Creatures of Accident: The Rise of the Animal Kingdom

CREATURES OF ACCIDENT (Chapter One)Hand Luggage Only\ Human thinking about the nature of life has been constrained by dogmatic ideological stances, not just religious and political ones, but also adherence to the scientific orthodoxy of the day. Such "philosophical baggage" needs to be jettisoned at the outset of our journey so that we open our minds to the maximum range of possibilities. Science should be all about open-mindedness and questioning. It is an honest search for the truth about the nature of "life, the universe and everything," to use that famous phrase introduced by Douglas Adams in his book The Hitchhiker's Guide to the Galaxy.\ THIS book, like the animal kingdom, had an accidental origin. The seed from which it eventually germinated was sown by a newspaper article I read a few years ago. The retention of this seed of information for several years is a bit odd, because newspapers, unlike books, are ephemeral things. If you have already read today's paper, how many articles can you remember? What about yesterday's? Or last week's? We pass our time on trains and planes perusing the papers, but we don't commit much to memory—or at least to the long-term version of that everyday miracle.\ However, occasionally something sticks. The article whose message stuck with me was in the color supplement of one of the Sunday papers. It was about evolution. A particular sentence is all that I retain, and doubtless in imperfect form. But, to an approximation, here it is: "Complex creatures, like humans, are a mere epiphenomenon in the history of Life."\ What did its author mean by describing us as an "epiphenomenon," that is, a sort of blip on the periphery of something altogether more important? What was the deep philosophical point he was trying to impart? Well, his point was something like this. Since life began about four billion years ago, the vast majority of the creatures that have lived out their lives in every corner of our planetary home have been bacteria. This in itself is hardly surprising, because bacteria are very small, and small creatures tend to be much more numerous than large ones. You and I are both individual humans, but we are also both roving vehicles transporting millions of bacteria from place to place—most of them in our guts and on our skin.\ But the point goes further. Not only are there far more bacteria than animals or plants; there are also more different types of bacteria. In other words, species. We all recognize different species of animals, whether very different, like humans and houseflies, or only slightly different, like horses and donkeys. In contrast, different species of bacteria require more than the naked eye to see, let alone distinguish. Indeed, the whole concept of a species—bounded by its members' inability to breed with other than same-species partners—is hard to apply in the bacterial realm, where reproduction is hardly sex as we know it.\ Such difficulties aside, it is probably true that both now and at all other points in evolutionary time the living world has been dominated by bacteria, both in numbers of individual creatures and in numbers of types of creatures. This fact is at odds with a curious human practice—naming particular periods of the earth's history after a particular kind of animal, as in the Age of Fishes. Such names have a rationale but also serve to mislead. They are normally used to refer to a type of animal that diversified rapidly in the period concerned and consequently contributed much to its fossil record. But unlike fish, most bacteria have no hard parts and rarely fossilize. So fossil frequencies are a poor guide to the dominant creatures of the past. In reality, the whole of evolutionary time could be labeled the Age of Bacteria.\ Under this view of life, we humans and our animal allies are indeed a "mere epiphenomenon," or, in more graphic terms, a tiny molehill on a vast bacterial lawn. A molehill that may one day disappear, whether by self-inflicted nuclear radiation or other means, leaving our simpler but more robust progenitors to go about their bacterial business in a molehill-free manner, as they were doing three billion years ago.\ I'm going to call this lawn-with-molehills perspective the left-wing view of life. The reason for this label is that, in this perspective, attention is focused on diversity—that is, creatures being merely different from each other within a level of complexity—rather than on complexity, and on evolutionary increases in this over time. There is a reluctance to think of any local corner of this variety as being better or higher in some sense than another. So it is an egalitarian view of life. We life-forms are all comrades in our struggle for existence, whether we are bacteria, beetles, blue whales, or bus drivers.\ So what is the right-wing view? This is where we go from lawn to ladder. A long time ago in Germany—about two centuries back—a group of philosophically minded folk interested in the nature of life came up with the idea that all creatures could be arranged in a line. And the line was a vertical one: it was effectively a sequence of increasing complexity and sophistication. Microbes at the bottom, humans at the top. This is something of a caricature, but it captures these philosophers' fundamental point.\ This vertical line needs a name. It is often referred to by the Latin scala naturae, meaning, as you might expect, "natural scale." Whatever we call it, any overall view of life based on it is fundamentally at odds with the lawn-with-molehills view. If our central metaphor shifts from horizontal to vertical, we change our focus from diversity to complexity. From humans as molehill to pinnacle. From bacteria as all-dominant life-forms to lowly dwellers on the bottom rung.\ Now, each of these views captures an element of truth. It is always unwise to regard thinkers of the past as ignorant or deluded. The further back we go, the less the information at the disposal of the thinkers concerned. But while lack of some kinds of information constrains thinking, it does not necessarily distort it. That is, it does not prevent a rigorous analysis of whatever information was available at the time. So I am not interested in being critical of last decade's Sunday newspapers or of worthy tomes written in the eighteenth and nineteenth centuries in a language that regrettably I cannot read. Sure, my criticism is implicit, but I will not develop it. Instead, what I will develop is a Middle Way, to borrow that famous Buddhist concept, between the left-wing and the right-wing views—that is, between life's lawn and life's ladder. However, it will be a decidedly biased Middle Way—more on that in the following chapter.\ Today, the worldview of most Europeans, and many (but not most?) Americans, is sufficiently infused with evolutionary thinking that the idea of evolution in general causes an adverse response only in the creationist movement. There are now many strands of this movement, the most recent being intelligent design, or ID.\ If all that phrase implied were an intelligent Creator causing the big bang through which the universe was born some fourteen billion years ago and then standing back to let nature run its course, it would seem no great threat to an evolutionary worldview. But unfortunately it goes further. I will return to this point toward the end of the book (in Chapter 20). Most of the book is not devoted to attacking ID, but rather to building the case that ID is unnecessary by explaining, in a way that is readily accessible to a general readership, how the rise of complex creatures can be explained in terms of natural processes. I will take the view that we should all accept evolution regardless of whether we do or do not see religious implications, because of the mass of accumulated evidence in its favor, some of which we will see in subsequent chapters. But accepting evolution was not so easy in the late eighteenth century.\ We need to be careful here. What exactly was the state of human thinking 220 years ago? How uniform or varied were people's views of life? It is easy to make ill-informed statements about such times that historians of science could disprove in a single sweep of the pen. Belief in evolution did not begin with Charles Darwin and his 1859 masterpiece, On the Origin of Species. That book may indeed have been the biggest single milestone along the way from a dominance of creationist myths to the triumph of evolutionary arguments, but it was not the first. European views on evolution can be traced back at least half a century earlier, and probably further. And who knows what the Chinese were thinking about life way back centuries before Darwin's time, when they scooped Copernicus with their early ideas on a heliocentric solar system?\ But caution should not go too far. Although you and I have little insight into the philosophical conversations that took place in late-eighteenth-century Germany, we can be fairly sure that evolution was not center stage at that time. I'd guess that some folk had thought of it. Regardless of whether this is true, evolution still did not feature in the prevailing European worldview by 1800.\ This fact raises an interesting question. What was in the minds of those late-eighteenth-century proponents of the right-wing view? If life had a ladder, did creatures not climb up it? No, they emphatically did not. The ladder, which now so readily permits an evolutionary interpretation, was at the time very abstract—a pattern in the mind of God.\ Now, in post-Darwinian times, creatures most certainly are seen to climb the ladder—but only some of them. Many creatures have remained on the lawn of design simplicity ever since life began. That is, they have remained as single cells. Others have climbed ladders that led to greater complexity, in some cases to designs that involved many millions of cells. Exactly how they climbed the ladders we'll get to later, and clearly Darwinian natural selection is a part—though only that—of the answer. For now, the main thing to notice is that the ladder has been pluralized. There has not been a single escape route from the realm of the simple; rather, there have been several. Some led to greater heights than others. After all, even some bacteria have climbed a short ladder to what are called filamentous designs. These involve cells sticking together in strings rather than all existing as entirely independent creatures. And plants climbed a very different ladder from animals.\ It gets worse. Evolution is more like the child's board game Snakes and Ladders than it is like ladders alone. Once you have ascended far enough, it is possible to go down as well as up. As one line of ancestors and descendants (a lineage) is going up, another is going down. Now we get into a terminological and pictorial morass. Darwin called one of his books The Descent of Man. More recently, the Polish-British-American scientist Jacob Bronowski wrote a book, and presented a TV series, called The Ascent of Man. In biological texts, evolutionary trees are usually pictured as going up; but occasionally they are shown as going down, or even sideways. Human family trees, and diagrams of genetic crosses, are usually shown as going down, though they are mere microcosms of those bigger evolutionary trees that usually go up.\ The morass in which we now find ourselves is sometimes referred to as philosophical baggage. Every individual thinker carries into every stage of his or her thinking a set of views (good) or prejudices (bad) that have been born of previous reading, thinking, and talking. We are by nature reflective creatures, albeit some more so than others, and we cannot help forming pictures of the world as we learn about it. But these pictures, although they are essential tools in the learning process, can lodge too firmly in our minds at too early a stage and inhibit shifts to different pictures that may become appropriate as we learn more. And this problem characterizes the whole human learning endeavor at the community level as much as it does our own individual mental journeys through life. We must be eternally vigilant about the need to discard our philosophical baggage when it becomes a hindrance rather than a help. Nobody should underestimate the difficulty that lies in maintaining this constant vigil, and in using it to good effect.\ A particular kind of philosophical baggage lies at the heart of the morass of ascent and descent metaphors that we have just reached. This baggage concerns our particularly human identity crisis: our views, which tend to be strongly held because they matter so much to us, about what kind of creatures we are, how we got here, where we are going, and whether there is any ultimate purpose in our journey, save that which we make up ourselves as we travel.\ This is the worst kind of baggage because it is the most important and thus the most difficult to discard. But discard it we must, even if only temporarily, in order to reach an unprejudiced view of the nature of life, a view based on facts rather than on ideology. The first step in discarding it is to inspect it carefully so that we can see its full extent. You can't discard baggage that you don't know you're carrying.\ So back to the left-wing and right-wing views of life, or the lawn and the ladder. This time I'll take them in the opposite order, which is the order in which they historically arose.\ The right-wing view emphasizes differences rather than similarities. Moreover, it emphasizes a single dimension along which (or, more accurately, up and down which) the differences among creatures are manifest. But what is this dimension? I referred to it earlier as being complexity or sophistication. Other terms could be, and have been, used. Some creatures can be described as of a higher grade or as being more advanced than their lower or more primitive relations. It is probably reasonable to describe humans as being more advanced in some sense than flies. Is it equally reasonable to describe flies as being more advanced than snails? Probably not—they're just different. What if we replace snails with their brighter molluskan cousin the octopus? Most people would describe the octopus as more advanced than the fly, largely because of the octopus's greater capacity for learning.\ Notice, however, that learning is not the same as complexity, though they are related. It is probably true to say that no very simple animal has a large capacity for learned, as opposed to instinctive, behavior. There are no brainy flatworms. But there certainly are many animals that are structurally complex yet limited in the realm of learning. This is true, for example, of sea urchins and centipedes.\ Worms lead us to another problem. Not only are structural complexity and behavioral complexity different, but complexity of life cycle is different again. Some parasitic worms, most famously tapeworms, are structurally quite simple as adults but exceedingly complicated in terms of the life cycles through which their adult forms are reached. These life cycles can consist of several stages that bear little resemblance to each other. The difference between a caterpillar and a butterfly springs to mind here. But some parasitic worms make the insect life cycle look simple, as they have more stages than just two—or three, if we count the butterfly's chrysalis as a stage.\ The complexity of creatures should not condemn us to complexity of thinking about them. So let's now try to emerge from our linguistic confusion. Paradoxically, complexity is simple to define. A commonly used definition of the complexity of a creature is its number of different types of constituent parts. But as ever, the devil is in the detail, and in this case the detail concerns the nature of the parts. Life stages, like larvae, are parts of life cycles. Organs, like hearts, are parts of organisms—at least of some organisms at some stages of their life cycles. And, at a lower level than organs, cells are also parts of organisms.\ Different axes of complexity run in parallel to varying degrees. Creatures with a greater range of cell types will often have a greater range of internal organs, simply because, for example, it takes different types of cells to make a brain and a heart. But creatures with more organs do not necessarily also have more life stages, as we have just seen. What this means is that in a comparison between two creatures, it is sometimes possible to say in a general way that one is more complex than another. Humans, for example, are more complex (by far) than bacteria. However, we sometimes have to be more specific, and say that creature X is more complex in some ways than creature Y, but simpler in others.\ Now to the left-wing, or lawn, perspective. Here the vertical axis, whatever its precise nature, is downplayed. Instead of emphasizing that some creatures are higher than others on this axis, it emphasizes that most creatures have retained a very simple body form—the unicellular one—but have diversified within this constraint. That is, they have become very different from each other, but without any of the different forms being significantly more complex than any others.\ Given that both lawn and ladder views capture elements of the truth, how can we best picture the evolutionary process? Think of a three-dimensional space, such as a room. Let's imagine that complexity starts at the floor and heads for the ceiling. The length and breadth of the room can be thought of as two variables, each representing some aspect of being different without differing in complexity. Three billion, two billion, and perhaps even one billion years ago, our room of life was carpeted but unfurnished. Or, if you prefer not to multiply the metaphors, it had an indoor lawn. There were no complex creatures of any great height on the vertical axis. Today's world, however, is very different. The carpet (or lawn) is still there. It may even, paradoxically to anyone familiar with real carpets, be deeper-pile and have fewer holes than before. But the room is now characterized by multilevel furniture. It contains everything from footstools to bookshelves. The vertebrates, arthropods, mollusks, and others stand tall, but they don't outnumber the fibers of the bacterial carpet.\ Problems are melting away fast. A Middle Way between lawn and ladder views is not only possible but necessary. It is a more accurate view of how our biosphere came to be populated by the creatures we see around us. And in terms of representation, it doesn't matter whether we choose to look at our room the right way up, or upside down, or even lying on its side. Nor does it matter that different measures of complexity will result in different arrays of furniture—indeed this is to be expected.\ But a problem still remains. Where has time gone? We have pictured two rooms separated by at least a billion years. In such a comparison, time is represented only by the contrast between two (or more) rooms; it is not represented at all within any one of them. Recall that "up" means more complex. Therefore, although there may be a tendency to think of "up" as also meaning later, this cannot be true. If particular lineages can go up or down (or stand still) in complexity terms as time proceeds, then an axis measuring complexity cannot also be an axis of time.\ The conflation of time and some measure of complexity, advancedness, or grade, regardless of what exact form this takes, has in my view been a major contributor to unwanted philosophical baggage in much thinking about the evolutionary process. And since evolutionary trees are usually drawn on pages, posters, or screens with a mere two dimensions, as opposed to our imaginary room of life with three, the problem is even more acute. The horizontal axis becomes "degree of difference within a level of complexity," the vertical one simultaneously grade and time, thus producing the false notion that evolution is an ever-upward process.\ An important step in solving this remaining problem was made by the German biologist Willi Hennig in the mid to late twentieth century. Now, Hennig is not a household name in the way Darwin and Wallace, or Watson and Crick, are. But perhaps it should be. For the advance that Hennig made is one of those brilliant, obvious-in-retrospect ideas. He sought to separate two things: (a) the connectedness of ancestor-descendant lineages in time, together with the splits that occur in such lineages; and (b) the evolutionary modifications (in terms of changes in the structure and function of the organisms concerned) that were happening in each lineage.\ Hennig chose to concentrate on the former, so the vertical axis in his branching diagrams was time—not in an absolute sense but in the form of a temporal sequence of lineage splits. Picture a V based on one of the branch tips of another V to get an idea of one lineage divergence preceding another. His horizontal axis was only there to allow lineage splits to be depicted at all—something that can't be done if you only have a single axis (picture an I stacked on another I). The horizontal axis emphatically does not represent degree of difference, as it does in other kinds of evolutionary trees.\ If we return to imaginary rooms, it becomes clear that we could design a better type of room than the one we were considering earlier. Effectively, the way it was designed wasted a dimension. Suppose we redesign it as follows. Change the vertical axis to "time." Change the length axis to "complexity." Retain the breadth axis as a single compound variable (for surely only one is needed) to refer to "changes within a level of complexity." Any lineage is now constrained to go up, but it can meander in both directions on the other two axes. Time and complexity can increase together or move in countercurrent; or complexity can stand still as time moves on. Strangely, I have never seen such a picture of evolution in print—but given the volume of the relevant literature, it may well be out there somewhere.\ This book, like others, is a mental journey. If you and I, and our fellow travelers, are weighed down with too much philosophical baggage, such as an obsession with lawns because of the previous excesses of the ladder proponents, we will burn up most of our fuel just dragging this baggage along, and will reach a destination not far from where we started. But it needn't be like that. People cannot discard all their prejudices, whatever they may think. I don't ask the impossible of you, or indeed of myself. I don't ask that we travel naked as the day we were born. Rather, I ask for the flight to be restricted to those who can travel with (philosophical) hand luggage only. That way we at least have a fighting chance of flying to an exotic destination.\ CREATURES OF ACCIDENT Copyright 2006 by Wallace Arthur

1Hand luggage only32The biased Buddhist153Sandcastles and their children274A deafening silence405Cork prisons556Building a castle of cells677Dances with genes798Life through time899The embryo wars10110Darwin and his legacy11411Possible creatures, probable creatures12612Duplicate and diversify13613The best box of all15014From simple to complex16115From complex to even more complex17216Acquiring your head18317Crossing the threshold19418Dinosaur blues20519Beyond Pluto21620Big questions225

\ Publishers WeeklyA core tenet of the intelligent design movement is that some organisms are simply too elaborate and complicated to have evolved by chance. Arthur, a professor of zoology at the National University of Ireland, Galway, aims to render this strain of creationism unnecessary by "explaining, in a way that is accessible to a general readership, how the rise of complex creatures can be explained in terms of natural processes." Creatures of Accident makes this case through a series of easily intelligible, chatty chapters, offering a way of understanding the emergence of animals (the most complex life form) without resorting to either the relativist idea that all life is essentially the same (with animals being, as Stephen Jay Gould once put it, "a mere epiphenomenon") or the teleological view that if animals are uniquely complex, then some intelligent designer must have made them so. Drawing ideas and examples from the large (zoology) to the small (cellular biology), Arthur popularizes recent breakthroughs in the field of evolutionary development-the trendily dubbed "evo-devo"-to make the paradoxical case that complexity can, in fact, happen quite simply. (Sept.) Copyright 2006 Reed Business Information.\ \ \ \ \ Kirkus ReviewsA short, reader-friendly discourse on the accidental rise of creatures great and small-emphasis on accidental. Which is to say that invertebrate-zoologist Arthur (Zoology/National University of Ireland, Galway) tackles the issue of complexity. How do we account, he asks, for the development over eons of bigger organisms with diverse cells and organs? Before Darwin, the answer was God and nature's ladder, which had a rung for every living thing from lowest bacteria to highest human beings. Darwin went far to discredit the ladder, but it has taken modern genetics and embryology to come up with a nonreligious explanation for why evolution has produced complexity over time. This happened initially, Arthur notes, by a random mutation that allowed single cells to clump together to become multi-cellular. Then came mutations that changed simple critters' shapes from round to bilateral, which allowed for left and right halves of the body, a rear and a head end that oriented the creature forward. Over millions of years, some creatures got bigger, and their heads got packed not only with mouths for food, but with eyes and ears and noses, the better to explore the environment, and a brain to control them. Copying mistakes could account for all these changes, Arthur observes, particularly duplications of genes that create redundancies of function. Fish have four gill slits on each side but don't need that many to breathe. So in some species, the forward pair evolved to become jaws. Besides examples from nature, Arthur points to the commonality in the sets of master genes that program the development of the embryo, from fruit flies to humans. He emphasizes that evolution is messy, neithergradual nor saltatory, in prose that is always gentle and professorial-except when it comes to today's creationists and Intelligent Design advocates, whom he blasts as blatantly dishonest. Elegantly and persuasively makes the case that "evolution is part lawn, part bush, part tree, part ladder. It defies simple models."\ \