Kitabı oxu: «Wonders of Life»

The Nobel Prize-winning physicist Richard Feynman used to tell a story about an artist friend who challenged him about the beauty of a flower. ‘You as a scientist, oh, take this all apart and it becomes a dull thing’ he said. Feynman, after describing his friend as ‘kind of nutty’, went on to explain that whilst the aesthetic beauty of nature is surely open to everyone, albeit not in quite as refined a way, the world becomes more beautiful as our understanding deepens.

The flower is made up of cells, single units with identical genes. Hidden within are a multitude of biochemical machines, each highly specialised to perform complex tasks that keep the cell alive. Some contain chloroplasts, once free-living bacteria, co-opted into capturing light from the Sun and using it to assemble food from carbon dioxide and water. There are mitochondria, factories that pump protons up energy ‘waterfalls’ and insert organic waterwheels into the ensuing cascade to assemble ATP molecules – the universal batteries of life. And there is DNA, a molecule with a code embedded in its structure that carries the instructions to assemble the flower, but also contains fragments of the story of the origin and evolution of all life on Earth, from its beginnings 3.8 billion years ago to the endless forms most beautiful that have transformed a once-sterile world into the grandest possible expression of the laws of nature. This is beauty way beyond the aesthetic that, as Feynman concluded, ‘only adds to the excitement and mystery and the awe of a flower. It only adds; I don’t understand how it subtracts.’

I confess that, when we began thinking about filming Wonders of Life, my knowledge of biology was a little dated – I gave it up as an academic subject in 1984. As I recall, the idea for the series came from an off-hand reference I made to Andrew Cohen about a little book I had read as a physics undergraduate.

What is Life? is an account of a series of lectures given by the physicist Erwin Schrödinger, published in 1944. Schrödinger was a Nobel Prize winner, one of the founders of quantum theory, and a deep and high-precision thinker. In the book, he poses a simple yet profound question: ‘How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?’ This question is beautifully phrased. Most important is the word ‘How’ at the beginning. Without this word, the question is metaphysical, in the sense that the answer may be ‘No’ – a complete understanding of life may be forever beyond the natural sciences because there is something inherently supernatural about it. The word ‘How’ transforms it, and provides a significant and important insight into the mind of a scientist. Let us find out, by studying nature, developing theories and testing those theories against our observations of the living world, how life can be fully explained by the laws of physics and chemistry, as it surely must be. This, I submit, is an excellent description of the science of biology.

Wonders of Life might be best described as a series exploring our current understanding of Schrodinger’s ‘How’ question. I enjoyed making the films immensely, because virtually everything in them was discovered after I gave up biology in 1984. The rate of discovery, driven by powerful new experimental techniques such as the exponentially increasing ease and decreasing cost of DNA sequencing, is quite dazzling and, Higgs Boson notwithstanding, I might be convinced that the 21st century has already become the century of the Life Sciences; but only ‘might’.

A truly wonderful exception to the modernity is Darwin’s theory of evolution by natural selection, published in November 1859 and spectacularly verified as a conceptual framework to understand the diversity and complexity of life on Earth. To understand Darwin’s genius, look out of your window at the living world. Unless you are in the high Atacama Desert, you will surely see a living world of tremendous complexity. Even a blade of grass should be seen through Feynman’s reductionist prism as a magnificent structure. On its own, it is a wonder, but viewed in isolation its complexity and very existence is inexplicable. Darwin’s genius was to see that the existence of something as magnificent as a blade of grass can be understood, but only in the context of its interaction with other living things and, crucially, its evolutionary history. A physicist might say it is a four-dimensional structure, with both spatial and temporal extent, and it is simply impossible to comprehend the existence of such a structure in a universe governed by the simple laws of physics if its history is ignored.

And whilst you are contemplating the humble majesty of a blade of grass, with a spatial extent of a few centimetres but stretching back in the temporal direction for almost a third of the age of the Universe, pause for a moment to consider the viewer, because what is true for the blade of grass is also true for you. You share the same basic biochemistry, all the way down to the details of proton waterfalls and ATP, and much of the same genetic history, carefully documented in your DNA. This is because you share a common ancestor. You are related. You were once the same.

‘How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?’

Erwin Schrödinger

I suppose this is a most difficult thing to accept. The human condition seems special; our conscious experience feels totally divorced from the mechanistic world of atoms and forces, and perhaps even from the ‘lower forms’ of life. If there is a central argument through the five films and chapters in Wonders of Life, it is that this feeling is an emergent illusion created by the sheer complexity of our arrangement of atoms. It must be, because the fundamental similarities between all living things outweigh the differences. If an alien biochemist had only two cells from Earth, one from a blade of grass and one from a human being, it would be immediately obvious that the cells come from the same planet, and are intimately related. If that sounds unbelievable, then this book is an attempt to convince you otherwise.

I write this in full appreciation of the so-called controversy surrounding Darwin’s theory of evolution by natural selection. My original aim was to avoid the matter entirely, because I think there are no intellectually interesting issues raised in such a ‘debate’. But during the filming of this series I developed a deep irritation with the intellectual vacuity of those who actively seek to deny the reality of evolution and the science of biology in general. So empty is such a position, in the face of evidence collected over centuries, that it can only be politically motivated; there is not a hint of reason in it. And more than that, taking such a position closes the mind to the most wonderful story, and this is a tragedy for those who choose it, or worse, are forced into it through deficient teaching.

As someone who thinks about religion very little – I reject the label atheist because defining me in terms of the things I don’t believe would require an infinite list of nouns – I see no necessary contradiction between religion and science. By which I mean that if I were a deist, I would claim no better example of the skill and ingenuity of The Creator than in the laws of nature that allowed for the magnificent story of the origin and evolution of life on Earth, and their overwhelmingly beautiful expression in our tree of life. I am not a deist, philosopher or theologian, so I will make no further comment on the origin of the laws of nature that permitted life to evolve. I simply don’t know; perhaps someday we will find out. But be in no doubt that laws they are, and Darwin’s theory of evolution by natural selection is as precise and well tested as Einstein’s theories of relativity.

If this sounds a little strong, then perhaps it reveals my genuine excitement in learning about the sheer explanatory power of Darwin’s theory when coupled with recent advances in biochemistry and genetics. Modern biology is close, in my view, to answering Schrodinger’s ‘How’ question. There are unknowns to be sure, which is what makes the subject of these films doubly exciting. Some parts are speculative, but that is nothing to be ashamed of in science. Indeed, all science is provisional. When observations of nature contradict a theory, no matter how revered, ancient or popular, the theory will be unceremoniously and joyously ditched, and the search for a more accurate theory will be redoubled. The magnificent thing about Darwin’s explanation of the origin of species is that it has survived over a hundred and fifty years of precision observations, and in that it has outlasted Newton’s law of universal gravitation.

My favourite moment in the series is the final scene of the final film, which unusually, was filmed on our final evening; television shows are rarely made in chronological order. We found a tiny rocky island off the coast of northern Madagascar, no bigger than the average suburban garden, isolated in the warm waters of the Mozambique Channel. The idea was to sit down and chat about the experience of making the series, and film the result. I won’t tell you what I thought and said, because that should wait until the end of the book. But I do want to say one thing here in the introduction. I recall a conversation in March 2009, just before we started filming Wonders of the Solar System. Andrew, my co-author and executive producer, said that we would have achieved our goal if those who watched never again looked at the night sky in quite the same way. This is in the spirit of Feynman’s flower. Deeper understanding confers that most precious thing – wonder. A sky filled with tiny, twinkling lights is one thing, but a sky filled with other worlds is quite another. I have known this for virtually all my life, because I have always been an astronomer at heart. Perched on my island, thinking about what to say, I realised that I now felt precisely the same about a single blade of grass.

On Christmas Eve 1968, Frank Borman, Jim Lovell and William Anders became the first humans in history to lose sight of their home planet as they orbited the Moon on board Apollo 8. As Borman looked into the crystal dark, pitted by the faint light of a billion worlds untarnished by atmospheric gases, framed by a virgin lunar surface unseen since its formation 4.5 billion years ago, he described his universe without Earth as a ‘vast, lonely, forbidding expanse of nothing’. On the ninth orbit, the crew made a scheduled live television broadcast in which they chose to read the Genesis creation story back across the quarter of a million miles to Earth.

‘We are now approaching lunar sunrise and, for all the people back on Earth, the crew of Apollo 8 has a message that we would like to send to you:

In the beginning God created the heavens and the Earth.

And the Earth was without form, and void; and darkness was upon the face of the deep.’

The act of reading from the Bible proved controversial, and was challenged in court as a violation of the 1st amendment of the United States Constitution, which prevents the promotion of religion by the federal government, of which NASA is a part. The Supreme Court dismissed the case, on the grounds that it had no jurisdiction in lunar orbit.

While the Genesis story is a myth, I have always found this broadcast moving; not merely because the King James version of the Bible contains some of the greatest prose ever written in the English language, but because it speaks to an ancient, resonant desire to understand our origin and the origin of our home. Why is the Earth a living oasis amid, as far as anyone can tell, a forbidding expanse of nothing? What is special about our pale blue anomaly of a world that makes it home to life?

These questions are complex, and we do not yet have all the answers, but there is a scientific consensus on at least some of the ingredients a planet requires to allow the emergence of life and the evolution of complex organisms capable of taking their first, faltering steps into a wider Universe. Many of those ingredients are common throughout the Solar System and beyond, but we have as yet no evidence for life, simple or complex, beyond Earth. That may be because the emergence of living things required a significant slice of luck and billions of years of relative stability; spacecraft builders may be a rare and precious commodity.

This thought may have been adrift somewhere in Frank Borman’s consciousness, catalysed by his feelings of isolation 400,000 km from home, when he ended the 1968 Christmas broadcast with a phrase I have always found overpowering in its simplicity and depth of meaning. To me, it was an instinctive plea to all of us to value our home – the absolutely necessary platform for the continued existence of, just possibly, the only living civilisation in the Universe:

‘And, from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.’

‘Earth rise’, first observed from Apollo 11 in 1969, gave us a totally new perspective on the planet we call home.

In early September each year, monarch butterflies gather in their millions east of the Rocky Mountains before migrating south to the evergreen forests of central Mexico.

RETURN OF THE KING

With its vivid orange colour and beautiful markings, the monarch butterfly (Danaus plexippus) is a striking example of the simple aesthetic beauty of life. But as is so often the case in the natural world, the superficial beauty of these butterflies is immeasurably enhanced by a deeper scientific understanding of their life cycle and biochemistry, and the reasons for their form and function.

Each year, as autumn approaches across Canada and the northern United States, millions of monarch butterflies begin preparations for an arduous expedition. To survive the harsh northern winter, they embark on one of nature’s great migrations, travelling up to 4,000 km to warmer domains in the south. It is a vast distance for such a small and seemingly fragile creature to travel, and requires the birth of a special generation of butterflies. An average adult monarch has a life span of little more than four weeks, but, when faced with the journey south, a ‘methuselah generation’ emerges; a generation that lives nearly ten times longer than its parents and grandparents.

Living for up to eight months, these butterflies carry with them the privilege of a longer life and the responsibility of carrying their genes through to the following year. As autumn begins in the forests, fields and meadows of the north, preparation starts for travel. The fading of the northern Sun, a result of Earth’s journey around the Sun coupled with the 23-degree tilt of its axis, causes temperatures to fall and food to become scarce. By early September, the young butterflies sense the shortening days and begin to gorge themselves on nectar, laying down extra layers of fat to increase their resilience. When the temperature approaches the very limits of their tolerance, they take flight. This is no random journey south. Covering up to 100 km a day, half a billion monarchs head towards a very specific location. None of them has travelled the route before, yet their destination has remained the same for thousands of years.

Of the many possible solutions to this annual challenge, the monarch butterflies have evolved into skilled navigators, using time and a star as their guide. From their starting point east of the Rocky Mountains, they journey across the great plains of the central United States into the damp humidity of the south. Along the way they face the same dangers as all long-distance travellers; illness and infection, bad weather and storms are a constant danger, and predatory birds will pick off thousands before they come close to completing their annual voyage.

But every year, despite the daunting distance and difficulties, millions of monarchs arrive in a single small area of evergreen forest in the heart of central Mexico. Populations of monarchs that were living west of the Rockies will have made a similar, though shorter, voyage to safety in southern California.

USING SPECTRAL GRADIENTS TO FIND THE POSITION OF THE SUN: Long wavelengths (green light) dominate the solar hemisphere, and shorter wavelengths (violet) dominate the anti-solar hemisphere.

The monarchs navigate like eighteenth-century explorers, using the position of the Sun in the sky and an internal clock to guide them. Taking a southerly bearing using the Sun is simple if you know the time. At noon in the northern hemisphere, the Sun will always be due south. This can be taken as a definition of noon. You can take a southerly bearing at other times of day if you have a watch. Point the hour hand at the Sun, and the line halfway between the hour hand and the 12 o’clock mark will point due south. The monarchs use a sophisticated version of this technique – known as a time-compensated Sun compass – to maintain their southerly orientation during their migration.

This magnified image of the head of a butterfly clearly shows its long, segmented antennae, its two segmented eyes, and its tightly coiled proboscis – the three most important sensory organs.

The butterflies measure the position of the Sun using their sophisticated eyes, which can detect the polarisation of sunlight, enabling them to ‘see’ the position of the Sun, even through cloud. They are also thought to use ‘spectral gradients’, whereby the precise mixture of colours in any given patch of sky depends on how close it is to the Sun. This is due to the way that different wavelengths of sunlight scatter in the atmosphere, an effect that is most familiar in the reddening of the sky at sunset and sunrise.

The nature of the monarch’s clock is more elusive. Biological clocks are ubiquitous in nature and thought to be a very ancient evolutionary invention. Circadian rhythms, which require the beating of an internal biological clock, are found in every corner of the biosphere, from the most complex of mammals to the simplest of bacteria. It is possible that biological clocks could have emerged as a form of protection against the destructive effects of the Sun’s radiation. An organism’s DNA is most exposed to damage at the point of replication, so restricting cell division to the hours of darkness would have been advantageous. This requires a clock that is synchronised to the rotation of the Earth.

Until recently it was assumed that, in common with other animals, the monarch’s clock must reside in the brain. But an experiment conducted by neurobiologists at the University of Massachusetts Medical School in 2009 revealed that it is instead located in the delicate structure of the antennae. The reason for this unusual location is not known. Timing information from the antenna clock is combined with information on solar position from the eyes in dedicated regions deep within the butterflies’ tiny brains, and this allows them to maintain a southerly bearing on their journey to central Mexico.

For the next five months, a handful of Mexican valleys are home to a billion butterflies, clustering on the firs in such numbers that the forests are painted with a magnificent orange glow. The monarch migration is a powerful example of the way that an organism’s home is not a fixed place, but rather a set of conditions that enable it to survive. If those conditions change, it may be necessary to move.

The monarch is an evocative example of a deep truth in biology. The form and function of an animal cannot be understood in isolation. The monarch’s behaviour and biochemistry are intimately connected with its habitat, the behaviour of countless other animals and plants, and the constantly shifting seasons driven by the dynamics of the Solar System. I find it simultaneously trivial and wonderful to observe that there would be no monarch butterflies as we know them if our planet’s spin axis were not tilted; there would be no seasons, and no evolutionary imperative for migrations. The reason for the tilt is undoubtedly pure chance – a relic of our planet’s formation and history stretching back over 4.5 billion years. Jupiter and Mercury have virtually no tilt, while Uranus rotates on its side.

This poses a series of interesting questions: What are the factors that make Earth a home to such a bewilderingly rich and complex ecosystem? What is the minimal set of ingredients necessary for life to evolve, and how widespread are these ingredients in the Universe beyond Earth? Is the emergence of complex living things such as monarch butterflies, fir trees and human beings an inevitable consequence of the laws of physics, or does it rely on a home whose existence is so improbable that Earth and its living ecosystem is a rare, even unique, corner of the Milky Way galaxy, itself one of billions of galaxies in the observable Universe?

The behaviour and biochemistry of monarch butterflies cannot be understood in isolation either from their habitat or from the shifting seasons.

A VERY SPECIAL HOME

It is difficult to do justice in a few short paragraphs to Mexico – or, as it is more correctly known, the United Mexican States. An intense and colourful country of contradictions, it is both welcoming yet occasionally frightening, peaceful yet troubled. It has a striking veneer of colonial architecture and customs, but the magnificent architecture of its great indigenous civilisations is intact and imposing, and their ancient mythology makes a vibrant contribution to twenty-first-century global culture. What schoolchild isn’t fascinated by the Aztecs, and which New Age conspiracy theorist doesn’t read infinitely too much into the Mayans’ fascination with the creation of complex and far-reaching calendars?

Physically, Mexico covers almost 2 million sq km and is home to 112 million people. Bordering the United States of America to the north, the Pacific Ocean to the south and west, the Gulf of Mexico to the east, and Guatemala, Belize and the Caribbean Sea to the southeast, it is a land of tremendous geological and climatic variation – from lowland rainforests to pine savannahs; from fertile grasslands to high volcanic mountain ranges. Its position – straddling the Tropic of Cancer and bounded by two of the world’s great oceans – also makes it one of the most biodiverse countries on Earth. Even though it covers only 1 per cent of the land area of our planet, it is home to over 200,000 different species – at the last count, 10 per cent of Earth’s bank of life. There are 707 species of reptiles, 438 species of mammals, 290 species of amphibians and over 26,000 species of flora. This is why we chose Mexico to tell the story of the ingredients that make our world such a comfortable home for life.