6
THE PACE OF LIFE

EVERY LIVING THING grows, changes, and, if it is among the lucky few, reproduces. Then, if it doesn’t first get eaten, freeze, drown, starve, contract a lethal infection, fall out of a tree, or encounter any of the other fatal hazards of life in the wild, it begins to fall apart. And then it dies. Listen closely to the hum of metabolic rate, and you will hear this rising and falling melody of life.

Any organism that just kept ticking at its resting metabolic rate wouldn’t get very far. All must obtain a surplus of energy and then invest it wisely. Natural selection keeps score of how well organisms exchange one currency of life, energy, for another, offspring. Successful investment strategies multiply; bad ones go out of business.

Evolution has smiled on an extraordinarily diverse portfolio. Consider animals. They are cold- and warm-blooded, herbivorous and carnivorous. Some start life as eggs in a nest and lay eggs themselves. Others gestate in their mother’s womb and bear live young in their turn. Some grow strong on their mother’s milk. Some are fed by their parents until they are adults. Others are shot into the plankton as unfertilized eggs and must fend for themselves from that point onward. Some are recognizable in youth as miniature versions of their adult



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature 6 THE PACE OF LIFE EVERY LIVING THING grows, changes, and, if it is among the lucky few, reproduces. Then, if it doesn’t first get eaten, freeze, drown, starve, contract a lethal infection, fall out of a tree, or encounter any of the other fatal hazards of life in the wild, it begins to fall apart. And then it dies. Listen closely to the hum of metabolic rate, and you will hear this rising and falling melody of life. Any organism that just kept ticking at its resting metabolic rate wouldn’t get very far. All must obtain a surplus of energy and then invest it wisely. Natural selection keeps score of how well organisms exchange one currency of life, energy, for another, offspring. Successful investment strategies multiply; bad ones go out of business. Evolution has smiled on an extraordinarily diverse portfolio. Consider animals. They are cold- and warm-blooded, herbivorous and carnivorous. Some start life as eggs in a nest and lay eggs themselves. Others gestate in their mother’s womb and bear live young in their turn. Some grow strong on their mother’s milk. Some are fed by their parents until they are adults. Others are shot into the plankton as unfertilized eggs and must fend for themselves from that point onward. Some are recognizable in youth as miniature versions of their adult

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature selves. Others go in for a complete remodeling before adulthood, breaking down their juvenile bodies and using the parts to make something very different. Some live for more than a century. The water flea Daphnia completes its life cycle inside a week. It’s easy to imagine what an all-conquering evolutionary super-beast—a Darwinian demon—would be like. It would grow to large size and sexual maturity in a flash and produce vast numbers of hearty offspring, forever. Such a paragon would monopolize the energy available to life. But, of course, this isn’t possible. Life’s resources are finite. Every feature of every plant and animal represents a choice made by evolution about how to invest energy. And every choice precludes other choices. Organisms must choose whether to put their resources into muscular legs or wings, gaudy tails or flowers, fearsome horns or juicy fruit. They must also choose what to invest in and when: whether to become sexually mature at a young age, and get on with mating and breeding before any mishaps befall them, or delay reproduction until they are bigger and better equipped to compete for resources or mates. They must choose whether to produce a few large offspring, each of which stands a good chance of surviving, or lots of little ones, most of which will die young. When they reproduce, they must decide whether to hold something back for future breeding seasons or fling everything on the one shot and die in the attempt. The result of all these decisions is called an organism’s life history. Choice—trade-offs, in other words—is fundamental to life’s design. And there are still deeper unities to be pursued. Even in a finite world, the diversity of life makes it look as if there are an astronomical number of biological options. In fact, metabolism controls what choices can be made, and understanding metabolic rate peels back the multiplicity of life histories to reveal one sleekly efficient mechanism for turning energy into offspring. The Form of Growth D’Arcy Thompson noticed that organisms grow quickly in their youth and then ever more slowly thereafter. Some, such as birds, mammals, and insects, stop growing when they reach adulthood and start

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature breeding. Others, such as fish and molluscs, seem to keep gaining weight throughout their lives, but this rate nevertheless decreases as they age. For all sorts of animals, plotting their size against their age creates an s-shaped curve, which starts shallow, takes a rapid turn upward, and then levels off. An animal’s growth rate seems to be some function of its age. Thompson keenly sought the laws of growth that controlled this trajectory, for he believed they controlled animal form. But in On Growth and Form, he had to admit defeat: For the main features which appear to be common to all curves of growth we may hope to have, some day, a simple explanation…. The characteristic form of the curve of growth … is a phenomenon which we are at present little able to explain, but which presents us with a definite and attractive problem for future solution. By the time the second edition came out in 1942, someone had offered that simple explanation. Born near Vienna in 1901, Ludwig von Bertalanffy got his Ph.D. at that city’s university in 1926, for a study of the work of Gustav Fechner, a nineteenth-century German psychologist and philosopher who studied human vision. Soon after, Bertalanffy concluded that the life sciences had reached an impasse. Knowledge was being churned out faster than ever before, but without theory the babble of unconnected facts drowned out understanding. “Today biology is in its pre-Copernican period. We possess an enormous mass of facts, but we still have only a very incomplete insight into the laws governing them,” he wrote. “Only if the multiplicity of facts is ordered, brought into a system, subordinated to great laws and principles, only then does the heap of data become a science…. The chief task of biology must be to discover the laws of biological systems to which the ingredient parts and processes are subordinate. We regard this as the fundamental problem for modern biology.” Bertalanffy saw the impasse as an opportunity as well as a crisis. He took inspiration from physics, then convulsing as quantum theory replaced the Newtonian worldview. Perhaps an equally radical shift in biological thought could reap similar rewards. He began to ponder the fundamental properties of life. The two he settled on are themes we have already encountered repeatedly: hierarchical organization and the continual exchange of matter and energy with the environment. Just as

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature quantum theory turned subatomic particles from rigid, fixed points into waves, so Bertalanffy sought to replace the idea of organisms as static, self-contained entities with something more dynamic and fluid. “Living forms are not in being, they are happening,” he wrote. “They are the expression of a perpetual stream of matter and energy which passes the organism and at the same time constitutes it.” Hierarchical organization meant that one could not hope to understand organisms by considering any aspect of their biology in isolation. The processes and structures of life were inseparable from, and only made sense in the context of, the organisms in which they were housed. Bertalanffy called this research program “organismic biology.” Through the 1930s and 1940s—following Alfred Lotka’s lead—he expanded the scope of his program beyond biology, coming to believe that all systems made up of many interacting elements shared similar properties, regardless of whether the system’s elements were physical, biological, psychological, or social. The task, Bertalanffy argued, was to work out the common principles that controlled these systems. In the process, biology and physics would unite. But instead of explaining the former in terms of the latter, he imagined a new science, based on logic and mathematics, that would provide a higher level of explanation and of which all other sciences would be a subset. It would be called General Systems Theory. These ideas should sound similar to what is done at the Santa Fe Institute, and indeed Bertalanffy helped found what has become the study of complex systems and emergent properties. But while he pursued such ambitious syntheses, Bertalanffy also carried on lab work, studying growth and metabolism. In his experimental work, just as in his theoretical pursuits, Bertalanffy sought to find the general principles amid the mass of detail. Growth involves myriad constantly varying chemical reactions, and every individual of every species will be different at every moment. But, he reasoned, just as we regard a company’s statement of profit or loss as a meaningful generalization, even though it conceals a host of individual transactions, so it ought to be possible to make generalizations about animal growth. In 1934, Bertalanffy suggested that an animal’s growth rate was proportional to its metabolic rate. He reasoned that the speed of an

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature organism’s growth must depend on the speed at which it gets and uses resources from the outside world. Bertalanffy thought there were several different metabolic types, some matching the surface law, others not, that created several different growth trajectories. Since then other measurements have shown that—with the usual caveats about variation—the range of metabolic types that Bertalanffy saw was an illusion and that growth rates, like metabolic rates, are in general proportional to body mass raised to the power of 3/4. Small animals grow relatively quickly, big ones relatively slowly. Bertalanffy believed that the link between metabolism and metabolic rate could explain why growth took the form it did and why it eventually stopped. He suggested that the net rate of growth was a simple balance between income and expenditure, reflecting the difference between the energy needed to build and expand bodies and the energy needed to maintain, repair, and replace existing parts. Because big animals metabolize relatively slowly, their maximum growth rate is also slower than that of small animals. But, he argued, this isn’t true for the costs of upkeep: The amount of energy needed to keep what you’ve already got in working order grows linearly with body mass. So growth becomes a function of size, rather than time. Growth slows with age because as an animal grows, it comes to spend all its resources on maintenance, leaving nothing for further expansion. Bertalanffy came up with a simple equation expressing this balance between growth and decay, which fits the s-shaped curve very nicely. It has since been used in fisheries and forestry to calculate the best stage at which to catch fish or cut down trees. Roughly, this is the point at which growth has slowed so much that it is no longer economical to leave a pine in the ground or a cod in the sea, but rather is more profitable to harvest it and start over with a smaller, and so faster growing, plant or animal. If a tree is adding wood, or a fish flesh, at high speed, it is becoming more valuable, and it pays to leave it be. If it is growing only slowly, it pays to asset strip, and invest the money in something more profitable. This is why industries based around harvesting slow-growing animals, such as whales or old-growth forest, or hunting large primates for meat, are hard, if not impossible, to sustain—from a coldly economic viewpoint, the best strategy would

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature probably be to slaughter every whale and buy Google stock, or something, with the proceeds. But without an explanation for why metabolic rate slows with size, this equation, although useful, says nothing about the biology of growth—it just fits the curve. Why should big animals find it harder to make a metabolic profit than small ones? They might need more food, but they are also better equipped to get it: They can cover more ground more quickly than small animals, tackle bigger food items, and, if they are herbivores, digest it more fully. And their slower metabolisms are also presumably cheaper to maintain. The network theory offers a new perspective on growth. It builds on Bertalanffy’s reasoning to show why increasing size eventually halts an organism’s growth. The difference compared with Bertalanffy’s theory is that where he focused on demand, West, Brown, and Enquist are more concerned with supply. Their theory predicts that the amount of tissue needing to be fed rises more quickly than the network’s power to deliver food. As an animal gets bigger, each of its capillaries must supply more cells. Eventually, the network can supply only enough energy for maintenance, and growth must stop. The bigger a body gets, the harder it is for its networks to supply the distant outposts of its body, and, like an overstretched empire with failing supply lines, further expansion becomes impossible. What this means is that, as an animal grows, the proportion of its resources that it puts into growth must decline. Proportionately, the growth trajectory for all animals is remarkably constant. An animal that is one-fifteenth of its final weight, whether it is a 40-kilogram calf or a 1.5-kilogram cod, will invest about one-half of its resources in growth. Once it reaches half its final weight, this proportion has dropped to about 15 percent. Life span and the amount of growth, of course, vary hugely for different species: A 40-kilogram calf is a newborn, whereas a cod takes six years to reach 1.5 kilograms and will probably never reach its upper size limit of about 25 kilograms. What goes for animals also goes for plants. To pursue his botanical interests, after he completed his Ph.D. at New Mexico, Enquist began collaborating with plant biologist Karl Niklas of Cornell University. Niklas’s speciality is plant biomechanics—he tries to work out why

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature trees don’t fall down. Studying this question has made him into one of a relatively small group of plant biologists expert in allometry. Niklas was and is unsure about whether the fractal theory really explains the scaling of metabolic rate; he has written that understanding scaling is “as potentially important to the biological sciences as Newton’s work was to the physical sciences,” but he is one of those who sees the fractal model as untestable. Nevertheless, he was impressed by its generality. “I’ve gone from being an atheist to an agnostic” is how he describes his view. And whatever the mechanism, Niklas was convinced that scaling laws were powerful and general descriptions of plant biology, that the same laws applied to all plants from pond algae to forest giants, and that metabolic rate lay at their heart. He was well qualified to bring anew source of mathematical oomph to complement Enquist’s ecological expertise. It was time for another trip to the library and another bout of biblio-ecology. Enquist and Niklas compiled data on the growth rates of plants from unicellular algae, which you can watch dividing under the microscope, to the giant sequoia in California known as General Sherman—the most massive tree in the world, which you have to visit each year with a very long tape measure. The General is 20 orders of magnitude, or 100 million trillion times, larger than the alga. But regardless of their evolutionary ancestry or habitat, Enquist and Niklas found that all plants’ growth is proportional to the 3/4 power of their body mass. That is, all plants share the same basic strategy for turning solar energy into vegetation. And animals and plants show similar growth patterns: Trees grow at the same rate as animals of the same size. No central controller sets the growth rate. Growth emerges from the network, from the interaction between a body’s demand for energy, its network’s capacity to supply the body with energy, and the balance, set by evolutionary experience, between growth and other demands on energy. In a healthy body, cells do not grow at the expense of their neighbors, because they share genes—and so evolutionary interests. But sometimes a genetic mutation causes part of the body to rebel against its neighbors, and against the limitations of the network that serves it. Such cells increase their metabolic rate and stimulate new

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature blood vessels to grow to supply them with resources. Such cells that do not know when to stop growing form tumors and cause cancer. In 2003 a team of Italian and American researchers showed that tumor growth also follows the growth law described by West, Brown, and Enquist, implying that a tumor’s nutrient supply limits its growth. It was already known that any cancer cell had to be within 0.1 millimeters of a blood vessel to divide. The team hopes that relating a tumor’s size to its growth rate will permit calculation of the drug dosage needed to reverse a tumor’s growth and also to predict when malignant tumors will metastasize and spread around the body. Cancer is a diverse disease, however—every tumor is different in its genetic makeup and biological effects, and it is still unclear how discovering such general principles might help treat individual clinical cases. Changes of Life Getting bigger is only one part of life’s history. Many things besides size change as an organism ages. It moves from egg, or womb, into the world. It leaves its mother’s care and tries to feed itself. It becomes sexually mature. It may go from a larval stage into an adult body. All these changes in lifestyle have their attendant changes in biology and their own set of choices. The time that organisms spend in the egg or the womb, the length of time they depend on their parents, and the interval between birth and adulthood all vary enormously. But relatively, these times have much in common. The average mammal, for example, spends about 4 percent of its life in the womb—which means three weeks for a rat, 50 days for a fox, five months for a goat, and eight months for a hippo—and reaches sexual maturity a fifth of the way through its life. The duration of all these phases—the time spent in the womb or egg, the period between birth and weaning, and the time taken to reach sexual maturity—is proportional to body mass raised to the power of 1/4, the power law that describes biological times. Like growth, the course of an organism’s development is controlled more by its size than its age, and considering size reveals the unity between species. Correcting for size accounts for much of the variation in developmental rates,

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature and correcting for body temperature accounts for some of what’s left over. Together, the two variables explain more than three-quarters of the variation in hatching times for fish, insect, amphibian, and bird eggs. If a salmon egg could somehow be expanded to the same size as a hen’s egg, and then incubated at the same temperature, they would hatch after approximately the same length of time. The two would also take approximately the same amount of time to reach maturity. There is more to growth and development than energy. A builder with a generator but no bricks would build nothing. Likewise, organisms need materials as well as calories: A body won’t work properly without a balanced diet and what it can do depends on what chemicals it gets. Just as they vary in size and temperature, organisms also vary in their chemical composition, and this variation can explain another tranche of their biological differences. After carbon the two most important chemical elements found in living things are nitrogen, which is a large component of proteins, and phosphorus, which is found in ATP, the energy molecule, and also in the molecules that carry genetic information, DNA, and its close cousin RNA. In many organisms, such as plants, it seems to be the availability of these elements, rather than solar energy, that limits growth. It is these elements that chemical and organic fertilizers add to soils. Bone meal, for example, is rich in phosphorus. Different species contain similar proportions of protein and so nitrogen. The amount of phosphorus is much more variable. Fast-growing species have more phosphorus because RNA is part of the cell’s protein-making machinery, so fast-growing cells need more of it. RNA is also used in the process that turns the information in the genome’s DNA into protein, giving hard-working cells an extra demand for phosphorus. Animals with high growth rates therefore also contain a lot of phosphorus. And when the metabolic ecologists compared relative phosphorous content to growth rate, they found that this quantity explains some of the variation in developmental times not accounted for by body size and temperature. Chemistry, then, is a third factor, besides body size and temperature, that controls metabolic rate. To explain metabolic rate and how it sets life’s other rates, we must consider materials as well as energy and

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature organisms’ chemical composition as well as their size and temperature. Life both depends on and controls a supply of elements through the rate at which plants take up nutrients and the rate at which they pass up the food chain and ultimately return to the world via death and decomposition. This chain links the events in cells to processes that encompass the whole earth. In 1934, Alfred Redfield noted that the ratio of carbon to nitrogen to phosphorus in seawater around the world was the same as the average ratio of those elements in the bodies of marine plankton. There are 106 carbon atoms, to every 16 nitrogen atoms, to every 1 phosphorous atom. The dietary habits of unicellular plants, therefore, control the chemistry of the world’s oceans—although no one has been able to explain why the Redfield ratios should take the values they do. As Lotka predicted, chemistry, like temperature, is another controller of biology that humans are changing on a global scale. Since 1908, when the German chemist Fritz Haber worked out how to take nitrogen out of the air and turn it into ammonia, the quantity of nitrogen available to living things has doubled. Much of this nitrogen is washed into lakes and rivers—U.S. rivers contain between two and 20 times more nitrogen than they did before the Industrial Revolution. From rivers the nitrogen goes into estuaries and coastal waters, where it stimulates plant growth, sometimes leading to algal blooms. When these blooms decay, the bacteria that eat them use up oxygen, sometimes choking animal life. Each summer, several thousand square kilometers of the Gulf of Mexico becomes a low-oxygen “dead zone,” thanks mainly to agricultural runoff from the Mississippi River. Evolution’s Winning Post All growth and development are just means to the end of reproduction. It doesn’t matter how big you are, or how quickly you grow, if you don’t leave any descendants. Sometimes reproduction and growth are indistinguishable. When a single-celled organism such as a bacterium divides, a new individual is born. For multicellular animals reproduction is also growth by proxy, although in a more complicated fashion. Instead of investing its energy in itself, an animal puts its resources into a copy of itself (a partial copy, if reproduction is sexual). This

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature process presents a further array of choices: how much to invest in the offspring; whether to divide this investment between a few big ones or lots of small ones; and after birth, how long to go on feeding and protecting your progeny. Metabolism sets the boundaries of reproductive choices, too. The rate at which animals can seize energy for themselves limits the rate at which they can pass it on to their offspring. So, not surprisingly, many aspects of reproduction march in step with metabolism, and an animal’s size is the best guide to its reproductive biology. Bigger animals produce bigger young, but they devote a smaller proportion of their resources to their offspring. The clutch of a 100-kilogram ostrich consists of thirteen 1-kilogram eggs, totaling about one-eighth of the mother’s body weight. A 3-gram hummingbird can lay two eggs that each weigh a quarter as much as itself. Big fish and reptiles also lay proportionately lighter clutches than small ones. The mass of mammals’ litters shows a similar trend, and big mammals’ milk contains less protein than that of small species. All these properties are more or less proportional to body mass raised to the power of 3/4, although there is much variation about this number. One thing that does vary between groups is the way these resources are divided up: Birds and mammals produce relatively few large offspring, in which they invest considerable resources, both before and after birth. Many reptiles, fish, and invertebrates go for quantity rather than quality, producing many small young, each of which stands only a small chance of survival. Bigger animals also breed less often—the interval between litters or clutches is proportional to body mass raised to the power of 1/4—and so produce a smaller number of offspring over their lifetime. The 30-Tonne Gorilla The bigger you are, the more energy you can get hold of and the more offspring you can produce. One species, however, flies in the face of this apparent evolutionary no-brainer. An adult female gorilla weighs about 100 kilograms and will give birth to between three and six young in her life. The average European or North American woman weighs considerably less but will produce approximately two babies.

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature match is striking: metabolic rate per cell declines proportional to body mass to the power of −1/4, and life span increases at a rate proportional to body mass raised to the power of 1/4. This means that every cell, be it rat or rhino, burns approximately the same amount of energy in its lifetime. Heart rate also declines in line with relative metabolic rate, as the −1/4 power of body mass. At rest a mouse’s heart beats more than 500 times a minute. A shrew’s blood races out of its heart, around its body, and back to base in four seconds. Our blood takes nearly a minute to complete the same journey. An elephant’s heart beats about 30 times every minute. Other organs, such as the kidneys, also work much harder in smaller animals. Life spans vary proportionately. A mouse, even a pampered one in the lab, is unlikely to see three summers; an elephant can expect three score. This means that every mammal should get about the same number of heartbeats—about 1 billion. One recent, unserious, calculation put the number at 955,787,040. Likewise, each mammal will draw about 200 million breaths before it expires. This has become folk wisdom. The astronaut Neil Armstrong once remarked that he did not jog because he believed that God had given him a finite number of heart-beats and he would be damned if he was going to fritter them away running up and down the street. It almost looks as if every animal—every cell—has a certain amount of energy to burn, after which it dies. For many years, people thought just that. The notion that the rate of living determined the length of life was given scientific form by Max Rubner, he of the surface law, in 1908. Rubner used his calorimetry experiments to calculate the relationship between energy use and life span in horses, cows, cats, dogs, and guinea pigs. He found that each gram of guinea pig flesh burns 260 calories in its six-year lifetime and each gram of horse flesh burns 170 calories during 30 years of life. We now know that horses can live up to 50 years, which works out to 280 calories per gram per lifetime, even closer to the guinea pig. Other species fell somewhere in between. This fit well with Rubner’s view that biology was a matter of food and energy. With characteristic grandeur, he announced that the finding had “the unity of a great law.” Experiments following on from Rubner seemed to confirm that, if you slowed an animal’s metabolism, you extended its life. Cooler tem-

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature peratures had the same effect as increasing body size: Fruit flies reared at 30°C live on average 14 days, those at 10°C live for 120 days. Water fleas show a similar link between temperature and life span. And because water fleas are transparent, you can see their hearts beating inside their bodies, showing that both cool and warm fleas have the same number of heartbeats in their lives. Rats that led sedentary lives lived longer than those that were forced to exercise. All this evidence made a deep impression on the man who would become most associated with the idea that to live fast is to die young—the American biologist and statistician Raymond Pearl. Well over 6-feet-tall, loud and boisterous, funny and caustic, given to marathon bouts of French horn playing, and with apparently bottomless supplies of enthusiasm and self-belief, Raymond Pearl was perhaps the only swashbuckling statistician who has ever lived. His great friend was the journalist Henry (H. L.) Mencken, and during Prohibition he was part of the Saturday Night Club that met in Mencken’s cellar to booze it up and play music. Pearl was an enemy of fundamentalism and Puritanism and caused a minor scandal in Prohibition-era America when he argued that health records showed that moderate drinkers outlived teetotallers. When he died in 1940 at age 61, Mencken wrote his obituary. Pearl began his scientific career at the University of Michigan, earning a doctorate in biology for his work on flatworm behavior in 1902. He discovered the joy of statistics while working in London with the eminent statistician Karl Pearson in 1905. Pearson believed in laws of nature but of a rather different sort than those sought by Bertalanffy and D’Arcy Thompson. Pearson saw science as the classification of facts, and his scientific laws were the mathematical expressions that described the patterns in messy data. He saw no need to work out what might be causing those patterns. Such statistical techniques had barely reached America; Pearl resolved to introduce them. Statistical analysis, he argued, was not a piece of drudgery useful only for bringing recalcitrant experimental data into line—it was the foundation of biology. “The matters with which biostatistics concerns itself constitute some of the most fundamental and important problems of pure biology,” he wrote. “Bio-

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature statistics … is the sign, the symbol, and indeed in some respects the very essence of the biology of groups.” By the mid-1920s, Pearl’s charisma and newfangled skills had made him something of an academic celebrity—the highest-paid professor at Johns Hopkins University in Baltimore, free to study whatever he liked, and (many a researcher’s dream) relieved of all teaching duties. Besides churning out a huge number of academic papers and more than 20 books, he had a sideline in journalism, writing for the Baltimore Sun and Mencken’s American Mercury. He makes a cameo appearance in Sinclair Lewis’s 1925 novel Arrowsmith, about one scientist’s struggle to maintain his individuality in the face of large organizations and their money. Inevitably, some colleagues found Pearl insufferable, partly because he dismissed all criticism as the product of minds duller or more conservative than his own. Such a gung-ho approach to life and science led him into blunder as well as triumph. Thinking he had spotted a negative correlation between cancer and tuberculosis in medical data, he injected some terminal cancer patients with TB. The errors in his analysis (although apparently not the experimental treatment) did considerable damage to his credibility and later career. On the plus side, in 1938 he was among the first people to show that smoking is associated with reduced life expectancy. Pearl believed that an organism’s life span was its most important attribute and used his academic freedom to try and work out what determined its duration. In his laboratory he studied the effects of starvation, population density, temperature, and heredity on longevity in fruit flies and on cantaloupe melon seedlings. The plants that grew quickly, he found, died before the slowpokes. Every living thing, Pearl concluded, was born with a certain amount of “inherent vitality,” and the speed with which this was consumed determined its life span. “In general the duration of life varies inversely as the rate of energy expenditure during life,” he wrote in his 1928 book, The Rate of Living. But these experiments were only a means to Pearl’s goal of understanding human longevity. You can’t starve humans in the name of science, but in statistics Pearl thought he had the tool that would crack the problem. In 1924 he analyzed the death records of British men of various occupations and concluded that, thanks to the rate-of-life

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature effect, hard labor was the route to an early grave. Coal miners and blacksmiths died younger than lawyers and insurance salesmen, he claimed, even accounting for differences in diet and accidental death. This, he argued, also explained why women lived longer than men—they didn’t work as hard. In 1927 he wrote an article for the Baltimore Sun called “Why Lazy People Live the Longest.” Pearl never suggested what his inherent vitality might consist of or how it might get used up. We now have a good candidate for a link between metabolism and mortality, but instead of being a good thing that disappears, it is a form of damage that accumulates. Dangerous Radicals Oxygen is a mixed blessing, because it is highly reactive. This property makes it perfect for releasing energy from food but also makes it liable to react with and damage the rest of the body’s molecules—just as a fire can both warm your house and burn it down. When two oxygen atoms are joined in an O2 molecule, they are relatively harmless. The trouble starts when the molecule is broken up for use in respiration. Such reactions create free radicals, lone oxygen atoms with spare electrons. It is these spare electrons that make the free radical so reactive and so noxious. The free radical careens around the cell, damaging any protein molecule or DNA it bumps into. DNA damage is particularly bad because it is passed on to the cell’s descendents. And when a free radical does find a molecule to react with, it is likely to create more lone oxygen atoms in a chain reaction. This is why dieticians recommend foods rich in small molecules, such as omega-3 fish oils, that mop up free radicals before they can cause harm. In 1956, Denham Harman, a chemist at the University of California in Berkeley, suggested that free-radical damage could be one cause of aging. About one in every thousand oxygen molecules broken up in mitochondria during respiration produces a free radical. So the faster mitochondria process oxygen, the more free radicals are produced and the more damage is done—providing an explanation for the rate-of-life theory. Studies since Harman’s have reinforced the view that free-radical damage is a central process in aging and one of the most

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature important determinants of life span. As a cell ages, it gets worse at doing the things it is supposed to, such as dividing and making useful chemicals, and is more likely to die, attack other cells, or embark on a malignant growth spurt. Old cells shut themselves down or die as a means of preventing such mishaps. Free radicals can replicate many of the effects of aging, mainly by damaging DNA in the cell’s genome and the smaller mitochondrial chromosome. Free-radical damage is a vicious spiral—mitochondria produce free radicals and are damaged by them, which makes them more likely to produce free radicals, which increases the damage, and so on. Mice genetically engineered to have more mutations in their mitochondrial DNA show symptoms of premature aging such as declining fertility, curvature of the spine, and hair loss. Many of the genes linked to life span in the worm Caenorhabditis elegans—the animal of choice for many researchers who study aging—affect the animal’s ability to deal with free-radical damage. Free radicals are implicated in hazards of old age such as cardiovascular disease, failing eyesight, and neurological disorders, including Alzheimer’s disease. The case against free radicals is not closed. Scientists debate how much can be learned about the lives of whole animals from studying cells in a dish, and also about what aging in animals reveals about the process in humans—and we still can’t answer Geoffrey West’s question of why maximum human life span is about a century. But to quote an article published in Nature in 2004, current ideas of how aging works can be summarized as: “It’s the free radicals, stupid!” The Bat Paradox Metabolism produces free radicals. Free radicals cause aging. The faster you metabolize, the more free radicals you produce and the sooner you die. This does indeed seem like the unity of a great law. But it’s not as simple as that. For a start, many things other than metabolic rate influence life span. Contrary to what Neil Armstrong might think, people who exercise do not hasten their deaths—quite the opposite. Exercise can increase free-radical production, but the benefits to the heart, lungs, and vascular system more than outweigh the downside.

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature Still more troubling for crude rate-of-lifers is that many animals live far longer lives than their metabolic rates alone would predict. The most spectacular examples are bats and birds. A bat smaller than a mouse can live for 30 years or more. Hibernation, during which the animal burns much less energy, might account for some of this, but tropical bats, which do not hibernate, are similarly long-lived. Birds have higher metabolic rates than mammals but much greater life spans. Studies of wild birds have revealed that seabirds can live for half a century, and some parrots are thought to have reached twice that age. Marsupials, on the other hand, have relatively slow metabolisms but short lives. A captive kangaroo will be lucky to see its twentieth birthday. Hyenas, which are of a similar size, have been known to live twice as long. And if you work out how long a billion of your heartbeats would last, you will know that, at least in rich countries, most of us live for much longer than our size would suggest. Even at a healthy 60 beats per minute, 955,787,040 seconds lasts only 30 years. Supposing a strict link between size, metabolism, and longevity, a mammal as big as a human should live only 27 years. This might have been a ripe old age for our prehistoric ancestors, but human hearts and bodies are clearly good for at least several decades more. The experimental evidence for the rate-of-life theory is also ambiguous. One surefire way to extend an animal’s life is to starve it. Rats, mice, worms, and even yeast cells, fed a diet containing all the necessary nutrients but a third fewer calories than they themselves would choose if given free access to food, live about a third longer than those on full rations. Such underfed creatures do have lower metabolic rates—mainly because their body weight has fallen—but many other aspects of their biology change, and it is not clear what contribution slower metabolisms make to their extended life span. In 2004 a team led by John Speakman of the University of Aberdeen revealed that mice with faster metabolisms live longer than normal ones. If anyone still held to the simple idea of living fast and dying young as originally proposed by Rubner and Pearl—and by then few, if any, serious aging researchers did—these experiments should have changed their mind. But the links between metabolism, mitochondria, and aging, although more complicated than they once appeared, are still strong.

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature It now looks as though it’s not how quickly you burn energy that influences the aging process but the way that you burn it. Long-lived mice do not simply have faster metabolisms than their shorter-lived counterparts. They also have different mitochondrial chemistry. Mitochondria can do two things with food: turn it into cellular fuel, in the form of ATP, or burn it off as heat. The chemical reactions that turn food into heat produce fewer free radicals than those that produce ATP. Cells have proteins that switch their mitochondria from ATP mode into heat mode, a process called mitochondrial uncoupling. The long-lived, energy-hungry mice in the Aberdeen experiment had a higher level of this uncoupling. Humans use mitochondrial uncoupling to keep warm. Babies have a tissue called brown fat that is stuffed with mitochondria and with proteins that uncouple them, and which disappears around the age of one. And the indigenous people of cold climates have more mitochondrial uncoupling than temperate or tropical groups. This might be part of the reason why cold dwellers have lower rates of neurodegenerative disorders, such as Alzheimer’s, but are more prone to diseases of energy metabolism. Some researchers, including Speakman, are on the trail of drugs that might increase life span by uncoupling mitochondria. We already know some chemicals that do that, but they tend to have unfortunate side effects. One such compound is 2,4-dinitrophenol. Found in explosives and insecticides, this chemical’s metabolic properties were discovered during the First World War, when workers exposed to dinitrophenol in munitions factories began losing weight at startling rates—because their metabolic rates had shot up. Dinitrophenol was sold as a slimming drug in the 1930s but was later banned because an overdose makes the body cook itself. Some bodybuilders still use it as a means of crash dieting, and some have died from it. Another chemical that uncouples mitochondria is Ecstasy, or MDMA, which often causes an unpleasant and potentially dangerous rise in body temperature. The drug produces this effect by activating uncoupling proteins in the cells. Together, this evidence raises the thrilling prospect of a pill that makes you thin, high, and ageless, if rather flushed and sweaty.

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature Bending the Rules This doesn’t explain why some species live so much longer than others of the same size and metabolic rate, but thinking about longevity in evolutionary terms can. Like everything else, how long an animal lives is a trade-off. Maintaining and repairing a body takes energy. There are ways of limiting free-radical damage; for example, cells have enzymes that dispose of free radicals and repair their effects. But making these repairs diverts resources from other processes, such as reproduction. To register on evolution’s scoreboard, an organism must survive long enough to reproduce once. After reaching this goal, many species do not bother with a second attempt. Some marsupials, fish, most squid and octopuses, most insects, and all annual plants reproduce only once and then die. By investing everything in one breeding attempt, they can produce more offspring now at the expense of reproducing in the future. Of course, animals from albatrosses to zebras, and plants from grasses to redwoods, invest in keeping themselves healthy enough to breed more than once. They are gambling that this investment will pay off in the long run. Species are tipped one way or the other by a combination of the ease of reproducing—how much effort they must put into each breeding attempt—and the likelihood of living long enough to breed again. The more effort breeding entails, and the smaller the chances of long-term survival, the more evolution will favor the all-at-once approach. What’s a salmon going to do once it has spawned? Swim all the way back out to sea and all the way back in again next year? Forget about it—predators kill most fish before they can complete the journey once. The same reasoning probably explains why male redback spiders will go to great lengths to be eaten by their mates, even somersaulting onto the female’s jaws. Females are so hard to find, and a spider’s life so chancy, that it pays a male to put everything, literally, into the one attempt and also use its body to feed the female carrying its young. But not all animals lead such risky lives. If an animal has a good chance of avoiding predators or surviving the next frost to breed again, it might pay for it to slow the aging process and keep the body going. It is striking that both bats and birds have an excellent means of escaping

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature danger—flight. Investing in repairing free-radical damage is a waste if you stand a good chance of ending up in a cat’s jaws or a hawk’s talons before the week is out, but the chances of this fate befalling a pigeon seem much less than they do a rat. This is probably why a cosseted pigeon can live for 35 years, but a pet rat of about the same size will live only about four years. In a lab dish, mitochondria from pigeons’ cells produce less than half as many free radicals as those from rats. Perhaps early humans’ social networks and technology cut the rate at which the weather and wild animals picked them off and so led them to evolve cellular systems that slowed the damage of aging. Small rodents, on the other hand, are so likely to die from extrinsic causes that natural selection has never favored a slowing of their aging process. But no animal can lead a perfectly safe life. And simple probability dictates that the longer you live, the greater the chance of encountering a falling tree, a nasty infection, a poisonous mushroom, a man-eating shark, or a slippery step. No amount of investment in self-maintenance can eliminate these dangers, and this tips the evolutionary balance away from staying healthy and toward reproduction. At some age, depending on life’s risks, the value of repair diminishes to the point where the benefits of longer survival become invisible to natural selection. There can also be benefits to breeding early. An animal that breeds at age oneyear can make copies of its genes twice as quickly as one that waits until it is two. This means that evolution can favor traits that give an animal more offspring early in life but that cause damage later. Flies bred to live longer produce fewer offspring, and vice versa. The same relationship is seen in the genealogical records of the British aristocracy. Bodies age and die because, to evolution, once they have reproduced, they are disposable. Brian McNab, an ecologist at the University of Florida, has spent decades studying the interactions between an animal’s metabolism and its environment. He can probably account for a wild animal’s metabolic rate better than any other biologist in the world. Using body size, altitude (which is similar to temperature because higher places are colder), and diet (which is similar to chemical composition), he has explained 99 percent of the variation in metabolic rate for birds of

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature paradise and a group of bats. But he does not think that knowing an animal’s metabolic rate reveals everything about it, and, unlike Jim Brown and company, he doubts that simple, universal predictions can be made based on metabolic rate and body size. “Animals have a lot of ways to bend the rules,” he says. The relationship between metabolism and aging seems to be the best example of this rule bending. The quarter-power scaling of life span to body size shows that, at the broadest scale, animals live by the ticking clock of metabolism. But the fact that this rule is frequently so poor at predicting the life span of a particular species shows that evolution can favor animals that invest in slowing their metabolic clocks. Nevertheless, without an idea of how energy use and body size influence biology, we would not have realized, for example, that birds live unusually long lives. Nature’s Gamelan Gamelan is the traditional music of the Indonesian islands of Java and Bali, a mesmerizing sound world built around repeated patterns played on metallophones and gongs. Two nights a week I play in a gamelan orchestra. In a western orchestra each instrument’s part is written down and predetermined. Gamelan is not like that. The notation for each piece is minimal, just enough to point a few of the ensemble’s instruments in a certain direction. Gamelan notation is more like the rules of a game than a computer program; each musician takes the rules and constructs his or her part based on a set of conventions, handed down from senior musicians, about what one can play in any situation. The rules are flexible. There are often several options for every situation, and every group will interpret the rules differently, so the same piece sounds very different from group to group. Each piece is also flexible: What notes, or even pieces, one plays, how one plays them, and how long a piece lasts are not predetermined but arise from the musical dynamic between group members. A group of physicists recently concluded that Javanese gamelan was, in terms of rhythm and volume, the world’s most complicated music. I was surprised because I have been in workshops where a group

OCR for page 121
In the Beat of a Heart: Life, Energy, and the Unity of Nature of complete novices learned how to perform a piece in only 20 minutes. The complexity of gamelan is an emergent property, of many players following simple rules. The laws of metabolism are like gamelan music: an underlying structure on which living things can elaborate and improvise. Every place has its own unique conditions and history, and so every species, and population within that species, will be different than the rest, just as every Javanese village has its own musical traditions and conventions. And just as a gamelan group builds a world of music from a handful of notes, nature’s diversity takes the form of myriad variations on the theme of turning energy into offspring. The rules of metabolism become most obvious when you look at many species over a broad size range. The more living things you encounter, the more impressive the similarities and regularities become. This pattern points the way to an understanding of nature that goes beyond the individual—after all, every wild place contains lots of species of lots of different sizes. And once you understand how individuals use energy, and if all use energy in the same way, the patterns seen in forests, flocks, and herds stop seeming like a mass of unconnected examples and start to look like the product of underlying rules.