The Tree (8 page)

The snag is as follows—and, again, for simplicity I will use an example from animals rather than from plants, but the principle applies universally. Suppose you wanted to work out whether human beings were more closely related to horses or to lizards. Suppose you decided to count the number of toes—a perfectly good “character.” Then you would conclude that the human and the lizard are closer, because both have five toes. The horse, with one toe, is the odd one out. Yet everything else about horses, human beings, and lizards suggests that horses and people belong together (in the class of the mammals) and that lizards are the odd ones. This is the same kind of problem that Aristotle identified. Owen’s idea of homology is not all that helpful in this context. After all, the feet of lizards, horses, and human beings are all homologous.

One further idea is needed to sort this out, and this was pinned down formally in the 1950s by a German biologist (in fact an entomologist) called Willi Hennig. He distinguished between homologous characters that are “primitive,” and those that are “derived.” Primitive characters are those that are inherited from the very earliest ancestor of
all
the creatures in question. Thus lizards, horses, and human beings are all distant descendants of some ancient amphibian that lived about 350 million years ago—and that ancestor had five toes. For all the descendants of that amphibian ancestor, the default position is also to have five toes. But some of those descendants have lost at least some of the toes—as birds have done and so (quite separately) have horses. Horses have lost four of the five toes—all except the middle one. So too have asses and zebras. The point is that horses, asses, and zebras all inherited their one-toedness from the same ancestor, the first ever one-toed equine, who lived somewhat more than five million years ago. Although human beings have many derived features—including enormous brains—we happen to have retained the five-toed limbs of the first amphibian ancestor—the primitive feature. So have lizards. But the fact that lizards and people have such characters in common does not show any special relationship. However, our big brains and our forward-looking eyes are derived features, which were not present in that ancient amphibian ancestor, or indeed among the first ancestral mammals. They arose only among primates. They are among the characters that show our special, close relationship to chimpanzees.

By the same token, we can see that oaks, chestnuts, and beeches all belong together (in the family Fagaceae) because all enclose their seeds within very similar casings (the cup of the acorn, the shell of the beech and chestnut). This casing is a derived feature, one that shows their affinity. All three also, of course, have green leaves. But the leaves are primitive features, also found in magnolias and eucalypts, or indeed in pines and araucarias. The mere presence of leaves tells us nothing about the relationships of oaks, chestnuts, and beeches—beyond the fact that all three are plants.

Hennig provided a whole list of rules for deciding whether shared homologous features are primitive or derived, and his general approach is known as “cladistics,” from the word “clade,” meaning all the descendants of a common ancestor. Cladistics has become the taxonomic orthodoxy only in the past few decades. Much of the traditional classification in conventional textbooks does not incorporate Hennig’s ideas. Traditional taxonomists sometimes (quite often, in fact) treated primitive and derived features together, and so created many groupings that seem convincing—since all the creatures in the various groups do have plenty of characters in common—but, in fact, if you look closely, are actually no more convincing than a classification would be that placed humans and lizards together and excluded horses. In the chapters that describe the various groups of trees, you will find many instances of reclassification. This is partly because botanists are now revisiting old territory and distinguishing more clearly than was often done in the past between the shared, homologous characters that are derived, which denote true, close relationships, and characters that are merely primitive.

Thus taxonomy has advanced conceptually over the past few decades—and it has advanced, too, in technique. From earliest times taxonomists looked at the obvious, “gross” anatomy of creatures. From the seventeenth century onward they could refine their observations with the help of microscopes—which also helped reveal the insights provided by embryos. From the 1930s they could look even closer, with electron microscopy, and home in on microanatomy. The fossil record has grown wonderfully, too, these past few decades. Some recently discovered fossils, recovered by modern techniques, offer the same microanatomical detail as living tissue. Brilliant. Then, of course, there are DNA studies—exploring and comparing the detailed chemical structure of genes.

But all these approaches have their drawbacks. All are subject to the trap that has beset all taxonomists since Aristotle: divergence and convergence. That is, creatures that are very closely related may adapt rapidly to different circumstances and end up looking very different; and creatures that are not at all related may adapt to similar circumstances and end up looking much the same. Thus it transpires (when you look closely) that the family of oaks, beeches, and chestnuts (Fagaceae) is closely related to that of cucumbers, melons, and squashes (Cucurbitaceae): a fine case of divergence. On the other hand, as we have seen, many tropical rain-forest trees have leaves that look very similar even though they may not be closely related, simply because all are adapted to dryness on the one hand and downpours on the other: a striking example of convergence. Fossils can be wonderfully instructive—but although some fossils show fine detail, most are to some extent fragmented and the fossil record as a whole is, as the palaeontologists say, “spotty.” Only one in many millions of extinct creatures gets to be fossilized and then discovered, and whole vast groups must have gone missing. Thus everything we know suggests that flowering plants and conifers share a common ancestor, but it is very hard to find truly convincing links between the two within the fossil record, vast though it has become.

This, too, is why DNA studies do not provide the royal road to truth that was hoped for. Genes may diverge or converge just as anatomical features do, and so they can deceive. Even more to the point, different genes in the same organism may tell different stories. Thus studies in the 1980s suggested that the genes of red seaweeds were very different indeed from those of green plants, and that the two groups should be placed in different kingdoms that were miles apart on the grand Darwinian phylogenetic tree. But later studies in the 1990s, which looked at a different set of genes within red seaweeds and green plants, suggested that the two were very closely related—so closely that the two groups were, as taxonomists put the matter, “sisters.” The later studies are probably more accurate than the earlier ones—but it is always hard to be sure. Judgment and experience play as much part in modern taxonomy as they always did in the past, and there will always be disagreements about who is really related to whom. In taxonomy, as in science as a whole, there
are
no royal roads to truth. Some of the continuing debate is reflected in Chapters 5 through 10.

All these new approaches have caused taxonomists to modify Linnaeus’s original classification more than somewhat. In particular, modern taxonomists have greatly increased the number of kingdoms. Early-twentieth-century biologists decided that all single-celled creatures that are not green are “protozoa,” and put them in with the animals; and all single-celled organisms that are green they called “single-celled algae,” and put them in with the plants. Fungi and similar creatures such as slime molds were also rammed in with the plants. So were the brown seaweeds (wracks) and red seaweeds. Now it is clear that there is huge variation within the protozoa and the single-celled “algae,” so that they are now divided among about a half a dozen or a dozen different kingdoms (depending on who is doing the dividing); and some of those newly defined kingdoms contain both “protozoa” and “algae.” Fungi, the various groups of slime molds, and red and brown seaweeds now each have their own kingdoms; plants and animals are just two kingdoms among many, albeit by far the most conspicuous. Broadly speaking, all the kingdoms seem to divide into two great blocks, one including the plants (and red and green seaweeds and others) and the other containing the animals and fungi (and a lot of smaller types). In the early twentieth century, too, no one knew quite what to do with bacteria, although a few brave souls put them in a kingdom of their own. Now they are found to be so different that they are given their own “domain” (though, in truth, this is divided into two domains). The kingdoms of the plants, animals, fungi, seaweeds, and so on together form a third domain.

So now, the ranks that Linnaeus first described (species, genus, order, class, kingdom) have been increased to eight. The rankings now run: species, genus, family, order, class, phylum, kingdom, and domain (though botanists commonly substitute the term “division” for what the zoologists call class and/or phylum). Often these basic eight ranks are further subdivided or bunched together, as in “subfamily” or “superorder.” (But this can be overdone!)

All this may seem cumbersome, but it is all extremely useful. The family names make it easy to keep track—which is the most basic purpose of classification. Thus the 600 or so living species of conifer are now divided into eight families—and although 600 is too many for nonprofessionals, eight is straightforward; and if you can place conifers in their families, tell a pine from a swamp cypress, that is a lot better than nothing. The 300,000 or so species of flowering plant divide into 400 or so families—still too many for comfort; but the 400 or so are further grouped within about forty-nine orders, of which about thirty or so contain significant trees. It isn’t hard to get your head around this number (especially if you focus, for starters, on the top dozen or so and then work outward). Thus with just a small sense of modern taxonomy the whole bewildering world of trees, all 60,000 or so of them, begins to become tractable.

Then, too, the modern phylogenetic tree that includes all living creatures is, in effect, a graphic summary of their evolutionary history. If you know what group a creature belongs to, then you also know who its ancestors were, and who it is related to. We also know where at least some of the groups originated. Some began in the Southern Hemisphere—even way down in Antarctica, which once was forested. Some started life in Asia, and then spread west across Europe, and were blown as seeds across the Atlantic, and into America; or spread east across the Pacific, again into America. Some began in South America and tracked through the whole world. Most of this happened millions of years ago—long before human beings came on the scene (and stirred the pot even more). It is a wonderful thing to contemplate a living tree, or a fossil one, or any other creature. It is even more moving when we add the fourth dimension, of time, and see in our mind’s eye how the ancestor of the tree that grows in the field next door first saw the light in some remote corner of the globe millions or hundreds of millions of years in the past, and floated on its respective bit of continent as the continent itself circumnavigated the globe, and skirted around the glaciers of the ice age, and perhaps sweated it out in some primeval, long-gone swamp, with alligators around its feet and the world’s first hawks and kingfishers scouting from its branches.

This is why I have been so keen to root this book in phylogeny—in modern taxonomy. It is an aide-mémoire to be sure, but more than that, it reflects evolution; and evolution reminds us of the glorious past of all living creatures. Without it, as the Russian-American geneticist Theodosius Dobzhansky said, biology makes no sense at all. The next chapter looks at a few of the historical details: how, in practice, modern trees are thought to have come into being.

3

How Trees Became

A
TREE IS A BIG PLANT
with a stick up the middle—and a big plant with a stick up the middle is not an easy thing to be. Darwin spoke of evolution as “descent with modification,” and it took a lot of descending—several billion generations—and a great deal of modification to get from the world’s first life to the world’s first plants to the creatures we recognize as oaks and monkey puzzles and eucalypts.

This chapter runs rapidly through the key events. It isn’t meant to be philosophical, but a point of philosophy continues to absorb me nonetheless. For when all Western thought was dominated by theology, change over time (like all aspects of nature) was seen to be part of God’s plan. Late-nineteenth-century and some twentieth-century theologians and scientists who were unhappy with Darwin’s particular idea that human beings descended from apes consoled themselves with the thought that those early apes were destined to become us; that they were mere prototypes, and prototypes are inevitably crude. In the same way, descriptions of evolution sometimes imply that the first land plants, say, somehow
knew
that their descendants would be vines and roses, redwoods and oaks, and saw themselves as a rehearsal. Again, the notion was that the course of evolution had been prescribed—or, as the Muslims say, “It is written.”

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 complex 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.