Читать книгу Life on Earth - David Attenborough, Sir David Attenborough - Страница 5

PROLOGUE

I still recall, with great clarity, the very first time I went to the tropics. Stepping out of the plane and into the muggy, perfumed air of West Africa was like walking into a steam laundry. Moisture hung in the atmosphere so heavily that my skin and shirt were soaked within minutes. A hedge of hibiscus bordered the airport buildings. Sunbirds, glittering with green and blue iridescence, played around it, darting from one scarlet blossom to another, hanging on beating wings as they probed for nectar. Only after I had watched them for some time did I notice, clasping a branch within the hedge, a chameleon, motionless except for its goggling eyes, which swivelled to follow every passing insect. Beside the hedge, I trod on what appeared to be grass. To my astonishment, the leaflets immediately folded themselves flat against the stem, transforming green fronds into apparently bare twigs. It was sensitive mimosa. Beyond lay a ditch covered with floating plants. In the spaces between them, the black water rippled with fish, and over the leaves walked a chestnut-coloured bird, lifting its long-toed feet with the exaggerated care of a man in snowshoes. Wherever I looked, I found a prodigality of pattern and colour for which I was quite unprepared. It was a revelation of the splendour and fecundity of the natural world from which I have never recovered.

Since then, I have managed, one way or another, to get back to the tropics many times. Usually my purpose has been to make a film about some corner of that infinitely varied world. So I have had the luck to find and film rare creatures that few outsiders have seen in the wild, and to gaze on some of the most marvellous spectacles that the wild places of the world have to offer – a tree full of displaying birds of paradise in New Guinea, giant lemurs leaping through the forest of Madagascar, the biggest lizards in the world prowling, like dragons, through the jungle of a tiny island in Indonesia.

Initially, the films we made tried to document the lives of particular animals showing how each found its food, defended itself and courted, and the ways in which it fitted into the community of animals and plants around it. But then the idea formed in my mind that a group of us might make a series of films that portrayed animals in a slightly different way. Our subject would be not only natural history in the sense that those two words are normally used, but the history of nature. We would try to survey the whole animal kingdom and consider each great group of animals in the light of the part it has played in the long drama of life from its beginnings until today. This book originated from the three years of travelling and research that went into the making of those films.

The condensation of three thousand million years of history into three hundred or so pages, and the description of a group of animals containing tens of thousands of species within one chapter, compels vast omissions. My method was to try to perceive the single most significant thread in the history of a group and then concentrate on tracing that, resolutely ignoring other issues, no matter how enticing they may seem.

This, however, risks imposing an appearance of purpose on the animal kingdom that does not exist in reality. Darwin demonstrated that the driving force of evolution comes from the accumulation, over countless generations, of chance genetic changes sifted by the rigours of natural selection. In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way – that fish wanted to climb on to dry land and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds. There is no objective evidence of anything of the kind and I have endeavoured, while describing these processes in a reasonably succinct way, not to use any phrases that might suggest otherwise.

To a surprising degree, nearly all the major events in this history can be told using living animals to represent the ancestral creatures which were the actual protagonists. The lungfish today shows how lungs may have developed; the mouse deer represents the first hoofed mammals that browsed in the forests of fifty million years ago. But misunderstandings can come unless the nature of this impersonation is made quite clear. In rare instances, a living species seems to be identical with one whose remains are fossilised in rocks several hundred million years old. It happens to have occupied a niche in the environment that has existed unchanged for such vast periods of time and suited it so ideally that it had no cause to change. In most cases, however, living species, while they may share essential characters with their ancestors, differ from them in many ways. The lungfish and the mouse deer are fundamentally similar to their ancestors, but they are by no means identical. To underline this distinction each time with a phrase like ‘ancestral forms that closely resemble the living species’ would be unnecessarily clumsy and literal-minded, but that qualifying phrase must be taken as read whenever I have referred to an ancient creature by the name of a living one.

Since this book was first written, science of course has continued to make new discoveries that have illuminated and amplified the history of nature. New species – some living, some fossil – have been discovered that link different groups. Some discoveries have been truly sensational. Perhaps the most dramatic have been those made in China of small dinosaurs with the clearly identifiable remains of feathers covering many parts of their bodies. They have cleared up one of the great and most vehement arguments among evolutionary biologists about the origins of both flight and the birds. Another concerns the very origins of life itself. Fossils have been found not only in Australia but in many other parts of the world, including the Avalon peninsula in northern Canada where a seabed thronged with all kinds of hitherto unknown organisms and dating from around 565 million years ago has been preserved with astounding perfection. All these advances in knowledge and many more will be mentioned in the appropriate places in the text that follows.

Spriggina fossil, leaf-like impression on sandstone, Ediacaran era (575 million years old), Australia.

One whole new branch of science has in recent years spread a great deal of light on the history of life – molecular genetics. Nearly a century after the publication of Darwin’s book on evolution by natural selection, On the Origin of Species, Crick and Watson described the structure of deoxyribonucleic acid – DNA for short – the molecule that carries the genetic blueprint from which another individual animal can be developed. This explained the mechanism by which physical characteristics are passed from one generation to the next.

The first organism to have its version completely deciphered was a small worm. Once that was done, the next great target was to analyse human DNA. That took many years of both international competition and cooperation. Today, however, it is possible to establish genetic identity of a species in a few hours using a piece of apparatus no bigger than a mobile phone. With such knowledge and techniques. all kinds of things can now be deduced – the relationship between individual species, the date in its evolutionary history at which any particular characteristic appeared, and even the precise way in which it did so. So the connections between the various groups that appear in our story can now be determined and statements about ancestry made with real confidence. Such new insights will be described in this new edition in their appropriate places in the pages that follow.

I have used familiar English names rather than scientific Latin ones so that when an animal makes its appearance in this history, it is quickly recognised for what it is. Those who wish to discover more about it in more technical books will find its scientific name in the index. For the most part, I have expressed age in absolute terms of millions of years rather than use the adjectival names of periods coined by classical geology. Lastly, I have made no reference by name to those many scientists whose work has provided the facts and theories on which the following pages are based. This has been done solely to try to maintain clarity in the narrative. I intend no minimisation of the debt owed to them by all of us who take pleasure in watching and thinking about animals. They and their researches have provided us with that most valuable of insights, the ability to perceive the continuity of nature in all its manifestations and to recognise our place within it.