who would explain to us, in terms of natural laws unordered by any intention, how even a mere blade of grass is produced. Kant believed that biology and physics occupied different intellectual worlds. This book has been the story of some of the scientists who

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In the Beat of a Heart: Life, Energy, and the Unity of Nature have tried to prove him wrong: Humboldt, Thompson, Max Rubner, Alfred Lotka, Ludwig von Bertalanffy, Evelyn Hutchinson, and Robert MacArthur, the modern generation of macroecologists and their physicist colleagues. Many have invoked Newtonian science as their model and goal, and many have sought to go beyond a single blade of grass to find and explain patterns in the living world that encompass the whole planet. No one now disputes that plants, animals, rocks, and galaxies are made from the same stuff and are subject to the same physical laws. This fact has become trivial. What is not trivial is whether we can find general laws, similar to those that physics applies to particles and planets, which explain how the living world works. Of course, biology already has one all-pervading principle. When Kant invoked Newton, he was arguing that the world only made sense “as the product of an intelligent cause.” Biology has already shown, through the theory of evolution, that this is not so. From the facts of genetic mutation, inheritance, and natural selection, evolutionary biology shows us how and why life came to be the way it is. Many biologists believe that, other than evolution, generality in biology is unlikely. They argue that biological systems are more complex than physical ones. Particles such as electrons and quarks, for example, can be described by their mass, velocity, and electrical charge. Geologists studying the earth’s crust must come to terms with a few thousand types of mineral. But there are millions of species, each differing in size, shape, behavior, habitat, range, and rarity. Within species, every individual is different, and all change over the course of their lives. Hindsight, these scientists argue, might show us how nature as we see it came about, through past events such as drifting continents and changing climates. And on a case-by-case basis, most biologists are confident that we can explain, by looking at factors such as the availability of food, the threat of predators, the fights between males, or what mates females prefer, why a particular species does what it does. But many also think that the interactions between living things and their environment and each other is too complicated, and too dependent on the circumstances of each moment, to allow generalization.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature From this viewpoint, trying to find physics-like laws of nature is like trying to track down the butterfly whose wing beat caused the typhoon. Most of the science I have written about sidelines history. Ideas such as D’Arcy Thompson’s “diagram of forces,” niches, network models, and energy equivalence assume that there has been enough time, and evolutionary flexibility, for evolution to find a solution to a problem—for nature to find a balance—that can be explained without reference to an organism’s past or the constraints on it. The successes of such an approach show that sometimes this assumption is justified. Robert MacArthur explicitly sought nonhistorical explanations: “The ecologist and the physicist tend to be machinery oriented,” he wrote, “whereas the paleontologist and most biogeographers tend to be history oriented.” But a historical viewpoint is as critical in understanding life today as it ever was. As an example, imagine the following thought experiment about how some animal groups become more diverse than others. You release a small animal species and a large one—a rat and a deer, say—onto an island with no other animals, and monitor the evolutionary results for a few million years, until many more species have evolved. Which of the original species will give rise to the most descendents? You might expect that the small animal would split into more new species because, as we have seen, there are more small niches and small animals are more diverse. The small animal would evolve more rapidly. Yet DNA evidence of some animals’ evolutionary history gives another picture. Andy Purvis and his colleagues have found that small, diverse groups such as rodents do not seem to have split into new species, and filled niches, any more quickly than large mammals such as primates have. But small mammals did emerge relatively unscathed from whatever killed the dinosaurs off 65 million years ago, and so the reason there might be more of them today is that they made up a greater proportion of the ancestral stock from which today’s mammals evolved. They won the race not by being faster but by having a head start. However much we might be able to explain about ecology using concepts that ignore an organism’s history, the historical perspective is still crucial to understand how today’s nature came to be.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Physics Envy? Biology’s would-be Newtons often get accused of physics envy. Why, say their critics, should biology be like physics? What reason is there to apply Newtonian goals and techniques to biology? Just because physics is an older science than biology does not mean that it should be a model for how to do science. Francis Crick wrote that “the ultimate aim … is to explain all biology in terms of physics and chemistry.” But others, such as Ernst Mayr, have argued that it is not being vitalist to believe that biology has concepts that physics cannot explain. There is no physical equivalent, for example, of inheritance, behavior, sexual reproduction, or immunity. Few would argue that these were not valid scientific concepts. Instead of feeling inferior, or trying to annex biology as a province of physics, perhaps we should just accept the difference and see the two fields as equally powerful and legitimate. Lawrence Slobodkin was a graduate student of Evelyn Hutchinson. He alerted Robert MacArthur to the potential for applying mathematics to ecology. But since the 1960s, Slobodkin has come to believe that the search for physics-like theories in biology is misguided. Biologists try to find them, he says, because their emotional attachment to nature makes them want to aggrandize their science. “Physicists are not in love with atoms, but ecologists are in love with organisms,” he says. “Devising theories gives biologists an excuse for the love affair.” Other biologists think that searching for generality diminishes their science more than it aggrandizes it—that it is arrogant to try and force the diversity of life, the story and traits of every species, into a simple uniform framework, and that if you do the result is drab biology. But then a hundred years ago, people used to think that the same criticisms applied to using mathematics to describe sea shells. Some physicists who have made the journey to ecology argue that the differences between physics and biology are not as great as they appear. Robert May trained as a theoretical physicist and then made the switch to ecology, during what he calls the “romantic phase” dominated by Robert MacArthur’s ideas. May’s mathematical tools were ideally suited to the ecology of the time, and he went on to become an eminent theoretical ecologist, discovering, among other things, that the size of a population could fluctuate chaotically even without any

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In the Beat of a Heart: Life, Energy, and the Unity of Nature environmental changes. Nature could look random even if we know it is not, and it can be unpredictable even if we know the rules. (He has since served as the British government’s chief science adviser and as president of the Royal Society and is now Lord May of Oxford.) To believe, he says, that physics offers the one pure, rational method of doing science, building a theory by moving from hypothesis to experiment to refutation, is a fiction. There are whole fields, such as astrophysics, without experiments and whole fields, such as string theory, without a single data point. “Ecologists who hanker for the precision of physics don’t have the faintest clue what physics is really like,” he says. Others, on the other hand, believe that it is not biology and physics that are different but biologists and physicists. The physicist Freeman Dyson wrote that scientists are split into unifiers and diversifiers and that biologists tend to be the diversifiers, “happy if they leave the world a little more complicated than they found it,” and that biology lacks generality because biologists do not look for it. What Makes a Theory This is getting out of hand. Trying to work out the differences between biology and physics, and biologists and physicists—and arguing whether biology is in its Copernican, Keplerian, Newtonian, Einsteinian, or whoever-ian phase—is a fun coffee-break conversation for academics. But more productive than asking whether biology is, or should be, like physics is to ask whether there are simple universal laws that apply to biological systems, and whether these laws can be rooted in principles and approaches borrowed from physicists, either directly (such as the theory that network structure controls metabolic rate) or by analogy (the idea that large groups of plants and animals behave like large groups of particles). And if there are, what should we expect from them? For starters, any law of nature should describe a pattern that would allow us to see generalities and shape our expectations. Kleiber’s rule predicts that if we encounter an unfamiliar mammal, we should expect its metabolic rate to be roughly 70 times its body mass raised to the

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In the Beat of a Heart: Life, Energy, and the Unity of Nature power of 3/4. This is equivalent to those laws of physics that are formal statements, such as “force equals mass times acceleration.” Biologists have spotted plenty of similar patterns, such as the latitudinal gradient in species diversity, and Bergmann’s rule; all have exceptions. But biological patterns such as Kleiber’s rule are descriptions—the pattern the rule describes is just a more mathematical version of the argument that, because every raven I have seen so far is black, all ravens are black. Experience has led me to believe that blackness is a general property of ravens—just as it has that mass-to-the-power-of-three-quartersness is a property of metabolic rate—but I have not worked out a reason why ravens have to be black. Formulating such a rule from observation and experience begs the question of why living things should be like this. Spotting a pattern therefore usually leads to a search for an explanation, perhaps in terms of evolutionary advantage, or the physical and historical constraints on evolution, or in what is statistically probable. With regard to metabolic rate, West, Brown, and Enquist’s fractal network theory looks at both the best solution to a particular problem and the physical limits on that solution. It argues that metabolic rate is proportional to mass to the power of 3/4 because this is the quickest and cheapest way for a body to supply its cells with energy, thanks to the way that the geometry of transport networks changes with body size. The network model also provides an abstraction. Theories are tools for thinking. As well as offering descriptions of the world, they seek to reveal something deeper that may not be obvious to observers and measurers. Physicists make theories by pretending that things are simpler than they really are. They understand their laws by thinking about imaginary systems, such as a universe containing one planet that orbits one star, or motion without friction, or collisions that rebound with perfect elasticity. Some ecologists argue that their science already has principles such as this. Lev Ginzburg, for example, argues that Malthusian population growth is similar to Newton’s first law of motion in that it describes what will happen if nothing else is happening. A population in an unlimited world will grow exponentially; a body given a push will carry on in that direction until some other force stops it.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature In biology such abstraction can have surprising consequences. Neutral ecology and the mid-domain effect show how far we can get by ignoring the things that have traditionally been the stuff of biology and hint that we may not need a full picture of the complexity of living things to understand the patterns in nature. Even in a featureless world, randomly filled with identical species, not all would be equally common or equally widespread. This is perhaps the part of physics-type thinking that comes hardest to biologists. And with good reason. Unlike planetary orbits, living things are responsive, flexible, and adaptive. They bend the rules. Combined with the importance of history and contingency, this property limits the predictive power of biological theories and creates the long-disputed territory between life’s generalities and its detail—and between the scientific lumpers and the splitters. The debate goes something like this: Macroecologist: Look, insectivorous mammals, such as shrews, have pointier faces than herbivores, such as mice. This seems like a general rule about nature. Skeptic: But what about the long-nosed potoroo, a Tasmanian marsupial that looks a bit like a cross between a large rat and a tiny kangaroo? It has a pointy face, but feeds mainly on fungus. Macroecologist: Nevertheless, my rule is good, even though it doesn’t work for the long-nosed potoroo—on average, if you use a large data set, most of the herbivores are flat-faced. Skeptic: Phooey, your rule is no good because it doesn’t work for the long-nosed potoroo. What’s so general about a principle that doesn’t apply to anything in the world as one finds it? Macroecologist: Well, perhaps my general principle can help you discover why, despite its fungal diet, the long-nosed potoroo has such a pointy nose. And so on. Much of the disputed territory is a question of taste: whether it is the diversity or unity in nature that we find striking, whether we think complicated problems are likely to have complicated answers, whether we enjoy describing details or dreaming up abstractions. I have concentrated on the search for generality in nature, but there is much to be said for the alternative view. As Robert May says: “The way that evolutionary forces interact with environments is so varied that it may be a mistake to look for general explanations. The

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In the Beat of a Heart: Life, Energy, and the Unity of Nature differences might be more interesting than a grand unified theory that isn’t particularly grand or unified.” Interesting is the key word—ideas persist because scientists find them stimulating and useful, rather than because they are grand. One consequence of biological theory’s limited powers of prediction is that we cannot expect the theories to provide catch-all prescriptions for conservation. Throughout this book I have tried to illustrate how ideas about how nature works might help us conserve nature. It has emerged that large-scale generalities such as metabolic scaling and energy equivalence are only good as rules of thumb, first approximations that can give conservationists an idea of where to start, suggesting what species might be most at risk and what conservation efforts might be most effective. “Large species are more at risk of going extinct” is useful but not decisive. A doctor confronted with a coughing patient would not make a diagnosis without a more thorough investigation. The analogy between medicine and conservation shows us the limits of prediction. Our knowledge of the human body, how it responds to its environment, and what goes wrong with it is far more detailed than our equivalent knowledge of nature. Biomedical research is much better staffed and funded than conservation. Yet none of us can know what our health will be like in five years’ time. Likewise, there is as yet no way of making precise local predictions about the effects of climate change or invasive species. On a larger scale, however, both medicine and nature are more predictable. Everyone knows someone who knows someone whose Uncle Bert smoked 60 cigarettes a day but lived to be 95. But, on average, smoking is a reliable way to reduce life expectancy. Similarly, using models of climate change’s affect on species’ habitats, ecologist Chris Thomas and his colleagues have predicted that, in a worst-case scenario, as many as a third of species with small ranges will be “committed to extinction,” (i.e., doomed) by 2050. But what species they will be, and the knock-on effects of removing species, are unpredictable. Finding a unity of nature would not make studying the details of nature obsolete. Indeed, finding unity depends on understanding the details. The variability of life means that in biology the ability to generalize is not enough. If you’ve measured one electron, you’ve measured

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In the Beat of a Heart: Life, Energy, and the Unity of Nature them all, but, as I saw in Costa Rica, to understand a forest you must be able to see the trees, and that takes a botanist. Thinkers such as Humboldt, Darwin, and Wallace gained their understanding of how nature works from years of intimate experience of nature in the flesh and the leaf. And yet they were not just interested in what their senses told them; they also tried to abstract and unify. This combination of attributes—intrepid and reflective, naturalist and mathematician—strikes me as rather rare, and becoming more so. These days scientific lone wolves such as D’Arcy Thompson are almost extinct, and it would take a truly awesome polymath to acquire the necessary suite of skills in natural history, ecology, mathematics, and physics to devise a theory as complex as fractal networks. It may be that, in today’s scientific landscape, groups of complementary specialists are better placed to find theories of nature. One thing that helps scientists to abstract and generalize is the ability to think on scales alien to human experience, to imagine the universe in terms of immense—or infinitesimal—distances, times, and numbers. Large scales, of area or differences in body size, are also those on which the patterns in nature seem most striking. Astronomers and particle physicists deal with this scale all the time, but again it is alien to biologists. Nature is all around us, and it is easy and fun to study. I could go back to Oxleas Wood tomorrow morning, watch some robins and come up with an idea about why they nest and feed where they do. This approach has gotten biology and ecology a long way. But it may not be the best way to find generalities. As Jim Brown observed, trying to work out how a large, complex system of interacting parts like a woodland works by looking at its constituent parts in isolation is thankless. It’s like trying to learn watchmaking by looking at a pile of dismantled cogs and springs. You might get an idea what each individual bit does, and some of the parts would fit together in suggestive ways. But it would be a long slog before you could build anything that kept time. Physicists know that systems with an intermediate number of parts are the most analytically intractable. In ones and twos, fine; in huge groups, statistical regularities emerge, although the behavior of each individual particle is unpredictable. Ecologists, or at least those who are trying to understand why species live where

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In the Beat of a Heart: Life, Energy, and the Unity of Nature they do and why some are common and some rare, have typically focused on the intermediate scale, which is the most difficult to understand. Biology resists physics-type theories partly because at human scales there are few physics-type patterns to theorize about. One of the reasons that general ecological ideas are in vogue is that experiencing nature on large scales now no longer needs great leaps of the imagination or self-sacrifice. To study the moon, or the rings of Saturn, one need only look to the heavens. But to truly appreciate what nature is capable of, you have to go to the tropics, which for biologists from temperate climates for most of history has meant spending months in a canoe. Now, you can jet into San Jose, rent a car, and see half a dozen different types of forest in a few days. And besides Costa Rica, the Enquist group is running projects in Mexico and Colorado. Simply staring out of a plane window is liable to set you thinking about nature on the grandest scales. Things look simpler from a distance and complex when they are in our faces. Similarly, biologists have a newfound ability to analyze nature on a large scale. Thanks to the efforts of researchers such as Al Gentry and Steve Hubbell, scientists can call up the vital statistics of millions of trees from around the world at the touch of a button. Theorists have bigger, better, and more accessible data sets to work with, more computer power, and more sophisticated statistical methods. They no longer need to spend years testing a model. Just as genomics researchers can mine sequence databases for ideas about how genes work, ecologists can mine data about forests or food webs and get an instant idea of how theory and fact match up. It is much easier, to paraphrase Alexander von Humboldt, to survey nature with a comprehensive glance and abstract your attention from local phenomena; or as Brian Enquist once said to me: “If I didn’t want to, I need never go into the field again.” Mutual Dependency Theory and fieldwork need one other. It’s obvious that theories should be tested by data. But facts also need theories, to provide a context for data, allowing facts to be linked and placed. An allometry line joins the

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In the Beat of a Heart: Life, Energy, and the Unity of Nature dots between data points, turning a mass of unruly facts into a tidy trend and a simple equation. Theories also filter facts, showing how much attention and credibility each piece of data deserves and whether you should perhaps mistrust your experiment or your theory. Unfortunately, wrong theories are almost as effective at filtering facts as right ones, and once a theory is established, it takes a lot of contradictory facts to bring it down. Max Rubner’s surface law relating metabolic rate to body size was dogma for half a century, and it took the combined efforts of Max Kleiber, Samuel Brody, and Francis Benedict to make it controversial again. But once the surface law had fallen out of fashion, nothing came along in its place. As far back as the eighteenth century Antoine Lavoisier, the first to measure metabolism, realized the dangers of a theoretical vacuum. He was writing about physics, but his words should be a warning to any scientist who thinks that a tricky problem will be solved by just one more experiment: “While the spirit of system is dangerous in the physical sciences, it is equally to be feared that the disorderly heaping up of a great many experiments will obscure science rather than clarify it, raise barriers to those who wish to advance beyond the first stages, and cause long and difficult research to lead to nothing but disorder and confusion.” Many of the biologists who studied metabolic rate ignored Lavoisier’s warning. More and more measurements accumulated that, without a theory to beat them into shape, became a formless, incomprehensible mass. Another thing that theories ought to do is make predictions, even if they are quite imprecise, either of what is impossible or of what will happen. Then, if the apparently impossible happens or the inevitable does not, the theory can be changed or discarded. Such predictions should apply to problems different from that for which the theory was originally devised. Yet contradictory evidence is rarely enough to do away with a theory, and biological hypotheses, such as those about metabolic rate or patterns in species diversity, are much harder to get rid of than they are to create. A biologist might create a theory to explain what he or she has seen on the prairie, or island, or rain forest, or in cows or trees. Inevitably, it will fit that case. But its failure to explain a similar pattern elsewhere is rarely enough to kill it off. Brian

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Enquist has not given up on metabolic ecology because it does not explain why forest respiration, as revealed by FLUXNET, does not vary with temperature. Instead, he and Drew Kerkhoff are looking for what else might be going on that can reconcile their theory with the data, in the same way that Chris Carbone and John Gittleman worked out how the rules of energy and population density apply to carnivores. But without some pruning, ideas will pile up, each good at explaining some things and weak at others. Life may actually be like this, with an atlas of ideas needed to describe the different mechanisms operating at different scales and in different places. But biologists will only find generality if they look for it. Could a biologist make a prediction like the one based on Newtonian theory, when wobbles in the orbit of Uranus were used to predict the existence and position of Neptune, or like Einstein’s prediction that large masses would bend starlight, later proven true during observations of a solar eclipse? What would such a biological prediction be like? Conservation efforts will help here: When trying to apply an idea in aid of a definite goal, it really matters whether that idea is right or not. On my suggested criteria—description, explanation, simplification, and prediction—the fractal network theory of metabolic rate does rather well. It takes a pattern in nature and argues that the pattern is a consequence of how the laws of physics limit the workings of living things. In other words, it uses mechanical principles to explain the internal possibilities of organized beings. It provides an abstraction: This is how animals and plants should work in the ideal case and in general, not in every specific instance. The theory then goes on to make many more predictions about other aspects of biology, such as anatomy, growth, development, reproduction, population density, species diversity, and rates of evolutionary change. You would hope that if these relationships were drastically different from the quarter-power laws that the fractal theory predicts, it would keel over, but none so far have been. The fractal network theory also exploits new mathematical tools that seem tailor-made for biology. Another reason that few biologists have risen to Kant’s challenge is that, even if they started thinking like physicists, classical Newtonian physics and mathematics are unequal

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In the Beat of a Heart: Life, Energy, and the Unity of Nature to the task of describing living things. As Lawrence Slobodkin says: “Almost any prepackaged mathematical structure used in biology was designed for something else. Biology wears these clothes like hand-me-downs.” The fields of chaos, complexity, and emergence being studied by researchers at the Santa Fe Institute and elsewhere came about through the efforts of biologists such as Ludwig von Bertalanffy to think like physicists, and of the biology envy of physicists such as Robert May and Geoff West. As a result of their efforts, we are starting to understand how large groups of simple units can interact to produce complex behavior. One of the fruits of this new science has been an understanding of the links between power laws and fractal geometry. Power laws run through biology—in metabolism and other allometries, and in patterns of species diversity, rarity, and commonness. The laws are equally ubiquitous in the physical processes that shape life’s environment. Many theories of biology and physics work at some scales but not others—but power laws show how the same principles can apply across scales from mitochondria to sequoias. Scientists have only just begun to exploit fractal geometry and power laws as ways of describing and unifying nature. The Other Pillar Attempts at deriving general laws for biology have always met with skepticism and controversy. Often this skepticism proves justified. Most of the patterns proposed as general laws of nature—such as Rubner’s surface law, Pearl’s rate-of-life hypothesis, Kyoji Yoda’s self-thinning in plants, and Evelyn Hutchinson’s ratios—have turned out to be not as general, or as accurate, as their advocates first thought. Metabolic ecology is science in the making, not received wisdom like the laws of natural selection, motion, and thermodynamics. The current state of the theory is probably not the last word on the matter, but then nothing in science ever is. “People end up being well known more for the questions they ask than for the answers they provide,” comments David Tilman. “West, Brown, and Enquist are asking the right questions.” West, Brown, and Enquist are not the first of biology’s unifiers to

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In the Beat of a Heart: Life, Energy, and the Unity of Nature focus on energy. It’s obvious why energy is a good place to seek generality: The chemistry of metabolism is universal, as is the value of energy to organisms. We cannot guarantee that evolution will make organisms faster, or smarter, or longer lived, but it will always reward those that can grab energy and turn it into copies of themselves. So this book has followed the trail of researchers investigating metabolic rate, from a calculation of how much to feed French cigar makers to a hypothesis about the rate at which new species come into being and spread themselves over the globe. How far could a focus on energy take us? Jim Brown thinks of life as resting on two pillars—energy and information. His work has focused on energy, in the form of metabolism, and he has sought to show how the large-scale structure of nature depends on how individuals get and use energy. There is already a theory dealing with how life uses information—genetics, which explains how the information in DNA is used to make living things. Biology is the product of the interaction between energy and information, similar to the way that a computer’s performance is the product of both its hardware and its software. How metabolism influences genetics, and vice versa, is not understood, but there are some hints. Creatures with more DNA, for example, have larger cells and their cells have slower metabolic rates. This has nothing to do with genes—most DNA (95 percent in humans) seems to be hitchhiking, performing no function in the organism that carries it. But cell size scales positively with the brute quantity of DNA, so creatures with lots of DNA, such as salamanders, have large cells and slow metabolic rates. Birds, on the other hand, have relatively small quantities of DNA and fast metabolic rates. Cells of large animals, which are burning fuel more slowly, also have less RNA, the chemical that life uses to translate genomic information into proteins, just as they have fewer mitochondria. The size and workings of a library depend on how many books it has, not what’s written in the books. Similarly, the amount of information a cell needs to handle seems to influence its physical structure and to place limits on the way it uses energy. There is lots that metabolic ecology will never explain, such as the structure of food webs, why species are common and rare, and why

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In the Beat of a Heart: Life, Energy, and the Unity of Nature they live in some places and not others. Other ecologists are working to find the general rules that might govern these patterns, in the hope that in a decade or two there will be a set of theories that can explain the patterns in nature. This begs the question of whether such theories could be linked into something even more general, something ecologists have already begun to explore. Drew Allen is combining metabolic theory and neutral theory to investigate the dynamics of evolution and extinction in foraminifera. Brian Enquist is teaming up with ecologists who study food webs and with those who study the patterns in how widespread and how common species are, to try and work out how size influences feeding behavior, and how animals use space. Steve Hubbell is working with physicists to incorporate energy into neutral ecology and to get a handle on species diversity in the process. Perhaps, he thinks, a metabolic view of mutation rates can help quantify the rate at which new species evolve, and provide a variable to plug into the formula for the universal biodiversity number. One day, Hubbell expects there to be a theoretical prediction of the number of species on Earth. Geoff West is dreaming of a grand unified theory of ecology that would bring all these ideas together, just as physicists seek links between quantum theory and relativity. Across the board, macroecologists are seeing where their ideas can take them, forming new collaborations, and creating a few antagonisms. The Silent Majority But we should also remember how little we know about nature. We have firm notions about how life works. It would be shocking—at least as momentous as finding extraterrestrial life—to discover a bacterium that did not use DNA to transmit genetic information or that had a genetic code radically different from the one humans use, or that didn’t use ATP as an all-purpose molecular fuel. It would be almost as surprising to find a mammal as small as a bee or an insect the size of a chicken. Yet we have almost no idea whether there are 5 million, 50 million, or 500 million species in the world. There are surprises to be had on even the best-known branches of the tree of life. The day I wrote this passage a new species of African monkey, the highland

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In the Beat of a Heart: Life, Energy, and the Unity of Nature mangabey, was announced. But, while we know a lot about primates—and mammals in general—there are many organisms about whose diversity we know very little. Biologists have described about 100,000 species of fungus and believe there are about 1.5 million in the world. There are thought to be a similar number of nematode worm species living in the sea, but only a few have been named. Estimates of the number of insect species vary hugely, but most biologists currently plump for about 5 million to 10 million. But it is in the world of the cells called prokaryotes—the species without mitochondria and other complex cellular organs—that things really start to boggle the mind. Until recently, there was no real way to get a handle on microbial diversity. Biologists could only study the microbes they could culture in the lab, but more than 99 percent of bacterial species will not breed in captivity, making them impossible to identify. DNA technology has given us a new way to identify microbes. We can fish bits of DNA out of the soil, sea, sewage, or wherever and amplify and sequence them. By looking at genes, biologists could get some idea of what was out there. The results have been challenging. There are, it seems, about 70 species of bacteria in a milliliter of sewage. This number rises to 500 species in the human gut, with the mouth, skin, and genitals all having their own distinctive and similarly diverse flora. There could be 2 million bacterial species in the oceans and 4 million in a tonne of soil. The biggest estimate for global bacterial diversity I have seen is 1 billion species. The overwhelming diversity of microbes asserts itself not just at the species level. The kingdom archaea, a group of unicellular organisms that look like regular bacteria but are genetically as different from them as we are, was first discovered living in hotsprings about 25 years ago. Currently, more than one new phylum of prokaryotes—the rank equivalent to molluscs or vertebrates—is being discovered every month. It’s not just in genetic diversity that microbes win. Worldwide, it’s been estimated that there are 5 × 1030 (i.e., a 5 followed by 30 zeros) prokaryotes. This is a billion times the number of stars in the universe. These cells contain the same mass of carbon as all the world’s plants and 10 times more nitrogen and phosphorus. There are microbes whose metabolism is based on burning not oxygen but uranium, and

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In the Beat of a Heart: Life, Energy, and the Unity of Nature others that use ammonia. They live in waters heated above boiling point and in streams and stomachs filled with concentrated acid. In terms of diversity, we large organisms can beat them on the variety of shapes we come in and our behavior, but that’s it. Most of what we think we know about ecology, and most patterns and explanations with claims to generality, have come about from studying birds, mammals, and plants. Yet any general theory must also apply to the vast iceberg of biodiversity that we cannot see. Some theories, such as Kleiber’s rule, seem to do so—although this is still disputed. And Brian Enquist’s team has found that patterns in the population density of marine plankton match the predictions of John Damuth’s energy equivalence rule; that is, the number of individuals in an area declines proportional to the −3/4 power of their body size. But microbes will present stiff challenges to any attempt to find general rules that explain diversity. For starters, belonging to a species is not the same thing for microbes as it is for larger organisms. Most microscopic organisms reproduce without sex, by simple cell division. Even more confusing, they can have sex without reproducing. Bacteria can swap DNA with distantly related fellows or even pick up stray bits of DNA from their environment and incorporate them into their genomes. We will need new definitions, and new measures, of diversity. For example, a common criterion for assigning two microbes to different species is whether one particular stretch of their DNA is less than 97 percent alike. For animals this would lump all the primates from humans to lemurs into the same group. Microbial ecology, and the patterns in microbial diversity, might not fit the theories devised for plants and animals. Large animals and plants are stuck in a certain place—wildebeest in Africa, mahogany in South America. But microbes have such huge populations and can travel so easily, on air and water currents, that some ecologists think any species can get to wherever conditions are right for it, so any saltmarsh will eventually contain all the world’s species that can live in salty, anoxic mud, and any hotspring will contain all the species that like hot, sulfurous water. It’s as if there were polar bears in Antarctica and penguins in Alaska. Some microbial ecologists are now challenging the view that every-

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In the Beat of a Heart: Life, Energy, and the Unity of Nature thing is everywhere, arguing that the genetic evidence is revealing patterns in the microbial life of hotsprings and saltmarshes that are similar to those seen in the trees or birds in a forest. For example, places become less biologically similar the farther apart they are—which might seem obvious but is still contentious. The science, and debate around it, is only just getting under way. It may be that some view of life can bring bacteria and other microbes into line with other species, or it may be that microscopic life needs new rules. Either way, the possibility that we might have to junk our general theories should not dampen excitement about what biologists are discovering about the microbial world. Ecologists seeking generality have only one data point, the earth. This makes testing ideas difficult. But the microbial world gives us somewhere to test a century’s worth of theories built by ecologists who have studied plants and animals. It’s like having a whole other planet to play with.