The Tree

by Colin Tudge

  But Darwin argued differently (which was one of the ways in which he irritated the theologians and the clerically inclined naturalists of his day). He suggested that evolution was merely opportunist; that each generation simply tried to solve its own problems as best it could; that as lineages of creatures unfolded (and evolution literally means “unfolding”) they might wander off in any direction. Thus a bear might evolve into a whale. The descendants of some of the apes that lived ten million years ago in the Miocene did become us—but it would have been impossible to know at the time which ones were going to do so; and with the flip of a coin the creatures that in fact were our ancestors might instead have become more apelike, or simply gone extinct (which is the fate of most lineages). Many late-twentieth-century biologists, not least the eloquent Harvard professor and writer Stephen Jay Gould, saw evolution as a “random walk.” Lineages of creatures over time, he argued, go every which way. There is no pattern to it; and there can therefore be no prescription, and nothing resembling destiny.

  Tree ferns once abounded. Some, like this Dicksonia, are still with us.

  Hard-nosed biology is at present more fashionable than theology, so notions of random walk now prevail over those of destiny. But fashion is a poor guide to truth. One stunning and undeniable fact of evolution is the phenomenon of convergence: the way in which lineage after lineage of creatures have independently reinvented the same body forms and, often, the same kind of behavior. There may be no literal prescription for how life should turn out—but any two creatures in the same kind of environment tend to evolve along much the same lines. Sharks, bony fishes, ichthyosaurs, and whales all independently reinvented the general form of the fish (and so, for good measure, did penguins and seals). Water poses its own particular problems, to which there is one optimal solution, which they all adopt. Among plants, lineage after lineage has independently reinvented the form of the tree. A tree, after all, is a good thing to be.

  So nature may not be literally prescriptive. But it is not random either. Living creatures are in perpetual dialogue with all that surrounds them—with the other creatures they encounter minute by minute, and with climate and landscape, which means they are in perpetual dialogue with the whole world, which in turn is subject to the influence of the whole universe. Whatever other creatures may do, however the world changes, each individual must take everything else into account. Each of us is engaged in this dialogue with other creatures and with the universe at large from conception to the grave. Furthermore, what applies to individuals also applies to whole lineages of creatures, as they evolve over time: all lineages of living creatures, whether of oaks or dogs or human beings, are engaged in this dialogue from inception to extinction. All creatures might in principle be able to evolve in an infinite number of ways as Darwin suggested, but if they are to survive along the way then each must solve the particular problems of its own environment at all times—and to each problem there is a limited number of solutions. There is something about the universe, at least as it is manifest on earth, that seemed to demand the emergence of fish and of trees (and perhaps—who knows?—of human intelligence). The physicist David Bohm spoke of the “implicate order” of the universe. Fish, like trees (and human intelligence) reflect this innate, implicit orderliness. They are its manifestation.

  What follows is an outline of what’s known about the historical (evolutionary) events that led to modern trees. I will discuss it as a series of what I will call “transformations.”

  TRANSFORMATION 1: LIFE

  The first transformation on the path to treedom was the evolution of life on earth—probably more than 3.5 billion years ago (the earth itself seems to have begun about 4.5 billion years ago). So how did life begin?

  In modern body cells, whether in people or in trees, the genes, in the form of DNA, sit in the middle, ensconced within the nucleus, like the chief executive in his office. They give out orders, which are relayed by RNA (a smaller molecule akin to DNA) to the rest of the cell outside the nucleus (the cytoplasm), where these orders are carried out. Accordingly, DNA and RNA are often taken as the starting point of life itself—as if there is not, and never could have been, anything that could lay claim to life before DNA and RNA came on the scene.

  Look closer, however, and we see that the flow of information within the cell is two-way: the genes themselves (the DNA) are turned on and off by signals from the cytoplasm, which in turn relays messages from the world at large. In short, DNA is in dialogue with cytoplasm, with all its intricate chemistry. Even at its most fundamental level, life is innately dialectic.

  It follows from all this that life could not have begun with DNA. DNA cannot survive by itself; it cannot function at all except in dialogue with cytoplasm and all that goes on in it. Furthermore, the DNA molecule is itself extremely intricate and highly evolved. It could not have been the first on the scene. RNA is simpler and can make a better fist of independent living—but RNA, too, is a highly evolved molecule. DNA and RNA were not the prime movers, therefore. We might as soon say that by the time these two aristocrats had come on board, the hard work had already been done. At least, the absolute beginnings had been left far behind.

  In truth, the essence of life is metabolism—the interplay of different molecules to form a series of self-renewing chemical feedback loops that go around and around and around. And they do this simply because, chemistry being what it is, such a modus operandi is chemically possible, and what is possible sometimes happens. The first life, so it is widely argued, was simply a metabolizing slime that spread over the surface of the earth, which in those early days was a very different place from now. Indeed, it was a nightmare world, at least by our standards: hot, steamy, volcanic, with an atmosphere absolutely devoid of oxygen and probably full of gases such as ammonia and hydrogen cyanide that would snuff out almost all life of the kind we know today in a trice. The hot springs of present-day Yellowstone, New Zealand, and Iceland and the perpetually outgassing vents in the depths of the great oceans give some idea of what that early world was like. An extraordinary variety of creatures live within today’s hot springs—most of which would be poisoned by oxygen if they were ever exposed to it. Anthropocentrically, we think of ourselves as “normal” and call the creatures of the hot springs “thermophiles”—heat lovers. But historically speaking, they are the normal ones. We— human beings and dogs and oak trees—are the highly evolved anomalies: the cold-loving “aerobes,” utterly dependent on the hyperreactive gas—oxygen—that would have laid our earliest ancestors flat.

  TRANSFORMATION 2: ORGANISMS

  Life today is not a continuous slime. For at least three billion years the substance of life has been divided into discrete (or fairly discrete) units, each known as an “organism.” Of course, we don’t know how, in practice, this separation came about—and never can, until someone builds a time machine. But we can speculate.

  For natural selection would have been at work within the original slime, just as it is today and always has been. Inevitably, some bits of the slime would have metabolized more efficiently than others. Some of the endlessly cycling chemical feedback loops would have harnessed energy and processed raw materials more rapidly than others. The bits that worked best would have been held back by the bits that worked less well. Natural selection would surely have favored the bits that were not only more efficient but also cut themselves free from the rest, surrounding themselves with membranes to monitor and filter all inputs from the world at large.

  So the first organisms came about: the first discrete creatures. After a time (probably a long time) these primordial creatures developed the general kind of structure that is still seen in present-day bacteria and archaea (pronounced ar-key-uh—creatures with a similar general form to bacteria which in fact have a quite different chemistry). We tend to say that bacteria are “simple,” not least because they are small. In truth, of course, nature is far more wondrous than anything we could cook up and bacteria in reality are more comple
x than battleships, and a great deal more versatile.

  TRANSFORMATION 3: MODERN-STYLE CELLS

  Compared with us (or indeed with mushrooms or seaweeds or flowering plants), bacteria are simple. In particular, they keep their DNA loosely packaged, hanging around the cell. In our own body cells (and those of mushrooms, seaweeds, and flowering plants) the DNA is neatly contained and cosseted within a discrete nucleus, cocooned in its discriminating membrane. Cells of this modern kind are said to be “eukaryotic” (Greek for “good kernel”). The nucleus is surrounded by cytoplasm, and within the cytoplasm there is a series of bodies known as “organelles” that carry out the essential functions of the cell. Among these organelles are “mitochondria,” which contain the enzymes responsible for much of the cell’s respiration (the generation of energy). These are found in all eukaryotic cells (apart from a few weird single-celled organisms that live as parasites, but they belong in another book). Plant and other green cells contain a unique kind of organelle known as the “chloroplast.” This contains the green pigment chlorophyll, which mediates the process of photosynthesis.

  I am treating all this in some detail because herein lies a tale of immense importance, which is crucial to all ecology, and is discussed again in Chapter 13. For the eukaryotic cell evolved as a coalition of bacteria and archaea. Broadly speaking, the cytoplasm seems to have originated as an archae. Either this ancient archae then engulfed some of the bacteria around it or the bacteria invaded it—or both. In any case, some of those engulfed or invading bacteria became permanent residents—and evolved into the present-day organelles. Mitochondria and chloroplasts both contain DNA of their own. The DNA of mitochondria most closely resembles that of present-day bacteria of the kind known as proteobacteria. The DNA of chloroplasts resembles that of the bacteria that still manifest as cyanobacteria (in the past erroneously called “blue-green algae”). Cyanobacteria, not plants, were the inventors of photosynthesis.

  In his notion of evolution by means of natural selection, Darwin emphasized the role of competition. Soon after Darwin published The Origin of Species, the philosopher and polymath Herbert Spencer summarized natural selection as “the survival of the fittest,” which was taken by post-Darwinians to imply that evolution proceeds by the stronger treading on the weaker. Two decades before Darwin, Lord Tennyson wrote of “nature red in tooth and claw”; and “Darwinism,” extended backward to embrace Tennyson and forward to Spencer, is commonly perceived these days as an exercise in the strong bashing the weak. But Darwin stressed, too, that we also see collaboration in nature; he made a particular study of the long-tongued moths that alone are able to pollinate deep-flowered orchids: two entirely different creatures, absolutely dependent on each other.

  Yet we see a far more spectacular illustration of nature’s collaborativeness within the fabric of the eukaryotic cell itself—the very structures of which we ourselves are compounded. For the eukaryotic cell is a coalition. It was formed initially by a combination of several different bacteria and archaea that hitherto had led separate lives (and others are probably involved, besides the proteobacteria and cyanobacteria). Over the past two billion or so years the eukaryotic cell, innately cooperative, has proved to be one of nature’s most successful and versatile creations. There could be no clearer demonstration that cooperation is at least as much a part of nature’s order as is competition. They are two sides of a coin.

  The ancestors of today’s plants arose from the ranks of the general melee of eukaryotic cells. These first ancestors contained chloroplasts and were green, and these can properly be called “green algae.” Many single-celled green algae are still with us (they often turn ponds bright green).

  It’s a reasonable guess that the first green algae appeared on earth about a billion years ago. Thus it took about 2.5 billion years to get from the first living things to single-celled green algae, and only another one billion to get from single-celled algae to oaks and monkey puzzles. Still, we tend to think of algae as “simple” and primitive. If we took the long view and considered all that life entails, we might rather argue that by the time the first green algae evolved, it was all over bar the shouting. (Though there was still an awful lot of shouting to be done.)

  TRANSFORMATION 4: ORGANISMS WITH MANY CELLS

  Organisms that have only one cell are doomed to be small. There are many advantages in smallness: there is more room for small organisms than for big ones, and a virtual infinity of niches to exploit. Single-celled organisms are easily the most numerous and always have been—living free wherever there is moisture, in oceans and lakes and soil, as inhabitants of bigger creatures’ guts, and as parasites of bigger creatures.

  But there are advantages in being big, too. A whole range of ways of life are open to big creatures, whether trees or people, that small ones cannot aspire to. To become large, organisms must become “multicellular.” Creatures like oak trees and us have trillions of body cells.

  Multicellular organisms must originally have arisen from single-celled organisms. At its simplest, a multicellular “organism” is little more than a collection of cells that have divided, but failed to separate. The real transition comes about when the different cells in the bunch begin to take on specialist functions—some producing gametes, some not; some photosynthesizing, some not; and so on. Then we see real division of labor, and real teamwork. Then you have what the great English biologist John Maynard Smith was wont to call a “proper” organism, with each cell dependent on all the rest, and groups of cells cooperating to form organs, such as lungs and livers or leaves and flowers. This degree of collaboration requires enormous self-sacrifice: to be a member of a bona fide organism, each cell must give up some of its own ability to live by itself. Each cell has to trust the others, so to speak. Any cell in the organism that goes berserk and tries simply to do its own thing destroys the whole, and ultimately destroys itself. In medical circles, such cells are said to be cancerous.

  In fact, there is a spectrum of compromise positions between cells that can live perfectly well by themselves (as single-celled organisms) and cells that are utterly dependent on those around them (like human brain cells). Thus many cells from many organisms (including many of ours) can be grown indefinitely in special cultures. Many cells from many plants, once cultured, can then be coaxed to develop into whole new organisms. Indeed, many plants (including many of the most valued trees, such as coconuts and teak) are now cloned by cell culture. On the whole, though, the generalization applies. True multicellularity is possible only because the individual cells give up their autonomy, each relying on the rest for its survival and for the replication of its genes.

  TRANSFORMATION 5: PLANTS COME ONTO LAND

  The first plants that can loosely be called “algae” ventured onto land around 450 million years ago. On land they faced, for the first time, the problems of gravity and desiccation. Some of the earliest algal pioneers evolved into mosses, liverworts, and hornworts, known collectively as “bryophytes.”

  None of the bryophytes has ever come properly to terms with the special difficulties posed by life on land. They duck the issue of gravity by staying squat and small, and hence extremely lightweight. They never solved the desiccation problem. They remain confined to damp places—but because there are plenty of damp places, they are extremely successful. Mosses in particular abound on damp walls and rocks just about everywhere. They are a huge presence in forests, as epiphytes. Some, particularly the sphagnum, or peat, mosses, form vast swards in the wet tundra and tend to prevent other plants from growing there. Mosses in general overcome desiccation not by resisting it, as a leathery-leaved holly tree or a spongy, water-packed baobab will do, but by putting up with it. They can be dried to a virtual crisp and yet spring back to life.

  An aside is called for on the reproduction of mosses—for they illustrate one of the fundamental phenomena of botany, and without some inkling of it, we cannot properly understand the reproduction of the plants that mainly concern us in this b
ook: the conifers and flowering plants. The phenomenon is known as “alternation of generations.” The moss that is a permanent presence on walls and tree trunks is called the “gametophyte generation” because it produces eggs and sperm (gametes), which fuse to produce embryos, which grow into the “sporophyte generation.” (It is odd to think of plants producing eggs and sperm, but that is what the primitive types do.) The sporophytes appear among the general background of “leafy” moss as little upright structures that commonly resemble tiny lampposts—the “lamps” at the top contain spores. Spores are little more than packets of unspecialized cells, encased in some protective coating. They are dispersed by various means (not least by water), and if they land in some comfortably damp spot, they multiply and differentiate to produce new mosses of the gametophyte type. The sporophytes, which produce the spores, cannot live independently. They depend entirely upon the gametophyte.

  Thus the gametophyte practices sexual reproduction, while the sporophyte practices asexual reproduction. Both ways of reproducing have their advantages and drawbacks—and plants practice both, in alternate generations. In this they are ahead of us. We (together with most but not all large animals) reproduce only by sex.

  Bryophytes could never have given rise to trees. Their overall body structure is too simple. They have no proper roots, merely anchoring themselves by projecting “rhizoids,” which have no special role in absorbing nutrients and water. Most mosses look as if they have leaves, but they are not true leaves, just green scales. Most important, bryophytes have no proper, specialist conducting tissue within them, to fast-track water and nutrients from one part of the plant to another (or at best they have very rudimentary conducting tissue). Lacking specialist plumbing, they are bound to remain small.