by Colin Tudge
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 moulds 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 protozoans 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 moulds, 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 non-professionals, 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 outwards). 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 ancestors 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 round 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
Tree ferns once abounded. Some, like this Dicksonia, are still with us
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 Moslems 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 10 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 ape-like, 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.
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 behaviour. 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 fish, 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 that they encou
nter 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 I: 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 relay 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 round and round and round: 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 out-gassing vents in the depths of the great oceans, give some idea of what it was 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 hyper-reactive gas, oxygen, that would have laid our earliest ancestors flat.
TRANSFORMATION 2: ORGANISMS
Life today is not a continuous slime. For at least 3 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 favoured the bits that were not only more efficient, but which 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 archaes (pronounced ‘arkeys’ – 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
Bacteria compared with us (or indeed with mushrooms or seaweeds or flowering plants) 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 which 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 12. For the eukaryotic cell evolved as a coalition of bacteria and archaes. Broadly speaking, the cytoplasm seems to have originated as an archae. Either this ancient archae then engulfed some of the bacteria around it, and/or the bacteria invaded it. Either way, 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 Origin of Species the philosophe
r 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 backwards to embrace Tennyson and forwards to Spencer, is commonly perceived these days as an exercise in strong bashing 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 archaes which hitherto had led separate lives (and others are probably involved, besides the proteobacteria and cyanobacteria). Over the past 2 billion years or so 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 mêlée 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 1 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.)