Cover image for The riddled chain : chance, coincidence, and chaos in human evolution
The riddled chain : chance, coincidence, and chaos in human evolution
McKee, Jeffrey Kevin.
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Publication Information:
New Brunswick, N.J. : Rutgers University Press, [2000]

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x, 280 pages : illustrations ; 24 cm
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GN281 .M396 2000 Adult Non-Fiction Central Closed Stacks

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Did human evolution proceed in an inevitable fashion? Can we attribute our origins solely to natural selection, or were more mischievous forces at work?

These are the questions investigated by anthropologist Jeff McKee. He argues that if we were to wind back the clock to our split from ancestral apes, evolution would proceed differently. Ever since our ancestors first stood up on two feet, natural selection undoubtedly was an important factor in guiding human evolution. But McKee shakes the standard notion that natural selection steered early hominids toward particular environmental adaptations. The fossil remains of our ancestors reveal a different story one of an adaptable hominid with no particular direction. It becomes clear that the evolutionary road to Homo sapiens was not paved solely by natural selection; indeed, there was no road to follow. There was just a dim path cut out by prehistoric coincidences and contingencies. Had any link in the evolutionary chain of events been slightly different, then our species would not be as it is today . . . or our ancestors may not have survived at all.

With equal doses of humor and awe, McKee illustrates how the chain of evolution has been riddled by chance, coincidence, and chaos. He uses familiar examples, noting that many of us exist as individuals because of chance meetings of our parents. From the present back through prehistory, chance is at the heart of our creationÑas is chaos. The classic example of chaos is the butterfly effect: a single butterfly, flapping its wings, causes a tiny change in the atmosphere, which in turn amplifies to affect the course of storms on another continent. McKee ties such examples of unpredictability to fossil evidence and computer simulations, revealing the natural coincidences that shaped our evolution. Although chaos exacted an evolutionary price by limiting the powers of natural selection, it also made us what we are. One can only conclude that human beings were neither inevitable nor probable.

Author Notes

Jeffrey K. McKee is an associate professor in the departments of anthropology and evolution, ecology, and organismal biology at The Ohio State University.

Reviews 3

Publisher's Weekly Review

Unlike most entries in the spate of recent books discussing human evolution, this book focuses on the processes that gave rise to humans rather than detailing the steps in our lineage. McKee, professor of anthropology at Ohio State, argues that evolution is much less directed than many people might believe. The dominant forces, he asserts, are chance, coincidence and chaos, coming together through the process of autocatalysis. McKee draws intelligently on his work on computer modeling to bolster his position. He shows, for example, that randomly removing just a single female from a simulated human population of 10,000 breeding individuals can have dramatic effects on the evolution of that population. In one such computer run, with a seemingly nondescript female removed, the population went extinct only 321 generations into the simulation. McKee also looks at the prospects for the future of human evolution, asserting that "the lessons of human evolution are important, have a broad value, and may even help us set the course for our future survival." We are continuing to evolve, he contends, in significant ways, from changes in eyesight to alterations in our immune system. Additionally, our actions are influencing the world around us to such an extent that our own continued existence is at risk. Although there's not much that's truly new here, McKee does an admirable job of presenting his ideas. (July) (c) Copyright PWxyz, LLC. All rights reserved

Library Journal Review

The role of chance in human evolution is often underemphasized by those who think in terms of purposeful change. As this book usefully demonstrates, chaos theory can give us clues about our own evolution. McKee (anthropology, Ohio State Univ.) uses computer simulations to show how apparently minute changes in the initial makeup of a simulated population cause enormous changes in the population after many generations. Environmental change does not cause evolution, he says; natural selection acts either to preserve or kill off mutations but is dependent on mutations occurring by chance to make any changes. McKee, who has excavated fossils in South Africa, is also a proponent of the role of autocatalytic evolution, in which an initial evolutionary change is affected by feedback loops. This means that the survival and spread of a mutation is affected by its impact on the functioning of the organism within its species and in the current environment and that it in turn can affect the survival of mutations in other parts of the organism. Most of McKee's ideas are not entirely new, but his working out of their implications leads to important and sometimes unorthodox insights. His clear, informal, and anecdotal writing style makes sometimes complex ideas interesting and understandable. For academic and large public libraries.DMarit MacArthur Taylor, Auraria Lib., Denver (c) Copyright 2010. Library Journals LLC, a wholly owned subsidiary of Media Source, Inc. No redistribution permitted.

Choice Review

McKee (departments of anthropology and of evolution, ecology, and organismal biology, Ohio State Univ.) undertakes a provoking and provocative reassessment of scientific processes based upon the premise that "natural selection gets more credit than it deserves in evolutionary theory." He challenges the concept that human origins are attributable solely to natural selection by considering whether other forces (contingencies, chance, and chaos) were at work. McKee contends that natural selection directed early hominids toward particular environmental adaptations, and he observes that the paleontological record documents the evolution of an adaptable hominid. Information is cited on South African hominid sites (Taung and Makapansgat), fossil genera (Australopithecus and Homo), and other fauna (butterflies, fruit flies, and cockroaches). McKee also considers evolutionary fact versus theory, the punctuated equilibrium concept, evolutionary processes (natural selection, genetic drift, gene flow, and mutation), variability selection, autocatalytic evolution, problems of bipedalism, paleoecological habitats, and extinction rates. McKee's delightful and thoughtful assessment of scientific processes is documented in nine chapters, augmented by 12 illustrations, 155 endnotes, 118 references, and a detailed 12-page index. He makes a case for an autocatalytic feedback loop of evolution in this lively, non-technical assessment. Both general and academic readers at all levels. C. C. Kolb; National Endowment for the Humanities



Chapter One Chance, Coincidence, and Chaos The evolution of life on earth has produced some exquisite and peculiar beasts, among which humans are a distinct curiosity. We tend to set ourselves apart from other animals, for sentient beings hardly seem to fit into nature's intricate order of life. But humans are really no less marvelous than other living species with whom we share a long and complex evolutionary history. All existing life forms endured a struggle for existence and evolved seemingly inspired adaptations. We are just another product of nature, yet we humans uniquely stride on two legs, carrying aloft a curious brain with which we can probe our origins and dare to foresee the future.     Our very nature makes us curious about how and why we evolved, but the particulars of our evolutionary creation have been somewhat elusive to traditional science. We understand some key principles, rooted in Darwinian biology, but sometimes the simple means of evolutionary progress do not seem to offer a full explanation of our biological gifts. How could an aimless evolutionary process, patching together random biological novelties and oddities through trial and error, lead to Homo sapiens ?     Much of our difficulty in probing our origins is due to the tremendous time scale required for the process of evolution to unfold, something that is not easy to comprehend. Only a few sparse fossils from the distant past, and keen observations of life on earth today, can spur the scientific imagination toward testable hypotheses of how evolution led to the menagerie populating the planet today--and how we were afforded the luxury of questioning our place in nature. Yet even with the fragmentary fossil evidence we have, our understanding is limited solely by our imaginations.     One can only dream of the grandest biological experiment: restarting the earth at the inception of life and watching the evolutionary story play itself out. One wonders how our history and prehistory might differ if we were to start again. Would birds and baboons alike ultimately find a place on the planet after billions of years of evolution from the simplest of living organisms? Would our place in nature be the same, or would humans even have any place in nature at all? Most likely not. Initial life forms would have evolved into an equally astonishing array of creatures, to be sure, being aimlessly self-propelled through a tortuous evolutionary history with no particular destiny. It is unlikely that life was destined to have included us. This is because the evolution of life is subject to fates wantonly dictated by three ubiquitous and mischievous forces: chance, coincidence, and chaos.     Chance I really should not be here. It is not that I have no right to be here in Ohio or anywhere else, but that I probably should not be anywhere at all. Biological principles made me what I am, but my good fortune in being present on earth is the highly unlikely consequence of a long series of chance events. Indeed, chance itself may be considered as a key biological principle.     Chance played more of a role in my immediate origin than I should care to admit. When I was born, the youngest of four boys, it was largely because my mother wanted a daughter. By having four children the odds for at least one daughter were on my mother's side, but the less likely outcome of the birth stakes played a little trick on her. Lucky me, for otherwise I would not have been conceived.     So how did all this happen? Let's assume, for convenience' sake, the chances of having a boy or a girl at each birth are exactly even--a 50 percent chance for the birth of Susan Gayle McKee. [Yes, her name had been chosen.) Had the firstborn of my family chanced upon an X chromosome from my father, and thus become a girl, I never would have been conceived years later. But my mother's peculiar saga had begun, much to my ultimate benefit. It was a Y chromosome that started a chain of events by producing a boy.     The second child also could have been a girl. The laws of probability tell us that chance is multiplicative, so the chance of my poor mother having two boys in a row was only 25 percent. We understand in retrospect that the third son was not exactly planned, another chance event, but to keep things simple, he too had a 50 percent chance of being the daughter my mother so desired. Multiply him in, and there was a 12.5 percent chance, or just one chance in eight, that there should have been any need for my conception.     Just a different gleam from my father's eye, and I too could have been Susan Gayle McKee, perhaps a distinguished suburban lawyer rather than an itinerant anthropologist. My mother would have been pleased, but it was not to be. Taking matters back further, now that we are thinking along these lines, none of the McKee boys would be here at all if my father had not been born a boy, and my mother born a girl. Put all these chance events of two generations together and there is less than a 1.6 percent probability, merely one chance out of sixty-four, that I should be here on earth. Keep in mind that I am considering only gender selection here. The fragility of the human developmental process, along with a myriad of biological and social factors, makes me even more unlikely.     Yet here I am, digging up ancient fossils and studying the remote human past, despite the improbability of my recent past. To what can we attribute this sequence of events? Some would call it fate; others may invoke manifest destiny. I prefer to look at it more scientifically, in which case, at least statistically speaking, the events leading to my birth were a culmination of nothing more than dumb luck--a product of chance. These things happen.     Some may look at my analysis of my own improbability and quibble with my mathematics. Fair enough, for they may look at things from a population perspective: it was likely that a boy such as myself would have been born, and probable that some families would consist of four unruly boys. True, but I cannot help thinking that if time were rolled back and the sequence started over, with just one tiny difference in the fertilization of any egg that became a member of the McKee family, I would not be here and you would not be reading this book.     As we go back generation after generation, the probabilities of coming up with exactly me become diminishingly small. Certainly my ancestors hoped that their lineage would carry on, but the precise way in which things happened would have been well beyond their prognostication. One chance event led to another and, for better or worse, here I am.     Certainly you must realize that you are equally unlikely. The same laws of probability apply to your family as well as mine. The same genetic principles determining gender and a host of other characteristics have been filtered through the generations to come up with something that is uniquely you, unless of course you have an identical twin. You are lucky to be here, for a plethora of scenarios could have played out over the years. Every human being, as well as every monkey, bird, and insect, is the uniquely improbable result of past combinations of genetic material. It is that specific improbability of every individual that makes each of us unique, and gives evolution something to work with.     Not only is the probability of your own individual existence very tenuous, but your species--our species, Homo sapiens --is also a fluke of familial descent. We are lucky to be here, and we are exceptionally lucky to be able to think and read and write about our good fortune. Baboons, pigeons, and cave crickets were not afforded the same modicum of cerebral capabilities, no matter how unique and well-adapted they may be. And they were bred by the same basic principles that led to us. We are just, as Thomas Henry Huxley described us in 1854, an "aberrant modification" of an evolutionary theme.     Now this may be a bit disconcerting to some people. Sentient beings, sapient human beings, have always thought that there was something inevitable about them. Even devout Darwinian evolutionists tend to put our own immodest species at the top branch of the evolutionary tree, as if we were somewhat better and more evolved than other living species. But, as we shall see throughout this book, chance has played a role in putting every living thing at the top of the present evolutionary tree.     Some have looked with discomfort at the unlikelihood of the evolution of a large-brained, intelligent species and proclaimed evolutionary theory a farce. Statistically speaking, our evolution seems to be too improbable even to consider. Our evolutionary probability is indeed minuscule. But such arguments invoke the same logical twist that I used above to claim my own improbability. Any specific product of biological evolution, be it human or giraffe, is unlikely. What is likely is that some sort of species would have descended from our own remote ancestors. Just as contemporary statisticians could have predicted that a boy such as myself would be born in Ohio in 1958, a prehistoric prognosticator could have foreseen some sort of peculiar species emerging from the animals then on earth. The boy did not have to be me, and the species did not have to be us. It was a matter of chance, intermingled with other principles of evolutionary biology.     Chance is one of the most horrifying words in the lexicon of a scientist. It strikes terror into our hearts, for we want rational, principled explanations of the world as we know it. Scientists try to package the phenomena of physics and biology alike into neat little explanatory boxes driven by deterministic processes in which chance plays no role. This is a pity, because by embracing the powers of chance, rather than balking at the use of the word, scientists may find that some very interesting things can happen by chance, including the evolution of the human species. We may even discover the keys to what increased our chances of surviving and evolving.     So how did all this happen? How did a series of chance events lead to complex sentient beings? Charles Darwin gave us the answer, or at least a large part of the answer, about a hundred years before one of his adamant supporters chanced to be born in Ohio. He called the directional force of evolution "natural selection"--the way nature "selects" among the variants born of chance. In the words of Sir Ronald Fisher, "natural selection is a mechanism for generating an exceedingly high degree of improbability."     Natural selection can shape and mold populations by allowing the fittest individuals to survive and reproduce. Among those individuals, in the last 3 or 4 million years, were our own ancestors as well as the ancestors of every creature on earth today. Our ancestors made it through natural selection. What luck.     But that luck may have been shaped by a series of timely coincidences.     Coincidence The events of one's life, or of many lives, often coincide in ways that invite the human imagination to conjure up intriguing but tenuous meanings to simultaneous circumstances. Coincidence plays a tremendous role in the way we perceive and interpret our universe. But the meaning of coincidence can be viewed in many different lights.     Many people firmly believe that tragic events, particularly deaths, always occur in threes. Thus, once two friends or relatives have passed away, such believers in this rule anxiously await the third demise. It is truly amazing how often we can observe such packages of three coincidental misfortunes, but more often than not the package is an artificial product of our minds' attempts to rationalize what we see. People die every day, and other things are always going wrong, so it is easy to choose three people we know or three personally significant circumstances to fulfill our own prophecies. Such sequences of events are thus mere coincidence, with no greater meaning.     Sometimes, however, events coincide for distinct reasons. One circumstance may cause another, either directly or indirectly. For example, your purchase of this book may cause my income to rise, and I am truly grateful for that direct effect, as is my growing family. More indirectly, what I am writing today may cause you to view life differently, a coincidence that is my primary purpose for writing this book. But up to this point, our lives have been largely independent.     Likewise, distinct events may share the same cause. The end of a war may cause an increase in the birthrate in many families; for this coincidence I hold a measured amount of gratitude, as my birth was at the tail end of the baby boom that followed World War II. But it is not always easy to differentiate between mere coincidences, caused by chance or perceived artificially, and significant coincidences that stem from recognizable causes.     It is no mere coincidence that I began writing this particular section of my book on the twelfth day of February. This day coincides with a particular anniversary, in fact two anniversaries, and my choice to write was made with forethought and eager anticipation. On February 12, 1809, Charles Darwin, the great naturalist and founder of modern biological thought, was born in Shrewsbury, England. On this day, also in 1809, Abraham Lincoln, the great American statesman and sixteenth president of the United States, was born in Hardin County, Kentucky. Both Darwin and Lincoln wrote words that have found their way into my personal library, and indeed my library contains biographies of both. The words and deeds of Darwin and Lincoln, two unrelated people born an ocean apart on the very same day , shaped human life and thought on earth for well over a century, and will continue to do so. What a coincidence!     Charles Darwin and Abraham Lincoln shared more than just birthdays, greatness, and long-term impact on human thought. Both lost their mothers at an early age, Darwin in 1817 and Lincoln in 1818. In 1831 Lincoln began life on his own as Darwin set sail on the Beagle , also leaving his former family life behind. Darwin published his first paper in 1836, a rite of passage for any academic, as Lincoln paralleled such an achievement by attaining his license to practice law. The following year, Lincoln moved to Springfield, Illinois, and presaged his future career with his first public statement on slavery; Darwin moved to London and began his all-important notebook on the transmutation of species, inscribing thoughts that soon led to the concept of evolution through natural selection.     The year 1858 was particularly significant for both Darwin and Lincoln. Darwin announced the theory of evolution through natural selection, beginning a debate on the nature of our biological origins that has lasted to this day. The following month, Abraham Lincoln held the first of his famous debates with Stephen Douglas on the issue of human slavery, changing forever the course of political events and human race relations.     From 1858, the lives of Abraham Lincoln and Charles Darwin diverged. Lincoln became the president of the United States in 1861, fought for the preservation of the Union and the emancipation of slaves, and briefly knew victory before his life was cut tragically short by an assassin in 1865. Darwin, on the other hand, lived a fruitful and longer but not healthful life, publishing significant works until his death in 1882. Both did die for their convictions: Lincoln for being perceived as a tyrant by John Wilkes Booth, and Darwin, so it is said, from a slow nervous illness brought on by his dedication to an unpopular view of life. However one looks at it, none can deny that besides the coincidence of their birth, the lives of Darwin and Lincoln contained many significant parallels. Or can it be denied?     One way to test for the possible significance of coincidence, such as that of Darwin and Lincoln, is to look for a common cause. Certainly the social influences of the nineteenth century shaped the courses of their lives. The circumstance of one human enslaving another went across the grain of Lincoln's thought, as it did Darwin's, and catalyzed a heartfelt and eventually effective reaction. But Lincoln had to work against established opinion; so the immediate cause of Lincoln's approach to life was not clear. Likewise, in the England Darwin knew, the works of geologists and economists influenced Darwin's interpretations of the living and fossilized life forms he so keenly observed. But his deductions of evolution through natural selection flew in the face of ensconced Victorian thought. So whereas they were influenced strongly by their times, both Lincoln and Darwin saw through the indoctrinated views of life and strove toward new light. Perhaps their strong-willed independence was more than mere coincidence, with a very specific and principled cause.     For a possible explanation of coincidence, one can turn, as people often do, to the influence of the stars and planets. As an amateur but enchanted stargazer, I marvel that my computer can take me to any place and any time and exhibit the sky in its full glory on a small screen above my keyboard. According to the computer program, the sky on February 12 was a sight to behold in 1809, from England or Kentucky. On the day on which Lincoln and Darwin joined the ranks of the living, Mercury, the messenger of the gods, was in their zodiac birth sign of Aquarius. Maybe this foretold the impact their messages would have. Indeed, Mars, the god of war, was in the constellation Virgo, the virgin; astrology is neither my forte nor interest, but one could interpret the sign in terms of the battles both men were to fight upon breaking fresh new ground. I have no idea of the possible significance of Uranus abutting Libra on that particular night, but certainly some astrologer could enlighten us as to the mystic significance of the event.     Such astronomical observances, or astrological fantasies I suspect, reach much too far for an explanation. The point is that one finds coincidence when one looks for it, and the human mind constantly searches for just such correlations. This habit of searching for patterns is ingrained in our nature for good reason--our curiosity carved the path of our evolution. But our ruminations may not be of any significance; the coincidences may be real, but they are not necessarily important. One must neither abandon principled rules of causation nor ignore the viewpoint that comes from greater circumspection. To put it into perspective, by some very rough calculations I estimate that around ninety thousand people were born on February 12, 1809. Despite the same planetary influences across the globe, only two, Charles Darwin and Abraham Lincoln, led lives that still resonate consciously in our thoughts today.     The critique of coincidence can be taken further. I chose the parallel dates of Darwin's and Lincoln's significant life events at the expense of other, possibly less interesting, noncoincidental occurrences. Poor Lincoln was rejected in his 1836 proposal of marriage to Mary Owen; Darwin married and had a first child in 1840, beating Lincoln to betrothal by well over two years and to fatherhood by three. Such discrepancies are easily overlooked in a well-played game of coincidence-seeking.     But a game it is. My entirely subjective analysis of Darwin and Lincoln is reminiscent of an intellectual game imagined by Hermann Hesse in his classic 1943 novel, The Glass Bead Game . As Hesse wrote: "Then there were the tempestuous letters from abstruse experimenters who could arrive at the most astounding conclusions from, say, a comparison of the horoscope of Goethe and Spinoza; such letters often included pretty and seemingly enlightening geometric drawings in several colors." Both scientists and pseudoscientists alike fall into the same playful habits of searching for and finding coincidence. It is, after all, good intellectual fun.     Coincidence, be it by cause or caprice, is part of all lives. Interpretations of coincidence can be fraught with error, but they are worth finding when a true, verifiable causal agent can be established. The distant past, heralded by the fossil record, is as subject to the dangers inherent in the overinterpretation of coincidence as it is to the rewards of discovering true causal agents of coincidental change. We may find from the sparse fossil evidence that one species arises as another declines, or even that evolutionary changes among many species appear to have coincided. A good scientific mind is then inclined to propose that such apparently simultaneous events had a common cause. After all, cause and effect in evolution is one of the processes we want to grasp. The job of a scientist is then to test the hypothesis of cause and effect by looking at the fossil record more closely and circumspectly. As it turns out, discerning the difference between mere coincidence and important causation is not only difficult but is severely hampered by the fragmentary nature of the fossil record.     The fossil record we extract from the ground is a notoriously incomplete sampling of past life. Fossilization and preservation of any individual animal is largely a matter of chance, using chance in the sense already explained. If the skeletons and imprints of every living being from the past had been fossilized and preserved intact, we would be walking upon a ground so full of incredible fossils that it would be an embarrassment of riches for even the most ambitious and assiduous paleontologists. But most plants and animals die and disintegrate without leaving a recognizable trace for posterity. Once the remains of a life are preserved, subsequent discovery of the resultant fossils is also largely a product of chance, as we shall see.     Our biographical information on the lives and deaths of ancestral forms is thus a mere sketch, and sometimes as lacking as for the ninety thousand or so humans born on February 12, 1809. Had the life histories of Darwin and Lincoln been subjected to the vagaries that characterize fossilization, we might not have been able to note the incredible coincidence of their simultaneous births. They would have appeared in the historical record at roughly the same time, but there would have been a lack of precision with which we could have identified their births, and the potential significance of February 12 would have been glossed over.     On the other hand, if historical records were like fossil records, births that coincided somewhat less precisely may have produced the illusion of simultaneity. Thus, Carolus Linnaeus, the Swedish botanist who founded our taxonomic system of naming and categorizing life on earth, born in 1707, would appear to have joined the human family at roughly the same time as Benjamin Franklin, the famous American statesman and philosopher who was born in 1706. Likewise, the birth in 1744 of Jean-Baptiste de Lamarck, an important French naturalist and early evolutionist, would seem to coincide with that of Thomas Jefferson, yet another American statesman of great historical significance, born but a year earlier in 1743.     One can be remarkably successful in roughly correlating the births of significant players in the history of European biological thought with those of great American statesmen. I tired of this little game after getting through only half of the alphabet, but I did happen to note that Erasmus Darwin, evolutionist and (biological) grandfather of Charles Darwin, and George Washington, statesman and (nonbiological) "father" of the United States of America, were born in 1731 and 1732 respectively.     My impatience with pursuing such historical coincidences and near coincidences stemmed from my firm belief that they hold no significance beyond the pleasures of gaming. I find no useful notions stemming from roughly simultaneous birthdays of biologists on one side of the Atlantic and statesmen on the other. Yet there may have been, and probably were, significant influences guiding each notable individual toward great thoughts and deeds. Such influences and perhaps general principles should not be lost to academic inquiry. Indeed, it is our scientific duty to test coincidences for possible meaning.     Just as it is easy to go astray with an imprecise or forced history, so it is with the fossil record. Paleontologists and evolutionary biologists must be wary of coincidences that are not real, such as close but not quite simultaneous origins or extinctions of different species. The simultaneous origin of a swifter impala with that of a more nimble cat to pursue it may be a clue of great evolutionary significance. Or it may be nothing but mere coincidence. Discerning the difference between mere coincidence and those events which reflect true cause and effect is one of the greatest challenges facing evolutionary biologists, and particularly paleontologists. Where true coincidences occur they must not be ignored; correlations between the expansion of the ancestral human brain and the appearance of stone tools, or even between changes in the brain and other parts of the body, could be important. The very essence of human success may have culminated from a fortunate set of significant coincidences. Scientists are constantly searching for clues to such coincidental events of true evolutionary significance.     Chaos In an incomplete fossil record, a cause of coincidental effects (or of any effect at all) may or may not be perceived. This can get frustrating. What if the cause, or at least a partial cause, of a significant evolutionary event was something as seemingly insignificant as a butterfly's passing by? A large carnivorous cat, destined for a successful kill of a complacent impala grazing upwind, may snap fruitlessly at a passing butterfly (as our pet cats and dogs often do). The action alarms the nearby animals, and the cat thus loses the chance to find substantive nutrition in the now escaped herd of impalas. For the lack of nutrition, the carnivore dies. Its potential offspring are never born, and grandcubs become an impossibility. Eventually another species is lost, not solely to caprice but specifically because of a passing butterfly. To an evolutionary biologist, as to a mathematician, such a sequence of events represents chaos . And it happens time and time again.     To understand chaos in the context of evolution, we must first define what chaos is, and fit the concept in with chance and coincidence. Chaos has many implications. To most people it is the opposite of order. Chaos may mean anarchy to a politician or unpredictability to a mathematician. It is often a cause for despair. But do not despair, for chaos is one of the reasons you are here. Indeed, in Greek mythology, Chaos is the yawning void from which all things come. Chaos can lead to order, if the chaotic system chances upon a lucky coincidence. But we must back up a bit to understand such nuances.     Unpredictability is at the heart of chaos theory, a branch of mathematics. Chaotic systems can be represented by the weather, which is notoriously unpredictable, or evolution through natural selection. Both systems rely upon very specific principles, such as the exact temperature and relative humidity that produce a rain cloud or the precise genes and environmental interactions that make up the lives of evolving animals subjected to natural selection. But there are many of these principles acting at once.     Chaos is derived from precise principles, exacting causes and effects. If precisely the same conditions start a chaotic system in motion, the result will be the same every time. That is the point: one must start with precisely the same conditions down to the most minute details. For the same storm to ensue more than once, all the weather conditions to the finest degree of temperature and the right accumulation of moisture in the air must exist. For the same species to evolve, every link in the chain must be precisely the same, down to the level of the genes of individuals in an initial population.     Chaotic systems are extremely sensitive to initial conditions. At a very fine level of analysis, chaos explains chance. If you were to go to a casino and roll a pair of dice from your hand, the result would appear to be the result of chance. But just imagine if you could control the roll down to the finest detail. With the dice accurately positioned in your hands, an exacting back swing of the forearm, a perfectly measured thrust forward, and precise timing of release at an optimally controlled angle, you could roll the same every time. You would be rich, and the casino would go broke. But you do not have that level of control, no matter how hard you try. On the level of control at which our nerves, muscles, and senses operate, the far more precise rules governing the physics of rolling dice come together in a chaotic fashion, and unpredictable manner, to result in the appearance of chance. Chance is thus the result of chaos, not vice versa.     Let's go back to the improbability of my birth. I attributed my familial sequence of male births to chance, but at a finer level of analysis the appearance of chance was the result of chaos. A very specific sperm had to follow a particular course toward an individual egg at precisely the right time. One unmentionable change of even the slightest proportion and you would not be reading this book today. Like rolling the dice, it was a matter of physical precision that resulted in me rather than anybody else. It was chaos, governed by laws of physics and biology but resulting in something unique every time. Perhaps these laws can provide some solace to those who fear the uncontrollable lawlessness implied by chance. As the great paleontologist William K. Gregory stated in 1949, "Chance should not be contrasted or logically opposed to Law, but both are merely different aspects of one continuous reality."     Many scientists have taken up the concept or concepts of chaos and shaped a definition to suit themselves. Here the term chaos will be restricted to its currently applied meaning: chaos represents unpredictability based on sensitivity to initial conditions. Historical contingencies of even the smallest proportion may eventually have a profound effect, even if every event is controlled by precise rules and principles.     Unpredictability is an easy concept. We do not know what will happen next, even with simple principles at work. For example, my grandfather, who presumably understood the basic mechanisms of human reproduction, could not have predicted that his son would have four boys. Likewise, as we shall see, the exact, long-term course of evolution is also unpredictable. One need not impose chance events on evolution to result in unpredictability. The impact of asteroids, for instance, may have skewed the course of evolution in unpredictable directions, but its course would have been unpredictable even if no asteroids ever disrupted an evolutionary trajectory. Evolving systems undergoing natural selection are so strongly affected by the nature of the plants and animals present at the beginning of any time we consider that they are intrinsically chaotic. Evolution depends on initial conditions.     Such extreme sensitivity to initial conditions is best illustrated by a hypothetical event called the "butterfly effect." Some people collect butterflies as a hobby; I collect butterfly effects, and you can view them at your leisure in the box below. The butterfly effect was originally conceived by Edward Lorenz, a researcher of meteorology, who saw the importance of initial conditions in trying to predict the weather. Thus, as you can see from the collection, most butterfly effects involve the weather in North America being changed by one or more butterflies in either South America or Asia. But one can have much more fun with the butterfly effect.     As an evolutionist, my idea of the butterfly effect is that described earlier, where the butterfly distracts a cat from its prey, condemning the carnivorous species to extinction. One can imagine a wealth of such chaotic foibles in the system. Even with the butterfly distraction, a more tenacious cat or dog in the wild may successfully catch its prey, live to reproduce, and pass on its genes of successful gazelle-hunting and butterfly-ignoring; in turn, this may lead to the demise of its prey, its food source, and ultimately but belatedly to the extinction of the carnivorous beast itself. That damned butterfly can have just about any effect, or lack thereof, and thus makes long-term prediction impossible.     More relevant to our subject of study, a single lost child who stumbled into a dark watery grave changed the course of science forever, nearly 3 million years after his premature death. The chance fossilization of the child's skull set the initial conditions for many of our concepts of human evolution. Indeed, butterfly effects are relevant not only to the evolution of life but also to the development of our interpretations of how life evolved.     Like coincidences--and the fossilization of a child's skull is nothing more than a chance coincidence--initial conditions and their unpredictably chaotic effects pose a problem to scientists who deal with the past. Sometimes when I talk about chaos and explain how it could have shaped human evolution, my colleagues toss their arms up in despair and ask why it is worth studying anything in evolution if some stupid butterfly could be the key to understanding everything. It gets worse when I include the role of chance and add insult to injury by pointing out that much of what we see in the fossil record is mere coincidence. But there is more to understanding chaos. And there is more to understanding evolution.     One need not despair at all. It is simply a matter of asking the right scientific questions. Although the initial conditions may help lead to the evolution or extinction of an animal, there still must be basic principles at work, and such principles are what scientists are after. Not everything can be attributed to the whims of a butterfly. The butterfly effect is simply like that of planting a seed. Sometimes the seed will grow into a tree, later to be used to build a house or to fuel a fire, whereas other times the seed will not germinate at all. At work are basic biological principles of plant reproduction and growth, and if a tree grows to be utilized, basic principles of architecture or combustion are still key considerations.     Trends in the evolution of animals also emerge from simple principles. Species origins and extinctions follow intriguing patterns, despite their chaotic unpredictability. Basic Darwinian principles such as natural selection, as well as ecological norms involving food supply and demand, shape and drive evolution. It is worth understanding those principles and how they apply to the evolution of a species such as our own. Yet there is also value in understanding evolution in the context of chaos theory, for as we shall see, chaos develops when the various forces of evolution combine. Furthermore, the initial conditions of the human form played a large role in determining the eventual outcome we know as Homo sapiens .     But if the evolution of life, like the course of an Atlantic hurricane, is shaped by basic principles and natural laws, why are things so unpredictable? Why do we perceive chaos and allow the thoughtless whims of butterflies into our equations? The answer takes us to the very heart of chaos theory.     Imagine a simple case of cause and effect: a leopard catches an impala; the impala dies. Admirers of the impala may cringe at the thought of the horribly violent death of this graceful antelope, but such is the nature of life in the wild. With a greater number of hungry leopards stalking the land, more impalas will meet their gruesome deaths. Fine, a simple principle. A mathematician would consider it to be a simple linear relationship--one could draw a straight line on a graph to represent the proportionate increase in leopards and decrease among impalas.     The equation of life, however, is not so simple and linear. Without enough food, in the form of impalas, the population of leopards may suffer. In nature, unlike capitalistic economic systems, the increase in demand does not ultimately result in an increased production of supply. Quite the opposite: increased demand almost always results in a decreased supply. So, based solely upon the simple principles espoused so far, the leopard population may decline due to lack of food as they deplete their food source of impalas.     What goes around comes around, or so it is said. And so it goes with the leopard and the impala. Fewer cats mean less threat to the impalas. Impala populations then rebound and fill the African landscape once again. This serves to restock the meat counter for the cats; they thrive upon the feast, and the impalas decline one more time. On simple principles, such a cycle of rise and fall can continue indefinitely. This simple yet ubiquitous ecological principle was noted by Alfred J. Lotka, a physical chemist, and his Italian counterpart, Vito Volterra, who formulated a mathematical model of the fluctuating relationships between predator and prey.     The laws of supply and demand, as envisioned in a simple Lotka-Volterra model, lead to a seeming balance of nature, albeit a cyclic balance. These cycles of population fluctuations do not follow a straight line but go up and down, depending on the densities of each population. They are nonlinear , as are chaotic systems. But if we add just one more principle, the picture becomes a bit more complex and the balance becomes a touch more chaotic.     A second principle could be this: a lion (rather than a leopard) catches an impala; the impala dies. And now two similar principles, those of leopard and lion hunting, both affect the fate of the impala. This is just the beginning of the chaos that is about to ensue for all the parties concerned.     One could complicate this dynamic model of life further, and an avid ecologist certainly would, to build an unpredictable chaotic system. Leopards, at least in southern Africa today, seem to have a strong preference for impalas, despite their fairly catholic dietary tastes. Lions, who ravenously consume everything from insects to buffalo, tend to opt for larger prey such as giraffes. Although this complicates our simple model of impala supply and demand, given the range of other options available, the impala situation would tend to affect the leopard more than the lion. The predictability of the system thus becomes considerably more complex and nonlinear, with population densities of each animal being dependent on the densities of all others.     Mathematical models of similar ecological relations have been constructed, and equations run for generation after generation in the simple mind of a computer. What happens? Well, it depends. Even a simple Lotka-Volterra model, as mimicked on a computer, can have many consequences. One can totally eliminate the element of chance with a strictly unwavering equation for the number of leopards, lions, and impalas, and still a plethora of scenarios may unfold--depending upon the initial numbers of each kind of animal. Throughout the generations, general oscillating trends may develop, and the system could go on forever. Alternatively, as we learn from both computers and real life, eventually the system may collapse, with one or more species going extinct. Such is the nature of chaos: unpredictability based upon the nature of initial conditions.     The true marvel of chaos, however, is that it can lead to ordered ecological systems and complex beings such as ourselves. Not just a few but thousands upon thousands of rigid ecological and evolutionary principles, perhaps complicated by chance and coincidental events, result in the balance of nature, or at least what we perceive to be a fairly stable situation. A chaotic system converges on a pattern known in mathematics as an attractor , and it can stay in that pattern for some time. Chaos then becomes an irony in and of itself, as the resultant system appears to be quite ordered. But the "balance of nature," if one wants to call it that, is a dynamic and nonlinear balance that can be frighteningly tenuous. We cannot predict the future behavior of the mathematical attractor, and it remains to be seen if we can predict what will happen next in nature. HUMAN EVOLUTION has been the product of many forces that together made us neither inevitable nor probable. The links of the human evolutionary chain were riddled with chance, coincidence, and chaos, and we cannot fit the links together without a full appreciation of these factors. We can explore the initial conditions of our evolutionary past through the fossil record, and we can test for principles of what made us human. We may learn how the capricious forces of nature acted constructively in driving our evolution, and how they played integral roles in the Darwinian forces of natural selection. Science can find the mischievous factors, dissect their component parts, and dare to make predictions.

Table of Contents

Prefacep. ix
1 Chance, Coincidence, and Chaosp. 1
2 Between a Rock and a Hard Placep. 19
3 A Tale of Two Sitesp. 34
4 Speeding Up the Pace of Evolutionp. 73
5 Rebels Without a Causep. 112
6 The Mother of Inventionp. 131
7 "You Can't Always Get What You Want ..."p. 168
8 Autocatalysisp. 201
9 The Beginning and the End of Evolutionp. 229
Notesp. 257
Literature Citedp. 263
Indexp. 269